Flood Basalts, Basalt Floods or Topless Bushvelds? Lunar

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 11 PAGES 1545–1651 2000

Flood Basalts, Basalt Floods or ToplessBushvelds? Lunar Petrogenesis Revisited

M. J. O’HARA∗DEPARTMENT OF EARTH SCIENCES, CARDIFF UNIVERSITY, PO BOX 914, CARDIFF CF1 3YE, UK

RECEIVED SEPTEMBER 2, 1999; REVISED TYPESCRIPT ACCEPTED MARCH 23, 2000

primary magmas. Mare basalt hand-specimen and pyroclastic glassThere is a conspicuous dichotomy in the conventional model ofbead compositions do not, however, display the required crystallizationlunar petrogenesis between the total intra-crustal differentiationsequence and cannot represent the required primary melt compositions.postulated for the products of feldspathic volcanism in the lunarThe true erupted lava compositions which gave rise to the regolithhighlands and the near absence of differentiation postulated for thecompositions across all the maria are much more feldspathic thanproducts of mare volcanism. Both the cumulate mantle model, andthe majority of large hand specimens and, in common with basaltsthe selenotherm postulated to accompany genesis of alleged ‘primary’on other planets, they are close to low-pressure plagioclase-saturatedmare magmas by remelting of those cumulates, imply supra-adiabaticcotectic residual liquids which have evolved by removal of gabbrosthermal gradients in near-solidus materials throughout the lunarin crustal magma chambers, or perhaps in giant lava lakes akin tomantle 4·3–3·2 Ga ago. This should have resulted in vigoroustopless Bushveld complexes. Any further debate could be resolvedconvective motion, which has not occurred. There is no positiveby a 100 m drill core in a few mare locations. Field provenanceeuropium anomaly in the average lunar highland crust. That crustof samples from Mars, a planet half covered by flood basalts andcannot, therefore, have formed by plagioclase flotation from a lunarproducts of central volcanoes, will be little better than for those frommagma ocean, for which there is no other requirement. There is nothe Moon. It will be important to encourage multiple workingnegative europium anomaly in the average mantle to be inherited byhypotheses, rather than to rush to a consensus.later mare basalts. Other rocky bodies of lunar size in the Solar

System have accreted at rates that allowed incorporation of plentyof volatiles and without forming global magma oceans. Partialmelting in the presence of water, followed by near-surface fractionationand volatile losses can explain the feldspathic character, high

KEY WORDS: lunar; basalt; highland; magma ocean; europiumincompatible element concentrations and lack of Eu anomaly in thelunar highlands. Volcanic eruption on the Moon must have beenaccompanied by selective volatilization losses of sodium, sulphurand other elements similar to the process seen on Io, which canaccount for the major differences between terrestrial and lunar

INTRODUCTIONbasalts. Siderophile element depletion in lunar lavas may reflectimmiscible sulphide liquid and metal separation, rather than global The background to this paper has been presented byimpoverishment in such elements, and large ore bodies may have O’Hara (2000) together with the substance of the ar-formed close to the lunar surface. Mare basalt volcanism appears gument in extended abstract form. The material ofto have been a protracted, low magma productivity event with few this paper is presented in a condensed narrative form,similarities to terrestrial ocean-floor, ocean-island, continental flood accompanied by detailed notes referenced throughoutbasalt or komatiite volcanism. At low pressure the crystallization by numbers in parentheses, and by references. Theof plagioclase well before pyroxene typifies those terrestrial mid- arguments to be presented are complex, with items andocean ridge basalt, ocean-island basalt and continental flood basalt groups of information utilized in, or relevant to, more

than one thread.magmas. A similar sequence is demanded of the postulated lunar

∗e-mail: [email protected] Oxford University Press 2000

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 11 NOVEMBER 2000

Evolving lunar volcanism 1553CONTENTS AND ROUTE MAPFeldspathic contribution from early mareLUNAR PETROGENESIS 1969–1999 1547

component also 1553Sulphur, carbon and their gases 1554

Petrogenetic background 1547 Io pyroclastic volcanism driven by theseConcepts in basalt petrogenesis 1547 gases 1554

Terrestrial basalts 1950–1980 1547 Lunar pyroclastic volcanism guaranteed 1554Terrestrial basalts 1980–1999 1547 Sulphide saturation and siderophileLunar petrogenesis—stasis and dichotomy 1547 depletion 1554

Role of trace element geochemistry 1547 Cerium anomalies and lunar oxygen fugacities 1554Equilibrium, not perfect fractional processes Eruption style on a small planet 1554

dominate 1547 Fire-fountaining, frothing and volatilizationPossible role for eutectoid PFC 1547 losses 1554Imperfect fractionation processes 1547 Effects of sodium loss 1554Model dependence and apparent Asteroidal basalts 1554

distribution coefficient 1548 Plagioclase saturation in lunar basalts 1555How much melting? 1548 No negative Eu anomaly in the lunar

Trace element requirement 1548 mantle 1555Major element requirement 1548 Imposed, not inherited, negative EuIntegrated melting regimes 1548 anomalies in mare basalts 1555

Simulating and exceeding effects of Low-F, moderate-P primary magmasEPM–EPC in magma chamber products 1548 precipitate plagioclase before pyroxene 1555Integrated crystallization simulates EPC 1548 MORB, CFB crystallize plagioclase early 1555Magma recharge and discharge 1548 Incorrect low-P phase equilibria of allegedSmall packet crystallization 1548 mare primary magmas 1555Extremes of magma chamber processes 1548 Alleged primary magmas do not display

Difficulties with primary magma hypotheses 1548 required moderate-P phase equilibria 1555Physical problems 1548 Lunar primary magmas and absence ofPlagioclase-saturated low-pressure cotectic tectonic deformation 1555

character 1549 Global magma ocean cumulates unstablePrimary magmas should be picritic 1550 from birth 1555Detection of superimposed melting and Supra-adiabatic thermal gradient still

crystallization 1550 required 1 Ga later 1555Thermal inertia problems 1550 Average compositions of mare basalts 1556

Caution in the use of trace element Quench crystal sinking 1556geochemistry 1550 Flow thicknesses 1557

Persistently feldspathic regolith compositions 1557Composition bias in hand specimens 1557Extra-terrestrial petrogenesis 1550Low-P plagioclase-saturated cotectic averageIrony of the lunar mafic hand specimens 1550

compositions 1557Solar System exploration post-Apollo 1550Misfits in experimental data 1557Accepted lunar petrogenesis 1550Petrogenesis of lunar high-titanium basalt 1557Volatiles plentiful in asteroids and otherCognate crustal cumulates 1560Moon-sized bodies 1550

Current Io volcanism a model forPrecambrian lunar volcanism 1551 Comparative petrogenesis 1561

Ancient feldspathic crust on at least three Meantime, back on Earth . . . 1561bodies 1551 Poor terrestrial controls 1561

Global melting or anorthosite flotation Magma production in large impacts 1561unnecessary elsewhere 1551 Bushveld complex a model for mare filling? 1563

Lunar highland crust 1551 Komatiite–greenstone analogies 1563Petrological composition 1551 Realities of mare basin filling 1563No positive europium anomaly 1551 Dubious continental flood basalt analogies 1563No plagioclase flotation 1551 Flood basalts and lava lakes 1563No lunar magma ocean 1553 An ocean-island basalt or mid-plate

volcanism analogy? 1563Wet mantle yields feldspathic melt 1553

1546

O’HARA LUNAR PETROGENESIS REVISITED

Back to the Moon 1563 interpreted as clast-free impact melts which formed fromLimited lunar field controls 1563 rehomogenized highland target materials. A further di-Gabbros at shallow depth in the maria? 1564 chotomy appears in the treatment of remote-sensing dataPyroclastic flow units 1564 (61), where lunar highland regoliths are assumed to bePyroclastic bead compositions 1564 representative of the chemistry of 40–120 km of under-

Finale 1564 lying crust, yet mare regoliths, which uniformly indicateMars, science and politics 1564 feldspathic average basaltic compositions, are assumedBasalt petrogenesis—a Solar System to be unrepresentative of even the topmost flow units

round-up 1564 (107–114).Anomaly of the established lunar

petrogenetic model 1565Predictions 1565

Role of trace element geochemistryNotes 1566Equilibrium, not perfect fractional processes dominate. Inter-pretations of trace element geochemistry (7–10, 13),however, apparently provided strong support for a

LUNAR PETROGENESIS 1969–1999 plethora of primary magmas. Erupted basalt sequencesPetrogenetic background have distinctive geochemical features (7), which are

incompatible (8) with those expected in products ofConcepts in basalt petrogenesisperfect fractional crystallization (PFC), despite theTerrestrial basalts 1950–1980. The lunar surface is com-evidence of a close approach to PFC seen in someposed of igneous rocks and their impact metamorphosedlarge peridotite–gabbro layered intrusion complexes.and impact weathered derivatives. Discussion of theirEquilibrium between liquid and crystals at low andorigins cannot be divorced from a consideration of thevariable mass fractions of melt, on the other hand,evolution of ideas in igneous petrogenesis as a whole (0).produces effects which match the gross variation inThe Apollo lunar samples were received into a scientificbasalts world-wide. All workers accepted partial meltingcommunity which had been brought up in the almostof the upper mantle as the ultimate source of mostunchallenged belief that the several types of abundantbasalts and the melting process could be modelled (9)terrestrial basalts were primary magmas (0–2). This com-as equilibrium partial melting (EPM). Equilibriummunity was ready to accept that eruption of unmodifiedpartial crystallization (EPC) is a process equally capableprimary partial melts of the mantle was a commonplaceof explaining the geochemical effects (10) but could beevent on the Moon. Contamination, assimilation andrejected as an explanation for reasons which still appearhybridization, which had been extensively explored assound. Appreciation that the actual process in themechanisms in igneous evolution before 1950 (1), wereupper mantle might approach perfect fractional partialbecoming less fashionable (2).melting (PFM) did not undermine these views (11, 12),Terrestrial basalts 1980–1999. Two assertions that hadbecause the accumulated mixed liquid products ofunderpinned the view that primary magmas aboundedperfect fractional partial melting (APFM) share moston Earth were that the common basalts were of greatgeochemical characteristics with the liquids of EPM oruniformity and that this feature would not survive variableEPC [the residues of PFM should, however, look veryamounts of partial crystallization. Both assertions havedifferent from most natural rocks (13) because of thebeen shown to be untrue (0, 5, 26–28) and the problemsanticipated extreme depletion of highly incompatibleattending both the definition (6) and eruption (31–34) ofelements]. Was it necessary to delve further into basaltprimary magmas have been appreciated.petrogenesis (14)?Lunar petrogenesis—stasis and dichotomy. In striking contrast

Possible role for eutectoid PFC. One of the distinctiveto the situation in terrestrial basalt petrogenesis, thegeneral features of EPM and EPC at low mass fractioninterpretation of lunar petrogenesis which was reachedof liquid is wide variation in incompatible trace elementin 1969–1970 (3) has changed little in the ensuing 30concentration with minimal change in major elementyears and displays a striking dichotomy (4). Almost everyconcentration in the liquid. Even perfect fractional crys-hand-specimen sample from the maria is supposed totallization can produce this effect in the special case ofbe close to a little differentiated primary magma ineutectoid crystallization (15), when the solid assemblagecomposition. By contrast, every igneous rock contributingseparating differs little in major and minor elementto the highlands is supposed to be completely differ-composition from the liquid.entiated to the point that no trace of its undifferentiated

Imperfect fractionation processes. The differences betweenparent magma remains. The only igneous compositionsclose to the lunar highland average composition are the products of PFC (8) or PFM (12, 13) and those of

1547


an equilibrium process evaporate rapidly if the process is Simulating and exceeding effects of EPM–EPC in magmaimperfect in the sense that finite rather than infinitesimal chamber productsincrements of solid or liquid are removed (16). Integrated crystallization simulates EPC. The process of melt

Model dependence and apparent distribution coefficient. The integration is not restricted, however, to the partial melt-conclusions from all modelling of trace element geo- ing process. Mixing of residual liquids from variablechemistry are wholly dependent on the use of appropriate amounts of PFC (24) yields aggregate liquids whosebulk distribution coefficients for each element. Crystal– geochemical properties approach and transcend those ofliquid distribution coefficient, d, itself may vary with melt liquids from an EPC process acting on the same parent

magma.composition or element speciation. The apparent bulkMagma recharge and discharge. Field and petrographicdistribution coefficient, dAp, i.e. that which should be

evidence establish the importance of magma rechargeutilized to obtain a satisfactory description of a processand discharge during partial crystallization processesor relationship in terms of a simple equilibrium (batch)(25). Geochemical evolution in a periodically recharged,model, can be greatly modified from the anticipatedperiodically tapped, continuously crystallized (RTXC)values of the simple crystal–liquid distribution coefficient,magma chamber (26) spans the range between PFC andd, by a number of factors. These include (a) changingEPC. The process facilitates contamination, imposes low-mutual solubility of crystalline phases at the site andpressure cotectic character on a body of liquid alreadytemperature of melting, and (b) zone refining, magmacollected close to the site of eruption, buffers outputchromatography or polybaric crystallization effects dur-composition against short-term fluctuations in any of theing magma ascent (18). Apparent bulk distribution co-inputs or the process parameters, and combines this withefficient may further vary because of (c) forceda simulation of the geochemical effects of EPC or EPMprecipitation of exotic phases on magma arrival andat low mass fractions of liquid. The ratios of recharge tomixing, (d) magma diagenesis of the growing cumulatedischarge and crystallization are the controlling factors

pile, (e) diffusive differentiation of the magma (19), and in this simulation. The feedstock for the chemical vari-(f ) trapping of melt in the cumulates (20) or residues (20, ations in the pseudo-equilibrium process (i.e. the bulk21). Above all, the apparent distribution coefficient is composition which appears to be undergoing the EPMheavily dependent on the choice of a relevant physical or EPC process) is, however, the average total input tomodel (17, and Fig. 1). the magma chamber including contaminants, not some

embarrassingly fertile upper-mantle source (27). Pondingand partial crystallization of new input magma may occur

How much melting? before mixing (28).Small packet crystallization. Recognition that large bodiesTrace element requirement. The inference of small mass frac-

of magma may solidify by repetitive partial crystallizationtions of melt extraction in terrestrial and lunar basaltof small magma batches (29), with mingling of the residualpetrogenesis is derived from one interpretation of theliquid back into the larger pool (SPC), introduced ad-trace element chemistry (7–13).ditional complexities. When combined with a variety ofMajor element requirement. The major element variationpartial crystallization–melt aggregation models in thein terrestrial peridotite suites (22), however, requires thatsmall packets and the whole inserted as the partiallarge mass fractions (>0·25) of partial melt have beencrystallization process in an RTXC magma chamberremoved. Few natural upper-mantle peridotite samplesstartling results can be obtained (30).are sufficiently low in mg-number, and rich enough in

Extremes of magma chamber processes. Concentrations ofincompatible trace elements, to support a single-stagehighly incompatible elements may be doubled in liquid

EPM origin for common terrestrial basalts (see 47) even products of SPC–RTXC magma chambers whereas theat low mass fractions of melting. concentration of a highly compatible element such as Ni

Integrated melting regimes. This contradiction between (d >10), although always decreasing in concentrationtrace and major element inferences was partly resolved relative to the parent magma, may achieve values asby recognition that partial melts must be integrated from great as four times that expected in an EPC process; 400regimes with high mass fractions of melting in the centre, times that expected in a PFC process (30)—and all thisbut in which small mass fraction melts contributed from with greatly enhanced discrimination between in-the periphery carry the dominant trace element signal compatible elements in a process whose cumulates would(23). Integrated melting regimes logically invalidate many everywhere be seen to be the product of local PFC!predictions about mantle source regions based on phase

Difficulties with primary magma hypothesesequilibria of a candidate primary magma compositionbecause there is in these cases no unique ‘primary’ liquid Physical problems. The movement of primary magmas to

the surface without modification of their compositions,which was ever in equilibrium with a specific residue.

1548


Fig. 1. Variation of apparent distribution coefficient with crystal–liquid separation process at any specific crystal–liquid distribution coefficient.The apparent bulk distribution coefficient (concentration in solid product/concentration in liquid product), which should be used in modellingan unknown process using the simple equilibrium (batch) relationship, varies by several orders of magnitude (17). Key to processes: APFC,perfect fractional crystallization, average solid/surviving liquid; APFM, perfect fractional melting, surviving solid/average liquid; EPC–EPM,equilibrium (batch) partial crystallization or melting, a straight line of 1:1 slope in this figure; IFM, imperfect fractional melting, surviving solid/average liquid; IPC, imperfect fractional crystallization, average solid/surviving liquid; IEPC, integrated linear equilibrium crystallization, averagemass fraction of surviving liquid 0·5 between limits of 0·0 and 1·0, average solid/average liquid; IPFC, integrated linear perfect fractionalcrystallization, average mass fraction of surviving liquid 0·5 between limits of 0·0 and 1·0, average solid/average liquid; SPC–EPC, small packetcrystallization, equilibrium crystallization in each packet, 0·1 mass fraction of each packet surviving as liquid, 0·5 mass fraction of original meltsurviving; SPC–PFC, small packet crystallization, perfect fractional crystallization in each packet, 0·1 mass fraction of each packet surviving asliquid, 0·5 mass fraction of original melt surviving.

posed as an industrial problem, would tax the ingenuity be broadened to picritic compositions by vesiculation(34).of large research teams of chemical and mechanical

engineers (31–34). Assimilation of cooler mantle and Plagioclase-saturated low-pressure cotectic character. The vastmajority of basalts erupted on the surface of the Earthcrust and partial crystallization are likely to be pervasive

(31). These effects are likely to be concentrated when have compositions and temperatures which conform tothose of liquids in low-pressure, plagioclase-saturatedmagmas arrive at the density contrast at the crust–mantle

boundary (32). It is doubtful whether any truly unmodified equilibria (35), an observation which admits three ex-planations: coincidence of high- and low-pressure equi-primary magma can ever be erupted (33). A window of

eruptability occurs in relatively magnesian basalt com- libria (36, 37); partial crystallization at low pressures withthe formation of crustal gabbro complexes (38, 39),positions around the condition that plagioclase is about

to start crystallizing at the liquidus, but this window could preferred in the terrestrial case; or partial melting of

1549


gabbro, troctolite or plagioclase–wehrlite at low pressures Caution in the use of trace element geochemistry(40, 41), which has been advocated for asteroid-sized Increased sophistication in modelling of igneous processesbodies with low central pressures. over the past 30 years has invalidated the facile use of

Primary magmas should be picritic. The partial melting trace element geochemistry as a definitive indicator ofproducts of likely mantle peridotites in all the terrestrial the presence or absence of low-pressure modification ofplanets should be picritic in composition (42) and they erupted basalt compositions (47, and see Fig. 1). Somemay be too dense to erupt unmodified through the combination of the field relations, petrology, major ele-crust of the Earth or Moon (43) unless assisted by ment chemistry or phase equilibria of erupted basaltsreduction of bulk density by vesiculation, an effect frequently suggests a possibility of modification by partialwhich can become influential only at low pressures. crystallization at some pressure within the crust or upperPicritic compositions also form readily from common mantle. In these circumstances, truly sympathetic con-basalt at low pressure by the accumulation of dense, sideration should be given to the possible effects of thoseearly-formed crystals in the magmas (44). Over the processes before projecting any geochemical features intolast 40 years the number of terrestrial magmas which the source region or the melting process.have been identified as picritic liquids (45) has increasedgreatly but this identification depends on possession ofthe field relations and an undisputed knowledge of the Extra-terrestrial petrogenesisaverage bulk compositions of the lava, both of which

Irony of the lunar mafic hand specimensare lacking for lunar mare basalts. Even fully accreditedLack of proper field relations, and lack of unambiguouspicritic liquids have generally undergone substantialknowledge of the average bulk compositions of the lunarpartial crystallization and some crystal accumulationmare lavas, underlies an important part of the debatesubsequent to eruption.addressed in this paper (48). It is ironic then that it hasDetection of superimposed melting and crystallization. Abeen necessary to argue over more than 40 years forcrucial point to keep in mind here, and at the end oflargely unseen picritic parental magmas on the Earthseveral previous trains of thought, is that whereas it[whose existence is still contested in mid-ocean ridgeis easy to detect the geochemical effects of a PFCbasalt (MORB), volumetrically the most important ter-process superimposed upon primary magmas developedrestrial domain of all], yet simultaneously to reject thein an EPM process, it is extremely difficult (46) topicritic samples which dominate the lunar mare hand-detect the operation of a partial crystallization processspecimen collections, as samples modified by crystalsuperimposed on the product of a partial meltingaccumulation.process, when the first process yields a liquid ap-

proximating to an EPM product of the true sourceSolar System exploration post-Apolloand the second process yields liquids which approximate

to EPC products of the EPM liquid. Accepted lunar petrogenesis. ‘Conventional’ lunar petrogenesisThermal inertia problems. A simplified interpretation of (3) is built around a Moon which was volatile depleted

a suite of samples from such a two-stage magma output from birth. It postulates the generation of a global magmamight opt for a single-stage partial melting process ocean during accretion with flotation of the anorthositicwith very small, but varying mass fractions of melting, lunar highland crust from the consolidating magmaor for an excessively enriched source region, or for ocean. It then requires the generation of mare basalts bysome compromise between the two. However, the remelting of the feldspar-depleted lunar mantle cumulatespartial melting regime is anticipated to be a large, formed by consolidation of the global magma ocean.thermally well-insulated volume (10–100 times that of After the Apollo program and the formulation of thethe derived magma) of material in which short-term ‘conventional’ interpretation of lunar petrogenesis influctuations of the melting parameters are unlikely. 1970–1971, a wealth of information, which does not yetThe environment of crustal magma chambers is much seem to have impinged on the interpretation of lunarmore susceptible to variations in thermal conditions rocks, has become available for more remote parts of thefrom one location to another, and to short-term Solar System.fluctuations in output chemistry, especially when rates Volatiles plentiful in asteroids and other Moon-sized bodies.of input and output are the critical factors in simulating Many asteroids (50) are small rocky bodies which acquiredthe appearance of small mass fractions of equilibrium plenty of volatiles during their accretion. There are nowpartial melting. The bulk of variation in basalt geo- six satellites known in the Solar System that have a rockchemistry within a single province is inherently more and metal content which would yield a roughly Moon-likely to be the result of varying partial crystallization sized body if all volatiles were removed (51). Four ofprocesses than varying conditions or compositions in these bodies retain thick ice crusts, demonstrating that

bodies of the same order of size and mass as the Moonthe source region.

1550


could, in principle, accrete with plenty of volatiles and ferroan anorthosite (FAN) component. There is alsoa substantial petrographically identified component ofwithout generating global magma oceans (52, 53 and see

77). magnesian gabbros and norites and a minor componentof petrographically identified mare basalts which are upCurrent Io volcanism a model for Precambrian lunar volcanism.

Io, a body closely comparable with the Moon in size and to 4·2 Ga in age. The relationship of Fe to Th/Tiratio, which can be obtained from remote sensing andmean density, was found 20 years ago to be the most

volcanically active body in the Solar System (54), losing calibrated by sample analysis, requires a much moresubstantial mare component to be present in the averagelarge amounts of sulphur and sodium to space. Surely

its style of violent pyroclastic silicate volcanism needs to lunar highland composition, raising a question aboutbe considered in relation to that of the Moon, yet two the possible mare parentage of some of the magnesianrecent authoritative compendiums on the Moon (Heiken gabbro–norite suite cumulates.et al., 1991; Papike et al., 1998) made no reference to Io and No positive europium anomaly. It had already been decidedpresented the ‘conventional’ view of lunar petrogenesis, that the Moon was volatile-poor from birth in the lightwhich excludes a significant role for reduction and vo- of the Apollo 11 basalt samples. A global magma oceanlatilization during eruption. in the Moon was proposed in the light of the Apollo 14

Ancient feldspathic crust on at least three bodies. Io, together highland material return because plagioclase flotationwith part of Mars, Venus and the Earth, has preserved from a large body of basaltic melt seemed the mostlittle or no early (>3·8 Ga) crust and we can learn little plausible way to generate a thick plagioclase-rich crustabout early planetary evolution from these bodies (55). in a volatile-poor body. The widely publicized largeThe Moon, Mercury and parts of Mars all developed positive Eu anomaly in the lunar highlands apparentlyand preserved an ancient, heavily cratered light-coloured supported this interpretation, but the remote-sensing dataand probably feldspathic crust (56). The early crust on confirm what has always been evident from the originalMars developed in the presence of water and other data (Figs 2 and 3). The positive Eu anomaly does notvolatiles, and appears to be of calc-alkaline and at least exist (62). This fact, well appreciated at least since 1988,partly of pyroclastic volcanic character (57). Neither the is not mentioned in either of the lunar compendiumspresence of volatiles nor the eruptive volcanism lends mentioned above. There is probably a small negative Euany support to the concept of formation of a global anomaly in the average highland composition. The bulkmagma ocean and feldspathic crust flotation during the of the rare earth elements (REE) in the lunar highlandsearly history of Mars. reside in the KREEPy component, which was apparently

Global melting or anorthosite flotation unnecessary elsewhere. excavated by the Imbrium impact and is localized in itsSome small bodies (58), particularly those of the inner vicinity. This leaves open the possibility that the highlandsasteroid belt, have undergone extensive partial melting, elsewhere do indeed have the low REE contents andbut none of these smaller bodies have developed anor- large positive Eu anomaly required by the conventionalthositic crustal materials (see also 64). Neither internal model. The latest survey of Th concentrations across theheating by short-lived isotope decay nor accretional en- whole lunar highland surface, however, shows valuesergy is appealing as the cause of this melting because which are predominantly in the range where small neg-similar-sized bodies (50) have manifestly undergone no ative or only small positive anomalies would be an-melting, and Callisto (4), a much larger body, may ticipated. The possibility that a deep-seated KREEPyhave evaded internal differentiation entirely. A localized component is more generally distributed but rarely ex-external heat source is required. Heating by tidal de- cavated has also to be considered.formation (59), which plays a significant role in Io, No plagioclase flotation. Plagioclase crystallization andEuropa and Ganymede today, may have been much accumulation takes place predominantly at the floor ofmore important in the evolution of small bodies such as terrestrial basic magma bodies (63). Plagioclase formedVesta and even the Moon in the early years of the Solar elsewhere in the magma cannot float or sink significantlySystem. If heat sources were marginally adequate for because of the greatly increased viscosity of magmas asmelting to occur at all it is reasonable to expect petro- they approach plagioclase saturation. Upward transportgenesis in those bodies which have been melted to be of suspended plagioclase by convection in a dry magmadominated by partial rather than total melting (60). ocean should lead to resorption of plagioclase, not for-

mation of an anorthositic crust. Terrestrial magmas areLunar highland crust richer in alkalis and more oxidized than lunar magmas,

but cumulates from the fragmented howardite–Petrological composition. Remote sensing of the compositioneucrite–diogenite and mesosiderite parent bodies (64)of the lunar highland crust (61) combines with petrologicalalso display accumulation of feldspar with denserdata from recovered samples and lunar meteorites toferromagnesian minerals, and provide no evidence ofshow that the average highland composition is anor-

thositic norite, an average that contains a significant plagioclase flotation or accumulation to form anorthosite.

1551


Fig. 2. Aluminium/silicon ratios in remotely sensed lunar surface materials suggest that average mare basalt compositions are significantly morefeldspathic than the hand specimens and that there is no significant europium anomaly in the average lunar highlands. The vertical axis in themain figure is the Al/Si ratio, which plots information by site, with mare sites to the left and highland sites to the right of the figure. In the left-hand box, histograms are plotted of the frequency of readings from the Apollo 15 and 16 orbital X-ray fluorescence experiments (Adler et al.,1973). Assuming 10 �m penetration in materials with a bulk density of 2 t/m3, each individual reading represents the average composition ofabout 72 000 t of material, i.e. about 3·5 × 106 t of mare surface and almost 6 × 106 t of highland surface. These reported values may beheavily influenced by the composition of the <10 mm fraction in the soils, which is known to be excessively feldspathic, but the general lack ofgradational mixing of mare and highland materials is clear. In the right-hand part of the figure Al/Si ratios in some analysed samples from arange of sites and occurrences are shown, with mare materials grouped to the left of the figure and highland materials towards the right.Randomly sampled mare basalts (clasts in breccias, meteorites, average compositions of lithic fragments in the regolith, small fragments recoveredby automated sampling missions) have higher Al/Si ratios than low-pressure plagioclase-saturated cotectic liquids produced in experiments onmare basalts (red crosses), the rock 12038 which is precisely cotectic, and the average groundmass of three Apollo 11 lithic fragments whichcontain plagioclase microphenocrysts (circle, filled red). Both the randomly sampled basalts and the low-pressure cotectic liquids have systematicallyhigher Al/Si ratios than the majority of the hand specimens, with the possible exception of low-K basalt samples from Apollo 11. The handspecimens in turn have systematically higher Al/Si ratios than the pristine pyroclastic glasses identified at many sites. The mare soils (squares,filled red) have as good a claim to represent average target rock in the top 5–10 m as do the highland soils (below). They have Al/Si ratioswhich are slightly enhanced relative to the randomly sampled basalts, consistent with a small percentage of observable added fragments ofhighland-derived materials (values calculated assuming the average hand-specimen composition for the basalt component are consistently higherthan the observable amounts). Vertical bars illustrate the differences in composition between the coarse fraction (lowest), the bulk (middle) andthe very fine grained fraction (top) in three representative mare soils and three selected highland soils. Mare soil and randomly sampled basaltcompositions are consistent with the average composition of the erupted mare magmas being close to those of plagioclase-saturated low-pressurecotectic liquids and even slightly biased towards appearance of plagioclase before pyroxene. The hand specimens and pristine glasses cannotrepresent the average erupted basalt compositions.

Rock samples (squares, filled blue) shown at the highland sites, except for the ferroan anorthosites (FAN), were selected on the basis that theywere impact mixed materials whose compositions were in each case likely to average those of large masses of target crust. The soils developedfrom them (squares, filled cyan) show a more restricted range of composition, entirely within the range defined by the probable components.All data are consistent with an average lunar highland composition which will not display a substantial positive Eu anomaly. Individual clastsof FAN do, however, display substantial positive Eu anomalies. Data sources predominantly Haskin & Warren (1991), McKay et al. (1991),Taylor et al. (1991) and Papike et al. (1998), with other data from sources referenced in the notes.

Al/Si is the vertical axis in the inset figure also, which plots this parameter as a function of log(Eu/Sm)N, a ratio which is slightly less thanthe true value of the Eu anomaly because of the slope of the chondrite-normalized REE patterns between Sm and Gd. The dataset is for allhighland materials of Taylor et al. (1991) and shows a reasonable correlation between the two parameters, suggesting that samples with Al/Si<0·65 are likely to display negative, not positive Eu anomalies.

1552


Fig. 3. Thorium concentration in lunar highland samples plotted as a function of the sum of concentrations of the rare earth elements, and ofthe magnitude of the europium anomaly in those samples, from the Apollo 15, 16 and 17 missions; all data are from Taylor et al. (1973a, 1973b),Bence et al. (1975) and Taylor & Bence (1975). The open circle shows the point corresponding to the average composition advanced by the twolatter groups at >65 ppm total REE with a suggested Eu/Eu∗ of >1·6 [estimated from Taylor (1975, figs 5·20 and 7·1)] in the right-handpart of the figure. The square symbol shows the plot of rock 68415, interpreted as a recrystallized impact melt rock of close to average lunarhighland crust composition.

No lunar magma ocean. Combining these thoughts with being a small average mass fraction partial melt of thewhole lunar interior, which would then have had to formthose of (62), an origin for the lunar highland composition

by plagioclase flotation is excluded (65) and there is, under moderate water vapour pressures (70). The smallnegative Eu anomaly inferred for the average lunartherefore, no petrological support or requirement for the

former existence of a global magma ocean in the Moon highland composition then suggests the presence of eitheramphibole or a trace of plagioclase in the residual lunar(66) and grave doubt (from 53, 57, 60 also) whether any

body the size of Mars or less developed a magma ocean mantle assemblage. Some might view this suggestion aspreposterous in the absence of any hydrous minerals induring its accretion (67). The manifest differentiation into

crust and mantle in the terrestrial planets is, in the present recovered lunar samples. There can, however, be fewmore efficient mechanisms for thoroughly dehydratingstate of knowledge, equally well accounted for by partial

melting and serial volcanism. materials than repeatedly spraying them as hot igneousor impact-generated particles into a vacuum.Wet mantle yields feldspathic melt. Leading on from the

pyroclastic, possibly andesitic character of early volcanism Evolving lunar volcanism. Combined with (61) the aboveleads to two complementary propositions. The early lunaron Mars (57), the partial melting of peridotites at mod-

erate (>0·1 GPa) pressures in the presence of excess highland crust may have formed by lower-temperatureserial water-rich volcanism yielding feldspathic partialwater yields liquids poor in potential olivine which will

have plagioclase as an early liquidus phase (68) on erup- melts (37), which had KREEP as one of its differentiationproducts. Activity may have evolved into water-poortion and water loss. This crystallization sequence abounds

in terrestrial calc-alkaline volcanics but might be com- basaltic volcanism as temperatures rose and the bodydegassed (71). The lunar highland crust is then theplicated in the lunar case by the simultaneous loss of

alkalis on eruption. We return now to the conclusions accumulated, differentiated partial melt product of thatevolving serial volcanism, incorporating a contributionthat partial melting is more probable than total melting

(60), that the Moon could have accreted plenty of volatiles from mare-related plutonics (72).Feldspathic contribution from early mare component also. The(51), and that some other origin than plagioclase flotation

is required for the lunar highlands (65). The high content mare components might have contributed further toplagioclase enrichment of the highland crust (see 93of highly incompatible trace elements in the average

lunar highland composition (69) points to the early crust below) if they were derived from a mantle which was

1553


close to saturation with an alumina-rich phase. Although crust might have evolved beneath an ice-layer like thatthis effect alone could have implanted a tendency towards on Io or Ganymede (52). If volatiles were relativelya positive Eu anomaly into the average lunar highlands abundant in the early Moon, and assuming no majorit would be much subdued if the average lunar highlands entrapment of hydrogen gas in the body, separation orcontain the pulverized residues of substantial pre-4·1 Ga losses of water from the silicate fraction would be anmare volcanism. oxidizing process (79). Subsequent losses of carbon and

sulphur as oxide gases will tend to be strongly reducingand the effects of any alkali loss are currently un-Sulphur, carbon and their gasesdetermined (80). Given the many orders of magnitude

Io pyroclastic volcanism driven by these gases. Gases in therange in the intrinsic oxygen fugacities of terrestrial basaltssulphur–carbon–oxygen system have driven the pyro-and the changes which might accompany eruption, thereclastic volcanism of Io (54), probably for the past 4·5 Ga.is no a priori reason to think that the oxygen fugacitiesThe contents of S and C even in consolidated lunar mareof lunar basalts reflect those of the lunar interior or thatbasalts are higher than in terrestrial basalts and wouldthe latter was necessarily uniform spatially or temporally.sustain volatile fugacities much higher than the lunar

surface confining pressure at magmatic temperatures (73).Lunar pyroclastic volcanism guaranteed. Pyroclastic vol- Eruption style on a small planet

canism similar to that on Io was guaranteed (81) on the Fire-fountaining, frothing and volatilization losses. A line ofMoon, yet this fact receives only a one-sentence comment thinking led to the postulate of a volatile-bearing, pro-by Papike et al. (1998) without reference to Io or to the gressively devolatilizing lunar mantle (71). Given thatprobable consequences for alkali contents of lunar basalts. liquidus vapour pressures even of the final (already muchThere is a fuller consideration by Heiken et al. (1991), degassed) consolidated mare basalts would have exceededagain without reference to Io, and tending to play down surface confining pressures (73) it is logical to expectthe role of reduction by sulphur loss and possible losses basalts erupted on small planets and asteroids to fire-of alkalis during pyroclastic volcanism (see also 80). fountain, to form ash- or droplet-emulsion flows which

Sulphide saturation and siderophile depletion. The lunar bas- would spread out with very low effective viscosities,alts were close to saturation with an immiscible sulphide and to froth at their top surfaces even once relativelymelt (74) and the widely publicized siderophile element condensed (81). Such eruptions would maximize thedepletion in lunar rocks is also predominantly a chal- surface area of the melt, maintain the surface temperaturecophile element depletion (75). Any cumulate gabbros of each droplet with minimal cooling in a black-bodyunderlying the lunar maria might contain sulphide-rich environment, and minimize the diffusion distances to behorizons which concentrate the chalcophile trace ele- covered by components seeking to volatilize (82). Thatments (78) as well as possible metal-rich horizons [a few this scenario probably affected lunar basalts is dem-fragments of both might then be expected in lunar onstrated by their high intrinsic vapour pressures athighland breccias if the speculation in (72) is correct]. liquidus temperatures (73) and by the observation thatOverall chalcophile element depletion in lunar surface even after eruption, flow and consolidation, some lunarrocks might owe much to this. Also relevant to the basalts are highly vesicular. Even in the waning stagessulphur–chalcophile element story is confirmation that of mare volcanism there were enough volatiles to powerthe Moon contains a small core which can be expected the pyroclastic eruptions which produced the dark mantleto be sulphur bearing if not sulphide rich (76). The strong glass bead deposits (83).influence of oxygen fugacity on wetting angles of sulphide

Effects of sodium loss. Loss of soda by volatilization hasmelts suggests that segregation of a sulphide liquid to a dramatic effect on the CIPW norm of the residual meltform such a core would have been difficult in a reduced (84), releasing alumina and silica from original albitelunar interior but easy in an oxidized Moon (76). molecule. These recombine with lime from augite, re-

leasing hypersthene molecule to join that being created byCerium anomalies and lunar oxygen fugacities reaction of released silica with original olivine molecule.

Terrestrial basic melts subjected to major volatilizationOxygen fugacities higher than have accompanied eitherlosses of sodium would be expected to be transformedterrestrial calc-alkaline volcanism or Martian volcanisminto anorthite–pigeonite basalts like those found on theare required at some stage in lunar evolution by theMoon (90).presence of small positive Ce anomalies in many lunar

Asteroidal basalts. The HED (howardite–eucrite–samples, another facet of lunar petrogenesis which hasdiogenite) parent body (?Vesta) has also yielded a range ofbeen largely ignored, yet never satisfactorily explainedanorthite+ calcium-poor-pyroxene basalts and doleriteswithin the ‘conventional’ petrogenesis (77). Similar an-which are low-pressure plagioclase-saturated cotecticomalies in a variety of Antarctic meteorites have fuelled

the wild?! (chess terminology) idea that the early lunar compositions (85). The geochemical features of these can

1554


be interpreted as products of either primary partial MORB, CFB crystallize plagioclase early. Terrestrial basalticmelting (40) or partial crystallization (47) processes. The partial melts have formed by larger average mass fractionspartial crystallization interpretation is favoured by the (>0·1) of partial melting than are supposed in the lunarpresence of cumulate-textured gabbro–norite samples case. They almost certainly separated from alumina-and abundant orthopyroxene cumulates (87), and perhaps undersaturated, feldspar-free harzburgites. Nevertheless,by the lack of identified olivine-rich types which might these melts are typically so rich in potential plagioclaserepresent the complementary residual mantle. Like lunar that precipitation of extensive olivine and plagioclase,basalts, HED basalts are relatively sulphide-rich (88), the common phenocryst phases in MORB and con-may have undergone considerable volatilization losses tinental flood basalt (CFB), precedes the arrival of theduring eruption and must have undergone major losses residual melt at the low-pressure cotectic equilibria whereof volatiles and sodium if the original body was related augitic pyroxene also begins to precipitate (94, 95).to chondrite in composition (89). The mantle residues Incorrect low-P phase equilibria of alleged mare primary magmas.from which the magmas evolved might not then be easily None of the lunar pyroclastic glass beads or allegedrecognizable. The character of the lavas is exactly what primary magma hand-specimen compositions displayswould be expected (90) if basic melts of familiar terrestrial the required crystallization sequence and most encounteror even more alkaline compositions had been subjected pigeonite saturation before the entry of either augite orto reduction and volatile losses on eruption at the surface plagioclase (96). If these had been the parental magmaof a small planet. compositions, and given (91), there would be no way of

generating the observed negative Eu anomalies (97).Alleged primary magmas do not display required moderate-P

Plagioclase saturation in lunar basaltsphase equilibria. A corollary of the issues discussed in (93)

No negative Eu anomaly in the lunar mantle. We return now to is that true primary magmas formed in the mannerthe complementary conclusion which arises from remote required in the ‘conventional’ lunar petrogenetic schemesensing of the average lunar highlands composition (61, should show simultaneous saturation with plagioclase,62). The average lunar mantle composition must reflect pyroxene and olivine at their pressure of formation (98),extraction of the highland crust (91) and must have a but none do.small complementary positive Eu anomaly if the bulkMoon has chondritic ratios of the REE. This conclusion

Lunar primary magmas and absence of tectonic deformationstands independent of the debate whether the lunarhighland REE signal is dominated by the KREEPy Global magma ocean cumulates unstable from birth. Deposition

(solidus) temperatures in cumulates from a global magmacomponent.Imposed, not inherited, negative Eu anomalies in mare basalts. ocean would decline towards the surface because of two

effects—the declining liquidus temperature of any dryMare basalts cannot then inherit their variable but inmany cases very marked negative Eu anomalies as a mafic magma with declining pressure, and the decreasing

mg-number of a differentiating magma ocean. Liquidusprimary magmatic feature (92) during partial melting ofsuch a mantle. Extensive plagioclase fractionation during thermal gradients in dry mafic and ultramafic materials

are typically supra-adiabatic, an effect which underwritespartial crystallization at low pressures is the most probablecause of these Eu anomalies. the partial melting of mantle plumes in the terrestrial

mantle. The thermal gradient at deposition of a thickLow-F, moderate-P primary magmas precipitate plagioclase beforepyroxene. The effect of elevated pressure on plagioclase- global cumulate would greatly exceed the adiabatic gra-

dient, promoting convective motion which would besaturated phase equilibria in the dry basalt–peridotitesystem is to displace the liquid compositions rapidly enhanced by the density inversion implicit in a plagio-

clase-free cumulate sequence of declining mg-number,towards higher normative plagioclase within the first 0·2GPa and less rapidly towards higher normative olivine especially where later cumulates contained ilmenite.

There is no tectonic evidence to suggest that the an-(Fig. 4). The implications are profound—plagioclaseshould precipitate before pyroxene from ascending prim- ticipated deformations took place.

Supra-adiabatic thermal gradient still required 1 Ga later.ary melts—and seem to have been overlooked in dis-cussions of mare basalt petrogenesis (93). If oxygen The two-phase (olivine + orthopyroxene in most cases)

saturation which occurs in the putative mare basaltfugacities were low and plagioclase were a residual phasein the lunar mantle during partial melting, the Eu an- primary magma compositions at a variety of pressures

and temperatures has no special petrogenetic significanceomalies of mare basalts might have been explained as aprimary feature, but the major element compositions of (99). If petrogenetic significance is imputed, the array of

co-saturation conditions would define a pressure–pyroclastic glass beads, hand specimens and even thefeldspathic basalts are too poor in plagioclase (93) to temperature gradient which is grossly supra-adiabatic

(100), has to be supposed to have persisted over at leastsupport this mechanism.

1555


Fig. 4. a1,2. Results of phase equilibria studies on compositions in the system CaO–MgO–Al2O3–SiO2 at pressures from atmospheric to 10GPa, redrawn and simplified from Herzberg & O’Hara (1998, fig. 1) where the two sub-projections, a1 from diopside and a2 from forsterite,are explained further. b1,2. Analogous results from experiments on natural basalts and peridotites, redrawn and simplified from Herzberg &O’Hara (1998, fig. 3) where the two sub-projections, b1 from a diopside-like component and b2 from an olivine-like component, are alsoexplained further. Bold lines in each figure record the positions of phase equilibria boundaries at low pressures. Fine lines record the loci ofliquid compositions in equilibrium with olivine, clinopyroxene, orthopyroxene and either plagioclase, spinel or garnet as the pressure increasesfrom low (0 GPa) to high (5 GPa). Liquids precipitating, or formed by small mass fractions of partial melting of assemblages of olivine, twopyroxenes and either anorthite, spinel or garnet at pressures below 5 GPa (approximately the central pressure in the Moon) will be expected toprecipitate olivine followed by plagioclase before pyroxene if erupted as unmodified primary magmas, as indicated by the dotted projection linein a1 from the olivine apex through the 1 GPa melt composition to the low-pressure boundary of plagioclase saturation. In a2 it is seen thatthis ‘projection’ from olivine will indeed result in the composition encountering plagioclase crystallization well before pyroxene, and that thiswill be true also for liquids formed anywhere along the locus up to close to 5 GPa. In natural multi-component systems there is a range of liquidcompositions which may form in equilibrium with the alumina-saturated lherzolite assemblages as mg-number, etc. varies, the extent of whichis indicated in the projections b1 and b2. This extra freedom does not alter the argument. Most of the change in composition along the locibetween 0 and 1 GPa is accomplished in the first 0·2 GPa.

a billion years through 500 km depth of mantle, and Average compositions of mare basaltsyet has produced no evidence of tectonics caused by Quench crystal sinking. The low viscosity of the alkali-poorconvective motion (101) even where the lunar crust is no mare basalt lavas, even in their condensed state, ensuresthicker than the continental crust of the Earth. Con- significant sinking of crystals, including the large zonedsequently, the glass-bead and hand-specimen com- metastable phenocrysts formed during quenching (103).positions cannot be primary partial melts of the alleged Settling rates may have been more rapid if quenchcumulate mantle (102). No problems with global supra- phenocrysts formed during the pyroclastic phase in theadiabatic thermal gradients need arise in the alternative eruption. Quench texture throughout a lunar hand speci-

men is no criterion of the former existence of its bulkpetrogenesis proposed here.

1556


composition as a liquid. Some, possibly many, of the Low-P plagioclase-saturated cotectic average compositions. In-formation and samples made available in a randomsamples must be enriched in ferro-magnesian phases by

accumulation of the quench phenocrysts (104). Petro- manner (110–116) indicate that the average mare basaltcomposition is close to that of a low-pressure cotectic,graphic variability at each site is more extensive than

was then known at individual terrestrial sites (but see plagioclase-saturated basalt which could be the residualliquid of a low-pressure gabbro–norite crystallization133).

Flow thicknesses. The general lack of flow fronts in the process (120) during which enrichment in incompatibleelements such as REE and titanium and negative Eumaria points either to very fluid flows (103) or to flow

thickness (105) comparable with or less than the regolith anomalies could be generated. These magmas mighthave been contaminated by highland crustal materialsdepth (>5 m), in which case the average regolith com-

position should be close to the average lava composition during that process.Misfits in experimental data. Objections to this in-(106). Estimates of cooling rates required to produce

the observed petrographic textures in hand-specimen terpretation based on small misfits in the experimentaldata, specifically small mismatches in the mg-number ofsamples, on the other hand, suggest much thicker cooling

units (107), in which case the regolith, being restricted liquidus olivines and the failure of armalcolite crys-tallization to overlap with that of plagioclase in someto the top 5 m, may preferentially sample a phenocryst-

depleted zone and not represent the average composition results, can be discounted because of the problems inprecise control of charge compositions and oxygenof the lava. Small impacts into the maria excavating to

>100 m depth (108) should, however, sample the average fugacities (119).Petrogenesis of lunar high-titanium basalt. The petrogenesislava composition accurately whichever estimate of flow

thickness is correct, and the same should be true of of the high-titanium basalts of Mare Tranquillitatis andTaurus Littrow collected by the first and last mannedmaterials exposed along the walls of the large rilles. There

is no evidence to suggest that the regolith surface is not missions of the Apollo program encapsulates the dis-cussion about mare basalts and is pursued at greaterrepresentative of materials to several tens, even a hundred

metres deep in the maria (109). length in Figs 5–9. These rocks are alkali-poor basaltsvery rich in TiO2, relatively rich in FeO relative toPersistently feldspathic regolith compositions. Remote sensing

of the mare surfaces (110) indicates average compositions MgO, relatively rich in incompatible trace elements (REE20–100 × chondritic and higher in Apollo 11 thanmuch more feldspathic than all but a few of the large

hand specimens from the mare landing sites (Fig. 2). Apollo 17 samples), with marked negative Eu anomalies(Eu/Eu∗>0·8–0·3). These magmas have been suggestedThese regoliths are expected to comprise about 95%

locally derived material. The remote-sensing results are to represent either (i) very small mass fraction partialmelts (1% or less by mass) of the postulated cumulatein good agreement with the limited ground truth es-

tablished by returned regolith samples (111). They also mantle, or (ii) late residual liquids (last 50–10% of theparent magma assuming a 10% initial partial melt) ofagree with the feldspathic basaltic compositions of lithic

fragments, breccia fragments, lunar mare meteorites and differentiation involving removal of plagioclase and otherminerals at low pressure.impact-generated glass groups in the regolith (112–116),

which leads on to the conclusion, reinforced by (103–109), The first hypothesis is incompatible with the high-pressure phase equilibria of the hand-specimen com-that the hand-specimen samples do not represent the

average consolidated liquid compositions (117). positions (93–100) and will also be incompatible with thehigh-pressure phase equilibria of the average eruptedComposition bias in hand specimens. The hand-specimen

compositions are inevitably biased towards materials ex- magma compositions advocated here (110–117), but lessstrikingly so. To validate the second hypothesis the phasecavated relatively recently from the top of the bedrock

at>5 m depth, as well as towards more cohesive samples equilibria of the average erupted magma compositionsare required to demonstrate near-simultaneous entry atfrom shallower depths, because longer exposed and more

friable materials have become preferentially comminuted the liquidus of all the mineral phases required to bepresent in the hypothetical cumulates, i.e. plagioclase,(118). It is quite proper to accord all such samples

equal weight when selecting them for investigation, but pyroxene, ilmenite, probably olivine, possibly armalcoliteand spinel as well. If the hand specimens representpotentially misleading to accord each of them equal

weight when arriving at an average mare basalt com- samples of average liquid enriched in phenocrysts ofdense olivine, ilmenite, armalcolite and spinel formedposition which ignores the regolith contribution. This

entire line of reasoning (103–118) reinforces the phase during quenching after eruption as advocated here then,subject to the points raised in (119), the hand-specimenequilibria, petrographic and geophysical arguments (93–

101) pointing to the conclusion that hand-specimen com- samples should show near-simultaneous entry of pyroxeneand plagioclase when the phenocryst phases are still inpositions cannot be primary partial melts of the alleged

cumulate mantle (102). equilibrium with the liquid.

1557


Fig. 5. Figure 5 displays the temperature and oxygen fugacity in experiments on 12 Apollo 17 hand-specimen samples for which phase equilibriaand phase composition data are reported in the Appendix. Figures 5–8 address the origin of the lunar high-titanium basalts, the first rocksreturned from the Moon, which are relatively rich in incompatible elements and display marked negative Eu anomalies. Interpretation of thepetrogenesis of this group encapsulates a more general problem common to many terrestrial basalts also. Does the incompatible enrichmentreflect large mass fractions of partial crystallization subsequent to magma formation, or low mass fractions of partial melting in the first instance?The negative Eu anomalies demand substantial prior removal of plagioclase from the system, which presents no problem if the basalts are low-pressure plagioclase-saturated cotectic liquids (see Fig. 2) whose compositions are controlled by gabbro fractionation within the lunar crust. Theconventional interpretation supposes, however, that prolonged plagioclase flotation from a lunar magma ocean had imparted a negative Euanomaly to a mantle cumulate pile. This then underwent small mass fractions of renewed partial melting to yield the parent magmas of themare basalts, represented by the hand-specimen compositions, a hypothesis which encounters difficulties outlined in Figs 4, 7 and 8.

A significant part of this debate has concerned the accuracy and relevance of phase equilibria studies on the natural samples, and is complicatedby the fact that at low oxygen fugacities precise control of oxygen fugacity and charge composition greatly influence the observed phase equilibria.Figure 5 shows the oxygen fugacity–temperature relationship of the Fe–‘FeO’ (wustite) equilibrium at which many experiments on Apollo 11samples and related materials were carried out in Mo containers. Circles show conditions of runs, reported in tables in the Appendix, in Mo-foil capsules, which are numbered as in the tables (unless otherwise shown, these numbers should be prefixed with a 5). Squares show conditionsof runs in Fe-foil capsules, similarly numbered. The intrinsic oxygen fugacity–temperature relationships of two Apollo 17 hand specimens (Sato,1978), and of one of these samples in contact with excess Fe metal are also indicated. An approximate boundary at which metallic iron appearedin sample 70215 in the experiments reported here is also shown. The approximate boundaries of plagioclase entry and armalcolite appearanceare indicated, but plagioclase was also present in 70275 in runs 381, 389 and 403. Spinel is present in most charges at oxygen fugacities higherthan those of 70017 + Fe metal. Plagioclase crystallization overlaps with that of armalcolite in a substantial number of these experiments.Conditions in these experiments are seen to be at least as relevant to the behaviour of lunar basalts as those carried out in ‘pure’ iron containersin ‘sealed’ silica glass tubes.

1558


Fig. 6. Schematic representation of atmospheric pressure phase equilibria anticipated in a representative high-titanium basalt hand-specimencomposition as a function of oxygen fugacity and temperature, based on available experimental data and inspired prediction. Crystallizationconditions observed in experiments which maintain charge composition, and are at an externally controlled or imposed oxygen fugacity, willfollow curves such as those shown for the iron–wustite equilibrium or an 80% H2–20% CO2 gas mixture (both dotted) or in the presence ofiron (parallel bold curve through B–F). A bewildering array of crystallization sequences might be observed according to the precise experimentalconditions achieved. Phases present in addition to liquid are: An, plagioclase; Arm, armalcolite; Fe, iron metal; Ilm, ilmenite; Ol, olivine; Px,pyroxene; Sp, spinel. Three bold boundaries mark the limits of stability of Fe metal, armalcolite and spinel, all of which are constrainedprincipally by the oxygen fugacity. Finer boundaries illustrate the appearance of olivine, ilmenite, pyroxene and plagioclase, which are constrainedprincipally by the temperature achieved. The sketch assumes that bulk charge composition has been maintained. The FeO content of the silicatecharge declines rapidly to the left of the steep bold boundary (B–F extended) marking the appearance of metallic iron in the composition withfalling oxygen fugacity. The FeO content of the charge decreases gradually to the right of this boundary as it is oxidized progressively to Fe2O3,and in both directions the mg-number of silicates will increase as FeO content decreases. Iron gain by the charges by oxidation of an Fe containerwill shrink the bold boundary of the armalcolite crystallization field (A–B–C–D–E–F) to lower oxygen fugacity, i.e. to the left, and lower thetemperature of entry of ferro-magnesian silicates in general. Iron loss to impure Fe containers by reduction, or to Mo containers at lower oxygenfugacity, will enhance the field of armalcolite crystallization and raise temperatures of entry of the ferro-magnesian silicates. The bold boundaryof spinel crystallization (G–H–C–J–E) is strongly dependent upon the oxygen fugacity because of its effect on the oxidation state of chromium.Spinel crystallization is inhibited in the presence of metallic iron as a container or as an equilibrium phase in the charge. The pyroxenecrystallizing in these compositions is calcic at and to the right of the boundary marking the entry of iron, but will become increasingly sub-calcicwith the possible appearance of separate pigeonite to the left of this boundary as potential olivine reacts with silica released by the reduction ofFeO. All these effects are believed to be displayed in available experimental data for the Apollo 17 hand specimens. The argument in this paperis that the natural hand specimens have been formed by accumulation of the dense minerals olivine, spinel, ilmenite and armalcolite into amultiply saturated cotectic liquid and erupted basalt composition close to that present at E, and that this interpretation should not be rejectedbecause of small discrepancies in some of the reported phase equilibria obtained by different groups in different laboratories using differenttechniques on different sub-samples of the rocks.

Data presented in (121), comprising with those pub- that the phase equilibria are appropriate for the secondhypothesis, making due allowance for the presence of alished by O’Hara et al. (1970a, 1970b) by far the largest

datasets for low-pressure crystallization of these rock small amount of highland component in the soils and ofsome pyroclastic bead material in the Apollo 17 soils.types, are interpreted in Figs 5–8, and demonstrate

1559


Fig. 7. High-titanium basalt hand specimens, groundmasses, average lithic fragments, soils and plagioclase-saturated experimental liquidcompositions compared in the isostructural equivalent weight projection FACKTS (O’Hara & Humphries, 1975), details of which aregiven below. Composition fields are indicated in which olivine or calcium-poor pyroxene (yellow), a titanium-rich oxide mineral (grey) oranorthite (blue) are early crystallizing phases. They are arranged about an intersection, the low-pressure plagioclase-saturated cotectic liquidcomposition, where all these phases and calcium-rich pyroxene may be simultaneously saturated at the liquidus at 1120–1140°C. Somehand specimens from the Apollo 11 low-K group and Apollo 17 samples 75035 and 75055 appear conspicuously close to the cotecticliquid compositions in this projection. Data at higher pressures for rocks 10017, 70017 and 70215 and for soil 10084 (O’Hara et al.,1970b; Longhi et al., 1974) are consistent with the equivalent cotectic liquid lying close to the point P at 0·5 GPa. True primary liquids,formed as required in the conventional interpretation, should have compositions close to P, should show early plagioclase crystallizationand should not project where the hand-specimen compositions fall. If the interpretation offered in this paper is correct, the hand-specimencompositions should plot in the triangle linking the plagioclase-saturated cotectics to olivine and to titanium oxide phase compositions,and with a little imagination they do.

Compositions are first converted to six projection components thus: F = (MgO + FeO + MnO + NiO + Cr2O3 + Fe2O3) ×71·846 (mol. wt FeO); A = (Al2O3 + Na2O + K2O) × 101·961 (mol. wt Al2O3); C = (CaO + 2Na2O − 3·333P2O5) × 56·08 (mol.wt CaO); K = 2K2O × 94·203 (mol. wt K2O); T = (TiO2 + Cr2O3 + Fe2O3) × 79·90 (mol. wt TiO2); S = [SiO2 − 2(Na2O +K2O)] × 60·085 (mol. wt SiO2); where all oxide symbols represent the number of moles present in the analyses. The C and K componentsmay be combined and treated as CaO, thus superimposing all feldspar components at a single point, when it is not desired to separatematerials on the basis of their potassium contents. The sub-system FM–SIL–T–CPX–FELS is calculated with FM = (F∗ + A∗ − C∗− K∗) × 71·846, SIL = (S∗ − 2C∗ − 2K∗) × 60·085; CPX = (C∗ + K∗ − A∗) × 248·1 (mol. wt hedenbergite) and FELS =A∗ × 278·2 (mol. wt anorthite), where the asterisked quantities are the molar, not equivalent weight, quantities calculated initially. Thecomponents of selected sub-groups are then scaled to 100%. The projection within the sub-system in Fig. 7 is from FM and CPX intothe plane of total feldspar (FELS)–titanium oxides (T, where armalcolite, ulvospinel and ilmenite project together)–available silica (SIL),where olivine and calcium-poor pyroxene project together. This projection conceals differences in calcium-rich clinopyroxene (not an earlyphase in these basalts) and displays essentially the differences in feldspar concentration between the various materials, and their ratios ofoxide phases to olivine and calcium-poor pyroxene (all but the last of which could be accumulating crystal phases). The central diagram,(a), compares the compositions of hand-specimen rocks [details in (c)], experimental and natural plagioclase-saturated liquids [details in(d)], mare and massif soils and the average compositions of basalt fragments in the Apollo 11 soils. (b) interprets the phase equilibriareported in a range of synthetic compositions (O’Hara et al., 1970b) which display a more restricted field of early oxide crystallizationthan the natural samples because of the lack of chromium in the system.

Cognate crustal cumulates. The presence of extensive crust beneath the mare surfaces (105, 122), some oldermembers of which may be sampled in the highlandbodies of cognate gabbro–norite and probably peri-

dotite–pyroxenite is predicted somewhere within the breccias (72).

1560


Fig. 8. High-titanium basalt hand specimens, groundmasses, average lithic fragments, soils and plagioclase-saturated experimental liquidcompositions compared in the isostructural equivalent weight projection FACKTS (O’Hara & Humphries, 1975) as in Fig. 7, but using adifferent sub-projection of the data which conceals differences in olivine and titanium-rich oxides (early crystallizing and possibly accumulatingphases in these basalts) and displays essentially the differences in feldspar concentration relative to pyroxene components (all three late crystallizingphases in these materials). Composition fields are indicated in which calcium-poor pyroxene (lime green), calcium-rich pyroxene (green) oranorthite (blue) are early crystallizing phases, with olivine and oxide phases. These fields are arranged about an intersection, the low-pressureplagioclase-saturated cotectic liquid compositions, where all these phases may be simultaneously saturated at the liquidus at 1120–1150°C. Allthe hand-specimen compositions project conspicuously close to the cotectic liquid compositions in this projection, consistent with the handspecimens being olivine + oxide mineral cumulates into low-pressure plagioclase-saturated liquids. Data at higher pressures for rocks 10017,70017 and 70215 and for soil 10084 are consistent with the equivalent cotectic liquid lying close to the point P at 0·5 GPa. True primaryliquids, formed as required in the conventional interpretation, should have compositions close to P, should show early plagioclase crystallizationand should not project where the hand-specimen compositions fall.

The sub-system OL–ILM–OR–FS–CPX–AN of the projection explained in the caption to Fig. 7 is calculated with OL = (F∗ + A∗ + C∗+ K∗ − T∗ − S∗) × 203·777 (mol. wt fayalite), ILM = T∗ × 151·746 (mol. wt ilmenite), OR = K∗ × 278·2; FS = [− F∗ − A∗ −3(C∗ + K∗) + T∗ + 2S∗] × 131·93 (mol. wt ferrosilite), CPX = (C∗ + K∗ − A∗) × 248·1 (mol. wt hedenbergite) and AN = (A∗ − K∗)× 278·2 (mol. wt anorthite), where the asterisked quantities are the molar, not equivalent weight, quantities calculated initially. The componentsof selected sub-groups are then scaled to 100%. The projection within the sub-system in Fig. 8 is from OL, ILM and OR into the plane of totalplagioclase (FELS)–calcium-rich pyroxene (HED)–calcium-poor pyroxene (FS). The central diagram, (a), compares the compositions of soils andhand-specimen rocks [details in (c)], experimental and natural plagioclase-saturated liquids [details in (d)], and the average compositions of basaltfragments in the Apollo 11 soils. (b) interprets the phase equilibria reported in a range of synthetic compositions (O’Hara et al., 1970b) whichyield a phase diagram closely similar to that deduced from the natural samples.

sparse and fragmentary, providing little in the way ofComparative petrogenesiscontrol over interpretations of lunar geology.

Meantime, back on Earth . . .Magma production in large impacts. Large basin-forming

impacts undoubtedly produce large volumes of impactPoor terrestrial controls. Most of the rock materials nowmelt (123), part at least of which may become clastforming the lunar highland crust were in place, the periodfree. In the small-basin-sized terrestrial impact basin atof heavy bombardment and large basin formation wasSudbury (124) the whole body of norite and gabbro withover, and the filling of the western maria complete by

3·7 Ga ago. The terrestrial record from that period is its associated nickel sulphide deposits may be a pool of

1561


Fig. 9. A ‘simple’ plot of Al2O3 against CaO in Apollo 11 samples to illustrate proximity of low-K group basalt compositions to the low-pressureplagioclase-saturated cotectic liquids and apparent controls of compositions within the high-K and low-K groups. This is actually a verycomplicated projection from whatever other components happen to be in each sample, in whatever ratios they happen to be present, with noregard to how these components combine in the permissible phases; i.e. each point is projected by a different method geometrically! The figuredisplays differences in the amounts of two components which are associated in plagioclase and are in the main excluded from the denseferromagnesian and oxide phases (with the exception of calcium-rich clinopyroxene) which may have sunk within the lavas. It conceals allinformation about relative silica saturation and titanium concentration and cannot be used uniquely to relate observed phase equilibria to thesample compositions, but in general plagioclase can be expected to crystallize before pyroxene from liquids to the CaO–Al2O3-rich side of theexperimental cotectic liquids. The composition fields of the high-K and low-K hand specimens [averages of groups B1–3 and D from Rhodes& Blanchard (1980), breccias from Rhodes & Blanchard (1981)] are shaded and labelled. Regolith breccias (small filled circles) and their average(large open circle) are significantly more aluminous, extending up to the open squares representing the soil compositions (which certainly containsome highland-derived component). The plagioclase-saturated cotectic liquids developed in 10020 and 10062 (low-K group), and those in 10017and a synthetic mixture simulating the composition of the average high-K lithic fragments (open triangles), lie within a restricted field. A fieldis also shown enclosing three analyses of the vitrophyric groundmasses enclosing plagioclase phenocrysts from lithic fragments in the soil (O’Haraet al., 1974) which also encloses the composition of monomict breccia 10056. This figure supports the inference from Figs 7 and 8 that the low-K Apollo 11 basalt hand specimens are close in composition to low-pressure plagioclase-saturated cotectic liquids. Also shown are two controllines (dashed). A–B passes through the cotectic liquid composition of 10017 and extends towards the compositions of plagioclase phenocrysts,An94, and reflects some strong control on the compositions of high-K hand specimens, breccias and soils. This control line is marked to showthe effect of adding or subtracting percentages of plagioclase (or subtracting or adding an alumina-free but lime-containing cumulate) to producethis displacement from the 10017 cotectic liquid composition. Control line X–Y passes through the cotectic liquids in low-K basalts and extendsbackwards towards a Ca, Al-free cumulus (olivine plus oxide phases only).

1562


impact melt which cooled slowly enough to undergo the western maria the oldest surface basalts are almostas old as the basins themselves (129), consistent with veryextensive fractionation. Mafic magma may also form

within the target body by partial melting triggered by a rapid initial filling of the basins. There is no associationwith plume-like motions in the underlying mantle. In-combination of pressure release accompanying the basin

excavation and shock-implanted energy. This internally dividual flow volumes must have been very large buttotal magma production was low.generated magma would have to rise through shattered

upper-mantle and crustal rocks in the breccia lens, when it Dubious continental flood basalt analogies. From the outset,majority opinion among Apollo scientists favoured anwould have a high probability of becoming contaminatedanalogy between lunar mare basalts and CFBs, partly(125). If it arrived soon enough, it would be liable tounder the mistaken belief that CFB provinces were arrayshybridize with the purely impact-generated melt in theof primary magmas. Terrestrial CFB events by contrastcrater.are strongly plume related, marked by very high magmaBushveld complex a model for mare filling? The 2·2 Gaproductivity and high total magma production (130) andBushveld complex of southern Africa has been put for-of very short duration (>2 Ma) although any moreward as a possible large terrestrial impact basin fillingextended igneous activity in CFB may be detached from(126) and it has several of the features one might seek: lackthe main centre by the onset of active spreading at aof tectonic association with an internal plume-generateddivergent plate margin and passive margin subsidence.event, extremely high magma production rate, pervasive

Flood basalts and lava lakes. Whenever eruptions aremagma contamination, and mare-like dimensions. Ex-sufficiently voluminous and sufficiently rapid, lava lakestreme differentiation by partial crystallization of noritesmay form and there could be no more convenient siteand gabbros in this body is complicated by many magmafor the low-pressure differentiation of basaltic magmarecharge events, accompanied by copious chromite and(131). Rates of irruption in komatiite–greenstone andsulphide precipitation. Large outflows of magma, whoseCFB provinces have manifestly been too low for lavacompositions were constrained to be multiply saturatedlake production but do seem to have supported formationwith plagioclase, pyroxenes and other phases at lowof large crustal or sub-crustal magma chambers (127).pressure, are deduced. These factors outline an en-

An ocean-island basalt or mid-plate volcanism analogy? Thevironment similar to that required to explain the lunarmare basalt samples recovered by the Apollo missionsmare basalts (122). The Bushveld lacks, however, anymust represent the final eruptions from waning volcanicunambiguous evidence of origin as an impact basin andcycles, whichever model for mare volcanism is adopted,there may be some differences in timing which distinguishand may not be typical of the average mare fill. In ait from the lunar maria. R. G. Cawthorn (personalnon-convecting Moon which provides no recharge ofcommunication, 1999) has, moreover, drawn attentionfertile source material into the melting regime, these lateto the impoverishment of the cumulate section in sulphur.eruptions might be expected to be thoroughly differ-Komatiite–greenstone analogies. Some of the terrestrial ko-entiated late-stage residual liquids in which volatile con-matiite–greenstone sequences date from a period over-tents might have built up to relatively high levels (132).lapping that of lunar mare filling (127) and it has beenThe late stages of terrestrial plume-related volcanismsuggested that they might be analogous features. Ex-producing ocean-island basalts yield very small volumespanding knowledge of both komatiite–greenstone andof magma rich in volatiles and incompatible elements,continental flood basalt provinces, however, has notoften relatively rich in titanium. As usual with terrestrialstrengthened the desired connection. The stratigraphic re-magma types, there is one body of opinion which regardscords in these provinces typically open with the eruptionthese as very small mass fraction partial melts of a fertileof more primitive magma types, which have a better claimmantle source. There is another which regards them asthan average to approximate to primary magma com-residual liquids from advanced high-pressure fractionalpositions. They then proceed through thick sequences ofcrystallization at>2·5 GPa because the requisite garnetbasalts whose compositions are controlled by partial crys-pyroxenite and eclogite precipitates are carried up intallization somewhere within the crust and terminate withexplosive eruptions. The much reduced pressure gradienttypes which may be seriously contaminated or hybridized.in the Moon would make such high-pressure evolutionRealities of mare basin filling. Mare basalt filling took placeimprobable but the possibility of an analogous evolutionover a period of about 2 Ga from >4·2 Ga ago (128)at >1·0 GPa, with the final stage magmas not directlyand was at least partly independent of the basin-formingrelated geochemically to the main mare fill by low-process because large basins are known which are virtuallypressure events, should be entertained.devoid of basaltic fill. Magma supply was sustained over

a long period of time. Even the last few surface flows atBack to the Moona single site may span as much as 250 Ma, but there are

few constraints on the time required to emplace some Limited lunar field controls. We are attempting to infer thefield relations of the lunar basalts from their experimental99% of the underlying mare fill, beyond noting that in

1563


petrology and trace element geochemistry, an unfortunate politicians who may legitimately feel that the interests oftheir constituents and their own chances of re-electionway to have to progress. Controls based on terrestrialare better served by spending closer to home. Vigorousexamples where the same had been attempted weredebate about interpretation, the democratic lifeblood ofinadequate (133). Rock exposures in the walls of Hadleyscience, can easily be portrayed as reprehensible disarrayRille (134) and the fallen blocks on its floor were con-and used as an argument for diverting that fundingfidently identified as basalt flows when the photographstowards more ‘worthy’ objectives. It is not always clearcame back from Apollo 15, but the upper 60 m is poorlywhere the balance of advantage will lie between qualityexposed. The underlying layered units and the fallenof debate and total quantity of science achieved.blocks appear much more massive and less conspicuously

The first returns of systematically collected, well-locatedjointed than typical basalts, where columnar jointingsamples from Mars, stamping ground of the largest centralcommonly develops in thick flow units. Other ob-volcanoes, largest calderas and some of the most extensiveservations from Hadley Rille and its surroundings pointspreads of basalt in the Solar System, are almost uponto the availability of very large volumes of fluid magmaus. It is to be hoped that there will be no Gadarene rushin a single event, and to lava ponding and magmato a consensus of interpretation. An extensive, protractedwithdrawal (see 107, 131).and dispassionate examination of a wide range of multipleGabbros at shallow depth in the maria? Although there isworking hypotheses would be appropriate.no indication of major compositional variations in the

Basalt petrogenesis—a Solar System round-up. Evidence re-uppermost layers of the maria (109), larger craters whichlating to basalt genesis from seven planetary bodies hasexcavate material down to >1 km depth do in someincreased greatly in the past 30 years, and that forcases excavate more mafic or more magnesian materialsextensive evolution of basaltic magmas between source(135) and at least one lunar meteorite, Asuka 881757,region and vent has multiplied. Central volcanic com-has sampled an ancient coarse-grained mare gabbroplexes and calderas on the Earth are associated with(136), further observations consistent with the presencehigh-level magma chambers, partial crystallization ofof gabbros at no great depth below the surface basaltsmagmas and eruption of residual liquids biased towards(122). The inferred rock assemblage would closely re-low-pressure cotectic compositions. High-level magmasemble that seen in samples from the HED parent bodychambers cannot, however, be the site of whatever low-(85–90).pressure modification has affected continental flood basaltPyroclastic flow units. The final eruptive events in thecompositions—subcrustal magma chambers are pre-maria were pyroclastic eruptions (83). The requirementsferred. The crust of Venus is riddled with central volcanicimposed by the morphology of most mare basalt flowscomplexes which suggest an abundance of high-level(105) and that of the earliest basalt flows on Mars (137)magma chambers in which partial crystallization of par-suggest that the majority of mare-filling eruptions mayental magmas might occur. The surface of Mars has thehave been mafic pyroclastic flows. Such eruptions, espe-largest central volcanic complexes and some of the largestcially those into low confining pressures, would havecalderas known in the Solar System, again potentialbeen accompanied by extensive selective volatilizationsites of advanced low-pressure partial crystallization. Thefrom the magma (138). Coupling this with the low-majority of lavas erupted on the surfaces of Mars andpressure cotectic character of the basalts (120) the con-Venus are likely to be extensively modified by partialclusion is reached that the mare basalt parent magmascrystallization and assimilation within the crusts andare not preserved at the lunar surface (139). Little cancentral volcanic superstructures on those planets;be reliably deduced from the lava geochemistry aboutmost Shergottite–Nakhlite–Chassignite (SNC) groupthe detail of the lunar mantle without first unravellingmeteorites derived from Mars are cumulates.the effects of modification at low pressures (33).

There is an abundance of central volcanic features andPyroclastic bead compositions. It has to be conceded by allcalderas on Io. The lavas erupting on Io are anticipatedparties to the debate that there is no simple mechanismto be evolved basalts on the basis of the existence ofof closed system magmatic evolution which can link thenumerous large calderas which imply extensive high-compositions of the late pyroclastic glass beads to thoselevel magma chambers and the probability of refluxingof the hand specimens (140) and the matter is best leftof the partial melt compositions at low pressure over awithout further speculation until the issue of the averageperiod of 4·5 Ga. The parent planet of the basalticcomposition of mare basalt (120) has been resolved byachondrite meteorites had a crust covered with low-the observations suggested (134).pressure cotectic plagioclase-saturated basic extrusives.These 4·5-Ga-old lavas have the sodium, volatile and

Finale siderophile element depletion and the high sulphurMars, science and politics. Supremely expensive science abundance of lunar mare basalts. They display a range

of negligible to moderately negative Eu anomalies. Theirprojects are ultimately funded from the public purse by

1564


geochemistry can now be interpreted as products of that of the mafic hand specimens, and that the same willbe true of the average regoliths on the steep sides of thecrust-forming, periodically recharged, periodically tapped

magma chambers perhaps afflicted by some form of small rille, which should be more mafic than the surfaceregoliths if the ‘conventional’ version were true. All thepacket crystallization. Complementary slowly cooled or-

thopyroxenite and gabbro cumulates are known among above can be tested with available samples or readilyaccessible measurements. Vents erupting the glass-beadthe meteorites and some (Moore County, Serra de Mage)

have the requisite substantial positive Eu anomalies. deposits may also have carried up xenoliths of the under-lying stratigraphy. High-resolution remote sensing of, or,Ancient igneous rocks from the Mesosiderite Parentbetter, a visit to the huge blocks on the floor of HadleyBodies have similar relationships, with one of the gabbroRille and the materials exposed in the slumped walls ofclasts containing the most extreme positive Eu anomalyCopernicus, or a few 100 m drill cores almost anywhereknown. Some very effective mechanism of volatile andin the maria, would settle the debate finally and un-sodium loss has to be found to arrive at these compositionsambiguously.from chondritic or carbonaceous chondritic starting

See also Notes added in proof, which follow Note 140materials.on p. 1627.Anomaly of the established lunar petrogenetic model. Yet the

conventional interpretation of lunar petrogenesis requiresthat the Apollo 11, 12, 15 and 17 missions to the Mooneach sampled, within a diminutive area, not one but

ACKNOWLEDGEMENTSmany near-primary quenched liquid samples which hadarrived unmodified through the volcanic plumbing system The author’s involvement in lunar studies began in 1967from depths of between 130 and 480 km, yielding a at the experimental laboratory of the Grant Institute ofgreater diversity of samples than would be expected from Geology, Edinburgh University, with his appointment asmost comparably small sampling areas on the Earth. a Principal Investigator for the Apollo missions andThis is a proposition worthy of the most careful re- survived a move to UCW Aberystwyth in 1978 withevaluation on the eve of planned sample returns from considerable but relevant and necessary digressions be-four localities on Mars in the next decade. Would the tween 1977 and 1998 into trace element modelling in asame conclusions about the Apollo and Luna samples variety of igneous processes. The research for this project

was pursued during tenure of a Sherman–Fairchild fel-have been reached if they had arrived in 1999, not 1969?Predictions. The essence of a satisfactory hypothesis is lowship at the California Institute of Technology 1984–

1985 and greatly advanced during an immenselythat it should make predictions which can be tested andproved or disproved. The alternative interpretation of stimulating two-year appointment 1988–1990 at Sultan

Qaboos University, Sultanate of Oman; somewhat de-mare basalt petrogenesis suggested here predicts that theaverage compositions of the small lithic fragments in the layed by a major commitment to development of com-

puter-assisted learning on return to AberystwythApollo 17 regolith will be found to be close to those oflow-pressure plagioclase-saturated cotectic basalts, like 1990–1993 and again greatly assisted by appointment

1994–present as a Distinguished Research Professor atthose reported from the Apollo 11 and 12 sites (butperhaps not those in the Apollo 15 mare regoliths, where Cardiff University. Each of these institutions in turn has

provided the facilities, time and stimulation without whichthere may have been magma drainage when the last flowwas partially crystallized, and least of all the winnowed this paper would not have happened. The final stimulus

was the opportunity to commemorate the scientific con-regolith from the edge of Hadley Rille). It anticipatesthat diligent search among vitrophyric lithic fragments tributions of Keith Cox, friend and guru of flood basalt

petrogenesis (see O’Hara, 2000).from the regolith at the Apollo 12, 15 and 17 mare siteswill find evidence of small plagioclase phenocrysts in an Some 80 copies of the submitted manuscript were

circulated to potential readers, and valuable commentsappropriate composition groundmass, similar to thosereported from the Apollo 11 site. It predicts that a few of varying detail and a wide range of levels of approbation

were received from N. Arndt, G. Cawthorn, L. Coogan,such plagioclase phenocrysts are present in small amountsin the vitrophyric groundmasses of some of the mafic J. Davidson, D. H. Green, C. T. Herzberg, K. Keil, P.

C. Lightfoot, H. Y. McSween, Jr, R. K. O’Nions, D. C.hand specimens but accepts that they might be verydifficult to find and even more difficult to prove con- Presnall, B. G. J. Upton, J. F. G. Wilkinson, D. A.

Williams and L. Wilson. My thanks are due to all theclusively absent. It predicts that a significant part of thegabbroic debris in highland breccias from Apollo 15, 16 above and to the referees, T. Grove, A. R. McBirney,

S. A. Morse, P. J. Wyllie and one anonymous, the lastand 17 can be linked to the evolution of earlier marebasalt magmas. It anticipates that remote sensing of the of whom nobly upheld traditional views, for their advice

and comments, which have significantly improved thislayers exposed in the walls of Hadley Rille will dem-onstrate compositions akin to the surface regoliths, not presentation. It became evident that several aspects of

1565


this paper may prove controversial. I am particularly The premise that the primary magmas were erupted inabundance was also widely accepted and was particularlygrateful to S. Delamont for assistance in locating theseductive to petrologists and geochemists because it im-Mitroff reference.mediately invested a sample with fundamental import-ance. As a primary magma, it is a stepping stone tomantle source mineralogy, pressure of formation and themantle dynamics leading to its partial melting. Breath-

NOTES taking vistas in planetology and cosmology unfold beforeNotes are numbered as in the text. the possessor of such a talisman. As a composition modi-

fied by low-pressure processes, that same sample has0. In 1950 petrologists were less than 20 years from much to say about crust-forming processes of immediate

contemplating the derivation of tholeiitic and alkaline interest to the petrologist, but little to say about thebasalts by the total melting of conveniently provided mantle until those near-surface processes have been un-glassy basalt layers of the appropriate compositions (e.g. ravelled and their effects stripped away. Entrants to theTurner & Verhoogen, 1951, p. 162). They were sim- profession were instructed:ultaneously less than 20 years from having a new planet ‘Probably the most satisfactory . . . [criterion by whichto explore, and closer to both events than we are today a primary magma may be recognized] . . . is a pronouncedto the first Apollo landing, over 30 years ago. During tendency for the magma to appear repeatedly throughoutthe preparations for the lunar missions and the receipt geologic time, in great quantities and in extensive in-of samples, the team at the Lunar Receiving Laboratory dividual bodies (lava floods, batholiths, lopolithic sheets,and the Principal Investigator group was dominated by etc.) over large sectors of the earth’s crust . . . The ultimategeochemists. Field and conventional petrologists had little criterion of a primary magma is its abundance in spacewith which to work, in contrast to the wealth of superb and time. . .. The case for world-wide development ofquality data generated by the numerous geochemistry primary basaltic magmas is now satisfactorily established’laboratories involved. Inevitably, the subsequent in- (Turner & Verhoogen, 1951, 1960, pp. 361–362).terpretation of the lunar samples was dominated by the Continental flood basalt was high on the list of ter-views of geochemists, whose opinions, like those of most restrial rocks conforming to these expectations. Im-workers in the project, had been formulated mainly in mediately before the Apollo missions, mid-ocean ridgethe 15 years following the middle of the century. The basalts were being discovered and proclaimed as theApollo lunar samples were delivered into a climate of primary magma (Engel et al., 1965), despite telling petro-petrogenetic opinion which accepted as commonplace the logical evidence to the contrary (Muir & Tilley, 1964)widespread eruption of little modified primary magmas, and later identification of phase equilibria impedimentswhose trace element geochemistry was believed to be (O’Hara, 1968a, 1968b; Stolper, 1980). Four quotationsdominated by the effects of small degrees of partial from Carmichael et al. (1974) convey the prevailingmelting of appropriate source rocks. This was a scientific mood among the community during and after the Apollocommunity which was temporarily disenchanted with program with respect to Hawaiian basalts, MORB, CFBthe role of assimilation and contamination in igneous and basalt petrogenesis generally:petrogenesis. ‘At many large volcanic centres—Mauna Loa for ex-

Postulating basalt layers of convenient composition, ample—a single type of lava has been generated awaiting to be melted to yield observed basalts, is the thousand times or more and erupted with only minorultimate in primary magma hypotheses. A minimalist subsequent modification . . . [6·0–10·0% MgO in theirstep from this is to postulate conveniently situated mantle quoted tholeiitic analyses, still more variable in practice!] . . . Thecompositions, also waiting to be partially melted to yield distinctive individuality of such a pile . . . can scarcely beobserved basalts without further modification of their due to repeated retracing of an identical course throughcomposition. Such a model has problems. There is a the maze of an ingenious model. . .. While a degree ofrequirement to get the right mantle to the right place at preeruptive fractionation can explain much of the major-the right time in a continuously and vigorously convecting element chemistry of a mafic series, it commonly fails tobody so as to match the observed correlation of magma account for more striking characteristics of the dispersed-type with plate tectonic environment. The view that element pattern . . . the chemistry of [a magma’s] in-basalts originated by the partial melting of peridotite compatible dispersed elements . . . will be determined(Bowen, 1928) was embodied in the standard petrological purely by that of the source rocks and by the degree andteaching text of the day (Turner & Verhoogen, 1951, regime of fusion’ (Carmichael et al., 1974, p. 649).1960) and the fundamental role of partial melting of ‘If, as seems likely from their great volume and regionalperidotite in producing basalt was accepted by all in- chemical uniformity . . . [5·4–10·2% MgO in quoted ana-

lyses!] . . . over very large areas, these lavas [abyssalvolved. Thereafter the community diverged.

1566


oceanic tholeiites] were generated as substantial melt (iii) That continental flood basalts were indeed uniformin composition. The uniformity of composition of floodfractions of source rocks’ (Carmichael et al., 1974, p.

651). basalts was progressively undermined (Cox et al., 1965,1967; Cox, 1971, 1972), and the accounts by Carmichael‘Continental tholeiitic flood basalts and related di-

abases, more than any other class of volcanic rocks, et al. (1974) were themselves sufficient to raise doubt.(iv) That partial crystallization in magma chamberssatisfy the two criteria . . . for magmas generated directly

by fusion—great volume and compositional homogeneity cannot impart uniformity of composition. Cox & Ja-mieson (1974) pointed out that arrival of compositionswithin each province. How else can one explain the

uniform character of more than 100,000 km3 of Yakima fractionating olivine at, and their diversion by, the pla-gioclase-saturated low-pressure cotectics provided one ofbasalts . . . [3·8–4·4% MgO in their quoted analyses] . . .

erupted within 5 m.y. in the Columbia River province, the mechanisms required to impart a degree of com-position uniformity. Other factors, connected with thenorthwestern United States? To derive this magma from

picritic basalt of deep-seated origin by low pressure buffering behaviour of periodically recharged magmachambers (26, 27), are comparably important.fractionation requires again and again that each suc-

cessive draught of magma must rid itself cleanly . . . of the None of these four assumptions was justified. Never-theless, Wilkinson (1982, 1991) has mounted a spiritedsame fraction of crystalline olivine along some identical

course of ascent. This seems highly improbable’ (Car- defence of the Carmichael et al. (1974) position.At the time of the Apollo missions a major role formichael et al., 1974, p. 654).

‘In general we hold the . . . view that voluminously partial crystallization was regarded by some as essentialto explain the major element compositions of basalts,erupted mafic magmas of uniform composition . . . appear

at the surface with their initial chemical character— coming to a head in studies by Cox et al. (1965, 1967),O’Hara (1965, 1968a, 1968b) and Cox & Hornung (1966).imparted by fusion of appropriate ultramafic source

rocks—unimpaired or at least still recognisable’ (Car- Partial crystallization of abundant basalt magmas wasregarded as irrelevant by other petrologists (Green &michael et al., 1974, p. 649).

At least four implicit assumptions underpinned the Ringwood, 1967), and as unnecessary or inadmissible bytrace element geochemists, coming to a head in the workCarmichael et al. (1974) interpretation:

(i) That each erupted magma comes as a discrete of Gast (1968). All but one of these protagonists wereappointed as Principal Investigators in the Apollo Lunarbatch from source to vent without intervention of staging

chambers. This was an oversimplification. There was Science Program in 1967. Major battlelines were drawnup and positions dug between selection of the Apolloalready abundant field evidence concerning the com-

plexity of volcanic plumbing (e.g. Harker, 1908; Brown, investigators and the arrival of the first samples. Withmost of the protagonists now involved on the same1956; Wager & Brown, 1968), which has been further

supported by postulated sub-crustal magma chambers project, what more natural than that the Moon shouldhave become a field of debate. Personalities and human(Cox, 1980), by the discovery of such sub-crustal ultra-

mafic complexes in the Ivrea zone (Quick et al., 1994), nature then played a part [see Mitroff (1974) for anenlightening view of the environment and times].by studies of individual active volcanoes (Ryan et al.,

1981) and by the presence of massive ultramafic cu- 1. The extent of the appeal of contamination, as-similation and hybridization in the petrogenesis of plu-mulates within ocean-island volcanoes (Watts et al., 1985;

Carress et al., 1995). tonic igneous sequences, predominantly those withsignificant acid and intermediate members, is apparent(ii) That the only physical processes of magma modi-

fication which needed to be entertained were PFC (perfect in the accounts in Turner & Verhoogen (1951) andearlier texts referenced therein. The field relations andfractional crystallization), EPM (equilibrium partial melt-

ing, also known as batch melting), PFM (perfect fractional petrography of these plutonic rocks are important partsof the evidence that many magmas undergo profoundmelting) and APFM (accumulated perfect fractional melt-

ing). With the discrete batch constraint removed, several modification of their compositions within the crust. Thecompositions of the actual liquids involved in these pro-plausible partial crystallization processes have been shown

to produce effects on both incompatible and compatible cesses, however, were sometimes speculative.2. The loss of appeal of contamination, assimilationtrace elements which would make their residual liquids

almost indistinguishable from small mass fraction equi- and hybridization in igneous petrogenesis generally isequally apparent from the reduced discussion of this issuelibrium partial melting products (see 15, 16, 20, 21, 23,

24, 26, 29, 30 below). Many geochemical interpretations in the study by Carmichael et al. (1974). This changingattitude reflected the shift of emphasis in petrological andof the period, conforming to the model conveyed by

Carmichael et al. (1974), called for specially fertile source geochemical studies away from plutonic rocks towardsxenolith-poor lavas, where there was less debate aboutregions from which the inferred small mass fraction

partial melts could be derived (see 22, 23 below). what were the liquid compositions, and towards oceanic

1567


basalts, where contamination by continental crust and meteorites except in some 12 fragments which are geo-chemically linked to the Moon itself. Relatively minorsediment was clearly minimal. Justification for this sub-

stantial change of attitude could be found in the belief basaltic igneous activity yielding smooth plains unitsbegan early (>4·2 Ga or earlier), continued vigorouslythat erupted basalts were very uniform (0) coupled with

an implicit assumption that the plumbing systems were for at least a billion years and less vigorously for perhapsas long again. It did not proceed to the generation ofsimple, with magma batches proceeding as discrete en-

tities from source to point of eruption. The extent of large central volcanic constructs, and there are no surfacefeatures which suggest an onset of large-scale resurfacingrehabilitation of processes of contamination, assimilation

and hybridization in basalt petrogenesis is reviewed below as a result of convection of the mantle. The Martian(55), and perhaps the Mercurian, lithosphere (56) was(25, 27, 28).

3. A detailed petrogenetic model for the Moon was strong enough to support large volcano shield-building,and sufficient total volumes of magma were erupted oncreated in 1969–1974 but essentially by mid-1970. This

interpretation has changed little in the following three the Moon. The lunar lithosphere would have supportedsimilar constructs but the rate and style of mare basaltdecades (Taylor, 1975; Vaniman et al., 1991; Shearer et

al., 1998; Shearer & Papike, 1999), whereas that of eruption was for some reason not suitable.The ‘conventional’ model (e.g. Taylor, 1975, pp. 318–terrestrial basalt petrogenesis has changed dramatically—

one might think we were dealing with two different 324, and fig. 7·2 in particular) holds that: (i) the Moonwas volatile and siderophile depleted relative to theplanets!

The origin of such a large yet relatively low-density, Earth and chondrite meteorites from its creation, (ii) an>500–1000 km deep magma ocean formed in the outermetallic iron depleted satellite as the Moon presents

special problems, with current opinion favouring an parts of the Moon during accretion, (iii) bottom-up frac-tional crystallization of this magma ocean occurred withoblique impact between a chemically differentiated proto-

Earth and a chemically differentiated proto-planet (Lis- development of an increasing negative Eu anomaly inthe REE patterns of the residual liquids and of allsauer, 1997). A large mass of iron-depleted mantle ma-

terial is ejected as vapour, melt and fragments into Earth ferromagnesian minerals later precipitated from themonce plagioclase had started to crystallize; (iv) plagioclaseorbit, where some accretes to form the present Moon

and the rest is either recaptured by the Earth or lost floating to form a 60 km thick cumulate crust where itmingled with a Mg- and Cr-rich chilled surface of thealtogether from the Earth–Moon system. Ida et al. (1997)

suggested that a Moon formed in this way would accrete original ocean and (v) mafic minerals settling to formlayered magnesian to more ferriferous and titaniferousin less than a year. Direct ejection of a disc massive

enough to form the Moon may require a differentiated ultramafic cumulates, to which marked negative euro-pium anomalies had been imparted, (vi) with the lastimpactor up to twice the mass of Mars but there are

residual problems concerning the angular momentum of residual liquids (KREEP basalts) becoming enriched inheat-producing elements and crystallizing deep in thethe resulting Earth–Moon system. Such a hypothesis

provides an opportunity for substantial cooling of the crust or erupting to the surface, after which (vii) eitherreheating by this concentration of heat-producing ele-condensing disc materials before reaccretion. Accretion

of the Moon would then be from particles and bodies ments (but see 101), or positive and negative plumeformation as a result of the inherent gravitational in-whose relative speeds at impact would have been much

lower than those of the bulk of the material accreted to stability of the upward decrease of mg-number in theultramafic cumulates, led to the renewed partial meltingform the two proto-planets, with adverse implications for

the postulated formation of a magma ocean. Substantial of these rocks, yielding picritic and ultramafic liquids at130–480 km depth, which (viii) erupted into the mariafurther accretion of cometary and asteroidal materials to

the Earth and Moon may have followed this major as thick basalt sequences, analogous to terrestrial floodbasalts, of unmodified primary magmas, (ix) whose com-impact event, restoring to the upper mantle the budget

of siderophile and volatile elements which might well positions, with only minor near-surface fractionation, arerepresented by those of the returned hand specimens,have been severely depleted in the earlier core-separation

and impact events, respectively. and (x) with negligible modification of all erupted mareand highland magmas by selective volatilization duringThe Moon has a small iron or iron–sulphide core and

is the smallest body in the Solar System known to have eruption. The principal later modification recognizes arole for serial magnesian anorthositic norite plutonisman ancient, heavily cratered crust enriched in plagioclase

beyond the level achievable by dry basalts in low-pressure in lunar highland petrogenesis.All elements of this internally consistent edifice arecotectic equilibria. Some type of comprehensive igneous

processing of the lunar interior is demanded. This anor- structural. Break one and the whole trembles, break twoand the structure is unsound. This paper summarizes thethositic crust composition is unrepresented among the

1568


case for doubting each and every element in that struc- boninites (Crawford et al., 1981), their possible elevationto a major magma type in the genesis of some of theture. An analogous model, which would permit most

terrestrial basalt magma compositions to be interpreted great layered intrusive complexes (Vogel et al., 1999),the recognition of the importance and style of ash-flowas primary magmas, might be devised, were it not for

the need to get the right cumulate to the right place at magmatism (Smith, 1979) and the exploring of the greatsub-crustal peridotite–gabbro complexes (Quick et al.,the right time in a convecting mantle.

4. When the current view of lunar petrogenesis was 1994). The correlation of terrestrial magma type withparticular plate-tectonic environments has been em-formulated, we knew nothing of the surface features and

volcanic activity of Io, Mercury, Mars and Venus, and phasized (Pearce & Cann, 1973; Pearce, 1987; Wilson,1989). Such correlations either demand exceptionallyvery little about the volcanic activity on the Achondrite

Parent Planet. Io is very similar in size, density and prescient mantle compilation four billion years ago orindicate that process, rather than the fine details of thepresumably major element composition to the Moon.

Whatever hypotheses are applicable to the Moon may source composition, is the controlling factor in eruptedmagma composition. The choice of a balance betweenbe required to apply at Io, and at Europa, whose rocky

portion is of slightly smaller size, with repercussions for partial melting process, partial crystallization process andassimilation effects remains but it is prudent to envisagethe giant impactor hypothesis. There is very little mention

of Io in publications about the Moon and still less of the a major role for all three.Terrestrial flood basalt provinces, not least throughMoon in publications about Io [e.g. internally within the

Basaltic Volcanism Study Project (hereafter BVSP, 1981), the studies of Cox and co-workers (BVSP, 1981, sections1.2.2 and 1.2.3), are no longer regarded as a source ofin the Lunar Sourcebook (Heiken et al., 1991), in Planetary

Materials (Papike et al., 1998) or in the latest edition of abundant unaltered primary magmas. In the waningphases of their activity, when partial crystallization inNew Solar System (Beatty et al., 1999)]. Few papers about

igneous processes on Io appear to have been contributed crustal magma chambers and probable crustal meltingand contamination become most important, their erupt-to the annual Lunar and Planetary Science conferences.

The vast literature interpreting results and exploring ive products are in general far removed in chemistryfrom parental magmas derived from the mantle.speculations in the light of the ‘conventional’ model (1)

should be regarded as propaganda rather than proof. BVSP (1981, section 1.2.5) provides a comprehensivereview of MORB petrogenesis, concluding that low-Equivalent effort has not been devoted to evaluating

radically different alternatives. The lunar mare basalts pressure fractionation is a major factor but noting thatit is often not possible to relate the trace element chemistrywere widely interpreted as flood basalts (e.g. Taylor,

1975) and primary magmas from the lunar interior, an of one sample to another from the same site by simplecrystal–liquid processes. Although this has been taken toidentification made by workers few of whom at the time

had much direct experience of terrestrial flood basalts. indicate separate evolution of magma batches in smallisolated magma chambers it is also a predictable con-Basaltic activity on the Earth, Venus, Mars, Vesta and

probably Io is marked by caldera formation or other sequence of the operation of some of the more soph-isticated crystal–liquid separation processes (26–30).evidence of high-level magma chambers, and is probably

accompanied by near-surface partial crystallization and The parental magmas of Hawaiian tholeiites shouldbe picritic in character if produced by partial melting ofmodification of the compositions of basic magmas on

their way to the surface. Eruptions of undifferentiated mantle peridotite at the depths commonly supposed(O’Hara, 1965, 1968a). The proposition was slow to beprimary magmas are infrequent in time and small in

volume on the Earth. accepted (Carmichael et al., 1974, pp. 418, 649), notleast because of the lack of picritic lavas unaffected byEven the lunar highlands are now believed by some

to be products of serial feldspathic volcanism with total phenocryst accumulation. This view denied the derivativenature of Hawaiian tholeiites and relegated to a minorfractionation at low pressure—no igneous rock with the

average magma composition is identified. It is implied role the low-pressure fractionation abundantly evidencedby pervasive disrupted dunite and gabbro xenolithsthat the lunar highland average is made up entirely by

impact mingling of plagioclase cumulates, late residual ( Jackson, 1968), and the petrographic and geochemicalevidence of magma mixing and differentiation in Ki-liquids of the magma ocean and early mare basalts (which

are feldspathic). Only the later mare basalts of the Moon lauean lavas (Wright, 1971, 1973; Wright & Fiske, 1971;Wright et al., 1975; Wright & Tilling, 1980). Todayare currently held to be radically different—thick piles

of predominantly little altered primary magmas from Kilauean tholeiites are interpreted as low-pressure differ-entiates of picritic parental magmas with 16% MgO orgreat depths.

5. During the post-Apollo period the number of differ- more (e.g. Clague et al., 1995), and Mauna Loa andMauna Kea lavas from the Hawaii Scientific Drillingent terrestrial magma types and associations which has

been recognized has increased with the discovery of Project are interpreted as derived from parental liquids

1569


containing at least 15–17% MgO (Baker et al., 1996; >91 ± 1 prevalent in the residual harzburgites ofophiolites and might encourage adoption of these glassesGarcia, 1996; Rhodes, 1996; Yang et al., 1996). Large

quantities of the requisite mafic cumulates have been as primary magmas. Viewed from the opposite end ofthe telescope, however, the fe-number of the olivinesdiscovered (Watts et al., 1985; Carress et al., 1993). How-

ever, current models concentrate on simple olivine or displays a mismatch of >20%, which leaves room forsubstantial amounts of differentiation in some modelsolivine plus clinopyroxene control and do not entertain

more sophisticated magma chamber models (18, 19, (30).In general, migrating melts cannot even be in local26–30) or possible refluxing of the erupted sequence

(O’Hara, 1998). Magma chamber processes buffer the equilibrium with the residues or source mantle throughwhich they migrate (31), hence chemical interactionserupted, modified basalt compositions (1, 26–28).

6. The less one thinks about the definition of a primary between magma and mantle in response to changes inpressure and temperature must have commenced evenmagma, the easier it is to be sure one knows what one

means! Achieving a suitable and robust definition has before the magma has segregated into a recognizablebody. Primary magma cannot, therefore, be definedtroubled many workers (e.g. Carmichael et al., 1974).

Taking a reasonable view of modern ideas about magma simply as the aggregated liquid from all those regionswhich are undergoing partial melting. During subsequentformation, segregation and movement, one might expect

melt to form, it is hoped in local (centimetre-scale) ascent of the magma body further interactions withsurrounding mantle are probably inevitable, a process inequilibrium with partially residual source rock, through-

out a melting volume in which the temperature and melt which the total mass of the liquid may initially increasewhen the country rocks are already hot and ‘fertile’ butfraction developed varies from negligible up to some

maximum, and through which the depth of burial, and will eventually decrease when the country rocks aresufficiently cool. Primary magma cannot, therefore, beconsequently the pressure, varies significantly. Melt ex-

traction is likely to be continuous and the melting process defined as the composition of the liquid when its massis at a maximum. A vague but useful concept on whichone of (imperfect) fractional melting with melt segregation

and mixing in some other location. The aggregated all might agree is that a primary magma composition isone which exists as a liquid close to the source region offractional partial melts are not in chemical equilibrium

with their unmelted residues, even in the absence of the igneous activity and is the product of the partialmelting and segregation events.integration of melts from different parts of a complex

melting regime, which further compounds the problems. 7. Common terrestrial basalts display relatively smallvariations in mg-number, moderate variations in con-Neither the methods of conventional experimental pet-

rology nor those of thermodynamic calculation derived centrations of highly compatible trace elements suchas Ni and Cr, and relatively large variations in thefrom the experimental data can be used directly. As-

sumption of equilibration of the major element com- concentrations and ratios of highly incompatible elements(e.g. Jamieson & Clarke, 1970; Hart & Allegre, 1980).position with a unique residue at a single mass fraction

of partial melting at a unique pressure is unjustified. The 8. Perfect fractional crystallization cannot explain thegood discrimination (variation of the ratios of almostabiding hope, to which most researchers clearly subscribe,

is that the errors introduced by this assumption are small equally incompatible elements) observed between highlyincompatible elements in flows from the same province.relative to the overall changes of melt composition as a

function of pressure. This hope may be justified for PFC predicts large variations in highly incompatibleelement concentrations only as the outcome of very largeelements whose bulk distribution coefficients are relatively

close to unity (e.g. >0·4–2·5 for most of the major and reductions in mass fraction of liquid, which in turndemand large variations in the concentration of highlyminor components) in situations where the pressure range

across which melt compositions are integrated is small compatible elements (Gast, 1968). These effects are notin general observed in basalt sequences. Moreover, Ring-relative to the rate of change of the equilibria with

pressure (e.g. >0·2 GPa below 1 GPa, >0·4 GPa wood (1975) evaluated the effects on Ni and Cr depletionduring fractionation, arguing that they should be de-between 1·0 and 2·5 GPa, and >0·7 GPa at higher

pressures). It is, however, readily demonstrated that this coupled because they are compatible in contrasted min-erals, but did not appear to be so:hope is not fulfilled for highly incompatible elements

during melting processes. Primary magma cannot be ‘Turekian . . . demonstrated the strong covariance ofCr and Ni in a suite of over 100 basalts [mostly tholeiites]defined as that liquid which is in equilibrium with the

source rock or its residue. from all over the world (Fig 4.11) in which absolute Crand Ni abundances varied by a factor of 50’ (Ringwood,The most magnesian MORB glasses with mg-number

>72 are clearly relatively primitive liquids and would 1975, p. 169).Figure 4.11 of Ringwood (1975) is a log–log plot of Nibe in equilibrium with olivines of mg-number >88·9 ±

1·0. This appears very similar to the values of mg-number vs Cr concentrations, of a type beloved of geochemists

1570


for sound representational reasons, but possessed of prop- to olivine crystallization)’ (Hart & Allegre, 1980, pp.121–122, 125, 131, 133).erties which require careful handling by the unwary. The

10. Implicit in the argument (8, 9) were the assumptionsvisually apparent strong covariance in the plot concealsthat processes only operated in chemical systems closeda variation in Ni/Cr ratio from>0·2 to>3·0, a variationto contamination and hybridization, and that an ap-by a factor of 10–15 at any chosen value of Ni or Crproach to equilibrium partial crystallization (which ex-concentration, and elbow-room for a substantial amountplains the geochemical data equally well) in magmaof decoupling if it is required! Mineral phases tend tochambers was impossible because of the difficulty ofco-precipitate in ratios determined by the phase equilibriaextracting the small mass fractions of melt implied andduring fractionation, thus naturally damping or elim-of maintaining equilibrium between residual liquid andinating the anticipated decoupling; covariation would notcrystals. Equilibrium between liquid and crystals wasbe a telling point in these circumstances—even if it diddenied by abundant petrological evidence of zoning,exist. Some factor has to account for the 50-fold variationfractionation and failure to maintain equilibrium in plu-in absolute abundances of Ni and Cr in rocks identifiedtonic rocks. Partial crystallization models more soph-as basalts. It cannot be a product of the partial meltingisticated than simple closed system perfect fractionalprocess at small to moderate mass fractions of partialcrystallization, however, can yield solid products whichmelting, because the Ni and Cr in the liquid would beshow zoning and cryptic variation and liquid productsbuffered by the compositions of the large mass of residualwith properties similar to those of the EPC process butperidotite minerals rich in these elements during anywith the residual liquid now readily available for tappingsuch melting process. Partial melting cannot explain thisas lava flows (16, 20, 26–30).without simultaneously postulating a matching extent of

11. See Maaløe (1982) and McKenzie (1984, 1985a,variability in the source peridotites. Partial crystallization1985b).processes must be welcomed because they offer a viable

12. The liquid end product of perfect fractional partialmeans of producing the observed spread of values frommelting, provided that the melt fractions are subsequently1000 to 20 ppm Ni.aggregated, would be very nearly the same as in the9. Equilibrium partial melting (Gast, 1968) predictsequilibrium case (Gast, 1968; Shaw, 1970; O’Hara, 1993).large variations in both concentration and ratio of highlyThe final residues are, however, very different.incompatible elements (such as the REE) at small but

13. Perfect fractional melting extracts most of thevariable (0·001–0·05) mass fractions of liquid in theincompatible trace elements in the first small mass frac-

system, which would be coupled with relatively high and tions of melting, imparting spectacular discrimination (7)stable concentrations of highly compatible elements (such among the very low residual concentrations in the residualas Ni or Cr). Equilibrium partial melting leading to the peridotites. Most peridotites exhibit higher concentrationsproduction of partial melts within the mantle was a and less discrimination than would be expected afterreadily acceptable concept. The whole trace element perfect fractional extraction of even a few percent ofargument was encapsulated in a review celebrating the partial melt.fiftieth anniversary of the publication of Bowen’s in- 14. The extremely influential conclusions of Car-fluential text: michael et al. (1974) gave most weight to the trace element

‘Partial melting is now thought to have a greater role geochemical approach to basalt genesis. Wyllie (1979)in producing the observed chemistry of erupted rocks, later provided an overview of the terrestrial petrogeneticrelative to fractional crystallization . . . The Rayleigh debate which gave more weight to the petrological–majorequation is commonly used to model fractional crys- element–experimental approach. Carmichael et al. (1974,tallization in a closed system (PFC), such as a magma p. 28) quoted Karl Popper (1972) as the greatest livingchamber; the Berthelot–Nernst equation is used to model exponent of the logic of scientific discovery: ‘Every goodpartial melting processes (EPM) . . . Crystallization of scientific theory is a prohibition: it forbids certain thingsvarious basaltic minerals is not effective in fractionating to happen. The more a theory forbids, the better it is’,the incompatible elements from each other; . . . In contrast and, in criticism of the more sophisticated view of ter-to this, the melt concentrations of compatible elements restrial basalt petrogenesis, ‘Its universal applicability ismay be drastically changed during crystal fractionation, an obstacle to rigorous testing by attempted disproof andso that melt concentrations of elements such as Ni, Cr, so is a weakness’. What would Popper have thought ofSr (Eu) and Sc (Yb) serve as excellent indicators of the hypotheses which postulate the existence of convenientextent of crystallization of the minerals olivine, clino- source compositions in an inaccessible, untestable loca-pyroxene, plagioclase and garnet respectively . . . Such a tion, to explain each basalt type which is sampled?process [of eclogite and olivine fractionation] would be The more sophisticated interpretation of basalt pe-expected to severely deplete the residual liquids in the trogenesis is, moreover, founded on two sweeping and

comprehensive prohibitions. The vast majority of eruptedheavy-REE (due to garnet crystallization) and in Ni (due

1571


magmas have compositions which should not be, and more relevant to lunar petrogenesis, but this involvesreaction between olivine and liquid—liquids will evolveare not, in equilibrium with upper-mantle mineral as-

semblages and compositions at the appropriate pressures in a eutectic-like manner only if the partial crystallizationprocess is one involving magma recharge and escape,(O’Hara, 1968a, 1968b; Stolper, 1980; Herzberg &

O’Hara, 1998). Sample suites from upper-mantle rocks which can lead to results not easily distinguished fromsimple equilibrium partial crystallization. Aspects of ol-which have undergone variable degrees of partial melting

should not show the extraction of commonly erupted ivine gabbro fractionation from liquids close to the ther-mal divide have been little explored.basalt compositions, and do not (O’Hara et al., 1975b).

Instead, they show the major element effects of the Liquid compositions which are in cotectic equilibriumwith three or more crystal species and are also close inextraction of olivine-rich magmas.

15. The liquid composition at a true eutectic solidifies composition to the liquid at a thermal minimum withinthat equilibrium may behave similarly, and without losson cooling without change of composition, precipitating

a fixed mineral assemblage in fixed mineral proportions of the eutectic-like behaviour as differentiation proceeds.However, no case of this which is likely to be relevantwhich is obviously identical in composition to the bulk

liquid. Add a trace amount of some incompatible element to basalt origins is yet known.Liquid compositions which are in cotectic equilibriumto that liquid, too small to influence the phase equilibria

(at least until very substantial crystallization has occurred) with a large number of crystal species may also exhibita close approximation to eutectoid crystallization pro-and the differentiation of the residual liquid will then be

marked by negligible change in major element com- vided the minerals include repositories for all or most ofthe major and minor elements in the liquid. The averageposition accompanied by increases in the highly in-

compatible element concentration which are inversely (not hand-specimen) high-titanium basalt compositionsfrom the Apollo 11 and 17 sites are close to cotecticproportional to the mass fraction of liquid remaining.

True eutectic crystallization is most unlikely to be en- equilibrium with olivine, calcium-poor and calcium-richpyroxene, plagioclase, spinel, ilmenite and armalcolite.countered in any natural basaltic composition but there

are three situations of eutectic-like crystallization which Crystallization of such an assemblage would lead toeutectic-like behaviour and is one possible factor in themay arise. These could lead to effects among the in-

compatible trace elements which might otherwise seem trace element contrasts between the low- and high-potassium basalt groups at the Apollo 11 site (O’Hara etto demand an origin by very small and variable mass

fractions of partial melting. al., 1974).Nodule suites in alkali basalts (O’Hara, 1965, 1969a),Liquid compositions which are in cotectic equilibrium

with three or more crystal species and are also close in kimberlites (O’Hara et al., 1975b; Cox et al., 1987) andperidotite massifs (Obata, 1980) demonstrate that eclo-composition to the liquid at a thermal maximum within

that equilibrium (i.e. close to a thermal divide) may gites and garnet pyroxenites have precipitated in pro-portions which would be significant in relation to theexhibit large amounts of crystallization with only minor

changes in the major element composition (e.g. increases amount of residual peridotite and probable mass of partialmelt which have developed. Eclogite separation is clearlyin Fe/Mg and Na/Ca) during both equilibrium and

fractional crystallization. Trace elements highly com- implicated in alkali basalt and kimberlite genesis and arole for eclogite–liquid partitioning or garnet pyroxenitepatible in one of the crystallizing phases may not have

a particularly high distribution coefficient in favour of melting in the periphery of melting regimes has beenargued on trace element and isotopic grounds ( Johnsonthe bulk crystal assemblage if the percentage of the

relevant phase is small in the eutectic-like precipitate. et al., 1990; Eggins, 1992; Hirschman & Stolper, 1995).Eclogite fractionation could not be a factor in lunarThe eclogite thermal divide (O’Hara & Yoder, 1963,

1967) at pressures above >3·0 GPa (Milholland & Pre- igneous petrogenesis within the outer 500 km, the depthof the postulated lunar magma ocean, because pressuressnall, 1998) and the olivine–gabbro thermal divide at

pressures below >0·8 GPa (O’Hara, 1969b) are such are too low—but it could in principle be a factor formagmas generated at greater depths within the coolingequilibria potentially relevant to terrestrial basalt pe-

trogenesis. Because the locus of compositions of liquids asthenosphere, which is now encountered only at depthsgreater than 1000 km (Taylor, 1975, p. 291). Eclogiteformed by initial partial melting of peridotites passes

through these two divides at >2·5 GPa and 1·0 GPa fractionation probably plays little part in evolution ofterrestrial tholeiitic basalt eruptives at mid-ocean ridges,respectively (O’Hara, 1968a; Herzberg & O’Hara, 1998),

it is probable that such liquid compositions do exist as in voluminous continental flood basalt provinces and inthe most active phase of ocean-island centres. Continentalparental liquids. It is not yet possible to exclude a role

for the olivine–gabbro divide in lunar petrogenesis. A flood basalt provinces such as the Parana, suggested tohave formed by thermal conduction from a plume headthermal divide also exists on the olivine+ calcium-poor

pyroxene + plagioclase equilibrium and might seem into the lithospheric upper mantle (Turner et al., 1996)

1572


may, however, provide opportunity for eclogite frac- That knowledge of distribution coefficients was not toolimited to prevent acceptance of sweeping conclusionstionation in tholeiite petrogenesis.

Eclogite fractionation was rejected (Gast, 1968; Ring- which did not agree with an abundance of field, ex-perimental and major element data. The volume ofwood, 1975) on the grounds of its predicted effects in a

closed system perfect fractional crystallization process publications in the quarter-century following 1968, eachreporting that some facet of the geochemistry of someon Yb concentrations and its probable effects on Cr

concentrations. This rejection, however, overlooked the suite of basalts required their origin by small mass frac-tions of partial melting of somewhat different mantle, islow mass fraction of garnet in the precipitating solidus

assemblage (O’Hara & Yoder, 1963, 1967). It also over- history.In modelling crystal–liquid separation events we arelooked the unknown distribution coefficients of Cr be-

tween liquid, garnet and the highly aluminous sub-calcic concerned with the apparent bulk distribution co-efficients; i.e. with the ratio of the concentration of anpyroxene actually present at the solidus, and it did

not of course consider more sophisticated crystallization element in the average extract produced divided by itsconcentration in the average liquid produced at the samemodels not then available (8).

16. Imperfect fractional processes of crystal–liquid sep- time. Extract is specified rather than solid to cater forescaped as well as trapped liquid and the possible lossaration, in which extraction takes place by small but

finite, rather than infinitesimal, increments, have effects of immiscible liquids and vapours. Process can greatlyinfluence the apparent bulk distribution coefficients ofon highly incompatible elements in fractional melting and

on highly compatible elements in fractional crystallization trace elements. Fractional partial melting processes (Gast,1968) rapidly relocate highly incompatible elements intowhich rapidly approach those of the equilibrium process

as increment size diverges even slightly from the in- the average integrated liquid produced and deplete theresidue. This causes the apparent bulk distribution co-finitesimal assumption embodied in Rayleigh frac-

tionation (O’Hara, 1993). efficient (concentration in solid/concentration in liquid) toalter away from unity relative to the simple crystal–liquid17. Experiment required that common basalts could

not be primary magmas and petrological observations equilibrium value, i.e. the distribution coefficient de-creases for incompatible elements and the effect is moreindicated that large mass fractions of partial melting were

involved. Petrology and field relations indicated that marked as the element becomes more incompatible.Fractional crystallization processes (Gast, 1968) have abasalt magmas were commonly fractionated in the crust

and when erupted generally bore the hallmark of low- similar effect, but acting this time on the highly com-patible elements where the apparent bulk distributionpressure modification by substantial mass fractions of

partial crystallization (essentially the major element– coefficient increases. Processes of melt integration duringpartial melting (O’Hara, 1985, 1995a; Eggins, 1992)sophisticated crustal process story). Trace element be-

haviour indicated that, of the models considered, contribute disproportionately high concentrations of in-compatible elements from the little melted peripheryequilibrium partial melting at very small mass fractions

of melting best explained the observed geochemistry of to the average residual solid whereas the large meltcontributions from the centre of the regime dilute thebasalts. Other aspects of basalt chemistry seemed to

exclude any significant role for closed system perfect high concentration in the average liquid. The final resultis that the apparent bulk distribution coefficient forfractional crystallization (essentially the trace element–

source process dominated story). The onus was on all incompatible elements during integrated partial meltingis changed towards the value of unity, but there is littleparties to demonstrate that (a) there were no processes

other than the two extreme closed system effects con- effect on the highly compatible elements. A similar effectis observed during integrated partial crystallizationsidered which were capable of producing the geochemical

effects—processes which might be more compatible with (O’Hara & Fry, 1996a) but in this case it is the highlycompatible elements which are most affected. The con-the partial crystallization requirements, and (b) to test

that there were no other feasible interpretations of the tribution of liquids which have been little fractionatedand so retain their highly compatible elements dominatesexperimental petrology results which might allow low-

pressure cotectic character to be a coincidence or of the budget of the integrated liquid. The contribution ofsolids which have been much fractionated and have theirlesser significance. In this the geochemists may have

felt under less pressure, as witnessed by the confidence budget of the highly compatible element diluted byfurther crystallization dominates the budget of the av-expressed by Hart & Allegre (9) and in the following:

‘A number of more complicated models have been erage solid, with the result that the apparent bulk dis-tribution coefficient is altered towards the value of unity,proposed . . . but have not yet been extensively used. In

many cases, our knowledge of partition coefficients is too i.e. decreased. Small packet crystallization (Langmuir,1989; O’Hara & Fry, 1996b) alters all apparent bulklimited to justify the use of more complex models’ (Hart

& Allegre, 1980, p. 125). distribution coefficients towards the value of unity and

1573


towards maximum or minimum values which are con- & Stolper, 1999), introducing geochemical complexitiestrolled by the parameters of the process. which it might be very difficult to invert from a knowledge

Apparent bulk distribution coefficients for trace ele- of erupted liquid composition only, and which add toments can be highly process dependent (Fig. 1). The the complications of defining primary magmas alreadyconclusion of Hart & Allegre above may be extended noted above. Maaløe (1999), however, argued that steady-to state that even when the equilibrium crystal–liquid state eruptions as at Kilauea volcano cannot be powereddistribution coefficient for an element has been de- directly by a percolative plume source because over-termined with high precision, there will be uncertainty, pressures generated would be too high. Episodic upwardperhaps of orders of magnitude, in the appropriate bulk movement as disperse multiple mini-intrusions is pre-distribution coefficients to employ when modelling pos- ferred and obviously gives opportunity for partial crys-sible melting and crystallization processes. Several of the tallization and assimilation but lessens the scope for zonecomplications lead to the apparent bulk distribution refining and chromatographic effects.coefficients being much closer to unity than the equi- Partial crystallization of a depressurizing magma flow-librium distribution coefficient. Many discoveries and ing in a conduit or ascending in a convective cell, or ofinsights into igneous petrogenesis have come in the years a pressurizing magma in a descending convective cell,following the Apollo program and have introduced great can lead to some surprising effects both in the suspendeduncertainty into any attempt to model the geochemistry crystal load and in any static cumulates formed on theof planetary basalts in the absence of a good knowledge of side wall ( Jamieson, 1970a; O’Hara et al., 1975b). (Thetheir field relations. This task cannot even be undertaken implicit assumption is that the situation is non-adiabaticreliably and unambiguously for more than a handful of and that heat is lost from a depressurizing dry magmaterrestrial examples where field control greatly exceeds to the conduit walls.) Related effects must also be a factorthat likely to be available for any other planet for decades in porous flow through peridotite whose importance willto come. increase as the depth of melt formation increases. A

18. At high temperatures and pressures the pyroxenes liquid which is in cotectic equilibrium with two or moreexhibit substantial ranges of crystalline solutions towards crystal species at the outset will, because of the changingeach other and towards the alumina-bearing components primary liquidus phase volumes as a function of pressure,which might otherwise appear as plagioclase, spinel or have to crystallize those minerals in proportions whichgarnet. The mineral proportions in a slowly cooled source may be very different from those appropriate underrock or cumulate assemblage may be a poor indication

isobaric conditions in order to stay in the cotectic con-of the crystalline phases which were present and able todition as the pressure changes. Let us consider a liquidpartition trace elements between solid and liquid at thein equilibrium with three crystal species at the outset.temperature and pressure of formation. Early examplesThe ratio of the minerals which precipitate in the magmaof this were the extensive solubility of potential garnetor on the side wall is in fact dependent on the ‘velocity’into the orthopyroxene structure at high pressure (Boyd(strictly, on the rate of change of enthalpy with pressure).& England, 1964) and into the clinopyroxene structureToo fast a decompression and the adiabatic gradient willin natural and synthetic eclogites (O’Hara & Yoder,carry the liquid above its liquidus and nothing will1963, 1967). These changes in modal mineral assemblageprecipitate; slower and only one phase will precipitate—can have dramatic effects on the calculation of bulkthe liquid composition will evolve but will not remaindistribution coefficients (O’Hara & Mathews, 1981), quiteon the cotectic; slower still and a second phase may joinapart from the effect of the changing composition ofthe first, but in a proportion which will increase towardsthe pyroxene phase on the pyroxene–liquid partitionan isobaric two-crystal phase cotectic ratio as the ‘velocity’coefficients.decreases—the liquid composition will evolve still closerZone refining effects during magma ascent were pro-to the three-crystal phase cotectic and may even attainposed by Harris (1957), underpinned the wall-rock re-it; slower still and the liquid remains on the three-crystalaction process (Green & Ringwood, 1967), were reviewedphase cotectic as pressure changes but the ratio of theby Cox et al. (1979), and have found an application incrystal species which must precipitate in order to stay onunderstanding the concentration of the platinum-groupthat cotectic will be biased away from the isobaric ratioelements into immiscible sulphide liquids from thetowards excess of the first and second phases to an extentNoril’sk flood basalts (Naldrett et al., 1996). Zone refiningwhich is still ‘velocity’ controlled. Factors of this natureeffects during the passage of mafic magma through theunderlie part of the discussion of polybaric melting andmantle would lead directly to trace element characteristicscrystallization by Niu (1997, 1999) and Walter (1999).similar to those observed in basalts. The interactionsContemplation of what happens in a magma flow under-between percolating magma and upper mantle maygoing an increase of pressure is left as an exercise for thealso exhibit the effects associated with chromatographic

separation columns (Navon & Stolper, 1987; Asimow reader.

1574


19. During mixing of two contrasted magma com- plagioclase with olivine compositions in ophiolite gabbrosis distinct from that among phenocrysts in MORB.positions, each saturated with crystal species, the resulting

liquid will in general have a composition within the Although there are many factors involved here, it maybe unwise to assume that the minerals saturated in theprimary liquidus phase volume of at most one of the

mineral species saturated in either of the end-member magma at the roof of a magma chamber are necessarilythe same in type or composition as those solidifying atliquids, and may lie within the primary phase volume of

a phase not even present in either of the end-member its base.The changing composition and structure of the liquidliquids. This can result in the forced precipitation of an

exotic phase which would not have been prominent or from which a phase is separating can change the dis-tribution coefficient dramatically. Ni, which has d >0·5even present in the closed system crystallization of either

end-member component. This effect also has relevance in the crystallization or melting of olivine in the forsterite–nickel–olivine system (Ringwood, 1956) must have d>2to hybridization of melts separated by thermal divides,

and to magma contamination (Chinner & Schairer, 1962; during the production of komatiitic melts yet has d>10–15 in silica-saturated tholeiitic basalt. Ni behaviour,Gribble & O’Hara, 1967; O’Hara, 1969b, 1980; McBir-

ney, 1979). The copious precipitation of near-mono- for this reason alone, is not a good indicator of olivinefractionation (Irvine & Kushiro, 1976; Hart & Davis,mineralic layers of chromitite in layered intrusions has

been ascribed to such effects in the Bushveld, Great 1978; Clarke & O’Hara, 1979).The speciation of the trace element in question mayDyke, Muskox and Rhum intrusions (e.g. Irvine, 1977).

Precipitation of an immiscible sulphide liquid has simi- greatly alter the bulk distribution coefficient, exemplifiedby the contrasting behaviour of iron in the ferrous andlarly been attributed to magma mixing in such bodies

(Naldrett, 1989). Such effects can dramatically alter the ferric states in basalts. Cr as Cr3+ is highly compatiblein spinel and clinopyroxene; as Cr2+ it behaves like Fe2+apparent bulk distribution coefficient for an element in

the total process. Also to be taken into account are and is mildly incompatible in all common ferromagnesianphases fractionating early from basalt. Eu3+ is highlypossible reactions between the top of the cumulus pile

and a new magma input. These cannot in general be in incompatible in all phases fractionating early from ba-saltic magma; Eu2+ is compatible in plagioclase (Morrisequilibrium with each other.

Partial crystallization of magma in small packets can et al., 1974; Drake & Weill, 1975; Schreiber, 1977).Platinum group element ions, PGE4+, are incompatiblecontribute geochemical signals, which imply the sep-

aration of ‘occult’ phases, to bulk residual liquids whose with respect to silicate phases but highly compatible insulphide; PGE0 are highly compatible in metal relativemajor element compositions are controlled almost ex-

clusively by the early separating phases (29). The resultant to silicate melt. The bulk distribution coefficients of allthese elements are functions of the oxygen fugacity andapparent bulk distribution coefficients cannot be pre-

dicted from the simple crystal–liquid values or knowledge will be significantly affected by the differences in oxygenfugacity between typical terrestrial and typical lunar lavas.of the parent magma composition. Oversimplified models

of the relationship between the bulk crystal extract and Within this range the behaviour of the Eu/Sr ratiomight be a sensitive indicator of oxygen fugacities duringthe residual liquid developed during the crystallization

of large magma bodies have been replaced by recognition crystal–liquid processes involving plagioclase. It may alsobe sensitive to those changes in oxygen fugacity in closedthat considerable diagenesis of the growing cumulate pile

may be caused by liquids which differ from the average basaltic systems which can be induced simply by changeof pressure, as well as by consequent changes in thesupernatant liquid in composition (Irvine, 1980; Sparks

et al., 1985), an effect which may invalidate all attempts speciation of C and S in the melt.20. The role of trapped melt in the partial meltingat geochemical modelling using equilibrium crystal–liquid

distribution coefficients. (dynamic melting) process was explored by Langmuir etal. (1977) and its role in both equilibrium and fractionalConsolidation in large magma bodies is frequently

accompanied by substantial assimilation of the roof and melting and crystallization processes further explored byO’Hara (1993). The principal effects are to modify thewalls, which is likely to affect the incompatible trace

element contents and isotopic ratios unpredictably and bulk distribution coefficients of all trace elements towardsthe value of 1·0, reducing the impact of all crystal–liquidfurther complicate all modelling (27).

Evisceration of substantial portions of the contents of processes on the concentrations and ratios of both highlyincompatible and compatible elements. Melt trapped inmagma chambers in ash-flow eruptions has established

the existence of large-scale compositional zonation within the cumulate pile during partial crystallization has similareffects on bulk distribution coefficients (O’Hara, 1993).acid magma bodies which cannot be explained by any

crystal–liquid process yet proposed (Hildreth, 1979). The The influence of periodic escape of partially differentiatedliquid from a chamber undergoing fractional crys-possibility of related effects being latent in basic magma

chambers has been little explored but the correlation of tallization without recharge was explored in connection

1575


with the behaviour of MORB magma chambers (Cann, when the plumbing systems which tap the melt zone arenot centred with respect to the plume axis (DePaolo &1982). The trace element geochemical effects in the

residual liquid composition are similar to those en- Stolper, 1996) adds a further complication.24. Integration of the residual liquids of perfect frac-countered when there is trapped liquid in the cumulates.

An additional phase is being removed for which the tional crystallization from partly solidified magma bodieswith temperature gradients can produce liquids whosedistribution coefficient of all elements is exactly unity;

apparent bulk distribution coefficients of highly in- geochemical characteristics will be closer to or identicalwith those of equilibrium crystallization of the samecompatible trace elements are grossly modified towards

higher values. This leaky fractionation, however, allows parent liquid—and can even yield liquids with higherretention of highly compatible elements while still con-the mathematical involvement of a much higher mass

fraction of apparent ‘trapped liquid’ than would be centrating the highly incompatible elements to the sameamount (O’Hara & Fry, 1996a, table 1). Integration ofpermitted by the porosity of the cumulate and is un-

affected by any diagenesis of the cumulate pile. The effect liquids from a process which locally simulates equilibriumpartial crystallization has even more dramatic effects.of magma escape on the geochemistry and petrology of

the cumulates is potentially very different from that of 25. Magma mixing is important in MORB (Muir &Tilley, 1964; Bonatti et al., 1974; Rhodes et al., 1979;trapped liquid. The cumulates might be pure adcumulates

with no trapped liquid yet the erupted liquids might Walker et al., 1979; Bloomer et al., 1989; Meyer et al.,1989). The Kilauea volcanic plumbing system has provedrecord a partial crystallization sequence which seemingly

required involvement of substantial trapped liquid. to be complex (Ryan et al., 1981) with ample opportunityfor recharge and fractionation at relatively low pressures.21. Maaløe (1982) and McKenzie (1984, 1985a, 1985b)

have argued that trapped melt fractions should be small Hawaiian basalt evolution involves much magma mixingand gabbro crystallization (Wright, 1971, 1973; Wrightduring mantle melting processes, and Thompson et al.

(1984) argued from observed incompatible trace element & Fiske, 1971). The potential importance of magmarecharge during the partial crystallization of magmadiscrimination in basalts that trapped melt fractions had

to be tiny and overall mass fractions of melting small bodies was first recognized in the Rhum gabbros (Harker,1908; Brown, 1956) and has assumed major importancethroughout terrestrial basalt petrogenesis. However, a

recharged mantle melting regime, such as is believed to in recent studies of the Bushveld complex (Cawthorn &Wallraven, 1998). Gabbro precipitation in ophiolites wasoperate wherever partial melting is the result of convective

decompression of hot mantle, does permit significant by partial crystallization during extensive magma re-charge and escape, leading to substantial major elementtrapped melt fractions to be present during more sub-

stantial mass fraction partial melting events (O’Hara, fractionation between gabbros, dykes and basalts (Nor-man & Strong, 1975; Browning, 1984).1995b).

22. Major element chemistry of peridotite suites implies 26. Modelling of the geochemical effects of a peri-odically recharged, periodically tapped, continuouslyremoval of >0·2 mass fraction partial melt in places

(O’Hara et al., 1975b). Trace element chemistry of di- fractionated magma chamber (O’Hara, 1977; O’Hara &Mathews, 1981) established that the liquids erupted fromopsides in residual upper-mantle peridotites suggests that

melting beneath mid-ocean ridges may involve pooling such a system could mimic in all geochemical respectsthe liquids produced during EPC of the total input (parentof liquids which have formed by fractional partial melting

up to 0·25 mass fraction over a range of pressures within magma plus contaminants) to the system. These productswould be indistinguishable from liquids produced bythe spinel peridotite and garnet peridotite stability fields

( Johnson et al., 1990). EPM of a similar source material. This apparent sourcematerial, consisting of the primitive partial melting prod-23. Integration of partial melts across realistic melting

regimes in which the mass fraction of partial melt varies uct of the true source plus some contaminants, would ofcourse appear to be a relatively fertile mantle source infrom zero to some maximum (McKenzie, 1984; O’Hara,

1985, 1995a; McKenzie & O’Nions, 1991) offers a re- trace element terms. All that was required to mimic theappearance of small and variable mass fractions of partialconciliation of the apparent conflict between the geo-

chemical features favouring high mass fractions of partial melting was a low but variable ratio of magma escapeto gabbro precipitation. These conclusions appliedmelting (contributions from the centre of the regime)

with those favouring low mass fractions of melting (trace equally to highly incompatible and highly compatibleelements, allowing the observed combination of traceelement geochemical signals inherited from the periphery

of the regime). It should be noted that (22), coupled with element behaviour to develop. The possibility of en-countering ‘perched states’ (Walker et al., 1979) in thethe widespread acceptance of an average melt mass

fraction of >0·1 in the generation of MORB, has sig- chamber means that the presence of olivine as soleliquidus crystalline phase in the erupted liquid doesnificant implications for the incompatible trace element

behaviour (see O’Hara, 1995a, fig. 14). Melt integration not preclude wehrlite or gabbro fractionation in some

1576


chamber below, another way of contributing a geo- SPC–EPC, or involvement of integrated partial crys-chemical signal of ‘occult’ phase removal. tallization within the small packets has even more

27. Provided the mass of the resident magma is large dramatic effects.relative to individual recharges and discharges, short- 31. In all the terrestrial planets the adiabatic gradientterm fluctuations in input composition, assimilants and with depth will be less steep than that of a dry basaltamount of fractionation between discharges will be liquidus. Ascending volatile-free magmas will be su-buffered, yielding considerable uniformity in the erupted perheated and should resorb entrained phenocrysts andproducts (O’Hara & Mathews, 1981). react with their surroundings especially if proceeding by

The role of assimilation in basalt genesis has been percolation or crack propagation (Shaw, 1980; Spera,considerably rehabilitated (McBirney, 1979; O’Hara, 1980). Eruption of primary magma (6) requires transport1980; DePaolo, 1981; O’Hara & Mathews, 1981), and without chemical interaction of a body of hot, potentiallyin particular that of assimilation of its own earlier eruptive superheated, and highly reactive magma through a chan-products by an advancing RTXC magma chamber nel of 130–480 km length in cooler, readily reactable(O’Hara, 1998). Such processes play havoc with the upper mantle with which the magma cannot be inincompatible trace element signal while contributing to chemical equilibrium as a result of the pressure changesthe pressure on basalts to conform to low-pressure cotectic alone. As a chemical engineering problem, this wouldliquid compositions. An estimated 21% bulk assimilation be a daunting task, which only becomes more difficultof crust has been inferred for the Hasvik layered intrusion, on arrival at the base of the crust. Here density contrastswith a mass ratio of assimilation to partial crystallization combined with composition contrasts inhibit escape ofof 0·27 (Tegner et al., 1999). Extensive and variable parental ultramafic magmas direct to the surface andcrustal contamination marks the magmas of the Bushveld favour assimilation and partial crystallization (Huppertcomplex (Cawthorn & Wallraven, 1998). Hybridization & Sparks, 1985a) to yield residual liquids whose densitiesbetween basic and acid magmas has long been known fit within a window of eruptability (33). Any hypothesisin terrestrial environments. Invasion of the breccia lens of primary magma eruption presupposes that these prob-below major impact basins by hot basaltic magma must lems have been solved.afford a uniquely favourable opportunity for assimilation Eruption of superheated ultramafic lavas on the Earthand even hybridization with any remaining impact melt. has been accompanied by extensive thermal erosion.

28. Huppert & Sparks (1980a) showed that mixing of Assimilation of several percent or more of substrate maynew magma input was not necessarily simple or intuitive

have occurred (Williams et al., 1999) in circumstancesand provided a process, supported by field observationsprobably less favourable to assimilation than those in thein the Rhum magma chamber and elsewhere, by whichconduit. In the case of the lunar lavas there is the risklarge mass fractions of olivine can be rapidly removedof contamination by KREEP, a material exceptionallyfrom picritic magma inputs under well-stirred (i.e. quasi-rich in incompatible trace elements and with distinctiveequilibrium) conditions—which would also favour theisotopic characteristics which might easily obscure thekinetic reduction of the apparent distribution coefficienttrue characteristics of the parental magma (see also 133).for highly compatible trace elements outlined by Hart &

Even workers who accept the argument that the majorAllegre (1980). This and the RTXC magma chamberelement compositions of most erupted basalts have beenprovide processes which would cleanly rid successivemodified by extensive partial crystallization at low pres-magma batches of their excess olivine without spectacularsure have frequently been fortunate to find at least onedepletion of their Ni and Cr contents.flow or hand specimen from a dredge haul in the deep29. Small packet crystallization [SPC; in situ crys-ocean which could be claimed as the local primarytallization of Langmuir (1989)] can contribute geo-magma. On the lunar maria we are looking at the finalchemical signals of ‘occult’ phase separation to eruptedoutpourings of extremely protracted igneous cycles—theliquids. These result from the removal, during advancedtype of situation where, from terrestrial experience, ad-crystallization in each small packet, of phases which arevanced fractionation and contamination are most likelynot close to the liquidus of the mingled resident magmato become important—yet almost every large hand speci-which is the source of any erupted basalts. Apparentmen is held to approximate in composition to a primarybulk distribution coefficients of all trace elements aremagma derived unmodified from depths several timessignificantly modified and depletion of highly compatiblegreater than those from which terrestrial basalts con-elements in particular is greatly reduced (Langmuir, 1989;spicuously fail to erupt unmodified.O’Hara & Fry, 1996b).

The situation for volatile-containing or volatile-sat-30. Combining fractional crystallization in small pack-urated magmas is much more complicated and wasets within an overall periodically recharged, periodicallyconsidered for the ‘closed’ system case of granitic meltstapped magma chamber (RTXC–SPC–PFC; O’Hara &

Fry, 1996b) can produce surprising results. RTXC– by Tuttle & Bowen (1958). The potential effects of evolved

1577


volatiles or equilibrating volatile activities on the country release sufficient to explain this low-pressure cotecticcharacter. Yoder & Tilley (1962) and Jamieson (1970b)rock also need to be taken into account.

32. Fluids will ascend by percolation or crack pro- established the conformity of Hawaiian basalt com-positions to those of low-pressure multiply saturatedpagation through solids of higher density. The liquids

will tend to migrate laterally on arrival at an interface cotectic liquids and further established that these com-positions were not in equilibrium with olivine at elevatedwith solids of lower density. Eruption of a melt then

requires modification of the magma composition and pressures. Low-pressure cotectic character may be ex-plained by coincidence (36), modification of parentaldensity by partial crystallization, assimilation or some

other process. The expansion of dry basic magma with liquids by partial crystallization near the surface (38) orpartial melting at low pressure (40).decompression is greater than that of mantle solids, hence

the density differential increases during ascent through 36. The compositions of dry partial melts in equilibriumwith peridotite minerals change particularly rapidly ata single rock layer.

The density of a liquid in equilibrium with peridotite pressures up to 0·2 GPa (>6 km depth in the Earth,>40 km in the Moon), and rapidly thereafter to >1·0minerals changes much more rapidly than this, however,

because of the rapid decrease in density as the normative GPa (>30 km depth in the Earth, >200 km in theMoon), all the time in equilibrium with plagioclase, twoolivine content decreases with decreasing pressure. Vol-

atile-free picritic magmas are low enough in density to pyroxenes and olivine, and with the composition movingtowards higher normative feldspar and less rapidly to-ascend through the terrestrial and lunar mantles but not

through the terrestrial continental crust, possibly not wards higher normative olivine. Once spinel replacesplagioclase as the mineral in equilibrium with olivine andthrough either the oceanic basaltic crust or the lunar

highland crust where the density relationships are finely two pyroxenes the liquid composition changes steadily butless rapidly up to >2·5 GPa (>75 km depth in thepoised, and certainly not through a lens of less picritic

basalt in a magma chamber. Earth, >500 km in the Moon) moving towards highernormative olivine and lower normative hypersthene con-33. The relationships between magma composition,

magma density and crustal composition are a major tents. At higher pressures garnet replaces spinel in theequilibria and the liquid compositions continue morefactor controlling the ability of basaltic magmas to erupt

(Huppert & Sparks, 1980b; Stolper & Walker, 1980). It slowly to become richer in normative olivine but rapidlybecome richer in normative augite and hypersthene.is difficult to erupt volatile-free picritic and ultramafic

magmas on the one hand, and more iron-rich fractionated There is no coincidence between the compositions ofmultiphase-saturated cotectic liquids at low and highermagmas on the other, because of their high densities.

The relatively magnesian basaltic magmas which have pressures in dry peridotite systems (O’Hara, 1968a; Stol-per, 1980; Thompson, 1987; Herzberg & O’Hara, 1998).evolved from their picritic parents to the point of plagio-

clase saturation have minimum density and are, therefore, 37. The effect of water on peridotite-saturated equi-libria is to decrease the normative olivine content of thethe most likely to erupt through crust of any composition.

This observation does not preclude the occasional erup- melts but simultaneously to increase greatly the normativeplagioclase contents (O’Hara, 1968a, 1972; Ford et al.,tion of more primitive liquids and many researchers have

claimed the discovery of examples of these (6). 1972, 1977). No coincidence appears in the first 1·0 GPaat least. The effect of carbon dioxide is small until34. The alleged copious outpourings of dense picritic

magmas of hand-specimen composition in the lunar pressures in excess of 2·0 GPa are reached, when meltsare then transposed towards higher normative olivinemaria appear anomalous. Reduction of bulk density by

vesiculation and frothing is the major factor which can and augite, and towards silica-undersaturated com-positions (e.g. Dalton & Presnall, 1998). There may,influence the conclusion of note (33). The abundance of

vesicles in consolidated lunar basalts (some are more than perhaps, be some special combination of specific pressure,H2O, CO2 and other volatile component concentrations25% vesicle space) and the widespread evidence for

pyroclastic volcanism on the Moon (83) can circumvent at which the partial melt product of peridotite, oncedevolatilized, matches those of the low-pressure pla-the simple density considerations. Given the debate about

the average condensed magma compositions and about gioclase-saturated liquids. This is not a satisfactory gen-eral solution to the problem.their volatile contents at eruption (73) it is doubtful if

any useful constraints can be developed at present from 38. Low-pressure cotectic character is an automaticconsequence of modification of primary or parentaldensity considerations.

35. Bowen (1928) was the first to recognize the low- magma compositions by partial crystallization at lowpressure (O’Hara, 1965, 1968a, 1968b), possibly ac-pressure cotectic character of erupted basalts and its

probable significance; he appears from his text to have companied by assimilation. Low-pressure modificationof parental magma compositions is the best explanationinclined to the interpretation that partial melting of

peridotite was accompanied or accomplished by pressure of this feature in planets of Moon-size and greater where

1578


the pressure gradient exceeds 0·005 GPa/km. The vary- gabbro fractionation. Phenocrysts also provide evidenceof mixing of contrasted magma compositions (25), aing cumulus assemblages of countless layered gabbro

bodies in the oceanic and continental crust specify the process easily envisaged in magma chambers which pro-vide a suitable environment for sophisticated crystal–low-pressure cotectic phenocryst assemblages which their

coexisting liquids might carry. Because of the large differ- liquid separation processes. Melt inclusions in olivinesand scarce hand-specimen samples provide evidence ofences in composition between low- and high-pressure

melts (36) extensive fractionation of much olivine, pla- the existence of the more picritic parental magmas whichwould be expected to be provided from the upper mantlegioclase and some pyroxene from the primary magmas

has to precede eruption of commonly observed tholeiitic (Sobolev & Shimizu, 1993).Thermal plumes like those of Hawaii and the Mar-basalts. Large mass fractions of partial melting (0·1–0·3)

were envisaged to produce an initial concentration of quesas give rise to large volumes of basic magma and tosubstantial oceanic island volcanic constructs in whichincompatible trace elements, followed by partial crys-

tallization to change major element composition and partial crystallization and formation of extensive olivine-rich cumulates is commonplace, accompanied by erup-variably enrich incompatible trace elements in the re-

sidual liquids. This mechanism circumvented the problem tion of low-pressure cotectic basalts which have had theircompositions modified by partial crystallization high inof collecting very small mass fractions of partial melt

from very large volumes of mantle which is encountered the plumbing system. Flows with MgO-rich compositionswhich might represent the parental magma compositionsby alternative models.

39. Three-quarters of the Earth is covered by ocean are scarce in volumetric terms or poorly exposed, al-though better represented in distal regions because offloor, an>8 km thick basic igneous province comprising

a volume of >2·5 × 109 km3 of magma formed within their greater fluidity.Terrestrial flood basalt provinces such as the Columbiathe past 250 Ma. This volume is comparable with that

of the entire lunar crust and is nearly a hundred times River, Farrar, Karroo, Deccan, Siberia, Keweenawanand the Coppermine–Muskox–Mackenzie dykes largethe volume of visible extruded basic magma on the Moon.

The view that the oceanic crustal section represents an igneous province are each associated with massive in-trusive as well as extrusive activity. Substantial crustalaverage >0·1 mass fraction melt of underlying mantle

has become well established. Geophysical data suggest assimilation, partial crystallization in recharged magmachambers, eruption of low-pressure plagioclase-saturatedthat the fractionated gabbroic products of mid-ocean

ridge magma chambers, rather than dykes or extrusives, cotectic basalts, and possibly still more extensive partialcrystallization in sub-crustal magma chambers (Cox,make up >0·5 mass fraction of the parental magmas

crossing the Moho (Christiansen & Salisbury, 1975). The 1980) mark these provinces. Lava compositions whichmight represent unmodified parental magmas are scarcevast majority of the extrusives are not primary magmas

but are low-pressure plagioclase-saturated cotectic basalts and concentrated mainly among the early, not the latestage eruptives. The West Greenland Tertiary eruptiveswhich have been partially crystallized and separated from

their crystals at low pressure within that magma chamber do, however, appear to contain a high proportion ofprimitive melt compositions (Lightfoot et al., 1997).(O’Hara, 1968b; Stolper, 1980). Decisive factors pointing

to extensive near-surface modification of erupted MORB Nevertheless, many teams even in the recent literaturehave interpreted the geochemistry of lava suites mainlyinclude the experimental petrology data showing that

olivine, which must be a residual mineral in the mantle, in terms of source composition and melting processvariations with little consideration of whether the majoris not on the liquidus in the common basalt compositions

even at the relatively low pressures of melt segregation element compositions of the samples could ever be inequilibrium with harzburgite–lherzolite or the possiblefrom the mantle. Orthopyroxene, which must also be a

residual mineral in the mantle, neither appears on the geochemical consequences of partial crystallization enroute to the surface.liquidus nor is saturated at the liquidus. The erupted

basalts are never in equilibrium with the magnesian 40. Partial melting of peridotite mantle assemblages atvery low pressures may occur in small bodies with lowharzburgite or magnesian lherzolite assemblage thought

to form the upper-mantle residuum. The geophysical central pressures (Stolper, 1977), such as the prospectiveparent body for the howardite–eucrite–diogenite groupevidence for a substantial gabbro layer beneath the basalts

almost throughout the ocean basins is supported by direct of meteorites (Vesta; central pressure >0·2 GPa). Thepressure gradient here is>0·0008 GPa/km, which wouldsampling on oceanic scarps. There is persistent and

widespread petrological evidence of eruption of basalts yield pressures of >0·04 GPa at 50 km depth. Theproblem with this interpretation is that the shift in thewhich are simultaneously saturated with olivine and

plagioclase and close to saturation with clinopyroxene, compositions of liquids in equilibrium with peridotitetowards plagioclase is most rapid in the first 0·2 GPa ofthe three major minerals of the gabbros and the minerals

which should be saturated in the residual liquids from increasing pressure (36). The basaltic eucrites are very

1579


precisely cotectic for plagioclase and pyroxene at 0·1 MPa can be demonstrated to be the products of local grav-itational accumulation of early formed olivine crystals(Stolper, 1977) and probably owe their present com-

positions to phase equilibria control within the outer within basic magma of more ‘normal’ composition. Ex-amples have multiplied since and the phenomenon can10 km of the body. Had they come from deeper, they

should have plagioclase on the liquidus detectably before be observed in the field in many places.45. The first lavas to have reasonable primary cre-pyroxene.

41. Partial melting of peridotite assemblages at very dentials came from flood basalt provinces and were theUbekandt and Baffin picrites (Drever, 1956; Drever &low pressure could be entertained as an explanation of

persistent low-pressure cotectic character in basalts if Johnston, 1957; Clarke, 1970), the picrites of the Deccan(West, 1958) and those of the Karroo (Cox et al., 1965).partial melting is invariably accompanied by, perhaps

even caused by, local pressure release (35). However, at Among ocean-island basalts picritic flows have beenfound in Iceland ( Jakobsson et al., 1978) and picriticthe ambient temperatures required, the pressure differ-

entials at the depth to the upper-mantle source regions distal flows have been identified more recently fromMauna Loa (Baker et al., 1996; Garcia, 1996; Rhodes,will be much greater than the strength of the rocks could

sustain other than transiently and is unlikely to explain 1996; Yang et al., 1996). Truly ultramafic komatiite lavaswere described for the first time (Viljoen & Viljoen,the observations for an Earth-sized, or even Moon-sized

body. It might be appealed to in bodies of asteroid size. 1969a, 1969b) from the Archaean.46. A wealth of evidence favours important roles for42. O’Hara (1965, 1968a) exploited published ex-

perimental data (Yoder & Tilley, 1962; O’Hara & Sch- both partial melting and partial crystallization as majorfactors in basalt petrogenesis. Superimposition of theirairer, 1963; O’Hara & Yoder, 1963, 1967; Green &

Ringwood, 1967) to show that true primary magmas geochemical effects is obvious in the major elementchemistry and strongly supported by field observations.produced by partial melting of dry upper-mantle peri-

dotite at high pressure would be picritic rather than Can this duplicity be detected and distinguished in thetrace element behaviour? When the melting process isbasaltic in character (36). Erupted basalts are not in

equilibrium with upper-mantle mineralogies at upper- one of EPM and the ensuing crystallization process PFC,this is relatively easy (Allegre et al., 1977; Minster et al.,mantle pressures. A succession of reviews of relevant

experimental data (O’Hara, 1968a, 1968b; Stolper, 1980; 1977; Allegre & Minster, 1978; Minster & Allegre, 1978).However, many partial crystallization processes canThompson, 1987; Herzberg & O’Hara, 1998) have dem-

onstrated that the majority of erupted MORB, tholeiitic produce residual liquids more akin to those of an equi-librium process. It is extremely difficult to detect andOIB and CFB cannot be equilibrated with residual upper

mantle. Few of the compositions advanced as primary distinguish the existence of, or the relative contributionsof, an EPC process superimposed on an EPM liquidpass the Roeder & Emslie (1970) constraints on mg-

number of melts in equilibrium with plausible upper- product in the trace element behaviour (O’Hara, 1994).The results for incompatible elements will be adequatelymantle olivines. The arguments have been explained with

clarity by Cox et al. (1979, Chapter 9). The conclusion has described by a single-stage EPM process with a putativemass fraction of liquid which is the product of thebeen substantiated in countless experimental studies in

many laboratories on a wide range of magma com- mass fractions of liquid in the two real processes; e.g.incompatible trace elements in a real 0·1 mass fractionpositions ever since it was first recognized, and is now

independently supported by studies of the ther- melt which is later subjected to 0·5 mass fraction EPCcan be adequately described by a single-stage 0·05 massmodynamics of the basalt–peridotite system (Ghiorso,

1994; Ghiorso & Sack, 1995; Ghiorso et al., 1995). fraction EPM process. The evidence of more sophisticatedevolution is not recoverable from the incompatible traceComposition variation in natural peridotite suites requires

the removal of a picritic, not basaltic, composition (22). element geochemistry. The compatible trace elements,which might in principle be utilized to identify the two-43. See (31). The solubility of gases in silicate magmas

increases very rapidly with the first small increments of stage nature of the process, are by their nature subjectto great uncertainty about the apparent bulk distributionpressure above the confining pressure exerted at the

surface of a small body such as the Moon. Solubility coefficients to be employed.47. Developments in geochemical modelling have pro-thereafter increases more slowly as the pressure rises. It

follows that vesiculation is a solution to the problem of ceeded far beyond the simple choices between perfectfractional melting, perfect fractional crystallization anderupting otherwise dense picritic magmas only if the

initial volatile content is high, or the magma has already equilibrium crystallization or melting which dominatedpetrochemical thinking when the first Apollo samplesbeen transported to shallow depth by some other mech-

anism. were returned. The subtleties (18–28) are far reaching.It would be an exhausting enterprise to prove conclusively44. Bowen (1928) observed that many occurrences of

picritic compositions among flows and minor intrusions that a particular set of geochemical data could or could

1580


not be the product of some plausible mantle composition from one region of space to another and could havedominated the thermal budgets of smaller bodies, evenvia some complex partial melting event followed by

some less-than-simple eruption and partial crystallization if the larger may have taken longer to accrete.There is no logical basis, therefore, to argue that aprocess—the more so when the nature of the source

mantle, the pressure of partial melting, the associated particular body should have developed in a particularway just because other bodies of similar size have donevolatile fugacities and the alkali contents of the melts

must also be regarded as undetermined variables. so. Nevertheless, a pattern of behaviour which appearsto be size dependent does emerge. Volatiles appear to48. This is the central question in mare basalt pe-

trogenesis. If the interpretation of mare basalt com- have been abundantly available even in the smaller bodiesformed, but their retention is restricted to those bodiespositions advocated in this paper is correct, the typical

cross-section of a mare basalt flow unit will show en- which have either not undergone prolonged igneousprocesses or were large enough to retain a substantialrichment of ferro-magnesian minerals near the base and

relative enrichment of feldspathic components and ves- atmosphere. Prevalence and duration of igneous activityincreases with size except where grossly modified by tidalicles towards the top. The lack of detailed field re-

lationships is keenly felt. factors. Early development of highly feldspathic crusts isnot a feature of the asteroidal-sized bodies but is in larger49. Intentionally spare.

50. Most of our knowledge of meteorites, asteroids, bodies (Moon, Mercury and Mars). Io, Venus and Earthhave all been subject to too much tectonic and igneousthe satellites of Jupiter and Saturn, and the other three

major terrestrial planets has been acquired long after the activity to preserve such crusts if they ever existed.Spectral reflectance data for asteroids have alloweddevelopment of views and interpretations about lunar

petrogenesis in the early months of the Apollo program. important conclusions to be reached about the processingof materials in small bodies (Gaffey & McCord, 1977;We have the luxury of reviewing this information before

reconsidering the information for the Moon. Com- Beatty et al., 1999). Carbonaceous chondrites and ironsappear to dominate. Chondritic materials, prominent inparisons are restricted to bodies which have a substantial

proportion of silicate materials in their present make-up. the flux of meteorites striking the Earth in recent time,are scarce in the main asteroid belt but relatively commonWhen attempting comparisons between bodies of similar

size, several additional factors need to be kept in mind. among objects in Earth-crossing orbits. Irons are prom-inent among asteroidal spectral types, which may in partThe bodies have not accreted at the same distance

from the Sun. They will not have had the same rates of be due to their greater resistance to fragmentation. Theirsilicate mantles may have been preferentially comminutedaccretion or access to the same pool of materials (although

much depends upon the balance between a ‘uniform’ by successive collisions over the life of the Solar System.The majority of bodies in the asteroid belt are volatilecometary input relative to local sources). Promptness of

accretion after the birth of the Solar System bears on rich. A minority have undergone post-accretionary heat-ing and differentiation; these bodies are concentrated inthe capacity to incorporate short-lived radioactive ele-

ments and enjoy their brief but major heat output. Speed the inner asteroid belt. All asteroids larger than 450 kmpresent diameter appear to have undergone some heatingof accretion may have a major influence on the internal

temperatures attained during the process and whether but the two largest asteroids, Ceres and Pallas, appearto be composed of an assemblage dominated by olivineor not magma oceans formed. Style of accretion, i.e. the

contrast between accretion from a multitude of small and magnetite, do not appear to have undergone igneousactivity, and are very different from the slightly smallerbodies as against one or more major impact events,

greatly influences the thermal budgets and geochemical Vesta.Mittlefehldt et al. (1998) reviewed the petrogenesis ofevolution. Promptness, speed and style may all be func-

tions of distance from the Sun (and Jupiter) as also may the irons, most of which can be viewed as products of ironmelt separation and subsequent fractional crystallizationbe post-accretional effects, particularly those involved in

the loss of volatiles from any atmosphere developed. from an iron–nickel melt, taking place within 50–60different planetary bodies. This is conventionally in-Present orbits of bodies may bear little relationship to

orbits during critical phases of their evolution. Tidal terpreted as the result of separation of a molten iron–nickel core during asteroid differentiation, followed byheating may have been a major influence in the evolution

of many bodies at some stage in their evolution (Vesta, differentiation during cooling of that core while thesilicate mantle evolved independently. The scale of theGanymede, Europa, Io and the Moon are all possible

victims). Heat production from the decay of long-lived exsolution textures in the metal phase is a function ofthe cooling rate. Radii of the various parent bodiesradioactive elements must today be about one-third what

it was soon after the formation of the Solar System and the calculated, with several assumptions, from the coolingrates range from 3 to 207 km (Mittlefehldt et al., 1998,amounts of short-lived radioactive elements incorporated

into the larger bodies are unknown but might have varied table 4).

1581


Metallic iron is in equilibrium with melts containing a fractionated and may be from two parent bodies. Mittle-fehldt et al. (1998) have reviewed ideas on the petro-slight excess of oxygen over that required to creategenesis of this group and Longhi (1999) has discussedstoichiometric FeO at low pressure (Bowen & Schairer,the development of angrite magmas in relation to the1935). The metal phase is in reaction relationship withol–cpx–plag–sp reaction point in the low sodium, highthe silicate melt during cooling and fractional crys-iron oxide system, a relationship which is also en-tallization. An erupted silicate magma which undergoescountered in the critically undersaturated portion of thereduction by carbon and sulphur loss (Brett, 1976) mayiron-free system CaO–MgO–Al2O3–SiO2 at low pressureprecipitate an iron-, nickel- and sulphur-containing(O’Hara & Biggar, 1969). The petrogenesis of this groupphase, probably as an immiscible liquid, which wouldis very different from that of the two preceding groups,settle and might then fractionate independently. Thebut there is again no suggestion of production of feldspar-silicate portion of the melt will not subsequently pre-rich cumulates by crystal flotation.cipitate metallic iron and will fractionate towards higher

The aubrites are igneous orthopyroxenites with tracesoxygen activities by the precipitation of phases containingof olivine, diopside, calcic plagioclase and a significantonly FeO. The inherent oxygen fugacities of the eucrites,amount of sulphide. They are extremely reduced withthe silicate portion of the mesosiderites and the lunaralmost no iron oxide in the pyroxenes and such lowbasalts are close to that at which iron metal would be inoxygen fugacities that both calcium and chromium be-equilibrium with the melt. Their present states mighthave partly as chalcophile elements. Basaltic membershave been achieved by partial crystallization followingof this suite are not known, nor are they linked to areduction and loss of a metal phase.particular group of irons, but links to the strongly reducedOther silicate magmas represented among the asteroidsenstatite chondrites are suspected. This group is im-have evolved at still lower oxygen fugacities where muchportant in indicating one extreme of the wide range ofof the FeO has been removed from the silicate magma.oxygen fugacities attending igneous processes in differentThe assumption inherent in Mittlefehldt et al. (1998), thatlocations within the Solar System.these oxygen fugacities have been imposed by cir-

None of the above groups of meteorites has beencumstances attending the accretion of the bodies and notreported to have a significant positive cerium anomaly,during eruption of magma at the surface, may not alwaysindicative of evolution under oxidizing conditions, re-be correct and some of the irons may represent near-gardless of where they were recovered.surface igneous precipitates. The hallmark of these should

51. Predominantly silicate bodies of lunar size andbe carbon and sulphur concentrations lower than would density (Moon, 1735 km radius, 3·344 g/cm3) havebe anticipated from closed system processing of chondritic formed elsewhere in the Solar System with an apparentstarting materials. surfeit of sulphur (Io, 1816 km radius, 3·55 g/cm3) and

The mesosiderites are a complex group of meteorites water (Europa, 1536 km radius, 2·99 g/cm3). The mech-which have evolved by igneous and impact processes. anics of accretion of a body of this size do not precludeThey are a mixture of metal phase (>30%) with silicate acquisition of volatiles (BVSP, 1981, table 4.2.1). Thereclasts similar to the howardite–eucrite–diogenite suite. are in all five other bodies in the Solar System—Titan,Eleven percent of large clasts are monogenetic basalts and Callisto, Ganymede, Europa and Io—whose com-5% quenched vitrophyres; 9% primary orthopyroxenites; binations of size and mean density suggest that they maythe basalts are comparable with eucrites with relatively have a silicate and metal portion whose size is comparablelow mg-number and have flat REE patterns with no with that of the Moon. All but the last of these containmarked Eu anomalies at concentrations 5–8 times chon- significant amounts of ice. This group has a non-volatiledritic. A high proportion of the clasts are gabbroic (i.e. rocky) portion intermediate in size between thecumulates which may have extreme positive Eu anomalies asteroids and Mercury, the smallest of the terrestrial(Rubin & Mittlefehldt, 1992, 1993; Mittlefehldt et al., planets. All are satellites and details of their features have1998). The high proportion of cumulate or plutonic been given by Beatty et al. (1999). Triton, Pluto andbasic igneous clasts may be a significant pointer to the Charon are also ice-rich bodies which have similar meanprevalence of high-level partial crystallization events in densities to Ganymede and Callisto but are much lessthe evolution of small bodies. massive and would have rocky cores much smaller than

Few angrite fragments are known (Mittlefehldt & Lind- that of the Moon but still significantly larger than Ceres,strom, 1990; Mittlefehldt et al., 1998). They are dis- Pallas or Vesta.tinguished by high Fe/Mn, high FeO, and very low (Na/ Titan, the major satellite of Saturn, is unique amongSm)N. Fassaitic pyroxenes are accompanied by calcic satellites in having a relatively dense atmosphere andolivines and spinel or anorthite in extremely silica-under- this atmosphere is methane rich. Callisto, the outermostsaturated assemblages. Volcanic outgassing is favoured Galilean satellite of Jupiter, has a very ancient heavily

cratered surface and was thought to have escaped internalover nebular fractionation. The samples are strongly

1582


differentiation, implying that neither the energy of ac- Where conditions of tidal heating have been sufficient tosustain some long-term volcanic activity in Europa, thecretion nor that from radioactive decay was sufficient to

even partially melt the interior, still less produce a silicate amount of ice retained is much less. Where tidally gen-erated partial melting is rampant in Io, ice and watermagma ocean. Recent results from Galileo indicate that

this body may be partially differentiated with a conductive have disappeared but there is still plenty of sulphur, andprobably carbon, too, although this is as yet unproven,layer under the surface (Showman & Malhotra, 1999).

Ganymede has an ancient but much tectonized surface, to provide volatiles to drive pyroclastic volcanism today.Io may originally have had a substantial budget of wateris composed of about 40% ice, and has differentiated

into a thick ice crust overlying a silicate mantle and a and other volatiles. Escape velocities from the surfacesof these bodies are very low. This water would have beensmall, still molten iron core. This difference in behaviour

between two otherwise rather similar bodies is attributed lost if the conditions of accretion had been such as togenerate a silicate magma ocean during their formation.to a tidal contribution to the internal heating of Gany-

mede. Europa, which is subject to somewhat larger energy Formation of silicate magma oceans was not, therefore,an inevitable accompaniment to accretion of bodies ofinput from tidal forces, retains an H2O layer 100–200 km

thick with an ice crust of perhaps as little as 1 km in lunar size. The existence of this class of bodies establishestwo fundamentally important points. They accretedplaces (Hoppa et al., 1999), the surface of which is

little cratered and relatively young. It is resurfaced and plenty of volatiles, and most did not generate globalsilicate magma oceans during their formation. Indeed,deformed in ways which suggest that the ice-rich crust

may overlie a water-rich ocean beneath which there may Kopal (1977) demolished the concept of a substantialmagma ocean at any time in lunar history on astronomicalbe silicate volcanic activity. Internal differentiation has

produced an iron or iron–sulphide core. grounds.54. Showman & Malhotra (1999) have summarized52. Thick ice covers the silicate crusts of Europa,

Ganymede and Titan (Callisto may be little differentiated) much of the new wealth of information relating to allfour Galilean satellites and even as this paper is beingand conceal whether those crusts are feldspathic like the

ancient lunar highlands. The silicate crusts on Europa revised for publication relevant new data are arrivingfrom the Galileo spacecraft (EOS 2000, 81, p. 1), includingand Ganymede are here predicted to be similar in some

respects to that of the Moon, but not volatile depleted observation of magma erupting on Io along a 25 kmfissure to heights of 1·5 km. The style of volcanismof course.

53. Small but still substantial bodies (Ceres, Pallas) observed today on Io is particularly relevant to con-sideration of ancient lunar volcanism.appear to be composed of carbonaceous chondrite and

are, therefore, liberally provided with the potentially Io, the innermost Galilean satellite, has only slightlygreater size, density and mass than the Moon and isvolatile hydroxyl, sulphur and carbon compounds. If

other comparable-sized bodies, such as Vesta and the arguably of closely similar bulk composition, although itslocation in a different part of the Solar System couldparent bodies of the mesosiderites, which have undergone

igneous processes are derived from the same geochemical support any alternative composition consistent with itssize and density. It is subject to major input of tidalpool as other asteroids, mechanisms must exist which

lead to massive volatilization losses, including losses of energy and is the most volcanically active body in theSolar System. A small iron or iron–sulphide core isthe alkalis. It is simplest to assume that the volatilization

losses are associated with the igneous processes. Even a postulated (Anderson et al., 1996). There is no water iceon the surface, which is young—volcanic resurfacingmuch larger body such as Mars, which clearly acquired

a substantial budget of volatiles during accretion, has probably takes place everywhere within 10 Ma and therewill be no trace left of the earliest formed crust of thislost a substantial part of the atmosphere and possible

hydrosphere originally produced and long sustained by body. The outer parts of Io are likely to have beenrefluxed many times through the eruption process andoutgassing. Venus, a much larger body with an abund-

ance of volatile components, with sustained and wide- its accompanying volatile losses. Initial accounts (Sagan,1979) stressed a major role for sulphur in the volcanismspread volcanic activity and a dense atmosphere, appears

to have lost water equivalent to a global ocean 75 m in but the height and steepness of some of the slopes pointto the abundance of strong silicate rocks rather than thedepth.

Plenty of water was available to accrete to bodies weaker sulphur as principal components in this topo-graphy (Clow & Carr, 1980; Greeley et al., 1990). Youngsmaller than the largest asteroids and it did accrete in

large amounts to bodies whose rocky portions are no (1984) argued that the false colour images on which theidentification of sulphur was based were artificially red;larger than the Moon. Present volatile contents of these

bodies and the Moon can be related to their surface real colours were greyer and bluer with the average coloura slightly greenish–greyish–pale yellow. Areas shown intemperatures through a combination of proximity to the

Sun or Jupiter and exposure to tidally driven volcanism. the false colour images as red are moderate greyish–

1583


yellow and may be greenish. BVSP (1981, section 5.8.2) more strongly represented among lower-trajectory part-icles. Volatilization losses from the body are extensiveleft open the question of whether the lavas were pre-(Spencer & Schneider, 1996; Kuppers & Jockers, 1997).dominantly sulphur or silicate. Nash et al. (1986) reviewedSulphur dioxide is prominent in the atmosphere andthe silicate–sulphur controversy and Carr (1986) con-much sulphur is being lost entirely from Io. The cloudcluded in favour of predominantly silicate volcanism.of sodium selectively lost from Io can be detected fromHapke (1989) considered that elemental S was absent onthe Earth in a region 500 times the radius of Jupiter orthe surface and that multispectral studies were consistent10 times the diameter of the Moon in the sky. Thesewith dark areas being fresh basalt and lighter areas beingobservations suggest that extensive selective volatilizationvariably coated with SO2, S2O and CSO frosts. Io maylosses of Na and S would also have accompanied thebe no richer in S than the Moon, whose basalts are stilleruption of the lunar lavas, which still contain morerelatively S rich after eruption into vacuum (73).sulphur than equivalent terrestrial lavas. Wilson & KeilTopographic features include many inactive volcanoes,(1991) and Keil & Wilson (1993) went further and sug-with five calderas >20 km in diameter per million km2.gested that on asteroidal-sized bodies the violence ofThese have steep walls and flat floors and the largest issulphur-driven volcanism may have accelerated basaltic200 km in diameter. There is up to 10 km of verticalmaterial beyond the escape velocity and led to netrelief at the poles. Shield volcanoes 80 km across andimpoverishment of the silicate fraction.2·5 km high with up to 10° slopes are present (Moore et

Showman & Malhotra (1999) quoted a current rate ofal., 1986) and require a 40 km thick lithosphere for theirloss of mass from Io of >1 t/s, which translates into asupport. Ra Patera is a 1 km high volcano with slopes ofcurrent rate loss of >756 t/m2 per billion years. This0·1–0·3°, high eruption rates and low-viscosity lavas.will be equivalent to loss of>250 m of basalt per billionIndividual lava flows are up to 250 km long, up to 15 kmyears or about 1% by weight of a crustal column 25 kmwide with deduced eruption rates of 60–2000 m3/s.deep in that time, and >100 km deep during the life ofThe case for magmatic differentiation of Io was madethe Solar System. The loss rate might have been muchby Keszthelyi & McEwen (1997), who inferred anhigher when the rocks were richer in volatiles. The>50 km thick crust of 2·6–2·9 g/cm3 density, which theypotential for significant loss of sodium and more volatileconsidered would be alkali rich with much feldspar andelements during volcanism over small bodies is clear.nepheline, and they predicted a forsterite-rich mantle.

The final phase of the Galileo mission has explored IoMagmas were considered to have temperatures <1100°C.at high resolution (McEwen et al., 2000). Observations

In contrast to this view, the highest observed eruption include active lava lakes (Pele, Loki patera); a lava curtain;temperatures require mafic or even ultramafic silicate active lava flows emplaced as channelized flows and bymelts with lava temperatures ranging from 1430 to flow under insulated crust; nested caldera chains up to1730°C and are tentatively identified as producing hy- 200 by 50 km in extent and several kilometres deep,persthene-bearing lavas (McEwen et al., 1998). An erupt- implying large near-surface magma chambers, and col-ive event of 1990 (Davies, 1996) has been modelled as a lapse of substantial shield volcano structures; and moun-large silicate lava flow erupting >105 m3/s of lava at tain chains up to 16±2 km higher than the general1200°C. Blaney et al. (1994) modelled IR outbursts as surface. Several plumes come from distal flows rathersilicate magma erupting at >3 × 105 m3/s, rates com- than eruptive sources (Keiffer et al., 2000; Lopes-Gautierparable with those on the Earth and early Moon. Almost et al., 2000). Gaseous sulphur has been discovered in thethe entire thermal output of Io comes from recent lava Pele plume where the sulphur–sulphur dioxide ratioflows. Consolmagno (1981) argued that the current ther- is consistent with equilibration with relatively reducedmal output of Io is much larger than the long-term input basaltic magmas having oxygen fugacities between thosefrom tidal heating and that the volcanic activity on Io may of the IW and QFM buffers (Spencer et al., 2000).consequently be episodic. The Galileo mission (Davies et Despite the eruption into hard vacuum in a gravity fieldal., 1997; Lopes-Gautier et al., 1997; McEwen et al., comparable with that of the Moon, despite the absence1997; Schenk et al., 1997; Spencer et al., 1997a, 1997b; of evidence for high-viscosity lavas and despite exhibitingStansberry et al., 1997) detected 12 previously known the highest rate of volcanic activity in the Solar System,and 18 new hotspots with temperatures consistent with the volcanic topography of Io, with its huge shields andsilicate volcanism. large calderas, is strikingly dissimilar from that of the

Volcanic plumes extend 70–280 km above the surface Moon.and spray pyroclastic material over regions 70–1000 km 55. Venus is very similar to the Earth in size, massin diameter. The mass fraction of silicate melt in these and probably in bulk composition but has a very differentplumes is debatable ( Johnson, 1990) but the large distribution of volatiles between rock and atmosphereamounts of sodium and potassium evaporated probably and may have lost a substantial amount of water. There

has been abundant volcanic and presumably intrusiveindicate a significant silicate component which may be

1584


igneous activity, much of it central in type. Abundant within the ocean basins. Central volcanism with de-flood basalts with distinct flow fronts have not produced velopment of large intrusive complexes is a widespreadmaria-like structures. Although no spreading axes have feature.been recognized, up to 10 000 km of subduction suture Nothing comparable with the large lunar impact basinshave been identified (Schubert & Sandwell, 1995) and has been preserved and there are no unambiguous maria-great lava flows analogous to terrestrial flood basalts in like structures (but see 126). Magma types appear tovolume and rate are seen with deduced eruption rates have undergone some secular evolution; the Archaeanup to 106 m3/s and volumes up to 104 km3 (Lancaster et granite–greenstone belts with their komatiite–picrite vol-al., 1995). Two large continent-like areas with apparent canism have not reappeared in that form in the latertectonic deformation are preserved but the proportional record, large bodies of calcic anorthositic gabbro alsoarea of continental material is lower than in the case of appear to be restricted to the early record, and therethe Earth. is a distinctive massif anorthosite association emplaced

There are no postulated meteorite recoveries from widely around 1·3 Ga ago (Ashwal, 1993). The vastVenus and no sample returns have been achieved. Un- majority of erupted basalts closely approach low-pressuremanned probes Venera 9 and Vega 1 and 2 landed on plagioclase-saturated cotectic character.vast plains units; Venera 8, 10, 13 and 14 on localized 56. Mercury, the innermost planet, is 2·75 times moreplains units (Weitz & Basilevsky, 1993). Volcanic features voluminous, 4·5 times more massive, and more than oneat Venera 8 and 13 are steep-sided domes, blocky lavas and a half times denser than the Moon, and it containsand ash beds, and have non-tholeiitic lava compositions. a much higher proportion of iron metal. It has anThe crust of Venus is inferred from limited sampling ancient, heavily cratered crust with highland, lowlandand geomorphology to be overwhelmingly mafic with and intercrater plains features (BVSP, 1981, section 5.5;aluminous (16–18·4%) basalts some of which are highly Kieffer et al., 1992; Beatty et al., 1999). Smooth plainsalkaline and potassic, others tholeiitic, with rather high units are downbowed, suggesting mascon-like features.MgO (8·3–12·8%) and relatively low iron (7·7–9·4%) No central volcanic constructs were seen on the 45% ofand low titanium (<1·6%) (Kargel et al., 1993). Large, the surface imaged by Mariner 10 but radar images ofpossibly multi-ringed, impact basins up to 170 km in the unseen part suggest the presence of a 500 km diameterdiameter, with smooth, perhaps basaltic fills, are present shield volcano topped by a 70 km caldera (Beatty et al.,on Venus where they probably formed during the past 1999). There is much less contrast in albedo on the500 Ma (Alexopoulos & McKinnon, 1994). In a planet-

surface than in the case of the Moon and mid-IR spec-wide review of large igneous provinces, Head & Coffintroscopy points to the presence of anorthositic breccias(1997) considered Venus to have exhibited a global largevery similar to those on the Moon, and to iron- andigneous province>300 Ma ago with possible total mantletitanium-poor (Fe + Ti <4%) basalts (Sprague et al.,overturn. There may be very little ancient crust preserved1994). Kiefer & Murray (1987) preferred a volcanic originon Venus.for Mercury’s smooth plains and viewed volcanism as aThe Earth’s surface is dominated by a two-fold sub-global process. Rava & Hapke (1987) confirmed a crustdivision. About one-fifth is thick, high-standing, broadlylow in Fe2+ and Ti and found no evidence for a secondcalc-alkaline and sedimentary continental crust, very littlewave of global melting, identifying local outpourings ofof which is more than 2·5 Ga old and almost none ofbasalt only. Some evidence was reported for late pyro-which has survived from the period of the late heavyclastic activity richer in iron and for the presence ofbombardment at >3·8 Ga. As in the case of Io andmore Fe-rich material underlying the surface. Rep-Venus, it may not be possible to directly observe theresentation of samples of Mercury among the meteoritescharacter of the earliest crust. About four-fifths of theis considered a low-probability event and none can besurface is the product of igneous activity at spreadingpositively identified (Love & Keil, 1995). Aubrites wereaxes, is covered by ocean and is broadly basaltic withconsidered the most likely group. Given that the planetminor sedimentary cover, all of which is less than 250 Mais rich in iron, the very low iron oxide content of probableold and most of which is less than 120 Ma old. Super-partial melt rocks at the surface points to a very lowimposed on this pattern are two smaller-scale effectsoxygen fugacity in the silicate mantle and possiblywhich are greatly influenced by plate motion. Subductionthroughout the body. If it accreted in its present positionzones at plate junctions are marked by extensive igneousrelative to the Sun without significant contributions fromactivity which is controlled by hydrous partial melting ofcomets, low volatile content, except for carbon andupper-mantle and crustal rocks. Plume activity on varioussulphur, and low oxygen fugacity could be envisaged asscales has resulted in the production of large basaltica primordial feature. If a lunar magma ocean is rejected,igneous provinces including oceanic plateaux, continentalhowever, similarities between the crustal compositionsflood basalts and great dyke swarms particularly as-

sociated with continental break-up, and long island chains on the Moon, Mercury and Mars (below) may favour a

1585


model of initial volatile accretion followed by losses during samples from Mars, a positive Ce anomaly is not reported(Laul et al., 1986).igneous processing in all three bodies.

The surface of Mars is now well surveyed and numerous Over one-third of heavily cratered Martian highlandsare ancient cratered plains units which are probablysummaries are available (BVSP, 1981, section 5.6; Carr,

1981; Kieffer et al., 1992; Beatty et al., 1999). Ancient volcanic outpourings and there are some volcanic featuressuggestive of ash-flow tuff eruptions in Tyrrhena Patera,cratered highlands cover the southern hemisphere; the

northern hemisphere has been extensively resurfaced Hadriaca Patera, Amphitrites Patera and other ancientvolcanoes within the highlands which have the mor-with basalt and has some of the largest central volcanoes

in the Solar System. The evolution of volcanism with phological characteristics of pyroclastic-containing com-posite volcanoes (Scott & Tanaka, 1981). McSween et al.time has been summarized graphically in BVSP (1981,

fig. 5.6.1b). The Mars global surveyor laser altimeter (1999) summarized the geology at the Pathfinder landingsite on ancient intercrater plains in Chryse Planitia,experiment has shown that the southern hemisphere

is on average >6 km higher than northern. Internal specifically on a flood deposit. Blocks of andesite, similarto icelandite rather than terrestrial orogenic andesite,convective processes are required to thin the northern

hemisphere (Smith et al., 1999). Crustal remnant mag- were analysed by Golombek et al. (1997) and Rieder etal. (1997). These workers viewed any inferences aboutnetism is mostly confined to the ancient heavily cratered

highlands, where there are broad E–W-trending linear the role of water and plate tectonics in the petrogenesisas premature.features up to 2000 km long, on a much larger spatial

scale than associated with spreading axes on Earth (Con- Sparsely cratered uplands plains are present in LunaePlanum, Syria Planum, Sinai Planum and Hesperianerney et al., 1999). Any internal convective activity was

greatly diminished before the growth of the great central Planum. Wrinkle ridges and inconspicuous flow frontsare identified. There are no recognizable vents. Layeredvolcanoes because there is no suggestion of development

of Hawaiian–Emperor type volcano chains. units are exposed in the walls of canyons and channels.Floor fractured craters occur, as on the Moon, associatedAt least a dozen samples from poorly constrained

locations on Mars are available from meteorite collections with the junction between the cratered highlands and thebasalt plains. They have been interpreted as volcanically(McSween & Treiman, 1998). Four extrusive or hypa-

byssal types include heavily shocked, relatively iron-rich modified craters with basaltic magma intrusion liftingand cracking the debris fill.‘tholeiitic’ andesine or labradorite basalts or dolerites

marked by pigeonites strongly zoned to augite. The Mars preserves three large impact basins (Hellas, Ar-gyre, Isidis) in the older, heavily cratered highland terrainsamples are possibly enriched in cumulus pigeonite, which

is foliated either by the flow or cumulus process. Oxygen of the southern hemisphere, a lower density of suchimpacts than on the Moon and consistent with either afugacities are moderate to low, there are no Eu anomalies

in the bulk materials and chalcophile elements are de- smaller number of impacts or more prolonged ancientcrust formation which has obliterated some basins. Thepleted. Three lherzolitic samples are conspicuously richer

in the ferrous components of olivine, orthopyroxene, largest, Hellas, is 9 km deep, and 2300 km wide with araised rim up to 2 km high extending 4000 km from thepigeonite and augite, and in the albite component of

plagioclase than most comparable terrestrial rocks. Crys- centre of the basin (Smith et al., 1999). Isidis has adiameter of 1900 km, Argyre 1200 km. Only the Isidistallization ages of all the above are in the range

330–180 Ma and they may all come from the same region basin appears to have been substantially flooded by basaltmagmas in a manner comparable with that seen on theof Mars and been ejected in a single impact event. There

are three 1·3 Ga old clinopyroxenite or wehrlite samples nearside of the Moon and the Isidis floor contains lowdomes possibly formed along eruptive fissures. Somecomposed of iron-rich augite, some pigeonite and olivine

with minor oligoclase, orthoclase and titanomagnetite. volcanic units are present on the floor of Hellas andArgyre; there are wrinkle ridges and possible flow frontsThey are thought to be pyroxene cumulates within a

lava flow. The samples have been subject to hydrous and in both. The Hellas basin is surrounded by old sprawlingvolcanoes. Hadriaca Patera on the NE rim has a 60 kmoxidizing weathering before ejection. Also 1·3 Ga old is a

cumulate of chromite and iron-rich olivine with interstitial diameter smooth-floored caldera. Several similar vol-canoes occur on the south rim, with large flow channelsaugite, pigeonite and sodic plagioclase. The final speci-

men is ALH84001, a relatively magnesian very ancient leading down into the floor of Hellas.The oldest basalt-like surfaces on Mars are the Old(4·5 Ga) orthopyroxenite cumulate showing later de-

velopment of carbonates and disputed evidence of life Ridged Plains units, which occur as small patchesthroughout the cratered highlands and also near Isidis.on Mars. Fe/Mn ratios in Martian samples are dis-

tinctively low relative to rocks of the Earth and Moon These basalts do not build central constructs and havelunar mare-like surfaces without visible flow fronts andand somewhat lower than in the eucrites. Despite the

relatively high oxygen fugacity evident in available with wrinkle ridges set on broad 10 km wide uplifts.

1586


Circular ridges occur on the margins of these plains units, fit inside some of the Martian calderas. Crater countssuggest activity extending over billions of years. Arsiapossibly as a result of settling of the crust of lava or of a

pyroclastic flow over the rims of preflow craters (black- Mons, with a volume of 106 km3, has a magma pro-ductivity as low as 0·0005 km3/year, much lower thanbird-like features). A large caldera-like feature is mapped

within this early basalt unit in Syrtis Major. that of Hawaii, which is 0·01–0·02 km3/year. These long-term average productivity rates imply >107 years to fillLater igneous activity largely resurfaced the northern

hemisphere with the production of vast basalt plains the collapse space of the largest caldera, which in turnsuggests an episodic supply and long intervals betweendistinguished by the presence of many flow units 10–80 m

thick, mostly 20–30 m thick, which are visible at res- eruptions, yielding a high probability of extensive low-pressure partial crystallization in magma evolution. Flowsolutions not as good as in available imagery of the lunar

maria (Plescia, 1981). A Northern Plains Unit may be a from embayments in the sides of volcanoes suggest thatupward growth had stalled, implying a depth to sourceproduct of sub-glacial volcanism. There is some evidence

of regional dyke swarms which do not extend into the of >160 km, and a thick strong lithosphere to supportthe edifice.older cratered highlands, and several highly conspicuous,

huge central volcanic constructs with very large calderas. Mars is distinguished by hotspots stationary for >1 Galeading to massive shield volcanoes of 1·5 × 106 km3,Even small volcanoes have large calderas and presumably

large shallow-seated magma chambers (Plescia, 1994). equivalent to a total large igneous province on the Earth(Head & Coffin, 1997). Plume activity has been suggestedLarge (up to 1300 km across) shield volcanoes and possible

stratovolcanoes, involving very fluid, possibly ultrabasic as the source of the large central volcanoes, but clearlyif there has been any plate motion on Mars the movementlavas or pyroclastic flows, have been identified around

the huge Hellas impact basin (Peterson, 1978). Central must have ceased before the bulk of the eruptions whichformed the large central volcanoes. Parallels have beenvolcanoes abound in the later volcanic history of Mars.

Early patera apparently formed by rapid eruption of low- drawn between the Mackenzie dyke swarm and possibledyke swarms around the Pavonis centre on Mars (McK-viscosity lavas which created large, low-angle shields

distinguished by huge central calderas, indicative of enzie & Nimmo, 1999). Conspicuously absent are anyindications of senescent alkaline undersaturated pyro-equally large high-level magma chambers in which partial

crystallization of parental magmas and imposition of low- clastic activity such as that which marks Mauna Kea.57. Volatiles including water were accreted to Marspressure cotectic character may have affected at least

the later erupted magmas. Alba Patera is 1600 km in in abundance and at some stage there was an atmospheresufficiently dense to have sustained surface floods. Bothdiameter, with a 100 km diameter caldera; Apollinaris

Patera is 400 km in diameter with a 70–100 km diameter the Northern Plains basaltic unit and parts of OlympusMons may have formed in sub-glacial conditions, sug-caldera. Patera heights are>3 km, with slopes of <0·5°,

implying lavas more fluid than those of Hawaii. Individual gesting much greater water loss (Beatty et al., 1999).Atmospheric loss to space and to freezing out in the soils,flows are >400 km3, small relative to the subsidence

volumes of the calderas. coupled with waning of outgassing by the igneous activity,has greatly reduced the density of the atmosphere. If theLarge central volcanoes reach heights of 17–20 km

above the surrounding plains units, 27 km above datum. ancient Martian crust proves to be as feldspathic as thatof the Moon, there will be no compelling need for aVolcano volumes are up to 100 times those of the largest

examples on Earth. The aureole of Olympus Mons may global magma ocean to provide for its genesis.58. Bodies the size of the largest asteroids clearly didbe formed from early pyroclastic flows and extends to a

1000 km radius, and a sub-glacial birth has been sug- not accrete with sufficient short-lived radioactive sourcesor sufficient accretional energy to power substantial par-gested. These structures again have huge central calderas;

for example, Arsia Mons is 110 km in diameter; Ascraeus tial melting, a conclusion reinforced by the suggestionthat the much larger Callisto may be a little differentiatedMons 40–80 km in diameter with four depressions, 4 km

deep at maximum; Olympus Mons is up to 90 km in rock–ice mixture. Other bodies smaller than the largestasteroids have undergone thermal metamorphism, partialdiameter with the youngest caldera 23 km in diameter;

Pavonis Mons is of 45 km diameter with an older flooded melting, possible core separation and igneous activity(50), which must be due to causes other than internalcaldera of 100 km diameter; even the smaller Elysium

Mons, which is 14 km high and 170 km in diameter, radioactive heating.59. Two external heat sources are proximity to thehas a 12 km diameter caldera. Wrinkle ridges occur on

intermediate age floor of the Olympus caldera (BVSP, Sun and heating by tidal deformation. The evidencefrom the four major satellites of Jupiter, all formed in1981, fig. 5.6.4), presumably on a consolidated lava lake.

Wrinkle ridges also occur in the circular depression (old the same region of space, points to tidal energy being amajor factor in determining the differences in thermalcaldera) of Pavonis Mons. Individual Hawaiian volcanoes

are>9 km high shields of <120 km diameter and would history among the satellites and asteroids. Proximity to

1587


the Sun may have been a major factor in the rate of regions is representative of the immediately underlyingbedrock, but that the regolith developed over the mariavolatile depletion once igneous activity had commenced.

The generation of four bodies with silicate portions is not. This paper, however, views mare regoliths as justas representative of bedrock as those from the highlandssimilar in size and density to that of the Moon, but

without any neighbouring planet to provide a differ- (110–120).Large-scale variations in the lunar highland crust areentiated silicate target, does nothing to support the large

impactor hypothesis for creation of the Moon. Williams evident in the vertical sense. Anorthositic regolith ispresent on higher ground and large impacts excavate(2000), however, inferred from geological evidence that

the Moon has never been in close proximity to the Earth, through to less feldspathic materials (Tompkins & Pieters,1999). Thorium contents in highland rocks are inverselybut this extrapolation may be vulnerable at 3·5–4·2 Ga.

60. There is an array of asteroids and satellites which correlated with elevation of the surface. The mega-regolith covering much of the lunar highlands is 1–2 kmrange from undifferentiated and unmetamorphosed,

through metamorphosed to partially melted, and even deep, and smooths out a certain amount of lateral vari-ation but large-scale lateral variations at greater depthperhaps to largely melted in their early evolution, most

of which provide no evidence for generation of a global are evident in the differences between materials excavatedand exposed in larger impacts (Pieters, 1986). This impliesmagma ocean or flotation of plagioclase-rich crusts. The

onset of partial melting should precede any more ad- that the Moon grew to much its present size before theend of the igneous differentiation which created thevanced melting in such a scenario. Based on a porous

flow model for magma segregation within the Moon, bulk of the highlands crust. Small-scale variations areconsistent with short-range heterogeneity or local lateessentially radioactive heating, and with radioactive heat

sources concentrated into the magma, between 5 and volcanism (Hubbard et al., 1978). Galileo imaging of largeareas of the Moon (Greeley et al., 1993; Pieters et al.,10% partial melting of the deep lunar interior has been

calculated (Turcotte & Ahern, 1978). In a survey of 1993b) displayed regional differences in the highlandscrust, with lowish-albedo material near the South Pole–accretional heating models, Ransford (1979) explored

large body impact effects and concluded that a Moon Aitken basin and relatively little high-albedo (anorthositic)material. Some of the highland light plains materialswith some melting in its outer layers but little towards

the centre was predicted. Generally <10% partial melting appear to be too young to be impact sheets and may bevolcanic.was predicted in the outer few hundred kilometres but

many assumptions had to be made to reach this con- A plot of Th/Ti ratio against Fe (wt %) in lunar samplesdistinguishes between ferroan anorthosites (Warren &clusion, prominent among them being the duration of

the main accretion phase—the faster, the hotter—and the Kallemeyn, 1993), mare basalts, and a variety of otherhighland rocks. All three quantities have been measuredmagnitude of the tidal input from the early Earth–Moon

system. for>18% of the lunar surface by the �-ray spectrometerexperiment. Each pixel in the results represents an av-Why should remelting of already consolidated cu-

mulates occur in the Moon, as is required by the con- erage of the concentration of the element or ratio ofelements in >104 t of material. These values, controlledventional hypothesis for mare basalt petrogenesis, in the

absence of an external heat source or internal convection? against ground truth, represent good average figuresand extend the composition estimates to much greaterLunar basalt generation is more easily explained if it is

viewed as the end stage of continuing partial melting volumes and surface areas of the lunar crust than aredirectly represented by the returned samples [see Taylorwith a geothermal gradient which first increased and

later may have decreased in a Moon which had not (1975, fig. 5·15 for Al/Si), BVSP (1981, plates 2.2 and2.3 for Fe and Ti, and plate 2.4 for Al/Si) and Spudisalready largely melted, fractionated and cooled.

61. Remote sensing of the lunar surface and upper & Pieters (1991, plates 10.2 and 10.3 for Fe and Ti)].These compositions could also be related less precisely tocrust has been provided by photogrammetry, laser al-

timetry, radar sounding, seismic studies, gravity meas- ground- and spacecraft-based optical and IR reflectancestudies to extend the findings over more than 50% ofurements, reflectance studies in the IR to UV, and X-

ray, �-ray and neutron spectrometry. Data have been the lunar surface, leading to the conclusion that thereflectance properties of the maria surfaces are those ofobtained from Earth-based telescopes, orbiting Apollo

command modules, Mariner 10, Galileo, Clementine the regoliths, not those of the hand-specimen com-positions. The plot has been used to survey petrologicaland Lunar Prospector. The geochemical data measure

the local surface regolith composition and are calibrated provinces across the Moon by Davis & Spudis (1987)with results also given by Spudis & Pieters (1991, plateagainst returned regolith samples from nine sampling

sites. Assumptions implicit in the interpretation of remote- 10.8). The bulk of the lunar highlands have compositionsintermediate between ferroan anorthosite and mare basaltsensing data to support the ‘conventional’ petrogenetic

model are that the regolith developed over all highland in composition. The mare input in the highlands has to

1588


be interpreted as older, reworked mare component which case for an initially hydrous Moon (Feldman et al.,1998b).it would be desirable to minimize because basaltic clasts

are not particularly abundant. Ferroan anorthosite (FAN) The distribution of TiO2 contents in lunar mare basaltshas been found to be unimodal and strongly skewed withclasts appear to be more abundant in examined breccias

than clasts representing the Mg-suite or KREEP. The a mode >2·2%, median of >2·5% and mean >3·2%(Giguere et al., 2000), rather than having the stronglyFAN component is expected to predominate in the

acceptable solutions. The other immediate constraints to bimodal distribution previously apparent from Apollosurface sampling. Although advanced as consistent withbe observed are an acceptable level of total REE, and

an absolute value of Th close to 0·91 ppm (Metzger et the conventional model, this distribution is equally con-sistent with a low-pressure, gabbro-dominated frac-al., 1977), the most precise of the individual parameters

measured from orbit. Solutions which incorporate the tionation trend leading from parental low-titanium,incompatible-element-poor basalts with low or negligiblepure KREEP component as end-member admit very

little KREEP component, otherwise Th rises too high. Eu anomalies, towards reducing quantities of high-titanium, incompatible-element-rich basalts with largeToo high a mare basalt component in the average

highland mixtures conflicts with the petrological ob- negative Eu anomalies.A recent brief summary ( Jolliff et al., 2000) has in-servations, unless a substantial part of the mare con-

tribution is in the form of plutonic gabbros and norites troduced a new Lunar Science Initiative, ‘New Views ofthe Moon Enabled by Combined Remotely Sensed andwhich have escaped separation from the Mg-suite rocks.

Solutions which incorporate the Th-poor Mg-rich suite Lunar Sample Data Sets’, initiated by the Curation andAnalysis Planning Team for Extraterrestrial Materialsend-member do not achieve the required absolute values

of Sm and Th in the average compositions. and supported by NASA’s cosmochemistry program, theLunar and Planetary Institute, Houston and the USSoon maps of the distribution of concentrations of H,

Fe, Ti, K, Th, Ca, Al and some REE over the entire Geological Survey, Flagstaff. Figure 1 of Jolliff et al.(2000) supports the inferences from the Al/Si ratios inlunar surface will be available (Binder, 1998), rather than

just for the restricted equatorial swaths provided by the Fig. 2 above—FeO values in the lunar maria, remotelysensed by Clementine, peak around 17% and tail offApollo 15 and 16 spacecraft orbits. Preliminary maps of

the global Th distribution on the lunar farside and at rapidly above 18·5%. Average hand-specimen high-ti-tanium basalts average >19%, low-titanium basaltsthe poles (Lawrence et al., 1998) confirm large areas of

low and very low concentration which may have been >20·2% FeO and those advanced as candidate primarymagmas contain 19·6–22·5% FeO. To produce averageunder-represented in the original limited coverage by the

Apollo 15 and 16 data, and a prominent area of higher mare regolith FeO concentrations of 17% FeO by ad-dition of average highland materials with >4·7% FeOvalues roughly antipodal to the Imbrium basin which

was also under-represented. The new global average for to such basalt compositions would require the presenceof 17·5–31% highland material in the regoliths, which isTh concentration in the lunar crust is unlikely to fall

significantly below the 0·76 ppm below which a sub- not observed, contravenes expectations from crateringdynamics, and is inconsistent with the diminutive re-stantial positive Eu anomaly in the lunar highlands might

be guaranteed (62). Definitive results for Fe and Ti are ciprocal scattering of mare basalt into the highlands.62. The geochemical ‘facts’ about lunar highland pet-awaited and there is some discrepancy between the

optical and �-ray results (Elphic et al., 1998; Lucey et rology have at times been in doubt (e.g. Anders, 1978)as they are now about mare basalts. This uncertaintyal., 1998). The thermal and fast neutron experiment

(Feldman et al., 1998a) has confirmed the association of afflicts the REE in particular. The REE are present inlunar rocks and magmas predominantly as trivalent cat-more mafic materials with lower altitudes in the lunar

highlands and in the immediate surrounds of the large ions and in this form behave as incompatible traceelements rejected into the liquid by all mineral phasesmare basins; of probable large-scale lateral heterogeneity

within the deeper highland crust; and of more feldspathic except the phosphate minerals. At these low oxygenfugacities europium is present substantially as the divalentmaterials with higher altitudes in the lunar highlands,

the latter particularly in an annulus surrounding the large cation and in that form is readily accepted into the crystalstructure of plagioclase (as are Sr and Ga). Consequently,South Pole–Aitken basin. The results also identify the

basalts of the Crisium and Smythii basins as com- there should be a preferential concentration of Eu inplagioclase and in plagioclase-enriched materials relativepositionally distinct from those surfacing the other maria.

Water discovered as shallowly buried ice beneath regolith to the amount of europium, Eu∗, which would notionallyhave been present had the element behaved as expectedin the permanently shadowed regions in craters at the

north and south lunar poles is less in amount than could from its position within the rare earth series as a whole,yielding a positive Eu anomaly (Eu/Eu∗ >1·0) in thebe expected to have accumulated from cometary impacts

over the past 2 Ga and has nothing to say towards the plagioclase itself. Progressive fractionation of plagioclase

1589


and its separation from the other crystallizing minerals europium anomaly in the average lunar highlands com-might, therefore, yield plagioclase cumulates with low position. Demonstration of a positive Eu anomaly inREE and positive Eu anomalies, REE-poor cognate mafic average lunar highland materials is thus essential (neces-cumulates with complementary negative Eu anomalies, sary but not sufficient—see 93) to the hypothesis ofand residual liquids with increasing REE and increasingly primary magma status for the maria basalt hand speci-negative Eu anomalies if the bulk extract has a positive mens because it would validate the requisite com-Eu anomaly. Cotectic crystallization of plagioclase with plementary negative Eu anomaly in the underlyingclinopyroxene and apatite in terrestrial anorthosites does mantle. Demonstration of this positive Eu anomaly innot, however, generate major Eu anomalies in bulk average lunar highland materials is equally essential tocumulate or residual liquid (Morse & Nolan, 1985) but the magma ocean hypothesis and its complementaryboth the oxygen fugacities and the alkali contents of the plagioclase flotation mechanism. The entire edifice ofmagmas are higher in this instance. conventional lunar petrogenetic interpretation rests heav-

The compositions of hand specimens of lunar maria ily on the alleged positive Eu anomaly in the averagebasalt mostly display significant negative Eu anomalies lunar highlands. This crucial matter is discussed at some(i.e. Eu/Eu∗ <1·0, often p1·0). Separation of liquid length here.from residual plagioclase in the mantle during partial An elegant logic underlies the estimate of average lunarmelting or from cumulus plagioclase in gabbros during highland composition, and was originally laid out bypartial crystallization processes could provide an ex- Taylor et al. (1973a, 1973b), Taylor & Jakes (1974) andplanation for this negative Eu anomaly in the magmas. Taylor & Bence (1975), and clearly explained by TaylorThe latter interpretation lies at the heart of the proposition (1975, pp. 249–253). Many returned samples from high-(110–120) that the aluminous basalts and average regolith land sources have been analysed. The concentrations ofcompositions, which are close to plagioclase saturation incompatible trace elements in these samples displayat the liquidus at low pressure, represent the true parent excellent linear correlations with one another in log–logliquid compositions of maria basalt flows. Those parent concentration plots. If the concentration of any one ofmagmas would be required to have undergone gabbro these elements in the average lunar highlands compositionfractionation or some other interaction with plagioclase can be obtained, then all the rest can be predicted withcrystals within the lunar crust before eruption. a high degree of confidence. Thorium, whose con-

However, the proposed primary magmas based on the centration in the surface rocks had been measured in ahand-specimen compositions have olivine, pyroxene and

wide equatorial swath of the lunar crust by its �-raytitanium-rich minerals as liquidus phases at all likelyemissions, provides the required global index to a plaus-pressures (BVSP, 1981, figs 3.4.3 and 3.4.5; Taylor et al.,ible average composition. Knowing thorium in the av-1991, tables 6.2–6.5). They are not in, or close to,erage composition, the concentration of the sum of theequilibrium with plagioclase crystals, at any pressure.REE in the average composition can be predicted. TheThe interpretation chosen by most workers is to postulateconcentration of Eu in highland rocks varies much lessthe existence of a ferro-magnesian silicate source regionthan the concentrations of the other REE. Anorthositicwhich has an in-built negative Eu anomaly which issamples with low total REE have excess Eu and a positiveinherited by the partial melts. That required sourceEu anomaly; as the concentration of total REE increasesis conveniently provided by the deep-seated cumulatein other samples the positive Eu anomaly declines, be-products of a postulated lunar magma ocean, from whichcomes non-existent and then becomes a pronouncedall the plagioclase and its accompanying europium hadnegative Eu anomaly in the KREEPy samples richest inbeen abstracted by flotation during crystallization.total REE. Knowing the total REE in the average lunarThe highly feldspathic compositions of the lunar high-highlands one can, therefore, predict the sign and mag-land crust cannot be derived as liquids by partial meltingnitude of the Eu anomaly in that material. This led toof any likely lunar mantle material in the absence ofthe confident assertion:water. Because the Moon was held to have been volatile

‘A consequence of the REE abundance patterns is thatdepleted from its birth, either a fortuitous accretion ofthe average highland composition has a positive Euhighly feldspathic material late in lunar history or physicalanomaly. The consequences of a positive Eu anomaly inseparation of plagioclase crystals from other lunarthe highland rocks are profound’ (Taylor, 1975, p. 251).materials was required. Hence the hypothesis that flot-

Application of the above procedure had resulted in aation of plagioclase crystals from a lunar magma oceanrepresentation of chondrite-normalized REE patterns increated the lunar highland crust, a hypothesis whichlunar highland samples which has been reproduced inarose from petrological need. The only compelling evi-numerous places (e.g. Taylor, 1975, fig. 5.20) and showsdence that plagioclase flotation really occurred and thea pronounced positive anomaly (Eu/Eu∗ >1·4) in theonly compelling evidence that a lunar magma ocean

actually existed, is to be found in the alleged positive deduced average highland composition at a total REE

1590


concentration of >65 ppm (>20 times chondritic) de- But EuN∗ may also be defined as the arithmetic meanrived from an average Th concentration of >1·5 ppm. of SmN∗ and GdN∗, orThe actual correlation between total REE and the mag-nitude and sign of the Eu anomaly, which underpins EuN∗ =

(SmN+ GdN)2

=Sm

2SmC+

Gd2GdC

=Sm

2SmC+

the vital step in this argument, was not presented forevaluation. This omission can be rectified using the dataof Taylor et al. (1973a, 1973b) and Taylor & Bence (1975) (1·168Sm+ 0·480)

2GdC= 4·9430Sm+ 0·9266.

(3)

in log–log plots of Th concentration vs concentration oftotal REE and of Th concentration vs Eu/Eu∗. Taylor

From this it follows thatet al. (1973a) showed a small negative anomaly. Tayloret al. (1973b, fig. 2) showed no anomaly. Taylor & Jakes EuN

EuN∗=

0·7044Sm+ 12·1554·9430Sm+ 0·9266

. (4)(1974, fig. 2) showed a positive anomaly. Bence et al.(1975), Taylor (1975) and Taylor & Bence (1975) showeda substantial positive anomaly. The combination of Th, Equation (4) is readily solved for the value of Sm attotal REE and value of Eu/Eu∗ in this latter average the condition (Eu/Eu∗)N= 1·0, when it is to be expectedcomposition falls outside the envelope of the data points that there will be no Eu anomaly in lunar highland(Fig. 3). At the adopted average of 1·5 ppm Th a small materials. The sum of REE and probable average Thbut definite negative anomaly should have been expected concentration at this condition may then be estimated.as originally deduced by Taylor et al. (1973a). This solution yields Sm= 2·649 ppm; �REE>48 ppm;

The case for a positive Eu anomaly in the lunar Th >0·763 ppm.highlands was, therefore, weak. Two developments have This value for Sm concentration when there is no Eupromised rehabilitation. The Th values measured from anomaly lies just within the limits of validity of theorbit have been revised downwards and it has become Eu–Sm correlation (Sm >2·5 ppm) quoted by Haskin &clear that the surface of the lunar farside contains a far Warren (1991). Regions of the lunar highland crust whichgreater proportion of Th-poor, REE-poor anorthositic have Th concentrations greater than 0·763 ppm can, onmaterials, in which positive Eu anomalies are more likely this basis, be expected to contain negative Eu anomalies.to be encountered, than do the lunar nearside highlands. At Th >1·5 ppm the new correlations imply (Eu/The new situation has been reviewed by Haskin & Eu∗)N = 0·65, a marked negative anomaly in contrastWarren (1991), who presented a revised representation to the (Eu/Eu∗)N = 1·4 presented by Taylor (1975); aof highland REE patterns which does not include an Th value of 0·6 ppm translates into Sm = 2·231 ppm,updated estimate for the average lunar highland com- slightly outside the valid limits of correlation, and aposition (Haskin & Warren, 1991, fig. 8.8). They also tentative value of (Eu/Eu∗)N = 1·15, a distinct butpresented revised linear correlations of incompatible ele- not startling anomaly compared with (Eu/Eu∗)N >30ment concentrations against the concentration of Sm reported from analysed lunar highland anorthosites.(Haskin & Warren, 1991, table 8.2) together with values Some words of caution are needed, however. Thefor concentrations in chondrites (Haskin & Warren, 1991, Eu–Sm log–log concentration plot given by Haskin &table 8.1): Warren (1991, fig. 8.10d) actually shows a range of Eu/

Sm in the region of ‘good’ correlation>0·2–1·1 althoughEu = 0·051Sm + 0·880; Gd = 1·168Sm + 0·480; most data points fall closer to the correlation given; andTh = 0·390Sm − 0·270; SmC = 0·186 ppm; among selected rock samples whose analyses are listedEuC = 0·0724 ppm; GdC = 0·259 ppm; (1) in the appendices to Taylor et al. (1991), the boundary

between the few specimens showing small positive orwhere EuC, SmC and GdC represent the concentrationsnegative Eu anomalies lies at Th between 1·03 and 1·26of these elements in chondrites. Then dividing by theppm. Europium is approximately constant in amount inproduct of EuC and SmC, and then replacing Eu/EuC,highland samples which contain less than >13·5 timesetc. by EuN, etc., where EuN is the chondrite-normalizedchondritic values of Sm (and total REE), but Sm (andconcentration of the element,total REE) contents are closely correlated throughout theconcentration range (Haskin & Warren, 1991). Highlandrocks with >0·76 ppm Th, corresponding to Sm >2·65Eu

SmCEuC=

0·051SmSmCEuC

+0·880

SmCEuC;

ppm, >14 times chondritic, with �REE >48 ppm areon average expected to have negative Eu anomalies, buta few alkali-rich rocks have flat REE patterns and no EuEuN=

0·051SmCSmN

EuC+

0·880EuC

(2)anomaly at 100 times chondritic REE (Papike et al.,1998). Some hand-specimen samples with slightly greater

= 0·131SmN + 12·155 = 0·7044Sm + 12·155. than 1·0 ppm Th have small positive or no Eu anomaly.

1591


It is convenient for the next argument to take 0·9 ppm in the crust. It follows that any hypothesis proposingsignificant plagioclase flotation from a reduced lunarTh as representing the boundary between highland rocks

which contain a positive Eu anomaly and those which magma ocean is excluded. The requisite substantial posi-tive Eu anomaly is absent, unless the bulk of the pla-do not, because orbital mapping of the �-ray flux and

inferred Th contents has been published; for example, gioclase now in the crust is related to later intrusiveactivity (Mg-suite or mare-related), not to the ferroanby BVSP (1981, plate 2.1), where pink was assigned to

regions with <0·9 ppm, purple to regions with 0·9–1·9 anorthosite suite (FAN). FAN could then be envisagedas relics of a much thinner primordial crust. Palme &ppm, blue to regions with 1·9–2·8 ppm, green to regions

with 2·8–3·8 ppm, yellow to regions with 3·8–4·7 ppm, Wanke (1977) independently deduced a probable positiveEu anomaly in the lunar interior based on trace elementorange to regions with 4·7–5·7 ppm and dark orange to

regions with >5·7 ppm. Even discounting the dark orange, correlations and in particular on the correlation betweenEu and Sr in KREEP and mare basalts.orange and some of the yellow areas which correlate

with the nearside maria basalts (but also exclude some 63. Solidification of a deep body of dry magma hasbeen the subject of intense thought and speculationhighland KREEP contributions), it is visually obvious

that the pink area is greatly exceeded by the purple, among the lunar science community because of its rel-evance to the solidification of the postulated lunar magmablue, green and yellow areas. A very rough calculation

indicates that, because of the way in which Eu and Sm ocean (Walker et al., 1975; Longhi, 1977; Solomon &Longhi, 1977; Herbert et al., 1978; Minear & Fletcher,vary in the rocks, the area of pink would need to be

greater than the sum of purple plus twice that of blue 1978; Minear, 1980; Morse, 1987). Minear (1980) con-cluded that solidification would be rapid (>60 Ma) withplus three times that of green plus four times that of

yellow (exclusive of maria) for there to be a likelihood of only a thin crust developed during most of this time.Solomon & Longhi (1977), among many, concluded thata positive Eu anomaly in the average highlands—and

this neglects altogether the contribution of nearside remelting of the magma ocean cumulates would requirethe retention of trapped liquid during solidification, aKREEP. It is also visually obvious from Haskin & Warren

(1991, fig. 8.7o and t) that a majority of analysed lunar conclusion with important consequences for the pla-gioclase saturation story (93, 98) although there is nohighland soils, regolith and polymict breccias, rep-

resenting a substantial mass of sample, contain >2·7 ppm necessity for this in major element terms because of theextensive crystalline solutions of basaltic components inSm and >0·9 ppm Th. Spudis & Pieters (1991) did not

quote results from the orbital �-ray experiment directly the pyroxenes at the relevant pressures. Morse (1987)has argued that, rather than flotation of actual plagioclasebut presented a colour-contoured map of the deduced

Th contents in the lunar surface (Spudis & Pieters, 1991, crystals, flotation of the less dense, more feldspathicresidual liquids from extraction of ferro-magnesian min-plate 10.1). This is seen graphically to have values >0·9

ppm across almost the whole of the nearside highlands erals may be the important mechanism. This allowssegregation of a potentially feldspar-rich material to thesurveyed and to lie close to or above that value over

about half of the farside highlands, but unfortunately the surface long before, and from a much greater volume ofmagma, than that which would have precipitated actualviolet coloration applied to all values of Th <0·9 ppm

embraces the boundary between highlands which are plagioclase crystals. If the Morse (1987) mechanism op-erates, a substantial part of the cumulate pile was pre-expected to have a negative Eu anomaly (Th >0·76 ppm)

and those which probably have a positive anomaly. On cipitated from magmas which were not plagioclasesaturated at depth, yet formed during enrichment inthe basis of these results it is impossible to be confident

that the average lunar highland composition across the potential plagioclase of the magmas from which thecrust would later form. This mechanism can explain awhole Moon will contain a positive Eu anomaly at all,

still less one large enough to support the hypothesis plagioclase-rich crust which has only a limited positiveEu anomaly. It does not implant a complementary neg-of massive plagioclase flotation into the crust. Latest

uncorrected counting results from Lunar Orbiter (Law- ative Eu anomaly in the cumulate mantle until plagioclaseis saturated in the crystallizing magma and so does notrence et al., 1998), however, indicate that the surface of

much of the lunar highlands away from the Imbrium ease the problem of generating the marked negative Euanomalies in the mare basalts. Nor does it ease theand South Pole basins is composed of rocks with low Th

contents which may have positive Eu anomalies. problem of generating magmas which do not have pla-gioclase precipitating as their second silicate phase at lowKorotev & Haskin (1988), in an important paper not

referred to by Haskin & Warren (1991) or Papike et al. pressure.The topic has long been the subject of detailed field,(1998), concluded that there is no significant positive

anomaly in the average lunar highland crust, that Eu/ petrological and geochemical studies of smaller, but stillvery deep and non-hypothetical terrestrial magma bodies.Al ratios are chondritic, and that most of the lunar Al,

REE and other highly incompatible elements are now The former concept of crystals nucleating in a static

1592


magma body as it cools and settling through the magma local magma density, adding to the density stratificationto accumulate at the floor fails to explain many features which opposes convective motion and changing the com-of the petrology of the cumulates (Campbell, 1978; position of the liquid towards compositions which shouldMcBirney & Noyes, 1979), although other features appear have plagioclase as liquidus phase on cooling. Doubleto rehabilitate the settling of crystals following nucleation diffusive layering may develop (Huppert & Sparks,in the upper boundary layer (Irvine et al., 1998). In a 1980a). The potential complications as this process con-deep body of dry magma the pressure gradient ensures tinues appear too extensive to be reliably predicted,that the liquidus temperature of a fixed basaltic com- especially if volatiles are also involved (Huppert et al.,position declines from the base of the intrusion to the 1982). Morse (1986b), however, argued that this processtop, at a gradient steeper than the adiabatic gradient for would not operate in the presence of crystals and in thea flow of convecting magma. If the magma is homo- absence of walls to an intrusion. It is necessary to turngeneous at the outset (Walker et al., 1975), magma cooled to the abundant terrestrial evidence to determine whatat the roof and descending to the floor is likely to the outcome might be, bearing in mind the possibleencounter the liquidus, become saturated and super- differences between the terrestrial and lunar situations insaturated with respect to crystals, nucleate them and then terms of further magma additions, stirring by impact,separate a residual liquid. Some form of small packet differences in initial magma composition, and the differ-crystallization (Langmuir, 1989) seems more likely than ences in the adiabatic and liquidus gradients when ex-whole body fractional crystallization. Hot magma rising pressed per kilometre of depth in the two situations.from the floor of the body is moving towards conditions The evidence from terrestrial layered gabbro com-above its liquidus temperature and will tend to dissolve plexes (e.g. the Bushveld, Kiglapait, Muskox, Skaergaardany plagioclase crystals and nuclei which it contains and Stillwater complexes) covers a considerable range of(Morse, 1986a, 1993). initial magma compositions from picritic through noritic

The viscosity of a magma increases as it cools and towards anorthositic, and exhibits a wide range of majorincreases dramatically in the temperature interval just and minor magma recharge events during consolidation.above the first appearance of plagioclase. The low alkali It indicates that plagioclase nucleates and grows pre-contents of lunar basalts ensure that, other things being dominantly at the floor of the body, with minor solidi-equal, lunar magmas will have much lower viscosities fication at the roof and walls. Plagioclase accumulationthan terrestrial basalts. Vertical segregation of pheno- at the roof is not a significant process. Morse (1982,crysts within lunar basalt magmas will have been more

1986b) noted that unzoned plagioclase of the type foundrapid than in terrestrial lavas in spite of the lowerin lunar ferroan anorthosites was indicative of near-gravitational acceleration. However, plagioclase, what-equilibrium conditions and probable adcumulus growthever its size, is likely to be almost neutrally buoyantat the floor of intrusive bodies. Lunar highland igneousin lunar and terrestrial basaltic magmas and may beclasts have been derived from many individual plutonsintrinsically buoyant in many mafic magmas (Scoates,in which plagioclase accumulated together with spinel,2000) although examples of plagioclase settling in Ice-olivine and pyroxene.landic pillow lavas are known, as at Litla Skogafell (map

Complex assimilation and refluxing of plagioclase inref. 329894), Reykjanes peninsula, Iceland. Unless itthe upper layers of a magma ocean was proposed as anundergoes froth-flotation by adhesion of gas bubbles,alternative mechanism for arriving at an anorthositicplagioclase is unlikely to float and probably will not sinkcrust from a chondritic source composition (Longhi &in lunar magmas after their eruption. Plagioclase crystalsBoudreau, 1979). Walker (1983), recognizing the in-might become relatively enriched in any part of the lavaadequate Eu anomaly in the lunar highlands, movedor magma, however, if ferro-magnesian phases wereaway from the magma ocean concept towards serialpreferentially removed.magmatism in the lunar highland crust. Longhi & AshwalThe residual liquid formed during crystallization at the(1985) went a stage further, proposing generation of bothfloor of the intrusion is likely to be less dense than thelunar and terrestrial anorthosites by accumulations withinmain body of liquid if plagioclase is absent from therhythmic layered intrusions followed by tectonic mo-crystallizing phases, more dense if plagioclase is amongbilization of the plagioclase-rich portions.the nucleating phases (Huppert & Sparks, 1980b; Stolper

64. The eucrite parent body and several other disrupted& Walker, 1980). Escape and mingling of the residualasteroidal bodies (87) have apparently developed deepliquid in the former case with the supernatant magmabasic magma bodies in which the fractional crystallizationwill promote convective motion. In the latter case, itsof magmas closer in character to lunar magmas has takenmingling will tend to set up density stratification whichplace, also apparently with bottom crystallization andopposes large-scale convective motion. If some plagioclasecumulate formation, but without development of anor-develops as suspended crystals and is carried upwards by

convecting magma, it will tend to dissolve, reducing the thositic materials by flotation [and see Scoates (2000)].

1593


65. The absence of an average positive Eu anomaly 68. The effect of increased water pressure and watersolubility on the phase equilibria of basic magmas is to(62) precludes any overall enrichment of the lunar crust

by movement of plagioclase crystals unaccompanied by a expand the liquidus phase volume of olivine at theexpense of those of plagioclase and pyroxenes, and thatKREEPy component. The enrichment of plagioclase in

the highlands must otherwise be accomplished by the of calcium-rich pyroxene at the expense of those ofplagioclase and calcium-poor pyroxene (37). Melts whichmovement of liquids containing high normative pla-

gioclase [see Morse (1987)]. The small negative Eu anhave been produced by small mass fractions of partialmelting in equilibrium with olivine, two-pyroxenes andomaly which may be present in the average lunar

highlands is consistent with plagioclase or amphibole plagioclase in a water-bearing regime will, on de-pressurization and water loss, precipitate anorthosites,crystals being residual in the upper-mantle source of

those liquids. troctolites and norites. These rock types dominate lunarhighland petrology.66. In the absence of a substantial positive Eu anomaly

in the lunar highlands (62) there is no other requirement 69. Partial melting of a planet is more probable thanglobal melting (60). The probability and consequencesfor a magma ocean to have existed even if one cannot

be excluded. Current thinking inclines towards a lunar of partial melting and differentiation during the final50% of growth, rather than after the final accretion ofhighland crust dominated by the effects of serial feld-

spathic magmatism whose products are marked by almost the Moon, were explored by Smith (1981) in a processwhich resembles partial melting with source materialtotal near-surface differentiation into cumulates and

KREEPy residual liquids. recharge (REXM, O’Hara, 1993) but is complicated bycumulate growth and the presence of large pressure67. The postulated global lunar magma ocean provided

for the petrogenesis of the feldspathic lunar highlands in gradients. Methods of generating an anorthositic crustfrom a partially molten magma ocean have been exploredthe assumed absence of volatiles. The four lunar-sized

rocky satellites which have conserved icy crusts and by Shirley (1983) and have led on to serial magmatismmodels.copious volatiles presumably accreted without developing

global magma oceans or their water and volatile contents The average composition of the relatively low-densitylunar highland crust, 10% by volume of the Moon,would not have survived (51). The former existence of

an accretionally generated lunar magma ocean remains contains about 0·9 ppm Th and about 12 times chondriticREE, consistent with the lunar crust representing anspeculative, therefore, as must the generation of magma

oceans in the evolution of Mars, Venus or the Earth. >0·08 mass fraction melt of the whole lunar interiorassuming the bulk Moon to have been of broadly chon-Whether or not a magma ocean will form during the

free accretion of a planet from near-infinite space is dritic composition initially. Such high concentrationsof elements which are incompatible in plagioclase arecritically dependent upon the accretion rate and on the

size of individual impacting projectiles (Wetherill, 1985; inconsistent with the highland crust representing a sub-stantial cumulus of plagioclase from a global magmaMelosh, 1989, section 12.3; Tonks & Melosh, 1993).

Kaula’s (1979) model for Earth accretion over a 25 Ma ocean, unless the Moon itself is grossly enriched in suchelements. The latter solution seems to be excluded in thetime-span from moderate-sized projectiles suggests that

the Moon would have been too small to generate a case of Th by the measured heat flow from the Moon,the known Th content of the crust and the measuredmagma ocean by impact heating. The currently popular

hypothesis of rapid reassembly of the Moon in orbit U/Th ratios. The positive identification of a small Fe orlarger FeS core segregated within the Moon suggestsaround the Earth from materials ejected by a giant

impact of a Mars-like body with the proto-Earth provides that the whole Moon, not just an outer 500 km thickaccretionally heated layer, has been involved in a globalyet another scenario. What is beyond question is that

partial melting of the outer parts of a planet by accretional chemical differentiation process.The Eu vs Sm plot given by Haskin & Warren (1991,heating becomes increasingly likely as it passes through

the maximum in accretion rate which precedes the decline fig. 8.10d) shows a scattered but in the main relativelyconstant value of Eu>0·8–2·0 in samples with Sm <2·5in available material. Assuming melt movement towards

the surface on a time-scale short relative to that of and a well-populated correlation beyond. This is theclassic pattern, familiar to petrologists examining cor-accretion, this would act as a modified process of zone

refining with an initially increasing, later decreasing par- relations between an element concentrated in a possiblycumulus phase and some index of advancing differ-tial melt fraction and a steadily increasing proportion of

melt in the outer parts of the accreting body. It is a entiation. The well-populated correlation is usually in-terpreted as a liquid ‘line’ of descent [Bowen, 1928;matter of opinion whether the Moon or any other ter-

restrial planet passed through this stage and on into the Turner & Verhoogen, 1951, 1960; Carmichael et al.,1974, pp. 46–50; Cox et al., 1979, pp. 6, 22–40, 166–173stage of generation of a magma ocean which then cooled

and differentiated as a single body of melt. (for a clear exposition of the complex issues involved);

1594


Wilson, 1989, pp. 14–17]. In the lunar case this would further, higher-temperature melts would contain less po-tential volatile material, mainly in the form of dissolveduse Sm concentration as the index of differentiation and

would be interpreted as showing liquid descent from a carbon and sulphur. Proving this postulate would requirean extensive experimental programme which paid dueparental liquid with >2·5 ppm Sm (13 × chondritic)

and a small negative Eu anomaly, i.e. a liquid close in attention to the speciation of these three elements andtheir compounds in solids and melt as functions ofthis respect to the average lunar highland composition.

Plagioclase (carrying up to 2 ppm Eu and negligible Sm) pressure and oxygen fugacity.72. The initial water-rich volcanism (70, 71) may haveand at least one other REE-poor phase (pyroxene and

olivine are both reported in possibly cumulus textures been short lived and provided only a thin feldspathiccrust. The extraction of water-poor basaltic melts fromwith plagioclase from highland rocks) accumulate in

variable amounts from those liquids to create the Sm- aluminous peridotites in the outer 400 km of the Moonwould supply melts to the surface which were oversat-poor compositions. The implied crystal–liquid dis-

tribution coefficient for total Eu in plagioclase is a little urated in plagioclase (93) and could deposit substantialamounts of gabbros and norites while evolving residuallower than that for Eu2+ alone, as would be expected.

Physiographic units identified as volcanic rocks before liquids which would be low-pressure plagioclase-saturatedcotectic residual liquids with negative Eu anomalies.the mission to the Apollo 16 site are chemically dis-

tinguished also in the orbital X-ray data (Andre & El- Mare volcanism took place over a time-span of at least1200 Ma, from 4·2 Ga or earlier to 3·0 Ga, and mayBaz, 1981). The Cayley Plains materials have closer

affinities to basalts than to typical terra physically, spec- have been more active in the period before the majorimpacts around 3·9 Ga than in the subsequent period.trally and chemically.

The above interpretation of the Eu–Sm variation fur- A high proportion of the materials occult in the highlandbreccia and regolith compositions have been assigned tother predicts that plots of bulk Cr or Sc against Sm, or

any other index of reduction in residual liquid volume, the mare basalt suite on remote-sensing and geochemicalgrounds but basalt fragments are scarce in recoveredwill display a related pattern. At the relevant oxygen

fugacities bulk Cr2+ is mildly incompatible in olivine, breccias. These observations are consistent with a sub-stantial presence in the breccias and soils of fragmentedcompatible in pyroxene and markedly compatible in

spinel, and overall mildly incompatible during cotectic coarse-grained gabbroic cumulates which are com-plementary to the older erupted basalts. A eucrite-likecrystallization of norite-like materials. Cr2+ appears to

follow Fe2+, whereas Cr3+ is the species strongly con- gabbro 61223/4 has been reported among pristinesamples from Apollo 16 (Marvin & Warren, 1980; Takedacentrated into pyroxene and spinel. This is not capable

of immediate testing from plots presented by Haskin & et al., 1981) but is currently grouped with the Mg-richhighland suite.Warren (1991). A plot of the relevant data from selected

individual rock specimens (Taylor et al., 1991) shows Model bulk lunar compositions which have been sug-gested, all predicated on the assumption of a magmaextreme scatter and is convincing of nothing.

70. The Moon may have acquired a complement of ocean and a plagioclase-flotation crust, range from 6·0to 27·2% Al2O3 and from 32·4 to 12·9% MgO (Kessonvolatiles during accretion (51) sufficient to support an

early calc-alkaline style of volcanism similar to that which & Ringwood, 1977, table 1). All of these compositionsare higher in alumina, and many much lower in magnesia,may have formed andesitic crust on Mars. Initial eruptive

activity on the Moon would then have been violently than chondrites. Thirteen estimates of the bulk com-position of the silicate portion of the Moon (Warren &explosive; later activity may have involved basaltic nuees

ardentes akin to those of Ulawun, New Britain (Melson Wasson, 1979, table 1) range from 3·7 to 26·6% Al2O3

and from 39·3 to 13·1% MgO, generally higher inet al., 1972), although the latter appear to involve frag-mented solid rather than liquid basalt. Early eruptions alumina and lower in magnesia than estimates of the

terrestrial upper-mantle composition. These results followon the Moon might have been accompanied or followedby spectacular carbon- and sulphur-gas driven basaltic naturally from the predicate that the highlands are a

feldspar-rich cumulate from a feldspar-saturated upperfire-fountaining similar to that displayed on Io. Even thefinal consolidated flows on the Moon are conspicuously mantle. Lower alumina and higher magnesia values for

the bulk Moon follow automatically from the postulatevesicular.71. It is postulated that in a peridotite system containing that the lunar highlands are a (wet) feldspathic partial

melt product.hydrogen, carbon and sulphur compounds and under-going progressive heating at pressures up to 2 GPa, 73. Wallace & Carmichael (1992) reported that MORB

are the most reduced terrestrial magmas and most ter-the onset of partial melting will be marked by initialdevelopment of a silicate melt with much dissolved water. restrial basalts are saturated with sulphur gases on erup-

tion. Sulphur, a potent volatile, is not depleted in lunarAfter removal of most of the water in this form, ac-companied by some carbon and sulphur compounds, surface rocks and is twice as abundant in lunar as it is

1595


in terrestrial basalts. Sulphur-bearing gas pressures will deposited during partial crystallization and magma re-charge events in large magma chambers in terrestrialhave been high (Sato et al., 1973; Sato, 1979). Sato (1976)

calculated vapour pressures in equilibrium with basalt igneous activity.The possibility that separation of sulphur-rich phases74275, assuming saturation with FeS and carbon, which

imply high volatility of S2 and SO2 with pressures up to has played a significant role in the geochemistry of thelunar rocks needs to be sympathetically re-evaluated. It106 times ambient and Na up to 104 greater than this

with CO as much higher again. CO and COS pressures was scarcely entertained in work summarized by Haskin& Warren (1991), where the term chalcophile is defined,would dominate the volcanism. Similar results were ob-

tained with glass 74220 (Sato, 1978). Some basalt speci- but not employed, and loss in the gas phase was notconsidered. Sulphur, together with the major elementsmens, whose textures suggest relatively slow cooling and

consequently relatively deep burial within the flow units, iron and oxygen respectively, has the capacity to act asthe controlling element in the formation of two highlyalso contain undeformed vesicles. Gas pressures in the

vesicles and hence volatile activities in the magmas must contrasted carrier-phases either of which might haveseparated from lunar silicate magmas. These are anhave been in the range 0·1–1·0 bar, >1012 times the

ambient surface pressure. The upper part of the flow immiscible sulphide liquid (into which many of thesiderophile and some of the volatile elements depletedmust have been a froth or ash-flow during emplacement

and initial eruption may have been powered by much in the lunar rocks would partition strongly) and a sulphur–oxygen-dominated vapour (into which potentially volatilehigher gas pressures (81).

Losses of sulphur gases from terrestrial basalts at much elements might be expected to partition closely in ac-cordance with their activities in the silicate melt). Whenhigher ambient pressures are conspicuous, as evidenced

following the Laki fissure eruption of 1783 and by sug- such carrier-phases are fractionally removed from thesystem even in small mass fractions, those trace elementsgested global effects in the wake of the Deccan and

Siberian flood basalt events earlier in geological history. which partition strongly into the carrier-phase are rapidlyremoved from the residual liquid [see O’Hara et al.Among the last eruptive products on the lunar surface

are areas covered in pyroclastic glass beads. Volatiles do (2001)]. The supply of such trace elements can be out-lasted by that of the major components S, Fe and O,not seem to have been in any shorter supply in the Moon

than in Io today. Volatilization losses of sodium and which condition the appearance of the carrier-phases.These major elements remain in the system in proportionssulphur gases during melt eruption into hard vacuum,

similar to that observed from Io, may have modified controlled by whatever cotectic equilibria are involvedin the continuing formation of the carrier-phases.important geochemical characteristics of the lunar surface

rocks during eruption (84). 76. Righter & Drake (1996) have discussed core for-mation in the Moon, Mars and Vesta. Levin (1979)74. Roedder & Weiblen (1978) reported early sulphide

droplet saturation in very low titanium (VLT) basalts. concluded that a small iron core perhaps with moderateamounts of FeS was required in the Moon. The improvedGrove (1981), seeking to explain the large chemical

variations among pyroclastic green glass beads from moment of inertia data from Lunar Prospector are con-sistent with a lunar Fe-rich core comprising>0·5–2·2%Apollo 15, found that removal of an FeS-rich immiscible

liquid was necessary. He further noted the sulphur-rich of the lunar mass, with a radius of 220–370 km, or anFeS-rich core comprising 0·9–5·4% of the lunar mass,coatings on the spheres, implying a sulphur-rich volatile

phase in the eruption process which powered fire-foun- with a radius of 330–590 km (Konopliv et al., 1998).Seismic data fix the maximum radius at 450 km, con-taining. His preferred interpretation involved production

of sulphur-bearing but not sulphide-saturated melts at sistent with either an Fe-rich or an FeS-rich core, butthe high sulphur content of the mare basalts might pointdepth under relatively oxidizing conditions, followed by

reduction on eruption accompanied by formation and to the latter. The simple existence of such a core mayrequire that the whole Moon has been differentiated, notsome separation of an FeS-rich melt.

75. Why is sulphur not strongly depleted in lunar just the outer regions involved in the postulated magmaocean. Mobility of an FeS-rich liquid may have requiredrocks and the basaltic achondrites when so many other

potentially volatile elements are? Many of the ‘vapour- relatively high oxygen fugacities (Gaetani & Grove, 1999).An argument based on the geochemical behaviour ofmobilized’ elements recognized by Haskin & Warren

(1991, fig. 8.1, specifically Cu, Zn, As, Se, Ag, Cd, In, tungsten has been advanced in support of the case thatthe materials of the Moon have ‘seen’ a substantial metalTe, Hg, Tl, Pb) are also strongly chalcophile. So also

are a majority of the non-volatile ‘siderophile’ elements phase separation, a metal phase not now preserved insidethe Moon. It is based on the coherent ratios of Wrecognized by Haskin & Warren (1991, fig. 8.1, spe-

cifically Ni, Mo, Ru, Rh, Pd, Sb, Re, Os, Ir, Pt and (siderophile) to La (refractory and highly magmaphile)in lunar and terrestrial samples and their 19-fold depletionpossibly Au) which are also depleted. These elements

are strongly concentrated into sulphide-bearing layers factor relative to carbonaceous chondrites (Rammensee

1596


& Wanke, 1977). A more oxidized lunar interior would in the ratios and concentrations of this group of transitionexacerbate the problem; a sulphide-rich core would offer metals and the two rare earth elements but the argumentno direct solution, but the mass fraction of metal with will be complicated by the lack of agreement aboutwhich the remaining silicates are required to have equi- parental magma compositions and the high compatibilitylibrated can be reduced if the iron-rich metal phase is in specific mineral phases of some of the species involved.poor in nickel. Martian meteorites have coherent W/La A speculative six-step scenario for the generation ofratios which are much less depleted relative to car- the positive Ce anomaly involves:bonaceous chondrites (Laul et al., 1986). However, dio- (i) formation of a proto-Moon about half the presentgenites, howardites and eucrites all have similar ratios to mass (Cameron & Canup, 1998; Halliday & Lee, 1999)the surface rocks of the Earth and Moon. There is as by giant impact on the Earth. This proto-Moon andyet no evidence of a substantial iron core within Vesta proto-Earth continue to accrete average carbonaceousand the coincidence suggests that there may be further chondrite material containing relatively large con-twists to come in this story. centrations of hydroxyl, carbon and sulphur compounds

77. Lunar highland anorthositic samples, including and with a relatively high inherent oxygen fugacity.lunar meteorites, contain a small (>10%) but persistent (ii) Partial melting sets in throughout and a smallpositive Ce anomaly (Masuda et al., 1972; Takahashi & iron–sulphide-rich core separates in the deeper portions.Masuda, 1978) which decreases as the REE content Separation of the sulphide liquid would have been greatlyincreases. Statistical correlations for lunar highland rocks promoted by a high oxygen fugacity. A relatively water-(Haskin & Warren, 1991) support this general observation rich melt, potentially feldspathic and with affinities tobut are consistent with there being only a very small terrestrial calc-alkaline basic magmas, is formed in thepositive anomaly in the average lunar highland com- outer parts under conditions of f O2 somewhat higher thanposition. KREEP and mesostasis has a small negative in the current terrestrial mantle at destructive margins. ACe anomaly but the positive anomaly is carried in the proportion of the Ce is present as Ce4+ and partitionsolivines, pigeonites, ilmenite and above all the calcium- more strongly into the liquids in this form than do therich clinopyroxene of the rocks. Basalt hand specimens trivalent REE, giving rise to a small positive anomaly inhave an even more marked positive anomaly. The pre- the bulk melt. Because of the oxidizing conditions muchvalent small positive Ce anomaly reported from many Eu is present as Eu3+ and there is no significant Eulunar highland and mare samples is consistent with an anomaly in the bulk melt.episode in their evolution in which REE were distributed

(iii) This water-rich melt migrates to the surface andbetween crystals and liquid under conditions more ox-there erupts and irrupts explosively under well-stirredidizing than have prevailed in the Earth’s mantle through-conditions because the Moon is still accreting fairlyout the past 3 Ga, even at destructive margins. Takahashirapidly and its radius increasing. A high degree of re-& Masuda (1978) attributed this anomaly to an alterationworking of earlier formed crustal materials may haveeffect, possibly hydrothermal alteration or interactionattended the eruptive process. Plagioclase is a liquiduswith ice early in lunar history. It cannot be whollyphase as a result of the diminished pressure and solubilityattributed to terrestrial alteration effects because theof water in the melts. Water and other volatiles are lostanomaly was first recognized in the Apollo samples,in large quantities and the magmas become reduced bywhich have not been exposed to terrestrial weathering.the losses of carbon and sulphur gases. Ce4+ is mostlyCanil (1999) has explored the partitioning of vanadiumreduced to Ce3+ and subsequently behaves like the otherbetween silicates and silicate liquid, which is very sensitiveREE. The small positive anomaly is handed on to theto the oxygen fugacity during crystal–liquid partitioning.rocks and minerals which crystallize, but some mineralVanadium becomes (more) incompatible as the oxygen(?clinopyroxene or phosphate) takes a slight excess of Ce,fugacity rises, with major changes around an oxygenleading to development of a small negative Ce anomalyfugacity three orders of magnitude lower than that ofin the late residual liquids. Eu3+ is substantially reducedthe equilibrium Ni–NiO (NNO× 10–3) where V3+ goesto Eu2+ as the oxygen fugacity falls and thereafter behavesto V4+ not far from where Cr2+ goes to Cr3+. Canillike Sr, becoming strongly concentrated in early pla-argued that Archaean komatiites developed at oxygengioclase and excluded from most other minerals relativefugacities close to that of NNO, younger komatiites andto the other REE. A substantial negative Eu anomalybasalts at NNO × (10–1–10–3) and boninites and arcdevelops in liquids which are residual from plagioclaserelated picrites at NNO × (101–102). Cr is expected tofractionation.be largely Cr2+ at the oxygen fugacities characteristic of

(iv) Partial melting declines in productivity and nowlunar basalts after their eruption, with Eu substantiallyproduces partial melts which are less water rich, possiblyas Eu2+ and Ti partly as Ti3+ (Schreiber, 1977). If thereboninitic, but still relatively rich in carbon and sulphurhas been an earlier episode in lunar petrogenesis marked

by higher oxygen fugacities, evidence for it may be found compounds, and still of relatively high inherent oxygen

1597


fugacity. The melts produced continue to develop sig- and to sustain internal radioactive heating for a prolongedperiod of time. Bodies the size of Vesta, whatever theirnificant positive Ce anomalies but are produced without

Eu anomalies (which there is no means of generating initial bulk composition, could be expected to be dom-inated by volatile-poor volcanism because of their lowboth because of the oxygen fugacity and the fact that

many melts are not plagioclase saturated on separation). central pressure. Feldspathic calc-alkaline melts couldnot develop and positive Ce anomalies would not beThese mare basalt precursors also erupt explosively, less

violently than previously, but more violently than is expected. There is no Ce anomaly in the basaltic achon-drites and mesosiderites thought to be the productsthe norm for modern terrestrial basalts. Surface flow

emplacement was predominantly as glowing avalanche of basaltic volcanism on small asteroidal bodies whereinternal pressures may have been insufficient to retaindeposits giving way to violent fire-fountaining as eruptive

activity and volatile contents waned. Extensive volatile water and other volatiles and the whole body may bereduced. Likewise, there is no Ce anomaly in freshloss and reduction by loss of carbon and sulphur com-

pounds takes place, with the effects noted above on the terrestrial igneous rocks which are derived from anevolved upper mantle with locally variable but overalloxidation states and geochemical behaviour of Ce and Eu.

Production of these water-poor mare basalt precursors moderately high oxygen fugacity, nor in the igneousrocks of the Martian meteorites, also derived from anoverlapped in time with continuing production of water-

rich melts from other parts of the mantle. evolved mantle with a lengthy evolutionary history. Thepossibility that relatively strongly oxidized mantles are a(v) Assimilation of, and interaction with, earlier crustal

materials as the outer parts of the Moon accreted even- feature of intermediate-sized rocky planets, large enoughto accrete and retain volatiles but too small to sustaintually imparted a volatile-poor, reduced character to all

the basic magma irrupting. Reduced melts formed shal- extensive subsequent mantle evolution, may be worthconsidering. It would predict that the surface rocks oflow intrusions and large lava lakes in which partial

crystallization led to gabbro fractionation and de- Europa and perhaps Mercury should also display positiveCe anomalies, whereas those of Io should not.velopment of marked enrichment of TiO2, up to the

point of ilmenite and armalcolite coprecipitation at>8% The alternative scenario calls for an initially ice-coveredMoon with early volcanic rocks being erupted and alteredTiO2 in the melt. Other incompatible elements, including

the REE with the exception of Eu, also became enriched. in a strongly oxidizing sub-glacial environment, whichwould terminate any debate about water-rich volcanismSulphide saturation and possible metal precipitation led

to chalcophile and siderophile element depletion. Most in the lunar highland crust (70).78. Processes akin to those which created the majormare basalts have been processed through this type of

geochemical filtering. PGE concentrations in the Bushveld, Stillwater and Sud-bury complexes, or in the vents during eruption of flood(vi) As volatiles in mantle regions were progressively

depleted and temperatures of partial melting continued basalts at Noril’sk, may account for some part of thedepletion of chalcophile and siderophile elements in lunarto rise, relatively small amounts of volatile-poor magmas

developed and erupt as visible conventional flows, dykes, mare basalts, although direct precipitation of moltenmetal alloys or metal–sulphide mixtures is also crediblesurface ridges of more viscous magma, domes and maars.

The bulk lunar mantle is predicted by this model still at these low oxygen fugacities. A higher production ofsuch a phase may have some bearing on the markedto have an inherent oxygen fugacity higher than that

typical of the modern terrestrial upper mantle. The lunar depletion of Au in lunar basalts relative to those of theHED parent body (Taylor, 1975, pp. 166–170, 176–177).mantle will have a negative Ce anomaly but the latest

partial melts may still carry small positive Ce anomalies Prolific ores of the noble metals may exist close to thelunar surface where they might be detected by high-because of the high oxygen fugacity at the source. The

same broad scenario of early evolution of water-rich resolution remote sensing in the walls and debris ofsuitable craters.highly explosive calc-alkaline magmas, succeeded by vo-

luminous volatile-rich basaltic materials giving rise to 79. Within the pressure range in the outer 500 km ofthe Moon, the potential solubility of water in silicatemare-like surface features, succeeded in turn by large-

scale basaltic flood volcanism with abundant visible flow melts exceeds that of carbon, sulphur and their gases.Water would be preferentially eliminated into the silicateunits and finally by localized central volcanism can be

fitted to the available evidence from Mars (57). Internal melts forming at the lowest temperatures duringradiogenic heating of an initially cool body of broadlyconditions in the Martian mantle may also have been

oxidizing and a positive Ce anomaly might be present carbonaceous chondrite bulk composition. These earliestwater-rich silicate melts which would have risen to formin the older volcanics at least.

This scenario will not be encountered in bodies too the early crust would have been biased towards highlyfeldspathic calc-alkaline and andesitic compositions (57).small to develop pressures of 0·1–0·2 GPa through sub-

stantial masses of material relatively close to the surface, On loss of water from such melts plagioclase is a very

1598


prominent and abundant early crystallizing phenocryst magmas at the lunar surface possibly followed by for-mation of lava lakes; or during final eruption of theas is seen in eruptives at terrestrial destructive margins.

A high activity of water in the lunar interior combined individual flows which gave rise to the samples recovered?Volatile depletion relative to the Earth might arise or bewith an oxygen fugacity within the range of terrestrial

or lunar magmas implies a relatively high activity of enhanced at any of these stages.Selective volatilization loss during eruption or selectivehydrogen also. Whereas water loss from initially hydroxyl-

bearing phases will be neutral in its effect on the oxidation failure to condense or be retained during accretion is acomplex topic. Each element may be present in thestate of the residual liquid, any loss of the highly mobile

hydrogen while water is still present will lead to some condensed and vapour phases as a variety of chemicalspecies. Transfer of material will attempt to equilibrateoxidation of the melt as further water dissociates.

80. Carbon, sulphur and hydroxyl compounds in sil- condensed and vapour phases by equalizing the activityof all possible species involving an element in the twoicate liquids are soluble at high pressures and volatile at

low pressures. The total effect of losses of dissolved water phases. In many circumstances the results of the processmay be controlled by kinetic as much as by equilibriumand water formed by reaction with escaping hydrogen

on the residual silicate melt is likely to be reducing. The phenomena. Even at equilibrium there are no uniquequantities to be defined such as the volatility of an elementactivity of water in any melt which had contained water

at elevated pressure in the Moon would probably greatly or the distribution coefficient of an element between twophases, and both properties are likely to be functions ofexceed that of the volatile carbon and sulphur compounds

and water would be lost rapidly from erupting magmas. both oxygen and sulphur fugacity. What happens indetail will depend upon the compositions of both phases,The evolution of carbon-oxide and sulphur-oxide gases

from silicate melts underpins the industrial process of the speciation of the element within each, and the kineticsof the system, especially if one of the phases (the vapour)iron-smelting and the reduction of oxidized metal ores.

Carbon and sulphur react with oxygen in the oxidized can readily escape from that part of the system which isrecovered for examination. The relative volatilities of theore, leading eventually to separation of immiscible molten

metal. At low pressures, even low activities of elemental pure elements are no guide to their relative volatilitiesfrom complex silicate melts.carbon and sulphur in the silicate melt imply very reduced

melts whose oxygen fugacities are close to or below those Little experimental work has been done to definethe behaviour of basalt magmas erupting under suchof the iron–wustite equilibrium. The effect of pressure

on the equilibria is, however, profound. An increase of conditions. Dobar (1965) reported ‘frothing’ of (partially)molten terrestrial basalt and granite on exposure to apressure would be expected to favour the distribution of

carbon- and sulphur-oxide gases into denser assemblages vacuum considerably higher in pressure than that am-bient at the lunar surface. Mackin (1969) anticipated theof elemental carbon, carbides or sulphides with the

oxygen combined with oxidized iron in the melt. High first Apollo mission with a prediction that the maria werefilled by nuees ardentes deposits, hence the ghost craters,activities of carbon and sulphur can then be sustained

in equilibrium with relatively oxidized silicate melts. and also asserted that lavas erupted on the Moon shouldfroth. O’Hara et al. (1970a, 1970b) reported violent vo-Graphite can be used as a container for terrestrial basaltic

melts in experiments at pressures as low as 5 kbar without latilization loss and rapid conversion of a molten ter-restrial flood basalt composition into lunar-likesmelting problems, and immiscible sulphide liquids are

in equilibrium with relatively oxidized terrestrial basic geochemistry in a vacuum furnace. Vinogradov (1971)also predicted spraying of basalt during eruption on themagmas at fairly low pressures. Ferri-ilmenites indicative

of very high inherent oxygen fugacities are a characteristic Moon, and Naughton et al. (1971, 1972) reported onvaporization from heated lunar samples. The large va-mineral in kimberlites carrying free carbon as diamond.

81. The lunar samples are strongly depleted, relative cuum facility at the Lunar Receiving Laboratory wasdecommissioned before experiments which would haveto the Earth and chondrite meteorites, in all of the

potentially volatile elements with the exception of sulphur, settled the debate could be carried out. The availablefield evidence from the Moon does little to constrainwhich is more than twice as abundant in lunar basalts

as in MORB. When did volatile depletion occur? Was basalt behaviour under such conditions, beyond requiringthat flows were exceptionally fluid and had very lowit during initial accretion of the Mars-size impactor

thought to have provided a substantial part of the Moon’s effective viscosities during the main period of formationof the maria (flow thicknesses estimated >10 m, vastmass; during final assembly of the Moon in orbit around

the Earth from the ejected and vaporized material; in extent, few flow fronts preserved) and were still low inthe final stages of filling of Mare Imbrium when verythe outer parts of the Moon during continuing impacts

into a lunar magma ocean formed in the latter stages of large flow units formed with flow thicknesses of >80 m.Less fluid magmas may have erupted in very smallreaccretion, if such ever existed; during irruption of

1599


quantities at a very late stage in the Marius Hills. Ter- important constraint. The surface area between melt andrestrial basaltic ash-flow eruptions are scarce (Melson et vapour would be huge, lithospheric pressure would notal., 1972) but if lunar magmas contained even small be an impediment to vapour development, and surfaceproportions of volatiles pyroclastic eruption and ash-flow cooling of droplets within the eruption plume would bemagmatism probably ensued. negligible during >100 s flight times (>10 km height

Eruptive activity took place into hard vacuum. Sulphur plume on the Moon) which would be required to producecontents in the mare lavas, like those in the basaltic significant losses of sodium at 0·1 mm diameter dropletachondrites, are higher than in terrestrial basalts and sizes. Fire-fountaining on the scale observed over Iowould sustain a vapour pressure many times the present would ensure significant losses from larger droplet sizes.confining pressure at the lunar surface. Fire-fountaining 83. Gas bubbles, vesicles, are a form of ‘phenocryst’certainly occurred at a late stage, producing the Apollo indicating early separation of a volatile phase from a15 green glasses and Apollo 17 orange and black glasses. magma. The vesicles present indicate no more than theMany returned hand specimens of basalt are vesicular, amount of volatiles evolved after the magma becamedemonstrating that gases were still evolving from the static and viscous enough to prevent their upward escape.basalt as flow ceased and the magma consolidated. Car- There is no easy way to estimate how much gas hasbon and sulphur gases are likely to have evolved, dis- evolved and been lost entirely from the system.rupting even fairly thick layers of basalt magma. Semi- 84. Volatilization of basaltic material by stepwise heat-continuous lava fountaining is favoured as the main type ing in vacuum proceeds by very rapid loss of soda, slightlyof lunar explosive volcanism (Horz et al., 1991) but it less rapid loss of potash, significant decreases in iron andmay have been the only type of volcanism during early magnesium oxides, and accelerating decrease in silica.mare formation. Models of flow of a gas–particle mix in Lime, alumina, magnesia and titania are enriched in thethe Earth and Moon point to much larger vents and residual liquid, with magnesia losses becoming significantmuch higher eruption velocities in the lunar case, other only when volatilization is far progressed, i.e. the residualparameters being equal (Pai et al., 1978) but Housley liquid is modified towards gabbroic anorthositic com-(1978) argued that lunar basalts would fire-fountain less positions (Yakovlev & Basilevsky, 1994). Volatilizationvigorously than those on the Earth. losses are extremely rapid and capable of converting

82. Two quotations encapsulate the ‘conventional’ terrestrial flood basalt compositions to lunar basalt-likeview: compositions, dominantly by sodium loss, in the equi-

‘Some petrologists appealed to loss of Na and K tovalent of 100 s flight time in a fire-fountain (Biggar et al.,account for the order of magnitude depletion [of lunar1972; Storey, 1973; Storey et al., 1974), when the changeslavas] in sodium compared with terrestrial lavas, perhapsare from basalt towards calcic norite or calcic pigeonitereluctant to believe that so fundamental a distinctionbasalt initially. The results of Yakovlev & Basilevskycould exist in basalt chemistry . . . The lunar lavas were(1994) even point to a possibility that the gabbroicextruded at temperatures approaching 1200°C into aanorthosite composition of the lunar highlands is a by-hard vacuum of at least 10–9 torr [>10–12 bar]. Althoughproduct of intense and long-sustained impact modi-such conditions might be thought to favor loss of volatilefication of more basaltic or gabbroic materials. Impactelements, the lithospheric pressure of the lava exceedsdriven volatilization yielded granitic condensates (Yakov-that of the vapor pressure of the elements at depthslev & Basilevsky, 1994) but such a component mightgreater than 10–3 cm. Thus loss could only occur fromhave been lost entirely from the lunar surface during thea thin skin . . . Very thorough stirring, mixing or bubblingperiod of heavy bombardment.would be needed to lose elements from depth . . . Loss

The conclusions of Melosh (1989), O’Keefe & Ahrensof volatiles is of course inhibited by rapid freezing of the(1994) and Stoffler et al. (1994) point to lunar highlandssurface of the extruded lavas. The consensus is thatin which target melt rocks abound, some of them fairlyvolatile loss from the maria basalts during extrusion ishomogeneous and some forming magma bodies largetrivial’ (Taylor, 1975, p. 150).enough to undergo internal fractional crystallization.‘Surface loss of volatiles during extrusion or meteoriteDoubt is thereby cast upon the identification of pristineimpact has not found much favour. During surface ex-lunar samples on the basis of their cumulus textures andtrusion of the lavas, freezing of the surface will rapidlylow noble metal contents, the more so if evolution of theoccur, and loss of volatiles will not occur’ (Taylor, 1975,impact melt magmas involved separation of a sulphidep. 166).melt, as it certainly did at Sudbury. Norman (1994)Taylor’s arguments and final assessment ignored thesummarized the case against Stoffler et al.’s (1994) in-specific proposal that volatilization losses occurred duringterpretation of Sudbury but did not refute the morefire-fountaining on eruption (Biggar et al., 1971, 1972).general conclusions of Melosh (1989) and O’Keefe &Diffusion distances to a surface would then be small

and diffusion rates through the melt would not be an Ahrens (1994).

1600


The parental magmas may have been subject to vo- excess alumina which we do not observe’ (Haskin &Warren, 1991, p. 415).latilization losses during their eruption as nuees ardentes

If one accepts the lunar magma ocean and plagioclaseor during fire-fountaining (81). Selective volatilization offlotation hypotheses for lunar highland origin, then in-sodium combined with release of iron originally presentdeed it follows that the entire lunar magma ocean mustas ferric oxide, iron sulphide or carbide would, however,itself have been Na depleted to crystallize the Na-poortranspose the residual silicate melt composition awayplagioclase which is typical of highland rocks. One mayfrom normative plagioclase and towards normative cal-question whether a lunar magma ocean, with its im-cium-poor pyroxene, yielding compositions which wouldplication of sustained bombardment into a growing pooltend to display the low-pressure crystallization sequenceof magma, accompanied by stirring and splashing on awhich is so common among mare samples. Petrologistsvast scale, could have formed without massive selectivewarned early in the Apollo program of the risk of Navolatilization of elements such as sodium, whatever theloss in particular (O’Hara et al., 1970a, 1970b; Brown &composition of the original material accreting to formPeckett, 1971) because of the experimental difficulty ofthe Moon. If, as argued here, the highlands are themaintaining the sodium content of silicate glasses atproduct of magmas formed as partial melts of an es-liquidus temperatures during syntheses and experimentssentially solid lunar interior which then irrupted to theeven at atmospheric pressure. Excess of alumina, releasedlunar surface, the appropriate question is whether Naduring sodium loss from albite molecule in basaltic com-and other volatiles were lost during eruption and beforepositions, manifests itself in the CIPW norm as extrathe crystallization of the plagioclase now seen in theanorthite and diminished diopside. Excess of silica fromanorthosites.the same source manifests itself in extra hypersthene and

The coherence and restricted range of ratios of po-diminished olivine (which might be tempered by antentially volatile trace elements (e.g. K, Rb) relative toincrease in FeO as a result of the reduction of anyinvolatile trace elements (e.g. U, Sr, La, Sm and Ba)oxidized iron during sulphur loss). The most evidenthave also been held to be strong arguments for indigenouspetrological characteristics of lunar relative to terrestrialvolatile depletion without subsequent selective vo-basalts are their high anorthite content in CIPW-norm-latilization in the lunar rocks. Taylor (1975, fig. 4.21)ative plagioclase and the high ratio of hypersthene toquoted a ‘good correlation’ between Rb and Ba, but thediopside in their CIPW norms, coupled with relatively lowfigure actually shows a variation of Ba/Rb from >0·01

mg-number in the normative ferromagnesian minerals.to>0·03. If the samples with the lowest ratio of Ba/Rb

These geochemical characteristics are precisely what have lost no Rb, those with the highest ratio have lostshould be expected if the true lunar parental magmas no more than >67% of their Rb.were more akin to terrestrial tholeiitic, or even alkali, Taylor (1975, fig. 4.23) also referred to the ‘closebasalts and had had their compositions severely modified association’ between K and La, but that figure shows aby selective volatilization during eruption. The scale of variation of K/La>50–125 and if the samples with thethe potential effects are such that this might be the end highest ratio of K/La have lost no K, those with theproduct even if the original parental magmas had been lowest ratio have lost no more than >60% of their K.strongly undersaturated nepheline-normative basalts. Taylor (1975, fig. 4.24) demonstrated a range in K/Until the issue of volatilization during eruption has been U from >8000 to 50 000 and if the samples with theresolved by experimentation it is optimistic to advance highest ratio of K/U have lost no K, those with theany lunar basalt as a primary magma. This effect would lowest ratio have lost no more than >84% of theirinvalidate all oversimplified inferences about the lunar potassium. Haskin & Warren (1991, figs 8.10 and 8.13),interior and its evolution derived from the chemistry or with a more extensive dataset, demonstrated a variationphase behaviour of the recovered materials. in K/Sm ratios of>40–500 in all lunar samples, 100–300

There are several specific geochemical arguments in mare basalts. If the samples with the highest ratio ofwhich have been adduced against selective volatilization: K/Sm have lost no K, those with the lowest ratio have

‘Another argument for originally low concentrations lost no more than >67–92% of their original K. Na/of lunar vapor-mobilized elements is the low ratio of Na Sm in the lunar samples varies from >50 to 2000. Ifto Ca in plagioclase feldspars from samples representative this variation is wholly due to selective volatilization ofof the bulk of the lunar highlands. If Na, which is a Na it would imply sodium losses of up to 97·5% of thatrelatively volatile element, had been as abundant in originally present.relation to Ca on the Moon as it is on Earth, lunar If the samples richer in the most volatile elementplagioclase would be more sodic. It is unlikely that the have already lost some of the volatile element, all theseNa could have evaporated from an anorthosite (with a maximum loss estimates must be increased. The ratioscomposition essentially equivalent to pure feldspar) or of volatile to non-volatile minor and trace elements in

lunar rocks do not preclude selective volatilization, evenfrom mare lavas without leaving excess silica or especially

1601


of trace elements whose activities in, and vapour pressures mass fractions of equilibrium partial melting. Subsequentdevelopments in modelling of sophisticated crystallizationover, the silicate melt would have been necessarily low

relative to those of sodium, carbon and sulphur gases. processes has invalidated these trace element arguments(18–28). The main conclusion to be drawn from theBoth Taylor (1975) and Haskin & Warren (1991) referred

to the systematics of the Rb–Sr isotopic system, which cumulate eucrites is perhaps that magma chambers whichcould act as sites of low-pressure partial crystallizationappears to preclude the Moon’s strontium having been

exposed to higher than present Rb/Sr ratios for any certainly existed on the parent planet. A direct linkbetween specific lavas and specific cumulates is not re-significant period of time, but even if there has been no

selective volatilization of Rb, this imposes no direct quired in a small sample set. There is evident uncertaintyabout the petrogenesis of this group of meteorites (Mit-constraint on possible losses of Na, S and O, which are

petrologically much more important. tlefehldt et al., 1998). The balance of evidence suggestsan equally viable alternative interpretation in terms of85. The howardite–eucrite–diogenite (HED) group

meteorites include many regolith breccias. Petrology and partial melting, perhaps of a carbonaceous chondriticsource, eruption with extensive volatilization losses, for-petrogenesis of this group has been reviewed by BVSP

(1981, section 1.2.8) and most recently by Mittlefehldt et mation of large magma bodies close to the surface,extensive partial crystallization and eruption of residualal. (1998). These groups stressed the low-pressure olivine

+ pigeonite + plagioclase-saturated cotectic character liquids to form a surface crust.87. Hubble Space Telescope observations of Vestaof the lavas and hypabyssal samples (Stolper, 1977;

Stolper et al., 1979); the presence of cumulate textures indicate a layered structure of eucrite overlying diogenitein turn overlying peridotite which has been excavatedin gabbro and orthopyroxenite samples; the scarcity of

vesicles in the eruptives; the development of small neg- by a 450 km (almost hemispherical) crater 8 km deepwith 8–14 km high rims and a 13 km high central peakative Eu anomalies in some of the lavas, and of distinct

positive anomalies in the cumulate samples which are (Gaffey, 1997). Other craters are observed up to 150 kmin diameter and a few kilometres deep. Miyamoto &consistent with plagioclase fractionation from reduced

basic magmas. The samples are siderophile depleted and Takeda (1994) inferred derivation of the Moore Countycumulate eucrite meteorite from>8 km deep in a 10 kmrelatively sulphur rich. There is no evidence for formation

of anorthositic materials by plagioclase flotation or any thick crust cooled from the solidus in <10 Ma. This depthwould be consistent with crust formation from an>10%other mechanism in the magma bodies (64). No high-

titanium basalts have been reported. Vesta, a possible melt fraction of the whole interior of a 525 km diameterbody. Gabbroic samples exist which are clearly cumulatesparent body of the HED meteorites, has a diameter of

about 525 km, and a poorly constrained density of>3·7 formed within large magma bodies differentiating at lowpressure. Orthopyroxenite samples are known which± 0·5 g/cm3 (Ghosh & McSween, 1998) based on

an H-chondrite model. The body has a differentiated, could represent an ultramafic layer at the base of thosemagma bodies. Diogenite (orthopyroxenite)-rich plutonsreduced basaltic surface (eucrite) of a type which is very

rare among the asteroids as a whole, although a larger were envisaged by Warren (1997).Ghosh & McSween (1998) argued for simple radiogenicnumber, particularly among the larger bodies, have the

reflectance properties of mesosiderites. Cruikshank et al. heating to power the volcanism of Vesta but a magmaocean has been proposed, with development of a core(1991) identified three possible small source bodies for

the available suite of HED meteorites other than Vesta between 5 and 30% of the body’s mass (Ruzicka et al.,1997). Righter & Drake (1997) went further and proposeditself. Hiroi et al. (1995) identified some small Vesta-like

objects in near-Earth orbits and 20 more in Vesta-like total melting of the body, core formation followed by well-stirred equilibrium crystallization to 0·20 mass fractionorbits.

86. Although plagioclase-saturated low-pressure co- of remaining melt, followed finally by gravitationallycontrolled fractional crystallization of the remainder.tectic character provides a strong case for low-pressure

partial crystallization in larger planets (35–41), Stolper 88. HED basalts are also relatively rich in sulphur andmay have separated both sulphide melts and sulphur(1977) concluded that it resulted from eucrite formation

as primary magmas at a pressure not too different from gases during their irruption (73–75, 78).89. Eruptions at the surfaces of small bodies withoutatmospheric, which in turn implied an unsampled calcic

plagioclase-bearing peridotite interior. Key arguments in permanent atmospheres can be expected to produceoptically dense fire-fountains in which particles will sufferreaching this conclusion were recognition that the mg-

number of minerals in the available cumulate samples little cooling and will fall back to form lava ponds andflows, with <1% surviving as discrete clasts (Wilson &could not be directly related to those of the available

lavas, and the presence of incompatible trace element Keil, 1997). These are the ideal circumstances for ex-tensive selective volatilization losses during eruption (82).features in the lavas inconsistent with perfect fractional

crystallization but consistent with small and variable Such losses are essential on a large scale if the HED suite

1602


and the mesosiderites are to have evolved within small reaction. In either event, the erupted basalt compositionsbodies from volatile-rich, somewhat oxidized parental would be required to be close to plagioclase saturationmaterials such as the carbonaceous chondrites. Sodium at low pressure and cannot be regarded as primaryloss in particular is essential because all the chondritic magmas.meteorites are relatively sodium rich and have actual or Walker (1983), recognizing that the lunar mantle mightpotential plagioclase in the oligoclase–albite range. not contain an adequate negative Eu anomaly to be-

90. Most of the meteorites which show evidence of queath to later basalts, proposed a mechanism for anearly igneous evolution within the asteroids are char- origin of the negative Eu anomaly in the mare basaltsacterized by low oxygen fugacities, extremely calcic pla- (assuming magmas of the hand-specimen compositions)gioclases, low sodium concentrations and high normative involving imposition of a cryptic plagioclase crys-orthopyroxene, characteristics also shared with the lunar tallization signature, analogous to the cryptic clino-igneous rocks. These are predictable results of extensive pyroxene crystallization signal seen in many terrestrialvolatilization losses of sulphur and sodium from basaltic continental flood basalts and MORB. However, althoughcompositions produced by partial melting of more chon- the clinopyroxene cryptic signal is readily explicable indritic peridotites. terms of a small depressurization of essentially dry

91. If there is no positive Eu anomaly in the average magmas between crustal magma chamber and surfacelunar highland crust (62), there can be no complementary eruption, the same change in the phase equilibria ex-built-in negative Eu anomaly in the underlying mantle pressly forbids generation of a cryptic plagioclase crys-[contrast Taylor & Jakes (1974, fig. 3)]. Positive Eu tallization signal in the lunar basalts by such a mechanismanomalies have been reported for some lunar metabasalts (93).and ophitic basalts (Laul et al., 1978), and only slightly If the mechanism of flotation of potentially feldspathicnegative anomalies for some ferrobasalts when the REE liquids (Morse, 1987) operates (see 63), a substantial partconcentration is about 10 times chondritic. When dealing of the cumulate pile was precipitated from magmas whichwith regolith samples, however, it is necessary to be were not plagioclase saturated at depth, yet formed duringaware of comminution and sampling effects. Crushing the enrichment in potential plagioclase of the magmasand mineral separation experiments on 70135,27 (Haskin from which the crust would later form. This mechanism& Korotev, 1977) established the extent of variability to can explain a plagioclase-rich crust which has only abe expected. Clinopyroxene is the major mineral carrier limited positive Eu anomaly, but it does not implant aof REE and, together with olivine, pigeonite and ilmenite,

complementary negative Eu anomaly in the cumulatehas a positive Ce anomaly. The mesostasis, which tendsmantle until plagioclase is saturated in the crystallizingto be preferentially comminuted, is 400 times richer inmagma and so does not ease the problem of generatingREE than the minerals and has a negative Ce anomalythe marked negative Eu anomalies in the mare basalts.[see also Blanchard et al. (1975)]. Haskin (1978) sum-Nor does it solve the problem of generating magmasmarized the trace element geochemistry of the Marewhich do not have plagioclase precipitating as theirCrisium fragments and concluded that the soil–rocksecond silicate phase at low pressure (see 93).differences mainly reflected the comminution effects,

93. The liquid compositions which form during lowwhereas inter-particle differences reflected mainly sam-mass fractions of partial melting of polyphase mantlepling problems. There may or may not be a small Eumineral assemblages are necessarily close to simultaneousanomaly in either sense and there is a small positive Cesaturation at the pressure of their formation with allanomaly in the average soil. The bottom line is thatthe crystal phases present at the solidus of the mantlethere is only a very small Eu anomaly in those basaltsassemblage. The phase equilibria of the liquid fractionpoorest in titanium.will show a close approach to simultaneous saturation in92. If there is no relative deficit of Eu in the underlyingthese phases at the liquidus at that pressure [subject tolunar mantle, then there is no inbuilt negative Eu anomalynote (6) and to the possible absence of a solid phasefor the later mare basalts to inherit at birth. Parentalwhich is in reaction relationship with the liquid at themelts would have contained no more than small Eutime of its formation]. The effect of pressure on theanomalies as is the case for some lunar VLT basaltsphase equilibria relevant to peridotite melting and basaltwhich are low in overall concentrations of incompatiblepetrogenesis in dry systems has been long establishedelements. The marked negative Eu anomalies in the(O’Hara, 1968a; Cox et al., 1979, figs 9.9–9.12). Dealingother mare basalts must, therefore, have been imposedfirst with the remelting of cumulates (without change ofon the residual liquids by fractionation of plagioclase,pressure on the cumulate before the remelting event),probably as gabbro, after arrival of the magmas at theliquids which are produced in equilibrium with olivine,base of, or within, the lunar crust. An alternative would beorthopyroxene, and clinopyroxene plus plagioclase in theextensive re-equilibration of the magmas with plagioclase

already located in the crust, i.e. by a form of assimilative range 0–1·0 GPa or spinel or garnet at higher pressures

1603


up to 2·5 GPa, can be expected to display the low- Other picrites which do display the low-pressure crys-pressure crystallization sequence olivine joined next by tallization sequence olivine–clinopyroxene, joined onlyplagioclase, then by clinopyroxene and finally by calcium- later by plagioclase, are voluminous among the earlypoor pyroxene as temperature falls. This predicted be- eruptives of continental flood basalt provinces (Karroo,haviour is displayed in phase equilibria diagrams for Deccan, Siberian Traps), in the Solomon Islands (Stantonprimitive MORB formed at >8–11 kbar (BVSP, 1981, & Bell, 1969), at Mauna Loa and Mauna Kea, and alsofigs 3.3.24 and 3.3.35), primitive Icelandic picrite formed occur in basalt sequences related to a destructive marginat>25 kbar (Maaløe & Jakobsson, 1980), primitive Ha- in Kamchatka (Kamenetsky et al., 1995), but none ofwaiian olivine tholeiite formed at>12 kbar (BVSP, 1981, these are required to have formed with the restrictionsfig. 3.3.25), tholeiitic CFB formed or equilibrated at>0·8 regarding their source region which apply to the lunarGPa (Thompson, 1972) and alkaline CFB formed or mare hand specimens. All can have formed at muchequilibrated at>1·6 GPa (BVSP, 1981, fig. 3.3.26). Real higher pressures and/or with much higher mass fractionsbasalts, furthermore, know about these constraints. The of partial melting.typical phenocryst assemblages in erupted MORBs, Ice- The desired lunar ‘primary’ liquids might also belandic and Hawaiian tholeiites, the Skye Main Lava generated by more advanced partial melting of the cu-Series (Scarrow & Cox, 1995) and in non-picritic CFBs mulates at pressures of 0·5–2·5 GPa leaving residues ofgenerally are olivine plus plagioclase sometimes ac- depleted harzburgites, but the required mass fractions ofcompanied by clinopyroxene. Pigeonite and ortho- melting required for all the ‘primary’ liquids if this solutionpyroxene phenocrysts are rare ( Jamieson, 1970b). A is adopted are likely to exceed those compatible withphase diagram for the forsterite–diopside–anorthite sys- the trace element imposed requirement for small masstem (BVSP, 1981, fig. 3.3.20) indicates that substantial fractions of partial melting. Phase equilibria for postulatedpartial melting of an alumina-saturated olivine–pyroxene lunar mare primary magmas (BVSP, 1981, figs 3.4.3 andassemblage might be required at 0·7–2·0 GPa, producing 3.4.5) exhibit temperature intervals between liquidus co-melts far advanced up the olivine + pyroxene phase saturation with olivine and orthopyroxene and theirboundary, to generate a primary liquid which would subsequent saturation with spinel or plagioclase, ofdisplay the low-pressure crystallization sequence of olivine 50–100°C, which suggests that the liquids are far from thefollowed by pyroxene. Terrestrial MORBs and CFBs are compositions of the initial primary melts of plagioclase-widely assumed to have been generated by >10% or saturated cumulates under those conditions.greater mass fractions of partial melting. Yet this more

Is the situation improved by allowing cumulates formedadvanced melting has either not proceeded far enoughat one pressure to be remelted at another? If plagioclase-for the liquid products to precipitate olivine joined bysaturated (but plagioclase-free) magnesian harzburgitespyroxene rather than plagioclase during low-pressureat pressures of close to 10 GPa plume upwards to lowercrystallization, or the melts have been modified towardspressures, alumina and lime dissolved in the or-more feldspathic compositions en route to the surface.thopyroxene will tend to exsolve to yield actual crystals ofThe commonly observed sequence in lunar mare handplagioclase. The initial partial melts will remain saturatedspecimens is olivine joined next by calcium-poor pyroxene withwith plagioclase to higher mass fractions of partial meltingor well in advance of plagioclase. Liquids with this typein consequence. These initial partial melts will migrateof low-pressure behaviour cannot be generated by smallin composition as the pressure falls so that they shouldmass fractions of partial melting of originally plagioclase-display less plagioclase crystallization before pyroxenesaturated cumulates even at very high pressures withinentry at low pressure, but most of the motion of thethe Moon. Addition of some trapped melt to the equationinitial liquid compositions takes place during the last 0·2exacerbates the problem.GPa decrements of pressure (BVSP, 1981, fig. 3.3.18),Clinopyroxene can be encountered as the second crys-i.e. within the thickness of even the thinnest parts of thetallizing silicate phase at low pressure in the initial partiallunar nearside crust, within the uppermost 5 km of themelts of mantle assemblages saturated with garnet, whenterrestrial crust and within the depth of some terrestrialthese are formed at pressures >4·5 GPa (Herzberg &shallow-seated layered gabbro complexes. There is noO’Hara, 1998) but this is not an admissible solution forresolution of the problem here. Advanced partial meltingthe Moon. Such liquids can also be generated at lowerinto the harzburgite + liquid equilibria continues topressures by more advanced partial melting yieldinghave the advantages and disadvantages already notedharzburgite equilibria. The picritic eruptives of Baffinbut affords no resolution of the problem because theIsland and Iceland (Clarke, 1970; Maaløe & Jakobsson,orientation of the locus of liquids in equilibrium with1980) may have formed by partial melting of garnet-olivine+ plagioclase+ clinopyroxene is almost coplanarsaturated lherzolite assemblages at 2·5–3·5 GPa andwith the olivine + orthopyroxene control planes [e.g.show the anticipated low-pressure silicate crystallization

sequence of olivine joined by plagioclase before pyroxene. O’Hara (1968a) and Fig. 4].

1604


If plagioclase-saturated (but plagioclase-free) ilmenite relatively low pressures. Whichever interpretation is ad-opted the compositions of these liquids have evolvedlherzolites, formed originally at pressures of close to 0·4

GPa, anchor downwards to pressures of 1·0 GPa and on under conditions where there was substantial ‘elbow-room’ towards compositions with high normative pla-to pressures of 2·5 GPa, then the initial liquids of their

partial melting are no longer required to be plagioclase gioclase content relative to pyroxene (93), allowing equi-libration of relatively feldspathic partial melts with olivineor spinel saturated on formation, but those alumina-

saturated liquids towards which the initial liquids will and pyroxenes at moderate pressures. Just being relativelyclose to equilibrium with plagioclase at pressures oftend are increasingly far displaced into the field where

olivine joined by plagioclase before pyroxene will be 0·2–1·0 GPa is sufficient to ensure that plagioclase willappear before pyroxene in the low-pressure crystallizationthe low-pressure crystallization sequence. A majority of

Apollo 17 hand-specimen compositions show plagioclase sequence.95. The anticipated sequence of olivine joined byentry virtually coincident with that of pyroxene at low

pressure (O’Hara & Humphries, 1975) and clearly lie plagioclase followed by clinopyroxene during low-pres-sure crystallization is pervasive in terrestrial MORBfar from the high-pressure alumina-saturated cotectics,

implying that substantial mass fractions of partial melting compositions and in many continental flood basalts.96. None of the putative lunar primary magmas basedwould be required to generate these compositions directly.

Again, the effects of pressure changes on the phase on glass-bead or mafic hand-specimen compositions dis-plays the olivine–plagioclase crystallization sequence atequilibria do not offer a resolution of the basic problem.

If these were true primary magmas formed in the manner low pressure. In these compositions the crystallizationsequence of liquidus olivine joined next by calcium-poorpostulated, they should display the crystallization

sequence olivine joined by plagioclase followed after a pyroxene is typical. There are in most cases extendedtemperature intervals at all pressures before the ap-significant interval by calcium-rich rather than calcium-

poor pyroxene. pearance of a third phase, which is usually Ca-richclinopyroxene. Such liquid compositions, if they areCan anything be salvaged? The primary character of

the major element composition might be maintained, indeed primary magmas, can only have formed fromsource rocks which were harzburgites or olivine py-and the trace element concentrations could be ascribed

to assimilation of KREEPy material during transit to the roxenites.97. The negative Eu anomaly cannot be inherited andsurface. This relaxes the requirement for low mass frac-

tion partial melts and so greatly eases the problem. must be generated by separation from plagioclase at lowpressure (92). Choice of the hand-specimen or glass-beadIntegrated melting (O’Hara, 1985, 1995a) can also com-

bine trace element features suggestive of low mass frac- compositions as the primary magmas leads to the impassethat plagioclase does not begin to precipitate until latetions of partial melting with major element features

suggestive of higher mass fractions of melting, but only in the crystallization sequence, whereas the anomalyis required to be well developed in the initial liquidat the expense of relatively high average mass fractions

of melting. In the absence of a positive Eu anomaly in composition. The hand specimens cannot represent theparental magmas.the average lunar highland crust, the requirement for

plagioclase saturation in the magma ocean (and indeed 98. If the lunar mantle has evolved from broadlychondritic material by extraction of a relatively smallthe requirement for a magma ocean at all) can be

abandoned and a harzburgitic mantle adopted (66, 67). mass fraction of partial melt to form the lunar highlandsand the mare basalts, the ultrabasic upper-mantle mineralThe problem of the low-pressure crystallization behaviour

is then solved at the expense of the primary character of assemblage probably still includes small amounts of analumina-rich phase. This would be plagioclase at depthsthe magmas. Plagioclase fractionation is required to im-

part the negative Eu anomaly (92) but is inadmissible less than >200 km, spinel at depths between 200 kmand 500 km, and garnet at greater depths. At the leastunless the hand-specimen compositions represent por-

tions of the erupted magmas which have become enriched the mineral assemblage is likely to consist of olivineand pyroxene crystalline solutions which are close toin ferro-magnesian and oxide phenocrysts (103, 104) or

have been translated in character by selective vo- saturation with the alumina-rich phases. This still seemsto be the case in the terrestrial mantle despite its havinglatilization (84). Either solution spells sudden death for

the primary magma hypothesis. undergone more prolonged and extensive partial meltingthan the lunar mantle. In the ‘conventional’ lunar model,94. Primitive terrestrial MORB compositions are

thought to have separated at moderate pressures by the outer parts of the mantle are required to have grownby accumulation of dense phases from an evolving magmaup to 10% average partial melting from plagioclase-

undersaturated lherzolite and harzburgite residua in one ocean which was simultaneously crystallizing and losingplagioclase by flotation to form the early crust. Even ifpopular view. In an alternative view they are thought to

have separated from plagioclase-bearing cumulates at the extraction of plagioclase is perfect and complete in

1605


this process, and there is negligible trapped liquid, it thermally driven convective motion from the moment ofdeposition. Ignoring this reservation, the pressures andwould still be the case that the ferromagnesian assemblage

would be plagioclase saturated. The first liquids to form temperatures of co-saturation of olivine and pyroxene inthe postulated lunar mare primary magmas have beenif it were to be remelted at the pressure of its original

formation would also be plagioclase saturated under those used to obtain points which are required to lie close topoints on the postulated selenotherms during mare basaltconditions.

99. The primary liquidus phase hypervolumes of generation. These confirm a required thermal gradientfar steeper than the adiabatic gradient. The lunar mantledifferent crystal species, i.e. the ranges of bulk com-

positions in multicomponent composition space from rocks would be required to be close enough to their solidustemperatures to provide little resistance to convection atwhich a given crystal species will begin to crystallize first,

are altered by changes in the pressure (O’Hara, 1968a; the time of production of the later mare basalts.101. There is little geomorphological evidence to sug-Herzberg & O’Hara, 1998). One of the consequences is

that in any fixed basaltic or picritic bulk composition it gest that there has been any substantial convective motionwithin the lunar mantle. In a non-convecting lunar mantleis likely that the liquidus phase at low pressure (olivine)

will be replaced as the pressure increases by one or more it is difficult to see how any cumulate formed from thepostulated global magma ocean can become remelted.other phases, typically a pyroxene first and later by garnet

at very high pressures. Co-saturation with olivine and The original accretional energy is a dwindling resource.So is the radiogenic heat from enclosed U, Th anda pyroxene somewhere along the liquidus as pressure

increases is an almost inevitable consequence of starting K, elements whose heat production was insufficient toprevent solidification in the first place, and which wouldwith a basaltic composition and need have no great

petrological significance with regard to origin of that bulk in any case be concentrated into the residual liquid, notthe cumulates. Once solidified, these cumulates shouldcomposition. This property is in no way diagnostic of

primary character and a very wide range of randomly stay solid, and gradually cool. Gravitational instability ofthe cumulate pile has been invoked to explain the re-chosen compositions will display this type of materials

behaviour. The data reported in table 6.5 of Taylor et melting. Deeper, hotter, earlier, less dense magnesiancumulates are postulated to rise and partially melt onal. (1991) and many of the entries in table 9 of Papike et al.

(1998), for example, have little petrogenetic significance. decompression. Shallower, cooler, later, denser ilmenite-bearing cumulates are postulated to sink and partiallyThe probability of primary character is greatly en-

hanced when it is found that the liquid displays sim- melt as they become heated by their surroundings. Theformer process should also have affected the earliest,ultaneous co-saturation at the liquidus for several silicate

phases, e.g. olivine, calcium-poor pyroxene, calcium-rich most magnesian and hottest cumulates, which would nothave been precipitated from plagioclase-saturated liquidspyroxene and garnet at upper-mantle pressures, as in the

case of Icelandic picrite liquids. The probability of gabbro in the conventional model and would have no Eu an-omaly. Although candidate compositions abound (99,fractionation is greatly enhanced when the liquid displays

simultaneous co-saturation with olivine, plagioclase and 100) the required primitive magmas lacking negative Euanomalies are scarce. This discussion would have to becalcium-rich clinopyroxene at deep crustal pressures as

in the case of Snake River flood basalt (Thompson, 1972). modified if it transpires that there was a substantial butdeclining input of energy from tidal deformation in theNone of the putative lunar primary magmas displays the

simultaneous co-saturation at high pressure which would early history of the Moon.102. The postulated eruption of unmodified hot, drybe expected if compositions were those of liquids gen-

erated by either primary partial melting or subsequent primary magmas from depths of 130–480 km within theMoon (or any other planet) presents a serious challenge indeep-seated differentiation, especially if mass fractions of

melting are constrained to be small by the incompatible chemical engineering, especially if those primary magmasare supposed to be volatile-poor, small mass fractiontrace element behaviour.

100. The adiabatic gradient within the probable partial melts with no initial superheat relative to thesolidus. Setting aside the problems of negotiating a longmaterials composing the lunar mantle will be lower than

that in the Earth’s mantle, as a result of the reduced channel through hot, potentially reactive mantle wallrock (31), there is the problem of negotiating the windowgravitational acceleration. The gradient of the solidus

temperature of the postulated cumulate pile (liquidus of eruptability (33) provided by the low-density highlandcrust without partial crystallization or contamination.temperature of the postulated residual liquid/postulated

initial remelt liquid) will greatly exceed this value, com- These problems have not been solved by the vastmajority of terrestrial basalts. Those terrestrial magmas,pounded by the additional decline in solidus temperature

with height associated with the evolving compositions of komatiites and picrites, which have the best claims todeep-seated primary character are typically erupted vo-liquid and cumulate. If a global magma ocean ever

existed, its cumulate pile should have been subject to luminously and in less than a few million years in the

1606


early stages of a volcanic cycle, to be succeeded by copious sampled were small (a few tens of metres around thebasalts with compositions which have been modified by Lunar Excursion Module at Apollo 11 and 12 sites, withlow-pressure crystallization and by assimilation of crust. a more extended set of small localities scattered overIn oceanic islands, late deep-seated eruptives, which some several kilometres at Apollo 15 and 17 sites), observingview as primary melts, but others as advanced high- that the diversity of chemistry and texture at each site ispressure magmatic differentiates, are typically very small greater than would be encountered at terrestrial sites.in volume, very rich in volatiles and alkalis, and erupt Vertical or horizontal mixing over long distances, par-explosively on top of very large accumulations of differ- ticularly of hand-specimen sized samples, can be excludedentiated tholeiitic basalts. Candidate lunar primary (110). Only a few flows are likely to have been sampledmagmas in the ‘conventional’ model would have had to at each site (e.g. Staid et al., 1996) and this very diversityhave been erupted sporadically over periods of hundreds points to the sampling of flows which are internallyof millions of years in the waning phases of the volcanic differentiated with respect to both chemistry and texturecycle. (see also 118). However, even in the thin Icelandic picrite

103. Primocrysts, i.e. phases possibly formed in equi- flows (Maaløe & Jakobsson, 1980), which must havelibrium with the magma at a fixed temperature in the cooled extremely rapidly, there is marked variation inmagma chamber or conduit before eruption, differ from phenocryst percentage (1–49%) ascribed to some com-phenocrysts, defined as large crystals in a finer-grained bination of gravitational and flowage differentiation.groundmass. Phenocrysts may include some primocrysts Some at least of the olivine and spinel phenocrysts inbut may also include materials which form during de- the glassy basal flow margins (which were probablyvolatilization and rapid cooling after eruption. Primo- quenched into ice) are partially annealed skeletal quenchcrysts, including plagioclase, identified in the most rapidly crystals, not partially resorbed phenocrysts or xenocrysts.quenched lunar samples, are very small (O’Hara et al., Quench phenocryst cumulates must have formed within1974). Quench products may nucleate on existing primo- the lunar lava flows (note 103). A further complicationcrysts, may grow to volumes and masses exceeding 103

is introduced by the possibility that quench crystals weretimes that of the primocrysts, and are typically strongly forming and segregating during the flow of the magma,zoned in their composition and often skeletal in form. leading to lateral as well as vertical differentiation withinAn excellent example from an Apollo 15 mare basalt is the flows. The higher viscosity of terrestrial basalts de-figured on the back cover of Planetary Materials (Papike mands higher cooling rates to form quench textureset al., 1998). Quench phenocrysts are not in chemical

similar to those in lunar rocks. The combination ofequilibrium with the average magma composition or anyhigh cooling rate with high viscosity prevents significantresidual liquid. Plagioclase, however, is reluctant to growquench crystal sinking in the case of terrestrial lavas,in the quench except in grossly supersaturated melts,except perhaps in the case of olivine spinifex within somewhich are most readily produced by rapid de-komatiites (Arndt, 1986) and a few other examples. Thepressurization of water-bearing feldspathic melts (e.g.pillow basalts of the Stapafjell quarry (map ref. 251880)Ford et al., 1972, 1977; O’Hara, 1972). Quench growthnear Keflavik airport, Reykjanes peninsula, Iceland, dis-of pyroxene can lead to the precipitation of metastableplay chilled margins which are almost phenocryst freealumina-rich pyroxene rims and lead to plagioclase su-but pillow interiors show basal accumulation of abundantpersaturation in the residual liquid, followed by generalolivine, sometimes skeletal, and very sparse plagioclasenucleation of plagioclase throughout the groundmassphenocrysts, both of which must have formed and thenwhich prevents the growth of large crystals (Lofgren etsunk in the quenching episode. The criterion that aal., 1974).quench-textured rock represents a chilled liquid com-Quench growth of phases is enhanced by low viscosityposition (other than for volatile content) may be validin the melt. Skeletal and strongly zoned phenocrystsfor most terrestrial lavas but cannot be extended to lunarof olivine, pyroxene and oxide phases are present inrocks.abundance in lunar basalts, and the low-titanium basalts

Recognition that the lunar lava hand specimens mayfrom Apollo 12 and 15 are remarkable for the size andbe related to one another and to the true average eruptedzoning of their phenocrysts. Walker et al. (1976a, 1976b)magma composition by differential movement ofstudied the relationship between cooling rate and crystalmaterials which are not in chemical equilibrium withmorphology in Apollo 12 hand-specimen compositioneach other or the liquid introduces serious complications12002, concluding that at the cooling rates in evidenceinto attempts to model the geochemical relationshipsfrom the petrography, observed phenocrysts would havebetween lunar samples (Biggar et al., 1972), further com-sunk 5 m during the cooling of the flows.pounded by the suggestion (Rhodes et al., 1977) that104. Taylor (1975, p. 124) commented on the manyincompatible element distributions indicate late residualdifferent varieties of mare basalt which were brought

back from the Apollo missions, although the areas liquid migration within slower cooled parts of the flows.

1607


The returned hand specimens contained numerous >10 m (106). The flows may have been exceptionallyquench textured picritic samples. It was subsequently fluid and tapered out rather than forming flow-fronts,demonstrated (Lofgren et al., 1974) that monotonic cool- yet in places may be rather thick, i.e. their rheology wasing of liquids of the compositions of these rocks could very different from that of most terrestrial basalt flowsreproduce these textures. Here at last were the picritic (81). Individual flows may have spread out over the wholeprimary magmas predicted by experimental petrology available surface of the maria.(42). They were, however, spurned by one group of The magma volumes involved in this third possibility,workers on the grounds (110–118) that the hand speci- and the implied volumes of source region processed, aremens did not represent the average magma composition shown in Table 1 and become impressive if thick overallerupted at each site and that unrecognized quench crystal units are demanded. Calculations are for a mare surfacecumulates had to be present in the sample set (O’Hara of >357 km diameter and 100 000 km2 surface area; aet al., 1970a, 1970b, 1974, 1975a; Biggar et al., 1971, fissure 100 km long extending uniformly to 300 km depth;1972; O’Hara & Biggar, 1972, 1975, 1977; Humphries a cylindrical pipe extending to 300 km depth; and for aet al., 1973; O’Hara & Humphries, 1975, 1977a, 1977b). regular cone of slope>5·71° (height 0·1× radius). The

A few of the picritic compositions from Apollo 12, but diameter of the required spherical source region has beenno other site, were truly glassy and could not have calculated assuming the basalt represents a 2% meltacquired their present combination of composition and fraction.physical state by any form of crystal accumulation. To If the magmas were higher mass fraction melts whichthose advocating hand specimens as primary magmas then underwent partial consolidation in the crust, verythese samples were proof of picritic liquids; to those substantial thicknesses, 2–50 times the average thicknessadvocating quench crystal accumulation they were of the basalts, of magma chamber contents are requiredformed by impact remelting of picritic target rock, splash- within the crust. A substantial part of the depth of theing and rapid chilling on the surface. maria might be the required magma chamber contents.

105. Rate of effusion rather than viscosity is the primary Crustal contamination of the incompatible element con-control on the landforms of accumulated lavas. At rel- tents could, of course, relax this requirement at theatively low rates of effusion Hawaiian type shields are expense of sacrificing the primary magma hypothesis.formed; at higher rates flat, ponded lavas are produced Another possibility is that the bulk of the maria are(Greeley & Womer, 1981). The distinction between a formed from lavas which do not have very high contentsmare-sized ponded lava and a lava lake then obviously

of incompatible elements. This implies higher averagedepends upon both rate of eruption and lava depthmass fraction partial melts in the earlier mare fill but, inachieved, i.e. upon the lava volume which is availablethe absence of convective recharge of the partial meltingfor rapid effusion and hence upon the magma storagezone, the basalts enriched in incompatible elementsfacilities beneath the maria. The view (Shaw & Swanson,should then be at the base of the pile and only depleted1970; Schaber, 1973; Walker, 1973) that very large, veryrocks at the top.long lava flows emplaced at low angles always require

Magma productivity during mare filling may havevery high eruption rates, combined with relatively lowbeen low (128). The irruption of large volumes of basaltviscosity to prevent the lava freezing before it has flowedmagma in a confined area within a short time-span,out fully, may need to be modified, both in the light ofhowever, may require source region melt productivitiesrecent studies on flow inflation and flow extension bywhich are much higher than those associated with thewell-insulated lava tubes (Cashman et al., 1998) andHawaiian plume (>5 m3/s, Wolfe et al., 1988). Somebecause of the possibility that lunar basalts may haveflood basalt provinces require melt productivities com-flowed as emulsions of gas and liquid droplets, the basalticparable with this, but the Deccan and Siberian Trapsequivalent of nuees ardentes. Zimbelman (1998) cal-may require productivities up to 20 times this valueculated most probable effusion rates of (0·5–1·0) × 105

(Richards et al., 1989; White & McKenzie, 1995), andm3/s for an Imbrium flow and a flow associated withthe Bushveld complex may require productivity a hun-Pavonis Montes on Mars (>100 000 times the Kilaueadred times greater again (Cawthorn & Walraven, 1998;rate), but with a wide range of possible solutions fromR. G. Cawthorn, personal communication, 1999). The100 to 40 ×106 times the Kilauea rate.terrestrial energy source for these high productivities isWell-marked flow-fronts are not observed over mostcurrently attributed to impingement of super-plumes onof the lunar maria (Mare Imbrium is an exception)the lithosphere, predominantly under previously longalthough mappable differences in optical reflectancestable continental areas (Cox, 1978, 1980; Richards etproperties indicate that flows of different types are present.al., 1989; White & McKenzie, 1989, 1995) with ac-This observation creates problems to which there arecompanying prominent crustal doming (Cox, 1989). Itthree possible solutions. The near-surface flow units may

be thin relative to the regolith thickness, i.e. less than may also occur under oceanic areas to produce such

1608


Table 1: Required dimensions of pre-eruption storage chambers

Flow thickness (m)

0·1 0·3 1·0 3·0 10·0 30·0

Volume (km3) 10 30 100 300 1000 3000

Fissure width (m) 0·33 0·96 3·33 9·56 33·3 95·6

Pipe diameter (m) 210 357 650 1128 2060 3568

Cone diameter (km)/height (m) 5·8/290 8·3/415 12·4/620 17·9/895 26·7/1340 38·6/1928

Source diameter (km) 9·85 14·2 21·2 30·6 45·7 65·9

features as the Ontong–Java plateau (Cox, 1991). Con- relating to morphology and size of crystals, particularlyplagioclase, may be vulnerable to how close the eruptionvective plumes are not admissible as an explanation of

the lunar phenomena. temperature is to plagioclase saturation and which av-erage magma composition is adopted, the very essenceThe alternative is that high effusion rates, intermittently

exercised, demand convenient magma storage facilities of the debate about mare basalt petrogenesis (48). Slowcooling and accumulation of olivine in cooling units ofrather than high production rates at the source. Large

volumes of single flows may demand very large storage 30 m thickness at the Apollo 12 site has been suggested(Walker et al., 1976b). Rhodes et al. (1977), who adoptedvolumes, perhaps 100–1000 times greater than the erup-

ted flow volume, i.e. >106–108 km3, which are volumes the rapidly cooled vitrophyres as the parental magmas,deduced extensive intra-flow fractionation related to cool-comparable with those calculated for the Bushveld

magma chamber (Cawthorn & Walraven, 1998). ing rate, with olivine, spinel and pyroxene settling inunits up to 40 m thick. Incompatible element distributionsTerrestrial picrite flows range in thickness from a few

centimetres (Haleyjabunga, Iceland) through a few metres were interpreted to indicate late residual liquid migrationwithin more slowly cooled parts of the flows.(Kamchatka, Baffin) to several tens of metres (e.g. Baffin,

Karroo, Siberia). These picrites are associated with dom- There is another way in which the feldspathic basaltcompositions of the average target rock for mare regolithinant feldspathic basalts in most locations, and are in

general a minor part of the whole basaltic assemblage and production can yet be reconciled with the hypothesis ofpicritic primary magmas. If the flows were thick, a formconcentrated in the early rather than the late eruptives

of, for example, the Karroo, Deccan, Siberia, Iceland, of compensated crystal fractionation (Cox & Bell, 1972)might have occurred after emplacement. The coolingSvartenhuk, Kamchatka and Solomon Island sequences.

Voluminous thick ultrabasic flows are specifically as- rates required to form the textures of the coarser-grainedmare basalts required flow thicknesses of>30 m (Walkersociated with active plume situations and incipient con-

tinental rifting, which is not relevant to the lunar situation. et al., 1976b) and the cooling times of such flows wouldpermit crystal settling of at least 5 m. This would have106. The mare fill might be a succession of very

thin units perhaps as little as 10–30 cm thick, each removed excess ferromagnesian crystals from the top>5 m, replaced lost phenocrysts throughout the nextdifferentiated (104), and then preferentially comminuted.

Only the basal parts of each flow, strengthened by an >20 m and allowed cumulus enrichment in the basal>5 m. Subsequent regolith formation mainly from theinterlocking mass of sunken quench crystals, provided

hand specimens. This could solve the problem of invisible top >5 m of such flows could then yield the observedpetrographic features of the regolith and the hand speci-flow margins but does not resolve the Eu anomaly

problem (92) or the need for slowly cooled environments mens, which might still represent the erupted magmacompositions. The minimum flow thickness which might(107). If mare flow units are less than >5 m thick, the

average regolith composition should, however, be close support the proposed reconciliation might seem to be>15 m. However, the margins of flows this thick shouldto that of the average magma erupted.

107. Thick layered units were observed in the walls of be clearly visible on the mare surfaces but are in generalnot detected (but see 105).Hadley Rille (Howard et al., 1972). Several groups worked

on the effects of cooling rates on textures in low-titanium Rhodes (1977) addressed the problem of the persistentenrichment of the mare surface regolith in feldspathicbasalts and thence attempted to deduce the thickness of

flow units (Lofgren et al., 1974; Donaldson et al., 1975; materials, accepting that lateral transport from adjacenthighlands, although demonstrable over short distances,Walker et al., 1976a, 1976b). All cooling rate studies

1609


was an inadequate explanation. He explored the sug- reflect the true average lava composition. There is asgestion that the feldspathic materials in the regoliths yet no indication that these materials are different in(20–60%) came from underlying basin-filling brecciated composition from the average mare regolith.highland materials, and that the mare basalts were every- 110. Some of the most important remote-sensing resultswhere a very thin skin (necessarily less than the thickness are verifiable with a good pair of binoculars or smallof the regolith) over highland debris. This proposal does telescope. The highlands are rough and heavily craterednot account for the mass concentrations associated with but the maria are smooth. The mare basalts do notthe filled mare basins, nor for the evidence from drowning drown many large craters nor have they been penetratedof large craters, nor for the failure of subsequent large by many. The edges of the maria are sharply defined incraters to excavate feldspathic material from beneath albedo against the highlands without significant gradationmare basalts, all three features being consistent with on either side of the contact. Consequently, neither lateralsubstantial thicknesses of mare basalt in the centres of nor vertical mixing has played a large part in determiningthe basins, as was recognized by Rhodes (1977). Any the compositions of the regolith after >3·8 Ga ago (seesuggestion that an explicit or occult highland-derived Fig. 2). The low extent of even very short range lateralfeldspathic component has been introduced into the transport of materials in the regolith over the past 3·8current mare regoliths by vertical mixing also predicts Ga is demonstrated at the Taurus Littrow (Apollo 17)that feldspathic character of the regolith should decline site by the low contents of readily identifiable mare basaltgradually away from the mare edge and away from any in the 2–4 mm size fraction in the regoliths ( Jolliff et al.,topographic highs on the basin floor as the basalt sequence 1996) from the South Massif (station 2: 1% of adjacentthickens. This is not observed. Sampled mare basalt high-Ti basalt, <2% VLT basalt) or the North Massifsurfaces differ by about 500 million years in age and (station 6: 13% mare basalts). This conclusion is supportedwere formed during a time of rapidly declining flux (a by the ability to map different basalt types across theten-fold decrease in cumulative crater frequency) of the maria by optical reflectance studies (Pieters, 1978).projectiles whose impacts lead to regolith formation (Neu- The idea that the soils developed over the lunar high-kum, 1977). This should lead to older mare basalt re- lands are chemically representative of underlying bedrockgoliths being much more feldspathic than younger ones is unquestioned (61). Observations from the same remote-if vertical mixing were an important mechanism, yet the sensing data sources persistently indicate that the mareputative process has been intelligent enough to self- regoliths have been developed from a source rock whosearrest once a low-pressure plagioclase-saturated basalt

average composition is much closer to that of a plagio-composition has been achieved. Vertical mixing of high-clase-saturated low-pressure cotectic liquid than the vastland derived materials into mare regoliths cannot bemajority of the analysed hand specimens. The agglutinatesustained as a major factor in generating the feldspathicportion of the soil, formed by repeated small impactscharacter of mare regoliths.into the regolith as it matures, may be chemically frac-108. Any process which randomly samples materialstionated and enriched in ferromagnesian constituentsto depths greater than the average flow thickness shouldrelative to the target material, but Hu & Taylor (1977)yield a regolith or debris apron whose composition isfound that agglutinate glass displayed no appreciableclose to that of the average target rock. Adopting 40 mfractionation of major or minor components and reflectedas the upper limit for average flow thickness and atarget regolith compositions. This maturation does, how-compensated crystal fractionation model, craters of aboutever, affect the response of the bulk soil to spectral500 m diameter and upwards should certainly excavatereflectance studies. Pieters et al. (1993a) found that thematerials approximating to average flow composition.<25 �m fraction dominates the optical properties of soils,109. Rhodes et al. (1977) argued that Middle Crescent,and elsewhere it has been demonstrated (McKay et al.,Surveyor and Head craters at the Apollo 12 site would1991) that the <10 �m fraction is systematically morehave excavated to depths of 80 m, 40 m and 20 m,feldspathic in the mare soils. The geochemical contrastrespectively. Two of these at least should have excavatedbetween hand specimens and average soil cannot, how-the whole thickness of a flow unit and their debris apronsever, be discounted as due to preferential comminutionshould have compositions reflecting that average. Thereof feldspar and mesostasis in the regolith. Returnedis no evidence that the regolith surrounding these orsoil analyses and geochemical remote-sensing techniquesmany other comparable craters in the maria excavateswhich sample the top several tens of centimetres ofmaterials different from the average regolith (C. M.the regolith yield the same conclusion as the spectralPieters, personal communication, 1999). The walls ofreflectance and X-ray fluorescence studies which sampleHadley Rille (Howard et al., 1972) expose at least 60 mmainly the shallow surface and might have been in-of mare fill at the top and a further 200 m of debris slopefluenced by the preferential comminution of feldspar intoformed from that and the underlying rocks. Regolith

developed on the surface of these slopes should also the ultrafine fraction.

1610


Hubbard et al. (1978) confirmed that the average ba- laterally or a few metres vertically. This does not, how-ever, exclude addition to every soil of a very far travelledsaltic target rock converted to regolith throughout the

maria is much more aluminous than the average hand component of average lunar surface composition whichis effectively randomly distributed into all the soils. De-specimens from Apollo 11, 12, 15 and 17. Spudis &

Pieters (1991, fig. 10.8c) showed that the properties of spite, or because of, the evident lack of lateral and verticalmixing, the soil compositions are considered to representthe mare surfaces are not represented by the hand-

specimen compositions. This version of the Th/Ti vs Fe the average target rock composition with no more than20% addition of very far travelled randomly distributedplot shows a peak of results implying mixtures of about

10–30% of the aluminous components into the hand- materials, much of them mare derived (BVSP, 1981).112. Bulk mare regolith analyses are those of aluminousspecimen compositions to create these mare soils. This

conflicts with the expectation that not more than 5% of basalts (McKay et al., 1991; Papike et al., 1998) yet thepaucity of aluminous basalt hand specimens is con-the soil at any site has been derived from more than

a few kilometres away and the absence of sufficient spicuous. However, three markedly feldpathic basaltshave been reported from the hand-specimen collectionpetrographically identifiable highland components. Data

were obtained from the Mariner 10 spacecraft on its way at the Apollo 12 site. Sample 12038 contains >12·7%Al2O3 and 3·25% TiO2 and is precisely cotectic at lowto Mercury in 1974. These cover the eastern limb and

farside, and detected neither very low titanium (VLT) pressure for plagioclase, olivine, Ca-poor pyroxene andspinel (Biggar et al., 1971, 1972). Rhodes et al. (1977)nor high-titanium basalts. Cryptomare were detected and

need to be abundant to explain the abundance of VLT found 12008/12045 to precipitate plagioclase after ol-ivine, pigeonite and spinel (the three prominent phe-basalt fragments in the lunar meteorites (Robinson et al.,

1992). Galileo imaging of large areas of the Moon (Gree- nocryst phases which may have been enriched by crystalsettling) at 1131 ± 6°C from a liquid containing aboutley et al., 1993; Pieters et al., 1993b) showed that none of

the maria surveyed contain basalts with the high TiO2 11% alumina, four crystalline phases appearing between1138 and 1125°C. This is an extraordinary number(9–14%) of the hand specimens from Apollo 11 and 17

although moderately titanium-rich basalts occur on the of crystalline phases to enter near simultaneously in arandomly chosen composition but is readily explicable ifwest limb and farside. Mare pyroclastic deposits were

also seen on the farside. the rock composition is derived by mafic phenocrystaccumulation into a low-pressure, plagioclase-saturatedMascons underlie many of the maria. Recently several

new mascons have been discovered, including four as- four-phase cotectic liquid. Sample 12031 contains12·63% Al2O3 and 2·88% TiO2, and 12074 containssociated with one nearside and three farside basins some

of which are apparently devoid of mare fill (Schiller– 11·6% Al2O3 and 1·8% TiO2 [calculated from modalanalysis by Beaty et al. (1979)]. Beaty et al (1979) gave aZucchius 55°S, 45°W; Hertzprung 2°N, 130°W; Cou-

lomb–Sarton 51°N, 120°W; and Freundlich–Sharonov general discussion of aluminous basalts and noted thatglasses of the composition of feldspathic basalt are com-18°N, 175°E). This may indicate shallow-level mare-

related peridotite–gabbro intrusive complexes, which mon (Reid et al., 1972; Reid & Jakes, 1974). The Apollo12 feldspathic basalts are, however, not prominent in themay also underlie some of the other mascon basins,

particularly those with sparse basalt fill. In the maria Lunar Sourcebook (Heiken et al., 1991), where 12038 ismentioned only as the source of a baddelyite crystal andHead et al. (1978) identified three types of basalt regolith

in Mare Crisium, distinguished by very low, intermediate as a numbered point in a figure and the other twosamples are ignored.and high titanium concentrations which appear not to

display an obvious age-differentiation sequence. This 113. The average compositions of small basaltic lithicfragments in the regoliths from the Apollo 11 and 12could point towards periodic magma recharging of what-

ever chamber was the site of the fractionation. Andre et sites in Mare Tranquillitatis and Oceanus Procellarumare those of feldspathic basalts (Prinz et al., 1971; Keil etal. (1978) identified a low-Mg, very low titanium basalt

layer overlying higher-titanium, higher-Mg basalts ex- al., 1972, table 14, analysis 5). These compositions areclose to those found experimentally for high- and low-posed elsewhere in Mare Crisium and with more mag-

nesian materials excavated from below 1400 m depth. titanium basaltic liquids which are low pressure, plagio-clase saturated, and cotectic also with respect to oli-Tompkins et al. (1994) reported a possible layered mafic

gabbro pluton excavated from beneath the basalts of vine, pyroxene and one or more oxide phases (O’Haraet al., 1970a, 1970b; Biggar et al., 1971, 1972). Mi-Mare Nubium.

111. Vertical and horizontal mixing in the post-mare crophenocrysts of plagioclase, olivine, ilmenite, spineland pyroxene were figured and analysed from vitrophyricregolith is expected to be minor. Regolith compositions

reflect the local, near-surface, target rock composition lithic fragments of high-titanium basalt in the Apollo 11regolith (O’Hara et al., 1974) and the groundmass was(Melosh, 1989, section 10.3) with only minor additions

of material from more than a few kilometres distant shown to have a composition similar to the average of

1611


lithic fragments and to that determined as the plagioclase- the contribution both from the upper layers of the targetrock and from extraneous sources, are >88·5% basalt,saturated cotectic liquid by experiment.

No determinations of the average compositions of the 4·6% KREEP and >6·8% highland sources (Griffiths etal., 1981). Mixing models applied to the soil compositionsbasalt lithic fragments in the regoliths from any of the

Apollo 15 or 17 stations have been published. However, using as input a ‘representative Apollo 15 (hand specimen)basalt’ recognize the station 9A soils as the most basaltdetailed studies have been published for the bulk and

modal compositions of the coarse fines sieved from the rich from all the Apollo 15 sites (Walker & Papike, 1981)but calculate a lower average percentage contributionApollo 17 drill core 70006–70009 in which the only

significant quantity not reported is the average com- from basalt, >82·6%, and a much higher contributionfrom aluminous highland sources,>9·2% KREEP, withposition of the basalt lithic fragments (Vaniman & Papike,

1977a, 1977b; Vaniman et al., 1979; Laul & Papike, >8·6% anorthosite and low-K Fra Mauro plus almost10% green glass.1980). The data show a wide variation in the proportions

of basalt and highland-derived components, allowing a Walker & Papike (1981) attributed these differencesentirely to the effect of the plagioclase-rich componentsregression of weight percent alumina and titania in the

bulk samples against the percentage of observed highland- which are preferentially enriched in the very fine fractionsof the regolith. The effect, however, is the same as thatderived materials. This predicts contents of these two

critical oxides in the basalt component, in the absence seen in the coarse fines from the Apollo 17 core wherethe plagioclase-comminution effect cannot have been aof highland component, which are identical to those

of plagioclase-saturated low-pressure cotectic liquids in factor. Lindstrom et al. (1977) reported average values of10·2% Al2O3 in smaller lithic fragments of olivine basaltexperiments on sample 70215 from the same site, and

significantly higher and lower, respectively, than in the and 10·8% in olivine-free basalt from the Apollo 15 deepdrill core 15003. In a study of 21 ‘selected’ Apollo 15average compositions of the hand specimens from the

Apollo 17 site. The material from three depths, 28, 38 rake samples 0·29–0·58 g in mass, the range of aluminacontent was 8·5–9·8% and the average 9·0%, only frac-and 47 cm, in the core is exceptionally rich in identifiable

basaltic material. Calculation of the composition of that tionally higher than for the average (8·4–8·8%) of largerhand specimens (Ma et al., 1978).basaltic component can be made with low errors because

the composition of all other components has been pro- 114. Very low titanium (VLT) basalt has never beencollected as a large hand specimen and is characterizedvided and their contribution is small. The alumina and

titania contents so calculated bracket the regression line by two fragments from the Apollo 17 site in TaurusLittrow at the edge of Mare Serenitatis, 70007,296 andand overlap those of the plagioclase-saturated low-pres-

sure cotectic liquids. 78526. Both are relatively aluminous basalts (Taylor etal., 1991, table A6.1).Laul & Papike (1980) calculated the percentage of

highland-derived materials present in each level of the Luna 16 and Luna 24 collected random samples ofregolith from sites in Mare Fecunditatis and Mare Cri-Apollo 17 core using the bulk chemical analyses and

assumed compositions for each of the modally analysed sium. There is very little highland-derived material inthe regolith at the Mare Crisium site (>2%, Bence &components. The composition assumed for the basaltic

component was that of the hand specimens, not that of the Grove, 1978). Basalts with sub-ophitic textures suggestiveof early plagioclase nucleation are present (Coish &actual lithic fragments present. The resultant calculated

percentages of highland-derived component greatly ex- Taylor, 1978). Analyses of lithic fragments from theregolith define an aluminous basalt as the principalceed the percentages observed. The simplest in-

terpretation is that most of the excess calculated highland component (Taylor et al., 1978). Basalt types at these sitescould be characterized only by analysis of small lithiccomponent is actually present as feldspar in the more

feldspathic basalt composition which should have been fragments because there were no hand specimens. Theresultant ‘low’-titanium basalt 21013,8 from Luna 16 (itused in the calculations.

The data published for the Apollo 15 cores are in- actually contains 5·1% TiO2 and is geochemically moreakin to the high-titanium group) and the two VLT basaltsadequate for a similar treatment. Walker & Papike (1981)

published modal analyses of bulk core, with modal es- 24174,7 and 24109,78 from Luna 24 are feldspathicbasalts (Taylor et al., 1991, table A6.1). Negative Eutimates necessarily restricted to the coarser particles, of

the core samples from two stations remote from the anomalies are significantly smaller in these feldspathicbasalts than in typical hand specimens and barely presenthighland front. None of the horizons were sufficiently

rich in basalt to allow a reliable calculation of the at all in the Mare Crisium VLT basalts. Large-ionlithophile trace elements are less concentrated in thecomposition of that basaltic component. The identifiable

lithic fragments in the soils from Apollo 15 station 9A VLT basalts than in all other types, and richest in thehigh-titanium basalts, which also contain the largeston the edge of Hadley Rille, where soils are relatively

immature (24–30% agglutinates) and mass wastage limits negative Eu anomalies.

1612


Armed only with remote-sensing data and regolith mare-type basalt from a >3·88 Ga old breccia 73255,containing 13·8–14·2% Al2O3. The basalt clasts in brec-samples, and in the absence of hand specimens, it has

been concluded that the average ferrobasalt composition cias generated in mega-impacts, where excavation depthsmust have vastly exceeded flow thicknesses, are also moreat the Luna 24 site was plagioclase saturated and close

to the plagioclase–olivine–Ca-rich clinopyroxene cotectic aluminous than the average hand specimens (Staid et al.,1996). Among lunar highland regolith breccia meteorites,at low pressure. Much the same conclusion, disregarding

the hand specimens, was reached earlier for the Apollo Lindstrom et al. (1986) reported analyses of two aluminousbasaltic quenched clasts in Y-791197. Bischoff et al. (1987)11 (O’Hara et al., 1970a) and Apollo 12 (Biggar et al.,

1971, 1972) sites, is reached in this paper regarding the and Goodrich & Keil (1987) reported frequent VLT andlow-Ti mare basalt clasts in Antarctic highland farsideApollo 17 site, and is here predicted to be reached for

the Apollo 15 site when adequate data for the average meteorites which are 3·9–4·0 Ga old.115. There are six lunar mare basalt meteorites whichcomposition of basalt fragments in the soils become

available. Taylor et al. (1978) concluded that there has may be taken as random samples of basalt from maresurfaces on the Moon. All are very ancient, aluminousbeen low-pressure fractionation of the Mare Crisium

basalts. The same result would be obtained working on low-Ti or VLT basalt. Y-793169 and A-881757 areunbrecciated gabbros or very coarse-grained basalts. EETthe regoliths from the Apollo 11, 12, 15 and 17 sites in

the absence of the hand specimens, and the procedure 87521 is a fragmental breccia and Y-793274 a regolithbreccia. A-881757 may have formed originally near thedoes give the same result at the Luna 16 site.

The Apollo 14 breccia 14321 contains random small centre of an uncommonly thick lava, as may have thetwo latter samples (Arai et al., 1996).samples of ancient basalts from an unknown, probably

destroyed, mare source—all are feldspathic basalt, as is 116. The preferred compositions (Reid et al., 1972;Reid & Jakes, 1974) of average target rocks giving rise14053, the only larger mare basalt fragment from that

site (Taylor et al., 1991, table A6.1). Mare basalt clasts to the population of impact generated glass beads at theApollo 11, 12 and 14 and Luna 16 sites are those ofin Apollo 14 breccias are relatively aluminous and define

five groups of broadly plagioclase-, olivine- and pyroxene- aluminous basalts. Delano (1975) deduced the presenceof three younger-mare derived glass components insaturated low-pressure cotectic liquid compositions with

moderate alumina contents (11·8–12·7%) which appear Apollo 16 soils which could have the compositions ofaluminous basalts with>11·5% Al2O3 and TiO2 varyingto be part of a differentiation trend from compositions

with higher MgO and higher mg-number, with low con- from 3·7 to 7·6%. Meyer (1978) has, however, questionedhow representative highland glass beads may be of theircentrations of highly incompatible elements (REE >12

× chondritic) and small negative Eu anomalies, towards target materials, on the grounds of 100-fold variations intheir correlated trace element concentrations.compositions with lower MgO and lower mg-number

compositions, with elevated incompatible element con- 117. The adoption of the hand-specimen and glass-bead compositions as primary magmas leads to seriouscentrations (REE >70 × chondritic) and pronounced

negative Eu anomalies. This trend is accompanied by a geophysical, phase equilibria field relationship and geo-chemical problems which are resolved by accepting andeep negative Sc anomaly and complex changes in the

Ba/La and Ta/Th ratios (Dickinson et al., 1985, fig. 2). average erupted lava composition approximating to thatof the regolith (120).A suite of high-alumina (11·1–14·2%) fine-grained basalt

clasts in breccia 14321 (Neal et al., 1989) extend still Let us postulate for the sake of argument [with manyworkers traceable through BVSP (1981) and Taylor et al.further this range in trace element concentrations. Neal

et al. discarded fractional crystallization from a common (1991)] that lunar mare basalts (of the types representedby hand specimens from the Apollo 11, 12, 15 and 17magma for the relationship between these groups because

of the great changes in the incompatible trace element sites in BVSP tables 1.2.9.1–1.2.9.5 and A6.1–A6.2 andspecifically samples 12002 and 70215) were liquid com-concentrations, unaccompanied by large changes in the

major element concentrations. This reasoning is valid for positions and were primary magmas which were pro-duced by partial melting deep (120–470 km) within theclosed system perfect fractional crystallization but may

not be for a range of more sophisticated magma chamber lunar mantle. It is then necessary to postulate that themass fraction of partial melting was small (>0·01–0·1)processes reviewed elsewhere (18–28). Contamination of

basalt by KREEP is a possible mechanism here, and because highly incompatible element concentrations inthese rocks reach values between 10 and 100 timessome of the more sophisticated partial crystallization

models which might be applied here demand con- chondritic and vary by factors of 4–8 within groups ofsamples which display little variation in mg-number andtamination coincident with partial crystallization on ac-

count of the space problem (O’Hara, 1977, 1980, 1998). MgO wt %.All the samples contain prominent negative Eu an-Blanchard & Budahn (1979) reported three clasts of

‘ancient’ low- to moderate-titanium, relatively magnesian omalies which all researchers ascribe to the separation

1613


of the liquids directly or indirectly from plagioclase. controlled by drilling and by sparse exposures on land.Initially, the lunar work was undertaken with poor con-Plagioclase, however, is nowhere near being a liquidus

phase in most of these compositions at any pressure. trols from well-studied terrestrial basalts (133). Inevitably,models and hypotheses have played a major role andConsequently, it is necessary to postulate further that

the source region itself carries the signal of plagioclase the ‘conclusions’ may be less than rigorous.The discrepancy between the compositions of the maficfractionation. This is achieved by postulating the former

existence of a deep magma ocean which fractionally hand specimens and the average target basalt has to beexplained. One possibility is that the relatively thin marecrystallized as it cooled and in due course attained

plagioclase saturation in the residual liquids. Thereafter it regoliths have not sampled deep enough to obtain anaverage of the whole flow unit and that the upper partswould have precipitated plagioclase-saturated cumulates

consisting of olivine and orthopyroxene, later joined by have been phenocryst depleted by gravitational settling(104, 107). Gravitational settling certainly took placeclinopyroxene and ilmenite. Most or all of the plagioclase

which was also crystallizing is postulated to have floated within the cooling time of these low-viscosity flows anddemands the existence of complementary phenocryst-or been selectively transported to the top of the magma

ocean and so separated to form the lunar highland crust. enriched cumulates at the base of the flows (103). Handspecimens from two to four flow units appear to haveThis plagioclase carried with it much Eu but little of the

other REE, and so imparted a pronounced and growing been collected at all four mare sampling sites yet thehand specimens chosen to represent the average com-negative Eu anomaly to the residual magma of the

magma ocean and thence to the cumulus ferromagnesian positions (and the putative primary magmas) are amongthe most mafic samples recovered from these sites.mineral assemblages which it was precipitating. This

model presupposes that the bulk composition of the Terrestrial basalt flows are often vertically zoned withvesicle-enriched, even frothy, tops passing downwardsmagma ocean is one which will have the crystalliza-

tion sequence olivine–orthopyroxene–plagioclase– into more massive basal portions. This should be true oflunar lavas also, with the added factor of quench crystalclinopyroxene–ilmenite under the rather complex poly-

baric conditions of crystallization. It must logically also sinking, which would have imparted extra cohesion tothe basal cumulate materials. The lunar lavas were thenhave included a process of declining recharge of the

remaining magma as crystallization proceeded by in- subject to extensive small-scale meteorite bombardment(gardening) which produced several metres of regolithfalling meteoritic parental material. For some geo-

chemical balances it is convenient to further postulate from the upper layers of the last flow at each site andpresumably some regolith on top of earlier flows giventhe entrapment within the cumulates of a small amount

of KREEPy residual liquid (although the logical liquid the long time intervals recorded between the last eruptiveevents at each site. The basalt component in the fine-to trap is the current residual liquid of the magma ocean).

The mare hand-specimen primary magmas are then grained lunar mare regolith will be biased towards thecomposition of the more vesicular, and more friable topspostulated to be the products of secondary partial melting

of selected parts of these later cumulates, thus imparting of the flows.The astronauts were expected to collect a variety ofan inherited negative Eu anomaly to the resultant primary

magmas. The major chemical variability of the erupted larger rock samples, in addition to the soil samples andregolith drill cores. The larger rock samples may bemare basalts is thus attributed to the variability of the

hypothetical cumulate pile, not to near-surface differ- representative of some combination of materials moreresistant to fragmentation and comminution duringentiation. However, although individual cumulate layers

might be highly contrasted in bulk composition as a regolith gardening and materials recently excavated byimpacts which penetrated the regolith and sampled theresult of variations in mineral proportions and species

accumulating, they are all required to have precipitated bedrock. If the original target material were in-homogeneous with respect to depth and cohesion thesequentially from a liquid composition evolving in some

coherent manner. The liquids formed by small mass assemblage of larger specimens would not be rep-resentative of average composition, mineralogy or texturefractions of remelting of those cumulates should also fall

close to that coherent trend. This is not evident in the of the target materials. The varied responses of rocklayers exposed in the walls of Hadley Rille to micro-compositions of the hand specimens and pyroclastic glass

beads advanced as primary magmas. impact ‘weathering’ (Howard et al., 1972) indicates thatsurface-parallel and, therefore, depth-dependent hetero-118. The situation regarding lunar mare basalts was

extraordinary in geological experience. For almost the geneity probably existed in the target rocks.On receipt at the lunar receiving laboratory samplesfirst time the community was endeavouring to reconstruct

the field relationships, field appearance and genesis of were examined, characterized, described and cataloguedbefore selection of samples for distribution to the numer-erupted lavas from the geochemistry of small, unlocated

fragments. Even in the deep oceans, interpretations were ous principal investigator groups. So far as possible, an

1614


example of each distinguishable category of basalt was arose where the quality and precision of experimentaldata became central. The first was the mass fraction ofselected and sent to each of the relevant investigators.

This procedure made no attempt to distribute samples crystallinity in basalt hand-specimen samples at the firstappearance of plagioclase during cooling, bearing on thein proportions that reflected either their numerical abund-

ance in the hand-specimen collection or the fraction of proximity of compositions to the low-pressure plagioclase-saturated cotectics and involving questions of chargethe mass of that collection which each constituted. If it

was different, it needed to be studied. Nevertheless, this preparation, path to run conditions and observationaltechnique. The second was the mg-number of liquidusprocedure added a further bias to the possible fabric and

depth bias introduced in initial sampling, towards small ferromagnesian silicates, bearing on which samples mighthave been erupted as liquids, and involving questions ofspecimens not representative even of the average mass

of the hand-specimen collection. control of charge composition, oxygen fugacity and choiceof container. The third was whether or not armalcoliteEach step in the sampling and selection procedures

was legitimate, but it is potentially misleading to then had reacted out of the charges before plagioclase ap-pearance, bearing on whether observed armalcolite phe-take a simple arithmetic average of the compositions of

the analysed hand specimens without regard to their nocrysts could have accumulated into a lava erupted asa plagioclase-saturated cotectic liquid, and also involvingrelative masses and abundance and represent this as the

average target rock or even as the average composition questions of control of charge composition, oxygen fu-gacity and choice of container.of the returned hand specimens. Any interpretation which

then selected the most mafic hand-specimen compositions The Geophysical Laboratory of the Carnegie In-stitution of Washington was not involved, yet had es-as representative of the parental magma compositions,

from which other samples might be derived by small tablished an international reputation for careful, precisework over the preceding 60 years. H. S. Yoder, notedmass fractions of fractional crystallization, is potentially

flawed because it implies that the conjugate cumulates for basalt experimentation and petrogenesis, was on itsstaff. The Pennsylvania State University laboratory withhave not been sampled at all. Particular caution needs

to be exercised when the average composition of the E. F. Osborn and A. Muan had established an inter-national reputation for careful work in iron-bearing sil-regolith is so different from that of the hand specimens,

as it is at all four mare sites sampled. icate systems, but was not involved in the earlyprogramme. The established Manchester experimentalThe regolith also has an equal claim to be rep-

resentative of the average target rock composition, but laboratory was not included. The 3-year-old Edinburghexperimental laboratory, initially the only experimentalin this case biased in composition away from that of the

material still locked up in the few larger hand specimens, petrology group which was approved for the Apollo11 sample return, contained staff who had previousand additionally biased towards a small input of highland-

derived materials. Both types of bias increase the like- experience in established laboratories (Wyllie et al., 1962;O’Hara & Schairer, 1963; O’Hara & Yoder, 1963, 1967)lihood of these regolith compositions showing an early

appearance of plagioclase during their crystallization. and was still in process of validating its techniques incontrol of temperature (Biggar & O’Hara, 1969a), startingData projections and direct experiments indicate that

soil compositions from all four sites have, or are likely to materials (Biggar & O’Hara, 1969b), oxygen fugacity(Biggar & O’Hara, 1972a), choice of container materialshave, plagioclase as an early crystallizing phase (e.g. Figs

7–9). for iron-bearing charges at low oxygen fugacity (O’Hara etal., 1970), precise experimentation in controlled chemicalThe alternative view supported here is that the lithic

fragments and a few hand specimens do represent the systems at low (O’Hara & Biggar, 1969) and high pres-sures (O’Hara et al., 1971), and interpretation of crystal–average compositions of the aluminous basalts actually

erupted. The more mafic hand specimens then represent liquid equilibria in tightly controlled multicomponentsystems (O’Hara, 1969b; O’Hara & Biggar, 1969).the phenocryst-enriched basal sections of the flows. Pet-

rographic interpretation is complicated by the very small Other groups invited to undertake experimental studiesin the Apollo scientific program from the Apollo 12size of the true primocrysts, which include plagioclase,

and by the relatively large size of the groundmass crys- return onwards did not in all cases meet these criteria.This led to publication of results for low-pressure equi-tallization in many cases. It is further complicated by the

enormous size of the phenocrysts, which are strongly libria in lunar basalts using a variety of techniquesfor temperature achievement and control without inter-zoned metastable crystals grown during the quenching

process. laboratory calibration (except on the lunar samples them-selves, whose homogeneity could not be guaranteed) and119. Choice of investigating laboratories and control

of experimental method within the lunar science pro- without publication of calibration against fixed pointson the international temperature scale. These varyinggramme was less rigorous in the field of experimental

petrology than in mainstream geochemistry. Three issues techniques may have led to inter- and intra-laboratory

1615


uncertainties in excess of 30°C (Biggar & O’Hara, 1969a). because the inherent oxygen fugacity of the self-bufferingexperiments was below that of the Fe–FeO equilibrium.Some workers converted samples to glass (to homogenize

the sample) and carried out runs on this material without Oxygen simply diffuses past the iron filings, leaving themuntarnished, and reacts at the inner charge–containerprior devitrification, a technique avoided at the Geo-

physical Laboratory because of the risk of generating interface, which is the best getter around. Use of moreeffective getters would solve the oxygen problem providedmetastable assemblages—the point is graphically ex-

plained in BVSP (1981, fig. 3.2.3). Even devitrification that their inherent oxygen fugacity matched that of thedesired run condition and that the iron container was trulybefore carrying out runs at low or high pressure is no

guarantee that such problems will be evaded (Biggar & pure. Bulky furnace inserts such as this can, incidentally,wreak havoc on temperature calibration and controlO’Hara, 1972b; Howells & O’Hara, 1978). Other workers

approached run temperature in their experiments by first (Biggar & O’Hara, 1969a).Subsequently Walker et al. (1976b) published a moretaking the charge well into the liquid field and then

cooling to the desired run temperature, a technique elaborate recipe involving thick silica glass tubes to kin-etically restrict oxygen diffusion, and purest iron capsuleswhich was demonstrated to delay the appearance of

plagioclase in lunar charges by as much as 60°C below machined under oil and pre-conditioned with multiplelunar samples to ‘get’ remaining impurities. This pro-the reversible equilibrium plagioclase entry (Biggar et al.,

1972; and see Walker et al., 1976b). Higher cooling cedure they found to maintain the iron content of chargeswithin 2% of the amount present (equivalent to a changerates can suppress even liquidus plagioclase crystallization

below that of both olivine and pyroxene (Grove & Bence, of about 0·5% in the mg-number of the equilibriumcoexisting olivine), but this level of care did not pertain1979).

Only one group attempted to relate returned sample to most results obtained by the sealed silica tube techniquebefore 1976. These experiments in sealed silica tubeschemistry to experiments in the wider chemical systems

surrounding them (O’Hara et al., 1970a, 1970b; Biggar were carried out at unknown oxygen fugacities definedby the equilibrium between iron metal and whateveret al., 1971, 1972; O’Hara et al., 1975a; O’Hara &

Humphries, 1977b). The desirability of this procedure in new composition had been reached by the charge. Self-buffering experiments such as these, in which the ironthe context of partial melting of the terrestrial mantle

has been stressed repeatedly (Bravo & O’Hara, 1975; capsule vastly outweighs the experimental charge, are atrisk that the inherent oxygen fugacity of the containerFalloon et al., 1999). The most difficult experiment to

perform on natural basaltic materials, and specifically on (controlled, for example, by the very small quantities ofcarbon or even manganese dissolved in the iron) maylunar basalts and basaltic achondrites, is the one which

you hoped you were going to perform when you started impose itself on the system, causing reduction of ironfrom the charge. A few early experiments used spec-(O’Hara et al., 1975a; O’Hara & Humphries, 1977b), and

underlines the case for carrying out more general phase troscopically pure iron capsules, which may contain un-controlled quantities of carbon and will smelt theirequilibria studies in adjacent parts of the complex oxide

systems involved (O’Hara et al., 1970a, 1970b; Biggar et contained charges with massive loss of iron to the con-tainer and establish an oxygen fugacity appropriate toal., 1971, 1972). The problems were acute in the case of

the lunar samples because the low oxygen fugacity makes the equilibrium between iron metal, the new silicatecharge composition and the CO–CO2 atmosphere gen-the melt potentially very reactive with almost any con-

tainer material chosen. Many techniques were used to erated. Experiments conducted on beads suspended incontrolled atmospheres on loops of Fe-soaked platinumcontain or support lunar basalt charges during low-

pressure experiments. The Edinburgh group used mo- wire kept good control of the iron content of the chargeand minimized the weight ratio of container to charge,lybdenum capsules at controlled oxygen fugacities close to

those of the iron–wustite equilibrium where molybdenum but at the expense of permitting unrestricted mobility ofalkalis between charge, atmosphere and furnace lining.pick-up by the charges and iron loss to the containers are

negligible. They also used the technique less successfully at The molybdenum capsule–controlled atmosphere tech-nique was reviewed discreditably by Kesson & Lindsleylower oxygen fugacity where iron loss to the capsule

becomes more significant, and explored the use of pure (1976), seemingly on the strength of bad experiences bythemselves and other workers who had not adhered toiron capsules at controlled oxygen fugacities.

Others used ‘pure’ iron capsules in evacuated sealed the precise technique used at Edinburgh, which specifieduse at the controlled oxygen fugacity of the iron–wustitesilica glass tubes, which do not prevent both alkali loss

from the charge and oxygen gain through the tube buffer. Subsequent studies (Holzheid et al., 1994) haveshown these conditions to be slightly more oxidizing thanleading to iron oxide gain by the charge. Attempts to

control oxygen gain by enclosing the silica glass tube those at which molybdenum in the melt changes frompredominantly Mo4+ to predominantly Mo6+. Ex-within a larger sealed silica glass tube containing an

oxygen ‘getter’ used pure iron as the getter, and failed perimental results from the Edinburgh laboratory for the

1616


lunar rocks have been largely ignored and sealed silica in a manner as professional as were the majority of thegeochemical measurements. The manifest discrepanciestube data preferred by later reviewers [see, for example,

BVSP (1981) and Taylor et al. (1991)], yet no such and dissensions had an outcome important for the evolu-tion of ideas on lunar basalt petrogenesis. Experimentalembargo has affected the results for basaltic achondrites

carried out in the Edinburgh laboratory by the same petrology as a whole, and to some extent the wider fieldof basalt petrology, became tarnished (with implicationstechniques at the same time under the guidance of

the same group of workers (Stolper, 1977), which was for the assessment of terrestrial basalts also) in the eyes ofthe geochemists, who were producing properly controlledfortunate to evade the assessment of Kesson & Lindsley

(1976). The Mo capsule technique in practice offers and calibrated data of outstanding quality.As in the case of terrestrial petrogenesis the key questionarguably the best compromise simulation of real lunar

basalt crystallization (O’Hara & Biggar, 1977), suc- was whether the erupted basalts were telling us moreabout crust-building processes than about the nature ofcessfully reproducing oxide phase compositions and as-

semblages of the natural rocks with minimal MoO2 uptake the underlying mantle. In the case of the lunar mariathe debate was further confused because most of theby the melt (>0·1%), by silicates and armalcolite (below

limits of detection), by ilmenite (0·1–0·2%) and by spinel hand specimens are clearly remote in composition fromthe average mare basalt which was the target-rock for(1·4–1·9%), which last phase strongly concentrates any

MoO2 present. These experiments reproduced the pres- regolith formation at each site and not representative ofthe emplaced magma composition. The debate here isence of spinel in the charges, a mineral persistent in the

natural rocks but appearing only sporadically in the more an issue of the average lava composition to beadopted than one of experimental precision. The majorresults of runs by the sealed silica tube techniques and

then only when there was suspicion of substantial gain element issue was clearly decided and not subject todebate. The regolith and lithic fragment compositionsof FeO by the charge. Because spinel crystallization was

not encountered at all in experiments on Cr-free synthetic were close to plagioclase-saturated cotectic compositions;the majority of hand-specimen compositions were not.simulations of high-titanium basalts when run using the

same techniques (O’Hara & Biggar, 1977), it was deduced Nevertheless, three minor issues which are related to thefine details of experimental control and technique havethat Cr content was the dominant factor and that MoO2

availability did not grossly modify the spinel crystallization assumed critical importance to the debate.Taking first the issue of crystallinity at plagioclasefield. The concentration of MoO2 in the spinel is, how-

ever, a function of the amount of spinel present and the saturation, this mineral is difficult to nucleate in coolingbasaltic melts, considerable supersaturation often beingextent of competition from Cr and Ti. In experiments

on basaltic achondrites with low Cr, Ti and low spinel necessary unless cooling rates are very slow. This hasgreat significance for experimental petrologists who areconcentrations, up to 5·8% MoO2 was encountered in

the spinel (Stolper, 1977). The low self-buffered oxygen attempting to establish the temperature of plagioclaseappearance on the liquidus especially if the startingfugacities in runs in iron capsules which had not gained

substantial FeO prevented the appearance of spinel al- material has ever been taken to supra-liquidus tem-peratures before final attainment of run conditions—together in such experiments (O’Hara et al., 1975).

The molybdenum capsule–controlled atmosphere tech- glasses and devitrified glasses have long memories! Theeffect is accompanied by an increase of magma viscositynique is superior in several respects to some of the

techniques preferred by Kesson & Lindsley (1976), who by up to three orders of magnitude over the temperaturerange of a few degrees immediately above the equilibriumdid not defend their views in the light of the comments

of O’Hara & Biggar (1977), did not withdraw their entry of plagioclase (Murase & McBirney, 1973) and thecontrolling kinetic factor in plagioclase nucleation duringassessments, and did not rectify the inaccuracies in their

review. BVSP (1981, section 3.4.2) adopted and con- cooling is probably the slow organization of complexnetwork structures within the liquid. Confusion reignedtinued the Kesson & Lindsey tabulation and implicitly

invalidated a large number of high-quality experimental over the issue of whether plagioclase was saturated in,and, therefore, could have been fractionating from theresults for the Apollo 17 basalts (O’Hara & Humphries,

1975), which are discussed further below. The fun- true mare basalt compositions before eruption.Two groups published figures for mass fraction ofdamental point to grasp (O’Hara et al., 1975a) is that no-

one carried out the experiment they would have hoped crystals at various temperatures based on visual estimationof crystallinity in their charges up to and beyond theto and that all the data are useful provided one understands

what actually happened in the different sets of ex- appearance of plagioclase. These results purported toshow that five Apollo 12 basalt hand-specimen com-periments.

In summary, experimental studies carried out on the positions would be 60–85% crystalline at plagioclaseappearance and, therefore, exceedingly distanced fromlunar basalts did not all match the best laboratory stand-

ards of the day and were not all calibrated and controlled low-pressure plagioclase-saturated liquid compositions.

1617


Because the percentage of phenocrysts in the hand speci- experimental charges which are representative of (a) thenominal hand-specimen composition and (b) the truemens was less than these high figures it was argued that

the hand-specimen compositions could not be created bulk composition, and (iv) accurate maintenance of theFeO content of the charges. Lesser factors are (v) theby sinking of phenocrysts within low-pressure plagioclase-

saturated liquids. However, visual estimation is a sub- achievement of correct oxygen fugacities and con-sequently correct activities of Fe2+ and Fe3+, and (vi)jective technique, best avoided because of its proneness

to gross error, and was demonstrated to have overstated correct maintenance of the alkali contents of the melt,which affects melt polymerization and hence the Mg andthe percentage crystallinity at plagioclase entry in these

lunar samples by factors of 100–200% (Biggar et al., Fe distribution coefficients.The samples provided for experimental studies of the1972). Nevertheless, the offending visual estimates of

crystallinity were reproduced without comment (Taylor, basalts were in many cases small uncrushed fragmentsof coarsely porphyritic rocks. Representative sampling1975, p. 196). The actual mass fractions of fer-

romagnesian crystals present in experimental charges of adequate to sustain this test may not have been achieved.The content of FeO in the individual experimentallunar mare basalt hand specimens at plagioclase entry

are comparable with the amounts of phenocrysts present charges is particularly sensitive to iron gain wheneveriron-bearing containers are used, and to iron loss in allin the natural samples.

Taking next the issue of matching the olivine com- cases if the oxygen fugacity falls below that of the actualrock. In a detailed analysis (O’Hara et al., 1975a; O’Harapositions, it has been argued that there is good match

between liquidus olivine compositions generated in equi- & Humphries, 1977b) it was shown that none of thetechniques used could guarantee the requisite tight con-librium experiments on mafic hand-specimen com-

positions and the most magnesian olivines reported from trol of the Fe content and oxygen fugacity of the charges,and that charges run under the iron capsule–sealed silicathe rocks. A corresponding mismatch between the most

magnesian olivines and the olivine compositions at pla- glass tube techniques in particular were vulnerable toserious, and more or less random, gains and losses ofgioclase saturation in experimental charges has been

argued to demonstrate that the liquids were not erupted FeO. O’Hara et al. (1975a) demonstrated variations inmg-number of up to 6% in charges of the same sample runin this condition. Let us suppose that a parent liquid

crystallized olivine of a particular mg-number and this by different techniques at the same condition (plagioclasesaturation). This is the same variation expected to resultolivine accumulated in part of the magma to yield phe-

nocryst-enriched samples. Then the experimentally ob- from >150°C variation in the temperature at fixedoxygen fugacity and iron content of the silicate phases.served liquidus olivine in the parent liquid sample should

match the mg-number of the phenocrysts in the phe- The total range in olivine composition recorded in ex-periments by different techniques at plagioclase entry innocryst-enriched sample. If the phenocryst-enriched

samples, which have gained more Mg than Fe from the sample 70215, for example, was Fo92–65. Despite thisinvalidation, the criterion of mg-number in liquidus olivineaccumulated phenocrysts, are taken to their liquidus

temperature in experiments, the mg-number of the new reappeared in BVSP (1981, section 3.4.2) as one of twoprincipal reasons for accepting the mafic hand specimensliquidus olivine should be greater than that of the ob-

served olivine in the natural samples. If, on the other as representative of the parental magma compositions.This criterion cannot be applied safely to any foreseeablehand, the phenocryst-enriched samples represent original

liquid compositions, the natural rocks will contain olivines dataset without much disputation.Finally, let us take the issue of the presence or absence ofof mg-number as high as that at the experimentally

determined liquidus. It was claimed that experimental armalcolite at plagioclase entry in experimental charges,which was the second reason given for disregardingliquidus olivine compositions in picritic mare basalts

matched the phenocryst core compositions, thus fa- the plagioclase-saturated source liquid argument (BVSP,1981, section 3.4.2). Longhi et al. (1974) argued that invouring the view that the porphyritic mafic hand speci-

mens represented liquid compositions. The test also their experiments armalcolite crystals formed at hightemperatures in Apollo 17 samples had reacted with therequires that the olivine composition at the temperature

of plagioclase saturation should be the same throughout liquids in the experimental charges at lower temperaturesand had disappeared at temperatures above the firstall samples from a given lava flow and should match the

most magnesian phenocrysts reported. appearance of plagioclase. Because the mafic hand speci-mens contained phenocrysts of armalcolite, the hand-However, this test is one whose validity depends upon

(i) the phenocrysts being equilibrated with the melt and specimen compositions could not, the argument runs,have been produced by phenocryst settling in plagioclase-not metastable quenching products which have never-

theless accumulated, (ii) the lava being of uniform oxygen saturated cotectic parent liquids. The point might beconsidered trivial because the temperature gap betweenfugacity throughout and not in process of further re-

duction by volatile release during cooling, (iii) the use of armalcolite disappearance and plagioclase appearance in

1618


the relevant experiments is less than 10°C (see BVSP, Crisium there had been low-pressure fractional crys-tallization of some more primitive Mg-rich basalt andfig. 3.4.5; Taylor et al., 1991, table 6.2) at which pointderivation of a multiply saturated ferrobasalt with highthe liquid must in any case be very close to plagioclaseAl2O3. Ryder & Marvin (1978), on the other hand, arguedsaturation and the observation very vulnerable to thefor a homogeneous primary magma from a plagioclase-precise experimental conditions.bearing source. A synthetic analogue composition of theAcceptance of this argument ignored data and analysisVLT basalt shows plagioclase on the liquidus followedfor the Apollo 17 basalts (O’Hara et al., 1975a; O’Haraby olivine, then by pyroxene, whereas a green glass& Humphries, 1975) which demonstrated that underanalogue composition showed extensive olivine crys-appropriate experimental conditions plagioclase crys-tallization followed by pyroxene and then by plagioclasetallization did overlap with armalcolite precipitation in(Grove & Vaniman, 1978). The latter authors concludedsamples 70017, 70215, 70275,6 and 70275,11, 71055,that the VLT basalts cannot be related to the green glass72135, 74275, 75075 and the orange glass 74220 andby low-pressure fractionation and opted for a source rockfive other soils. Crystallization of the two phases may notyielding a plagioclase- and pyroxene-bearing residue (thehave overlapped in 74235 and definitely did not overlapVLT analogue composition was cotectic for these twoin 75035 except in Fe capsules at low oxygen fugacities.phases at 5 kbar).An expanded dataset (see 121), which was provided to

121. Tables A1 and A2, in the Appendix, reportthe relevant BVSP team in May 1976, confirmed overlapthe previously unpublished results of experiments atin 74235 and reported it additionally in 71569 andatmospheric pressure in controlled oxygen fugacity at-74255, provided microprobe analyses of the relevantmospheres on samples from the Apollo 17 mission, con-minerals and glasses and cited 69 individual experimentsducted in 35 multi-charge experiments under 30 differenton rocks where the overlap was observed, three in thecombinations of temperature, oxygen fugacity and con-orange soil and six in other soil samples. The crys-tainer material (Fig. 2). Run products are reported intallization of plagioclase and armalcolite probably over-Table A1 for 214 individual experiments on 12 handlapped even when the correct charge composition andspecimens including 22–25 runs on each of the samplesoxygen fugacity were maintained (not achieved by any70017, 70275 and 74275 which have been widely studiedof the experimental techniques). Getting the conditionsin other contexts. Run products for 15 individual ex-right would also permit simultaneous crystallization ofperiments on the orange soil pyroclastic bead deposit,ilmenite and chrome-rich ulvospinel. The latter was also74220, and a further 36 individual experiments on six

reported as phenocrysts, and was produced in ex- more typical mare soils are reported in Table A2.periments showing overlap of plagioclase and armalcolite Phases identified are indicated as follows: a, armalcolite;crystallization. Spinel was absent from the experiments c, clinopyroxene; i, ilmenite; k, pseudobrookite, karrooite;(Longhi et al., 1974) which showed armalcolite absent at m, iron metal; o, olivine; p, plagioclase; s, spinel; z, glass,plagioclase saturation, yet was not held to invalidate liquid. Underscored symbols indicate small amounts onlyLonghi et al.’s choice of experimental conditions. of this phase. Results in Table A1 are presented in

Experimental petrology studies on lunar mare basalt declining temperature sequence in the first two sections,hand-specimen samples provide no critical evidence for which were run at different oxygen fugacities bracketingor against either of the competing interpretations of the those of the natural rocks (Fig. 5), and, apart frommare basalts. A few large hand-specimen samples do one higher-temperature result, in sequence of decliningexist which have bulk compositions close to those of of oxygen fugacity at temperatures close to those at whichlow-pressure, plagioclase-saturated cotectic liquids plagioclase and clinopyroxene first appear, into which(12038, and aluminous basalts 10047 and 75035). The sequence could be inserted runs 5·394 from section 1,discussion turns on the correct identification of the av- and 5·391, 5·323 and 5·323 from section 2 between theerage basalt composition emplaced at each site, and on second and third rows of section 3.the phase equilibria of those compositions and of those In Tables A1 and A2 the percentage of hydrogen inhand specimens which most closely approximate the the hydrogen–carbon dioxide gas mixtures used to controlaverage compositions. the oxygen fugacity are listed in the third column and

120. The average compositions of basalt textured lithic the nature of the container material in the fifth column.fragments in the regoliths from Apollo 11, 12 and 17, Most runs were held overnight; shorter runs were usedLuna 16 and 24, and five mare-derived lunar meteorites only where it was necessary to limit charge–containerare close to those of plagioclase-saturated cotectic liquids. reactions because of the chosen run conditions.The Apollo 11 regolith has plagioclase as its liquidus Every sample (bar one of the soils) is recorded assilicate phase and plagioclase is an early crystallizing displaying armalcolite present with plagioclase (69 in-silicate in six Apollo 17 regoliths other than the orange dividual experiments) in the presence of olivine, ilmenite

and frequently clinopyroxene and spinel at temperaturessoil, 74220. Norman et al. (1978) deduced that in Mare

1619


Table 2: Dimensions of the required source regions

Flow thickness (m)

0·1 0·3 1·0 3·0 10·0 30·0

Flow volume (km3) 10 30 100 300 1000 3000

Cylindrical source radius (km)/height (km) 3·17/31·7 4·57/45·7 6·83/68·3 9·85/98·5 14·7/147 21·2/212

Discoid source radius (km)/thickness (km) 14·7/1·5 21·2/2·1 31·7/3·17 45·7/4·57 68·3/6·83 98·5/9·85

Spherical source radius (km) 6·4 9·2 13·8 19·9 29·7 42·8

Armalcolite crystallization is favoured by higher mg-close to 1140°C. Armalcolite was not found in 70215number, i.e. by iron loss, and suppressed by iron gain.above 1143°C and pyroxene was not found aboveOverlap or armalcolite and plagioclase crystallization is1145°C. Plagioclase was present in most rocks at 1143°Cfavoured by lower f O2 provided there has been no ironbut appears at>1159± 3°C in both samples of 70275oxide gain from the capsule. Armalcolite crystallizationand at slightly higher temperatures again in the six valleyis limited in the sense of increasing f O2 by oxidation ofsoils of Table A2. Overlap of spinel and armalcoliteall Ti, until the crystallization of a pseudobrookite-likecrystallization at or close to the presence of plagioclasephase is stabilized by rising ferric iron at higher f O2.is reported from most rock samples in runs using Mo

Spinel crystallization is limited with falling f O2 becauseand five rock samples using Fe containers. Solidus tem-of the reduction of Cr3+ to Cr2+ ion. Spinels concentrateperature is close to 1095°C.whatever MoO2 has been oxidized into the charge andMicroprobe analyses of the glasses developed in thereach higher values (1·5–1·9%) in runs at the higher12 rocks at 1181°C and in 70215 at 17 other conditions,oxygen fugacities, but the concentrations are never high12 of them close to the first appearance of plagioclase,in these experiments. Olivine, ilmenite, clinopyroxeneare reported in Table A3; olivine compositions from nine(1140–1160°C) and plagioclase (1140–1160°C) entriesrocks at 1181°C and 70215 at 12 lower temperatures inare not strongly affected by varying f O2. Run 5·392, ofTable A4; spinel compositions in six rocks at 1181°Cshort duration in Mo at very low f O2, was probablyand 70215 at seven lower temperatures in Table A5;losing iron fast to the capsule. The Roedder–Emsliearmalcolite compositions from 10 rocks at 1181°C, fourpartition coefficient for Mg and Fe between liquid androcks at 1141°C and 70215 at five temperatures betweenolivine is 0·34 vs 0·28 ± 0·01 at 1181°C in all other1143 and 1137°C in Table A6; ilmenites from eightcharges at the same temperature and 0·295 ± 0·15 inrocks at 1144°C and 70215 at 18 temperatures betweenall other lower-temperature runs. Calcium and chromium1178 and 1097°C in Table A7; clinopyroxenes from sixare relatively high in these olivine analyses. Liquidusexperiments and plagioclases from five experiments onplagioclase compositions are close to the anorthite–70215 at 1143–1137°C in Tables A8 and A9, respectively.bytownite boundary.These data underpinned the analysis of the controls of

Results from run 5·415 in iron foil at f O2 very closearmalcolite, spinel and plagioclase presence in the studyto the appearance of Fe in equilibrium with 70215 (twoby O’Hara et al. (1974) and the rebuttal of criticisms ofof the other samples contained metal) in the top 12 rowsthe techniques used by the Edinburgh laboratory (O’Haraindicate the inter-sample variability of mg-number at& Biggar, 1977).1181°C (39·7–44·8). The next three rows indicate vari-Charges reported here which were conducted in Moability in mg-number of 70215 (41·7–44·4) at roughly thecapsules at oxygen fugacities a little below those of thesame temperature, as a function of oxygen fugacityFe–FeO equilibrium are expected to have lost some FeOaffecting the ferric–ferrous ratio and interactions withto the capsules (O’Hara et al., 1974). Charges run in ironthe container material; 5·405 is expected to have lost lesscapsules may have gained some FeO at the higher oxygenFeO than 5·382 but the difference between 5·382 (Mo)fugacities (run 408 in particular) but may have lost someand 5·415 (Fe) is barely significant and if anything theFeO to the capsules at lower oxygen fugacities (runs 399,reverse of what might have been expected. From 5·395401 and 413 in particular). The possible extent of thesedownwards to 5·401, temperatures vary little around thatchanges may be evaluated from glass analyses presentedof first entry of plagioclase but oxygen fugacity declinesin Table 3, below, where gain of MoO2 is seen to have

been minimal. from high (5·395) to close to the first appearance of iron

1620


Table 3: Dimensions of the required storage volume

Volume (km3)

10 30 100 300 1000 3000

Flow thickness (m) 0·1 0·3 1·0 3·0 10·0 30·0

Fissure width (m) 0·33 0·96 3·33 9·56 33·3 95·6

Pipe diameter (m) 210 357 650 1128 2060 3568

Table 4: Dimensions of the equivalent cone

Volume (km3)

10 30 100 300 1000 3000

Flow thickness (m) 0·1 0·3 1·0 3·0 10·0 30·0

Cone diameter (km)/height (m) 5·8/290 8·3/415 12·4/620 17·9/895 26·7/1340 38·6/1928

(5·392) all run in Mo capsules; 5·392 is expected to have long extending uniformly to 300 km depth, and a cyl-indrical pipe extending to 300 km depth. The dyke andlost some FeO, whereas the next two rows (5·408, 5·404)

were run in iron foil a little above the first appearance pipe solutions have several problems, not least being thecomposition, pressure and temperature gradients liableof iron and will have gained some FeO. The next four

rows show some loss of FeO by reduction to metallic to be set up in the magma if it has equilibrated withcrystals throughout its depth. The discoid volume re-iron at decreasing f O2.

122. The volume of mare lava eruptions is a pointer quired in an underlying magma body to accommodatethe required flow is, of course, the same as the flowto the probability of high-level magma storage chambers

within the lunar crust. Individual mare eruptions may thickness if the lateral extent of the magma body is thesame as that of the maria, i.e. of a size comparable withbe required to be very large and melt fractions may be

required to be very small. If the magmas are primary, the Bushveld complex, South Africa. The latter is theonly type of solution to the storage problem whichsuccessful melt aggregation must occur from very large

mantle volumes at low porosity. Conditions and melting permits the whole erupted magma to be equilibrated toapproximately the same pressure, should this be requiredparameters should vary across such melting regimes.

Table 2 shows the dimensions of the required source by the low-pressure cotectic character of the flow.Thicker, but still relatively thin layers in less extensiveregions of individual flows for three different geometries.

Calculations assuming the erupted basalt represents a magma chambers are equally acceptable solutions to thestorage problem. On Earth, sill and lopolith-like bodies1% average melt fraction are for a flow completely

covering a mare surface of >357 km diameter and of this type are the home of extensive gabbro fractionationand are frequently marked by numerous magma recharge100 000 km2 surface area and show the radius and height

of the required cylindrical source region at fixed aspect events. Sobolev et al. (1980) interpreted melt inclusionsin magnesian olivine crystals from the Luna 24 regolithratio of 1/10, the radius and thickness of the required

discoid source region at fixed aspect ratio of 10, and the as indicating magma mixing events at >100 MPa betweenultramafic and VLT basaltic magma.diameter of the required spherical source region.

Individual eruptions may be required to be very large Table 4 shows the dimensions of the conical super-structures required to accommodate the same volume ofand very rapid. A large storage volume immediately

before eruption may be required as shown in Table 3. magma. Calculations are for a regular cone of slope>5·71° (height 0·1 × radius). These would have beenCalculations are for a flow as above, a fissure 100 km

1621


conspicuous features of the lunar surface if eruption rates 124. At the Sudbury impact basin Grieve et al. (1991)suggested a transient cavity of 100 km diameter, and ahad been slow enough and viscosity high enough to

permit shield and cone formation. final crater of 150–200 km diameter originally containing>104 km3 of melt, 40–80% of which remains. TheThe dimensional problems are eased, as with all in-

compatible trace element enriched magmas on the ter- geochemistry, including the sulphide content, is consistentwith an origin entirely by crustal melting followed byrestrial planets, by assuming a larger initial average partial

melt fraction, say 5%, followed by concentration of the differentiation (Stoffler et al., 1994). Even the relativelysmall magma pool created in association with the Sudburyincompatible elements by partial crystallization in magma

chambers. The most primitive glasses and the basalts impact event is conspicuously fractionated by partialcrystallization at low pressure, a process which wouldwould then represent residual liquids after 50–85% and

80–98% solidification, respectively, in these chambers, have been reflected in the compositions of any lava flowsescaping from the chamber.processes which are known to have occurred repeatedly

in real magma chambers on the Earth. The problems 125. In the Earth, events such as those discussed innote (123) would be equivalent to >2 × 106 years ofevaporate if it is allowed that a significant proportion of

the incompatible element budget reflects contamination productivity per unit area of upflow in a fast-spreadingridge today, i.e. perhaps 0·05–0·1 mass fraction melt ofby interaction with the lunar crust.

123. Shock pressures in large impacts can reach several a mantle volume 100 km deep multiplied by the area ofthe 200 km diameter transient crater, orthousand GPa with corresponding temperatures of many

thousands of degrees, leading to vaporization of projectile >160 000–320 000 km3 of magma, all produced in ashort time, to add to the >1× 106 km3 of melted crustand some of the target, and melting of a part of the

target (Melosh, 1989). The ratio of melt produced relative and upper mantle which might have been instantaneouslycreated by the impact itself. Such events might yieldto the mass of vaporized and shattered rock increases as

projectile size and velocity increase, stabilizing with melt hybrid magmas formed mainly by total melting of crustwith some uppermost mantle. This surficial melt wouldmass about 10 times the mass vaporized and total pro-

duction proportional to the mass and the square of the have been modified by an input of endogenous partialmelt of peridotite mantle, which might have been equi-velocity of the projectile. In larger impacts the ratio of

melt produced to the mass displaced in crater formation librated initially with residual minerals of peridotite atmuch higher pressures. In the Moon the excavationrises, and may approach a value of 0·5 in impacts

which would produce hemispherical transient cavities in would have much less effect at the same crater sizebecause of the much lower gradient of pressure withmaterials similar to gabbroic anorthosite approaching

500–1000 km diameter on the Moon, or 100–300 km depth, but the larger basins may have had transientcraters as much as 500–1000 km in diameter providingdiameter on the Earth. Such impacts might melt materials

originally located at depths from the surface to over for significant pressure release nevertheless. In both Moonand Earth the probability is that the bulk of any melt250 km on the Moon, or 50–150 km on the Earth, i.e.

much deep crustal and some upper-mantle material formed as a direct result of the impact would be providedby total melting of a mixture of crust and uppermostwould contribute to the melt in both bodies, although

the ejecta and far-scattered jetted materials will be derived mantle and such melts would have been present beneaththe floors of the lunar multi-ring basins. As at Sudburyalmost exclusively from shallow depths. A substantial

part of the melt pool may be buried beneath material (124), this melt can be expected to cool and differentiatewith little extrusive activity in a relatively short periodslumping from the walls of the transient crater (Melosh,

1989, sections 4.5, 5.2.3, 5.5.3, 7.10; O’Keefe & Ahrens, of time, but in the case of large lunar basins there isevidence of escape of some impact melt lava flows (Spudis,1994).

Pressure relief melting of the underlying mantle 1994). Ryder (1994) interpreted the pre-mare KREEPvolcanic flows of the Apennine Bench formation as ex-resulting from impact excavation of up to 100 km of

overburden is not expected to be a significant contributor trusive impact melts from the Mare Imbrium impact.Head & Wilson (1979) interpreted dark-halo craters into the total melt produced in any body unless the geo-

therm in that mantle was already close to the solidus at Alphonsus as pyroclastic eruptions from mare basaltwhich had risen into the underlying breccia lens but notsome point beneath the crater (Melosh, 1989, section

9.4). This, however, may well have been true at the time high enough to flood the surface, coeval with the floodingof the adjacent lower Mare Nubium. However, the extentof formation of the large impact basins on the Moon

and during the early Archaean on Earth, and might yield of later cratering of the unflooded basins, the partialflooding of craters on the floors of the nearside basinsa melt productivity which would become geologically

significant if the body did not possess any other convective and the sparse impact record of the visible mare surfacesall point to a substantial time interval between basinmechanism to promote pressure release and was not

being heated internally by any other process. formation and the end of basalt flooding. Some other

1622


reason is probably required for the endogeneous partial section 7.4.1, pp. 974–989). Eruptive cycles were shortlived as in the case of continental flood basalts (see below)melting events which produced the mare basalts.

126. One of the larger basic magma events recorded and magma production rates were very high. Basalticand acid rocks predominate and fractional crystallizationin early Earth history formed the Bushveld complex in

southern Africa, a body marked by extensive crustal appears to have played an important role in their genesis,possibly with some crustal assimilation or melting. Thecontamination of the various parent magmas. It is not

clearly associated with typical flood basalt or plume majority of lavas which are not komatiitic are morealuminous and less titaniferous than lunar mare basaltsactivity and is best known for its spectacular evidence of

partial crystallization interrupted by magma recharge other than the VLT group. Almost all contain much lessFeO than typical lunar samples. Ultramafic lavas formand escape, accompanied by precipitation of chalcophile

(siderophile) elements in sulphide-enriched reefs and of <10% of the 10–14 km thick sequences and are con-centrated in the lower parts of the successions. They dolaterally extensive chromitite layers. It was probably

accompanied by equally voluminous basalt flows which not characterize the last eruptive events of the extrusivecycles. High-level intrusive bodies are not prominent,have not been preserved (Cawthorn & Wallraven, 1998)

but which would have had their compositions sub- hence any low-pressure partial crystallization has to berelegated to hypothetical magma chambers in or beneathstantially modified from those of the parent magmas and

would have been cotectic with respect to the cumulus the crust. High-temperature mantle plume activity isfavoured as a mechanism. Mare lavas share some featuresassemblages precipitating in the underlying layered gab-

bros. An impact origin has been debated (Hamilton, with the nearly contemporary terrestrial komatiite vol-canism, e.g. high Cr in olivine and small negative Eu1970; Rhodes, 1975). If this suggestion has merit then

the present acid roof rocks of the Bushveld must be anomalies in primitive rocks thought not to have beenseparated from plagioclase. Some komatiites are vol-interpreted as a floated, molten lens of crustal debris or

late eruptives from an unidentified source. caniclastic and probably erupt uninterrupted from greatdepth, as diamond has been reported (Capdevila et al.,127. Terrestrial analogues of the maria filled impact

basins must presumably have formed at the same time 1999).Short duration, very high magma productivity, mucharound 3·9–4·1 Ga as well perhaps as a few later examples

and should be represented in the terrestrial record (Ham- thicker sequences and the sparsity of very mafic magmasat the termination of the cycles distinguish this assemblageilton, 1970; Green, 1972, 1975; Rhodes, 1975; Glickson,

1976). The terrestrial record from the time of formation from lunar mare basalts. None of the komatiite–greenstone suites has yet been unambiguously linked toand filling of the great mare basins 4·1–3·2 Ga ago is

sparse and confused by alteration and metamorphism. impact basins.128. The tectonic setting, eruptive time-span, magmaIt has been suggested that the Barberton sequence may

represent the terrestrial equivalent of a lunar mare filling productivity and topographic expression of lunar marebasalts are very different from those of terrestrial mid-(Green, 1972). Beds of 30 cm–2 m thickness rich in silicate

spherules, originally of ultramafic and mafic glass, which ocean ridge, ocean-island and continental flood basalts;or from those of flood and central basalt provinces onpossibly originated in major impacts on the Earth, have

been reported among 3·2–3·5 Ga sediment and lava Io, Mars and Venus. The filling of at least some of themaria took place soon after the period of major impactsequences from Barberton and Pilbara (Lowe & Byerly,

1986), although Koeberl & Reimold (1995) preferred an at 3·9 Ga. Very few examples exist of craters whichformed on partially constructed maria and were thenorigin from terrestrial pyroclastic materials. These beds

occur both above and within the main volcanic sequences, partially buried by subsequent flows. Partly submergedcraters can mostly be interpreted satisfactorily as residingboth of which include komatiites near the base. A picture

emerges of widespread mafic volcanism proceeding in- on the floor of the original basin (see below). This pointsto the main episode of mare filling being very rapid, i.e.dependently of impact events, marked by sparse early

ultramafic lava extrusion and build-up of more frac- magma supply rate as well as individual effusion ratewas high. At a high enough rate of magma supply relativetionated lava and sediment sequences over 10 km thick.

The broad pattern of events resembles that seen in later to cooling rate, comparable for example with that evidentin the Bushveld complex, it could be expected that theCFB provinces and suggests production of high mass

fraction partial melts in a series of hot plumes which mare would become a lava sea. In that state it wouldnot preserve much evidence of impacts until consolidationpartially crystallized within and interacted with the crust

through which they passed. was far advanced. A lava sea, now preserved as a layeredgabbro body, formed in the relatively small terrestrialGreenstone belts and their associated komatiite erupt-

ives are among the earliest surviving records of basaltic impact basin at Sudbury (124), and large shallow-seatedmagma bodies gave rise to the low-Ca pyroxene (di-volcanism on the Earth and have been comprehensively

reviewed by BVSP (1981, section 1.2.1, pp. 5–29, and ogenite) and pigeonite gabbro cumulates of the HED

1623


parent body (87). Nevertheless, a long time-span (up to of matured sub-continental lithospheric mantle (Richardset al., 1989; White & McKenzie, 1989, 1995). The activity200 Ma) is recorded in the last few flows at several

mare sites. This would be remarkable in the context of is accompanied by very large scale crustal doming (Cox,1989), rifting, extensive dyke-swarm emplacement andterrestrial volcanism, where plate motion rapidly trans-

poses volcanic constructs away from the mantle source, widespread as well as centralized high-level intrusiveactivity. Massive intrusive complexes may form at thebut need not be so exceptional in a body lacking large-

scale convective motion in the mantle and corresponding base of the crust (Cox, 1980; Voshage et al., 1990; Quicket al., 1994; Sinigoi et al., 1994), and Head & Coffinlarge-scale plate motion in the crust.

Wall heights of impact craters are related simply to (1997) suggested that the Moon displays magma pondingat the base of the crust with dyking to the surface.their diameters. De Hon (1974, 1977, 1979) estimated

the total thicknesses of mare accumulations by measuring Ultramafic magmas derived from the plume interactgeochemically with the mantle lithosphere and then inthe remaining height of the exposed walls of partially

flooded craters relative to their diameters and subtracting many cases with the crust also (e.g. Cox & Hawkesworth,1984, 1985; Lightfoot et al., 1990, 1993; Wooden etfrom the expected rim height. The implicit assumption

was that most of the craters utilized were located on the al., 1993; Peng et al., 1994, 1998; Shirey et al., 1994;Hawkesworth et al., 1995; DePaolo, 1996; Turner et al.,floors of the basins. The technique gave coherent results,

i.e. few craters were developed on the partially filled 1996; Lassiter & DePaolo, 1997), and undergo substantialpartial crystallization. The latter imparts to the majoritymare surfaces and manifestly few large craters were

developed on the mare surfaces after completion of their of erupted lavas the close approach to low-pressurecotectic character which is the hallmark of their complexfilling regardless of their age. Given the very short interval

between the latest of the major basin-forming impacts origins and crustal modification. Large volumes of magmaare produced in a few million years, short durationsand the completion of filling of some of the visible maria,

this implies very low flux of impacting medium-sized which are being further telescoped by recent studies(Courtillot et al., 1988).projectiles after basin formation and very rapid filling,

at least of those basins with the oldest surface basalts, Although the majority of eruptives are basaltic, moreprimitive picritic magmas occur particularly in the lowerpossibly aided by the formation of lava lakes which were

incapable of recording impacts for some time interval. parts of the sequences and more acid eruptives maybecome dominant in the upper parts of the sequences.These results stand despite the reduction of the basalt

depths proposed by Horz (1978) on the grounds that Sharma (1997) reported Siberian Traps as very variablein the first 8% of eruptions, but in the late stage thefresh rather than degraded crater morphologies had been

utilized by De Hon. Horz (1978) argued that the total eruptions were very voluminous lavas which were veryhomogeneous, but showing variable amounts of as-volume of basic rocks in the visible nearside maria might

be as little as (1–2)× 106 km3. The total volume of mare similation. Meimechite and picrite, however, occur atthe top of the sequence in the type area and there arebasalt and related products may be much greater (61,

110, 114, 115) with many pre-3·9 Ga maria disrupted many tuffs. Hooper (1997) noted that the ColumbiaRiver province is much smaller than the Karroo, andor concealed by later impact debris sheets. Some dark-

halo craters in high-albedo plains units are evidence of erupted mainly within 3 Ma. Minor picrites occur at thebase. Earliest flows have a strong plume geochemicalolder mare surfaces excavated from beneath the debris

sheets thrown out by some of the large basin-forming signature and are strongly plagioclase-phyric, whereaslater flows are small volume and contaminated by crustimpacts (Hawke & Bell, 1981).

129. Snyder et al. (1996) recorded the oldest Apollo 11 or else have a strong geochemical signature of continentallithosphere. Single flows are up to 2000 km3 in volumebasalt as 3·896 Ga, making the age range in the two

flows probably sampled at this one site >250 Ma. The and flow up to 600 km without change in composition,implying very large shallow-seated storage chambers.oldest flow is 60 Ma older than samples identified as

clast-free impact melts of average highland crust at the Large areas of thick oceanic crust, like the Kerguelenand Ontong–Java plateaux, are now being interpretedApollo 16 site, and the hidden mare fill at this site is

presumably still older. as possible flood basalt manifestations above mantleplumes in non-continental environments (Cox, 1991).130. The balance of opinion among those involved in

the Apollo program in 1968–1974 favoured the in- The Ontong–Java plateau, however, is an Alaska-size,30 km thick large igneous province, the largest on Earthterpretation of the lunar maria as being filled by suc-

cessions of flood basalts, with the implicit understanding (Neal et al., 1997) which formed from 122 to 90 Ma,an unusually long duration. Two OIB-type sources areof the time that these were successions of primary melts

from the planetary interior. Terrestrial continental flood deduced with 20–30% polybaric melting. Most basaltsunderwent 30–40% fractionation yielding wehrlitic andbasalt provinces are now viewed as products of the

impingement of a hot mantle plume, usually on the base pyroxenitic cumulates forming the basal high-velocity

1624


crustal layer. Gabbroic and anorthositic xenoliths and eruptives might form from many isolated pockets ofmagma and could be expected to be very variable inplagioclase phenocrysts are common in erupted basalts.

The late Cretaceous Caribbean–Colombian oceanic plat- character. Such a mechanism could be consistent alsowith the late-stage explosive alkaline volcanism associatedeau (Kerr et al., 1996) has partially escaped subduction

and includes the Gorgona komatiites, and Bolivian mafic with the Hawaiian plume (see Feigenson et al., 1996)although the scenario where a plume may be deceleratingand ultramafic cumulates. Most MgO-rich lavas occur

near the base of the plateau and may have erupted before or the volcano drifting away from the plume axis iscomplicated by the possibilities for renewal of the geo-magma chambers formed.

There are specific features of flood basalts which are, chemical or energy resources.133. The NASA Apollo program’s interpretationalor may be, satisfied on Earth, Mars and Venus, but are

apparently not satisfied in the case of lunar maria basalts. procedure should have been validated by application toterrestrial flow fields such as Kilauea, Mauna Loa, theThese include the brief time-span (Marsh et al., 1997;

Sharma, 1997), very high magma productivity, large- Karroo and the Deccan. The exercise might with ad-vantage have been performed, in retrospect, on a set ofscale uplift and doming (Cox, 1989; Courtillot et al.,

1999), and large-scale crustal rifting. Ernst & Buchan small hand specimens and crushed mixtures derivedrandomly and provided out of context from the Hawaii(1997) cited seven cases where giant dyke swarms con-

verge towards known plume centres in the Mesozoic– Scientific Drilling Project (DePaolo & Stolper, 1996)where olivine phenocryst contents range from <1% toCenozoic and 14 pre-Mesozoic examples. Regional dyke

swarms are not identified in the lunar highlands and few 37% with considerable variation as a result of grav-itational settling within individual flows, MgO contentsbasalt flows can be traced back to adjacent high ground

in the highlands. Abundant extensive flows and multi- of bulk rocks range from 5 to 30%, and the interpretationof trace element and isotopic data is not simple evenlobed flow fronts mark terrestrial CFB but on the Moon

such features are established only for the latest flows in within the constraints provided by adequate field re-lationships. The lack of proper control during the ApolloMare Imbrium. In terrestrial CFB provinces there is

development of some large central complexes and cal- program is exemplified by the contrast between theassessment of Taylor (1975, p. 124) quoted in note (104)deras which are not conspicuous on the Moon. In CFB

provinces there is a lack of the aprons, bath-tub marks and what follows.In a subsequent detailed calibration study (Haskin et(Howard et al., 1972) and ‘blackbird’ effects which are

seen in the mare basalts. Whatever the final outcome of al., 1977) it was found that several mare basalt typesshow only slightly greater dispersion of chemical com-the debate about the lunar basalts, there is little reason

to anticipate a direct analogy with terrestrial flood basalts positions of the hand specimens than is observed in singlevertical sections of terrestrial basalt flows from Iceland,except insofar as common features are enforced by the

processes leading to magma transport through the crust Hawaii, Cascades, Columbia River Basalt Province, andDSDP Leg 37 basalts, where REE abundances cannotand eruption.

131. Any assumption that the surface basalts sampled be correlated even between holes only 100 m apartand the features reflect fractionation processes in small,by the Apollo and Luna missions are representative of

the underlying maria on a kilometre depth scale could shallow magma chambers rather than variations in sourcecomposition or melting process (Puchelt & Emmermann,be seriously misleading. Chemical differentiation between

flow units is evident in remote-sensing data, with more 1977). Apollo 12 and 15 basalts display greater variationsthan other lunar basalt suites, but not greater than lateralmagnesian lavas or cumulates being excavated from

beneath iron-rich mare basalts by larger impact craters variations within some large terrestrial flows. Haskin et al.(1977) concluded that there was no pressing geochemicalin Mare Fecunditatis and Mare Crisium. Many basalt

types detectable on the mare surfaces have not yet been need to presume more than one magma generation eventat each site and that the observed variations in lunarsampled and their variety, not least in inferred Mg/Fe

ratio, demands either very varied mantle source regions basalts could not yet be explained quantitatively.Rhodes (1983), by contrast, commented on the com-or tapping of serial magma chambers in different stages

of their evolution. positional uniformity of some Mauna Loan basalt flowsof variable length, areal extent and duration, yet the132. The last eruptives in a volcanic cycle in a non-

convecting body should begin as relatively advanced same cannot be said for two almost simultaneous MaunaLoan flows or for flows along the East Rift Zone ofpartial melts of a depleted source region. There would

be greatly reduced tendency to erupt in the absence of Kilauea, where subtle but significant variations withlocation and time are the norm (Rhodes, 1983; Wrightfurther partial melting at the source and the magmas

might be stored and partially crystallized at depth, erupt- et al., 1975; Wright & Tilling, 1980). Not even a detailedand comprehensive knowledge of field relations, in whating their residual liquids only when volatile contents and

pressures have increased substantially. These late-stage is petrologically and geochemically one of the most

1625


thoroughly investigated terrestrial basic intrusions, has a partially crystallized closed system lava lake at least500 m deep, or similar effects in a much deeper butprecluded recent vigorous debate about one of its most

fundamentally important features, the liquid line of des- recharged lava lake, or drainage of the contents of a lakeof ultramafic lava originally more than 100 m deep acrosscent in the Skaergaard intrusion (Hunter & Sparks, 1987,

1990; Brooks & Nielsen, 1990; McBirney & Naslund, this region of Mare Imbrium (in which case the residualsurface deposit has a high probability of being enriched1990; Morse, 1990).

134. The most informative surface-based observations in sunken phenocrysts, whether equilibrium or quench).An annotated selection of high-quality topographicwere those obtained at Hadley Rille by the Apollo 15

crew (Howard et al., 1972; Spudis et al., 1988). The rille photographs of lunar features (Schultz, 1976) includesvolcanic crater-topped mounds (plate 125) and tephrawas interpreted as an open or collapsed lava tube which

probably drained a large ponded lava or partially crys- rings (plate 132e); substantial pits resulting from magmawithdrawal (plate 243); possible nuees ardentes featurestallized lava lake. Photographs show that the rille has cut

down through horizontally layered rock units at least (plate 143); and numerous possible dyke-like features inor adjacent to basins. Other features include magma20 m thick in which the columnar jointing so common

in thick terrestrial flows and minor intrusions is con- drowned rings, some of which are craters but some ofwhich might be central complexes or calderas (platespicuously absent. The layering is not as marked as would

be expected if the rocks were a sequence of thin flows 109b); numerous ghost craters and potential ‘blackbird’effects (e.g. plates 75a, 111, 150 and 153); an overflowingseparated by appreciable regolith layers which could be

expected to have developed on them in the apparently cup feature (plate 148b); frequent aprons and bath-tubmarks around mare surfaces (plates 40b, 60b–d, 154–158,long intervals between eruptive events. Fallen blocks on

the floor of the rille are up to 30 m in diameter, an 197) suggestive of magma withdrawal or volume con-traction on solidification; and rille patterns in cratersimprobable product of fragmentation of thin flows sep-

arated by regolith layers, and appear to be composed of suggestive of contraction on consolidation of a substantialpool of magma. Vents for mare basalt eruptions aremassive, poorly jointed materials more akin to gabbro

than basalt. Some photographs of Hadley Rille in the rarely recognized, but have been noted between theHumorum and Procellarum basins (Greeley & Spudis,studies by Howard et al. (1972, fig. 7) and Light (1999,

plate 77) show outcrops and fallen blocks with narrow, 1978). Undoubted flow morphologies can be seen on thesurface of Mare Imbrium (Schaber, 1973), where theyvery dark apparently linear features which are sub-

horizontal when in situ. If these are not intense shadows represent the last effusions in the waning phase of activity,but the flows are nevertheless huge and extend forin planar cracks they look for all the Moon like chromitite

horizons in layered gabbro. 200–1200 km from their probable source. Identifiableflow front features are, however, not abundant on theThermal erosion by cutting down of lava tubes appears

to be significant below some but not all terrestrial ultra- lunar maria (Schaber et al., 1976) despite evidence thatseveral flow units were sampled in a small area at eachmafic lava flows (Huppert et al., 1984; Huppert & Sparks,

1985b; Williams et al., 1998) but less significant when Apollo site.135. Andre et al. (1978) identified more magnesianbasalt flows across a basalt surface (Greeley et al., 1998).

Hadley Rille is some 500 m deep, 1500 m wide and at materials excavated from below 1400 m depth at MareCrisium. The Theophilus impact far to the east of theleast 100 km long (Spudis et al., 1988). At one extreme,

it may have been formed by flow of, and downcutting Apollo 16 site created a 100 km diameter crater over2 km deep and has excavated magnesium-rich materialsby, large volumes of hot, possibly ultramafic, lavas which

would then be contaminated by assimilation of previously from beneath less magnesian flows in Mare Nectaris(Andre & El-Baz, 1981). Tompkins et al. (1994) reportederupted basaltic compositions and of intervening

KREEP-rich regoliths (see 31). Photographs of layers in a possible layered mafic gabbro pluton excavated frombeneath basalts of Mare Nubium by crater Bulliardus.the walls of Hadley Rille and of the massive overlying

unit which is the source of many large blocks and has 136. Antarctic lunar mare meteorite Asuka 881757 isa 442 g fragment of>3·9 Ga old, low-Ti gabbro (Yanai,been identified as a section through superimposed basalt

flows (Howard et al., 1972, fig. 7; Taylor, 1975, fig. 2.24) 1991), also described as a very coarse-grained basalt (Araiet al., 1996).can as easily be compared with photographs of layering

in the upper parts of the Skaergaard intrusion (Wager & 137. The styles of eruption in the bulk of the lunarmaria, although absent on the modern Earth, may alsoBrown, 1968, plate IV). High-water mark evidence for

subsidence of the mare surface at this locality by at least have marked the earliest basalt plains on Mars (57),where volcanic activity was sustained for a much longer100 m subsequent to eruption (Howard et al., 1972) can

be viewed as resulting from some combination of volume time than on the Moon and the later massive basalticeffusions are dominated by basalts erupted in the con-reduction during closed system crystallization of a lava

lake greater than 1 km deep, or draining of liquid from ventional style with clearly defined flow units and by

1626


large central constructs with impressive calderas. The (3) And what can we learn from Io about the supplyrates and eruptive conditions necessary to produce thelast erupted basalts on a largely degassed Moon appearflat-surfaced maria typical of the Moon, rather thanto have behaved more conventionally and have formeddistinct flows, low shields, huge calderas and up to 16 kmlarge, readily detectable flow units in Mare Imbrium andrelief which characterize Io?cinder cones in the Marius Hills (BVSP, 1981, section

(4) What are the consequences of an inadequate, if5.4.6, and section 134).not absent, positive Eu anomaly in the average lunar138. Basaltic nuees ardentes may have been the prin-highlands for the plagioclase-flotation model of lunarcipal eruption style of early, volatile-rich mare basalts.crustal origin?As volatile contents decreased a transition might be

(5) What are the implications of no, or even perhapsexpected to eruptions with spectacular fire-fountaining,a small positive Eu anomaly in the lunar mantle for theexceeding even that seen today on Io.primary basalt magma compositions?139. There are problems with transfer of melts un-

(6) What is the cause of the pronounced negative Eualtered through great thicknesses of mantle and crustanomaly in the TiO2-rich basalts if it has not been(31) and there is evidence for partial crystallization andinherited from the mantle source?sulphide separation from mare basalts at crustal depths

(7) Why are the low-TiO2 lunar basalts so much morebefore eruption (74, 75, 78, 120) and for crystal settlingabundant than high TiO2 basalts, when they are allegedlyafter eruption (103–104). There is a risk of hybridization,the products of lower-temperature partial melting ofcontamination and assimilation during irruption (27, 125)advanced mantle cumulates which are, moreover, muchand a high probability of extensive losses by selectivethe densest and most likely to founder within the pos-volatilization during eruption (54, 73, 80–84). It is, there-tulated cumulate mantle?fore, improbable that any mare basalt sample returned

(8) Why do the alleged mare primary magma com-from the lunar surface will be close in composition to apositions display extreme plagioclase undersaturation atprimary magma. It may be difficult even to establishlow and high pressures which is totally incompatible withprecisely the pre-eruption composition of the magmastheir proposed mode of origin?immediately parental to the surface flows.

(9) Why are the mare regoliths persistently more feld-140. The existence of predominantly feldspathic marespathic than the hand specimens, particularly those ad-basalts creates a major problem for the conventionalvanced as primary magmas, far beyond what can beinterpretation. Great as this problem is, the ultramaficaccounted for by later transport of highland material?green and orange volcanic glass deposits of Hadley Rille

(10) What is the real average basalt component con-and Taurus Littrow create as great an interpretativetributing to regolith formation—a question which can beproblem for the alternative approach. Their existenceaddressed with the existing sample base?underlines the need for direct experimentation on the

physics and chemistry of eruption of basalt into hardvacuum and the whole debate highlights our ignoranceregarding the field relationships of the mare basalt

REFERENCESsamples, knowledge of which would resolve several de-Adler, I., Trombka, J., Schmadebeck, R., Lowman, P., Blodget, N.,bates. A visit to the huge blocks on the floor of Hadley

Yin, L. & Eller, E. (1973). Results of the Apollo 15 and 16 X-rayRille, a traverse down its walls or a few 100 m drill coresexperiment. Proceedings of the Lunar Science Conference 4, 2783–2791.

almost anywhere in the maria would settle the debate Alexopoulos, J. S. & McKinnon, W. B. (1994). Large impact cratersfinally and unambiguously (maybe). and basins on Venus, with implications for ring mechanics on the

terrestrial planets. In: Dressler, B. O., Grieve, R. A. F. & Sharpton,V. L. (eds) Large Meteorite Impacts and Planetary Evolution. Geological Society

of America, Special Papers 293, 29–50.Allegre, C. J. & Minster, J.-F. (1978). Quantitative models of traceNotes added in proof

element behaviour in magmatic processes. Earth and Planetary ScienceJolliff et al. (2000) tabulated ‘key questions to be addressed’Letters 38, 1–25.with respect to lunar petrogenesis. Several additional Allegre, C. J., Treuill, M., Minster, J.-F., Minster, J. B. & Albarede,

questions emerge from this study as overwhelmingly F. (1977). Systematic use of trace elements in igneous processes.important: Contributions to Mineralogy and Petrology 60, 55–75.

Anders, E. (1978). Procrustean science: indigenous siderophiles in the(1) What should we learn from terrestrial MORB andlunar highlands, according to Delano and Ringwood. Proceedings ofOIB about the difficulties of erupting primary magmasthe Lunar and Planetary Science Conference 9, 161–184.even from depths as little as 30–50 km?

Anderson, J. D., Sjogren, W. L. & Schubert, G. (1996). Galileo gravity(2) What should we learn from recent observations onresults and the internal structure of Io. Science 272, 709–712.

Io, currently the most active volcanic body in the Solar Andre, C. G. & El-Baz, F. (1981). Regional chemical setting of theSystem, about the extent of volatilization accompanying Apollo 16 landing site and the importance of the Kant plateau.

Proceedings of the Lunar and Planetary Science Conference 12B, 767–779.eruption over a lunar-sized planet?

1627


Andre, C. G., Wolfe, R. W. & Adler, I. (1978). Evidence for high- lunar meteorites Yamato-82192 and 82193. Memoirs of the National

Institute of Polar Research, Special Issue 46, 21–42.magnesium subsurface basalt in Mare Crisium from orbital X-rayfluorescence data. In: Merrill, R. B. & Papike, J. J. (eds) Mare Crisium: Blanchard, D. P. & Budahn, J. R. (1979). Remnants from the ancient

crust: clasts from consortium breccia 73255. Proceedings of the Lunarthe View from Luna 24. Geochimica et Cosmochimica Acta Supplement 9,1–12. and Planetary Science Conference 10, c 803–816.

Blanchard, D. P., Korotev, R. L., Brannon, J. C., Jacobs, J. W., Haskin,Arai, T., Takeda, H. & Warren, P. H. (1996). Four lunar maremeteorites: crystallization trends of pyroxenes and spinels. Meteoritics L. A., Reid, A. M., Donaldson, C. H. & Brown, R. W. (1975). A

geochemical and petrographic study of 1–2 mm fines from Apolloand Planetary Science 31, 877–892.Arndt, N. T. (1986). Differentiation of komatiite flows. Journal of Petrology 17. Proceedings of the Lunar Science Conference 6, 2321–2341.

Blaney, D., Johnson, T. V., Matson, D. L. & Veeder, G. J. (1995).27, 279–301.Ashwal, L. D. (1993). Anorthosites. Berlin: Springer. Volcanic eruptions on Io: heat flow, resurfacing and lava com-

position. Icarus 113, 220–225.Asimow, P. D. & Stolper, E. M. (1999). Steady-state mantle–meltinteractions on one dimension: I. Equilibrium transport and melt Bloomer, S. H., Natland, J. H. & Fisher, R. L. (1989). Mineral

relationships in gabbroic rocks from fracture zones of Indian Oceanfocusing. Journal of Petrology 40, 475–494.Baker, M. B., Alves, S. & Stolper, E. M. (1996). Petrography and Ridges: evidence for extensive fractionation, parental diversity and

boundary-layer recrystallisation. In: Saunders, A. D. & Norry, M.petrology of the Hawaiian Scientific Drilling Project lavas: inferencesfrom olivine phenocryst abundances and compositions. Journal of J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special

Publication 42, 107–124.Geophysical Research 101, 11715–11727.Basaltic Volcanism Study Project (BVSP) (1981). Basaltic Volcanism on Bonatti, E., Honnorez, J. & Ferrara, G. (1974). Ultramafic rocks:

peridotite–gabbro–basalt complex from the equatorial mid-Atlanticthe Terrestrial Planets. New York: Pergamon, 1286 pp.Beaty, D. W., Hill, S. M. R., Albee, A. L. & Baldridge, W. S. (1979). ridge. Journal of Geophysical Research 86, 2593–2644.

Bowen, N. L. (1928). The Evolution of the Igneous Rocks. Princeton, NJ:The Apollo 12 feldspathic basalts 12031, 12038 and 12072; petrology,comparison and interpretations. Proceedings of the Lunar and Planetary Princeton University Press (reprinted by Dover, New York, 1956),

334 pp.Science Conference 10, 115–139.Beatty, J. K., Petersen, C. C. & Chaikin, A. (eds) (1999). The New Bowen, N. L. & Schairer, J. F. (1935). The system MgO–FeO–SiO2.

American Journal of Science 29, 151–217.Solar System, 4th edn. Cambridge: Cambridge University Press;Cambridge, MA: Sky, 421 pp. Boyd, F. R. & England, J. L. (1964). The system enstatite–pyrope.

Carnegie Institution of Washington, Yearbook 63, 157–161.Bence, A. E. & Grove, T. L. (1978). The Luna 24 highland component.In: Merrill, R. B. & Papike, J. J. (eds) Mare Crisium: the View from Luna Bravo, M. & O’Hara, M. J. (1975). Partial melting of phlogopite-

bearing synthetic spinel- and garnet-lherzolites. Physics and Chemistry24. Geochimica et Cosmochimica Acta Supplement 9, 429–444.Bence, A. E., Taylor, S. R., Muir, P. M., Nance, W. B., Rudowski, R. of the Earth 9, 845–854.

Brett, R. (1976). Reduction of mare basalts by sulfur loss. Geochimica et& Ware, N. (1975). Chemical and petrologic relations among lunarhighland rock types. In: Lunar Science VI. Houston, TX: Lunar Science Cosmochimica Acta 40, 997–1004.

Brooks, C. K. & Nielsen, T. F. D. (1990). A discussion of HunterInstitute, pp. 36–38.Biggar, G. M. & O’Hara, M. J. (1969a). Temperature control and and Sparks (Contrib Mineral Petrol 95: 451–461). Contributions to

Mineralogy and Petrology 104, 244–247.calibration in quench furnaces and some new temperature meas-urements in the system CaO–MgO–Al2O3–SiO2. Mineralogical Ma- Brown, G. M. (1956). The layered ultrabasic rocks of Rhum, Inner

Hebrides. Philosophical Transactions of the Royal Society of London, Seriesgazine 37, 1–15.Biggar, G. M. & O’Hara, M. J. (1969b). A comparison of gel and glass B 240, 1–53.

Brown, G. M. & Peckett, A. (1971). Selective volatilization on the lunarstarting materials for phase equilibrium studies. Mineralogical Magazine

37, 198–205. surface: evidence from Apollo 14 feldspar-phyric basalts. Nature 234,262–266.Biggar, G. M. & O’Hara, M. J. (1972a). Calibration of oxygen fugacity

at atmospheric pressure. In: Progress in Experimental Petrology: Second Browning, P. (1984). Cryptic variations within the cumulate sequenceof the Oman ophiolite: magma chamber depth and petrologicalProgress Report of Research Supported by NERC at Edinburgh and Manchester

Universities, 1969–1971. NERC Publication Series D 2, 99–102. implications. In: Gass, I. G., Lippard, S. J. & Shelton, A. W.(eds) Ophiolites and Oceanic Lithosphere. Geological Society, London, SpecialBiggar, G. M. & O’Hara, M. J. (1972b). Eight joins in CaO–

MgO–Al2O3–SiO2. In: Progress in Experimental Petrology: Second Progress Publication 14, 71–82.Cameron, A. G. W. & Canup, R. M. (1998). Continued accretionReport of Research Supported by NERC at Edinburgh and Manchester Uni-

versities, 1969–1971. NERC Publication Series D 2, 99–102. of the Moon and Earth. Lunar and Planetary Science Conference 29(Abstracts).Biggar, G. M., O’Hara, M. J., Peckett, A. & Humphries, D. J. (1971).

Lunar lavas and the achondrites: petrogenesis of proto-hypersthene Campbell, I. H. (1978). Some problems with cumulus theory. Lithos

11, 311–323.basalts in the maria lava lakes. Proceedings of the Lunar Science Conference

2(1), 617–643. Canil, D. (1999). Vanadium partitioning between orthopyroxene, spineland silicate melt and the redox states of mantle source regions forBiggar, G. M., O’Hara, M. J., Humphries, D. J. & Peckett, A. (1972).

Maria lavas, mascons, layered complexes, achondrites and the lunar primary magmas. Geochimica et Cosmochimica Acta 63, 557–572.Cann, J. R. (1982). Rayleigh fractionation with continuous removal ofmantle. In: Urey, H. C. & Runcorn, S. K. (eds) The Moon. Inter-

national Astronomical Union, pp. 129–164. liquid. Earth and Planetary Science Letters 60, 114–116.Capdevila, R., Arndt, N., Letendre, J. & Sauvage, J.-F. (1999). Dia-Binder, A. B. (1998). Lunar Prospector: overview. Science 281, 1475–

1476. monds in volcaniclastic komatiite from French Guiana. Nature 399,456–458.Bischoff, A., Palme, H., Weber, H. W., Stoffler, D., Braun, O., Spettel,

B., Begemann, F., Wanke, H. & Ostertag, O. (1987). Petrography, Carmichael, I. S. E., Turner, F. J. & Verhoogen, J. (1974). Igneous

Petrology. New York: McGraw–Hill, 739 pp.shock history, chemical composition and noble gas content of the

1628


Carr, M. H. (1981). The Surface of Mars. New Haven, CT: Yale University Cox, K. G. & Hawkesworth, C. J. (1984). Relative contribution ofcrust and mantle to flood basalt magmatism, Mahabaleshwar area,Press, 232 pp.Deccan Traps. Philosophical Transactions of the Royal Society of London,Carr, M. H. (1986). Silicate volcanism on Io. Journal of Geophysical

Series A 310, 627–641.Research 91, 3521–3532.Cox, K. G. & Hawkesworth, C. J. (1985). Geochemical stratigraphyCarress, D. W., McNutt, M. K., Detrick, R. S. & Mutter, J. C. (1995).

of the Deccan traps at Mahabaleshwar, Western Ghats, India, withSeismic imaging of hotspot-related crustal underplating beneath theimplications for open system magmatic processes. Journal of PetrologyMarquesas Islands. Nature 373, 600–603.26, 355–377.Cashman, K., Pinkerton, H. & Stephenson, J. (1998). Introduction to

Cox, K. G. & Hornung, G. (1966). The petrology of the Karroo basaltsspecial section: long lava flows. Journal of Geophysical Research 103,of Basutoland. American Mineralogist 51, 1414–1432.27281–27289.

Cox, K. G. & Jamieson, B. G. (1974). The olivine-rich lavas ofCawthorn, R. G. & Walraven, F. (1998). Emplacement and crys-Nuanetsi—a study of polybaric magmatic evolution. Journal of Petrologytallization time for the Bushveld Complex. Journal of Petrology 39,15, 269–301.1669–1687.

Cox, K. G., Johnson, R. L., Monkman, L. J., Stillman, C. J., Vail, J.Chinner, G. A. & Schairer, J. F. (1962). The join CaAl2Si3O12–R. & Wood, D. N. (1965). The geology of the Nuanetsi igneousMg3Al2Si3O12 and its bearing on the system CaO–MgO–Al2O3–SiO2province. Philosophical Transactions of the Royal Society of London, Series Aat atmospheric pressure. American Journal of Science 260, 611–634.257, 71–218.Christiansen, N. I. & Salisbury, M. H. (1975). Structure and constitution

Cox, K. G., MacDonald, R. & Hornung, G. (1967). Geochemical andof the lower oceanic crust. Reviews of Geophysics and Space Physics 13,petrographic provinces in the Karroo basalts of southern Africa.57–86.American Mineralogist 52, 1451–1474.Clague, D. A., Moore, J. G., Dixon, J. E. & Friesen, W. B. (1995).

Cox, K. G., Bell, J. D. & Pankhurst, R. J. (1979). The Interpretation ofPetrology of submarine lavas from Kilauea’s Puna Ridge, Hawaii.Igneous Rocks. London: George Allen and Unwin, 450 pp.Journal of Petrology 36, 299–349.

Cox, K. G., Smith, M. R. & Beswetherick, S. (1987). Textural studiesClarke, D. B. (1970). Tertiary basalts of Baffin Bay: possible primaryof garnet lherzolites: evidence of exsolution origin from high-tem-magma from the mantle. Contributions to Mineralogy and Petrology 25,perature harzburgites. In: Nixon, P. H. (ed.) Mantle Xenoliths. New203–224.York: Wiley, pp. 537–550.Clarke, D. B. & O’Hara, M. J. (1979). Nickel and the existence of

Crawford, A. J., Beccaluva, L. & Serri, G. (1981). Tectono-magmatichigh-MgO liquids in nature. Earth and Planetary Science Letters 44,

evolution of the west Philippine–Mariana region and the origin of153–158.

boninites. Earth and Planetary Science Letters 54, 345–356.Clow, G. D. & Carr, M. H. (1980). Stability of sulfur slopes on Io.

Cruikshank, D. P., Tholen, D. J., Hartmann, W. K., Bell, J. F. &Icarus 44, 729–733.

Brown, R. H. (1991). Three basaltic earth-approaching asteroidsCoish, R. A. & Taylor, L. A. (1978). Mineralogy and petrology of

and the source of the basaltic meteorites. Icarus 89, 1–13.basaltic fragments from the Luna 24 drill core. In: Merrill, R. B. & Dalton, J. A. & Presnall, D. C. (1998). The continuum of primaryPapike, J. J. (eds) Mare Crisium: the View from Luna 24. Geochimica et carbonatitic–kimberlitic melt compositions in equilibrium with lher-Cosmochimica Acta Supplement 9, 403–417. zolite: data from the system CaO–MgO–Al2O3–SiO2–CO2. Journal

Connerney, J. E. P., Acuna, M. H., Wasilewski, P. J., Ness, N. F.,of Petrology 39, 1953–1964.

Reme, H., Mazelle, C., Vignes, D., Lin, R. P., Mitchell, D. L. & Davies, A. G. (1996). Volcanism: thermo-physical models of silicateCloutier, P. A. (1999). Magnetic lineations in the ancient crust of lava compared with observations of thermal emission. Icarus 124,Mars. Science 284, 794–798. 45–61.

Consolmagno, G. J. (1981). An Io thermal model with intermittent Davies, A. G., McEwen, A. S., Lopes-Gautier, R., Keszthelyi, L.,volcanism. Proceedings of the Lunar and Planetary Science Conference 12, Carlson, R. & Smythe, W. D. (1997). Temperature and area con-1533–1542. straints of the Soputh Vorlund volcano on Io from the NIMS and

Courtillot, V., Feraud, G., Maluski, H., Vandamme, D., Moreau, M. G. SSI instruments during the Galileo G1 orbit. Geophysical Research& Besse, J. (1988). Deccan flood basalts and the Cretaceous–Tertiary Letters 24, 2447–2450.boundary. Nature 333, 843–846. Davis, P. A. & Spudis, P. D. (1987). Global petrologic variations on

Courtillot, V., Jaupart, C., Manighetti, I., Tapponier, P. & Besse, J. the Moon: a ternary diagram approach. Proceedings of the Lunar(1999). On causal links between flood basalts and continental break- and Planetary Science Conference 17. Journal of Geophysical Research 92,up. Earth and Planetary Science Letters 166, 177–195. E387–395.

Cox, K. G. (1971). The Karroo lavas and associated igneous rocks of De Hon, R. A. (1974). Thickness of mare material in the Tranquillitatissouthern Africa. Bulletin of Volcanology 35, 867–886. and Nectaris basins. Proceedings of the Lunar Science Conference 5, 53–59.

Cox, K. G. (1972). The Karroo volcanic cycle. Journal of the Geological De Hon, R. A. (1977). Mare Humorum and Mare Nubium: basaltSociety, London 128, 311–336. thickness and basin forming history. Proceedings of the Lunar Science

Cox, K. G. (1978). Flood basalts, subduction and the break-up of Conference 8, 633–641.Gondwanaland. Nature 274, 47–49. De Hon, R. A. (1979). Thickness of the western mare basalts. Proceedings

Cox, K. G. (1980). A model for flood basalt vulcanism. Journal of of the Lunar and Planetary Science Conference 10, 2935–2955.Petrology 21, 629–650. Delano, J. W. (1975). Petrology of the Apollo 16 mare component:

Cox, K. G. (1989). The role of mantle plumes in the development of Mare Nectaris ETC. Proceedings of the Lunar Science Conference 6, 15–47.continental drainage patterns. Nature 342, 873–877. DePaolo, D. J. (1981). Trace element and isotopic effects of combined

Cox, K. G. (1991). Earth sciences—a superplume in the mantle. Nature wallrock assimilation and fractional crystallisation. Earth and Planetary

352, 564–565. Science Letters 53, 189–202.Cox, K. G. & Bell, J. D. (1972). A crystal fractionation model for DePaolo, D. J. (1996). High-frequency isotopic variations in the Mauna

basaltic rocks of the New Georgia group, British Solomon Islands. Kea tholeiitic basalt sequence: melt zone dispersivity and chromato-graphy. Journal of Geophysical Research 101, 11855–11864.Contributions to Mineralogy and Petrology 37, 1–13.

1629


DePaolo, D. J. & Stolper, E. M. (1996). Models of Hawaiian volcano and late stage core formation. Earth and Planetary Science Letters 169,growth and plume structure: implications of results from the Hawaii 147–163.Scientific Drilling Project. Journal of Geophysical Research 101, 11643– Gaffey, M. J. (1997). Surface lithologic heterogeneity of asteroid 411654. Vesta. Icarus 127, 130–157.

Dickinson, T., Taylor, G. R., Keil, K., Schmitt, R. A., Hughes, S. S. Gaffey, M. J. & McCord, T. B. (1977). Asteroid surface materials:& Smith, M. R. (1985). Apollo 14 aluminous mare basalts and their mineralogical characterizations and cosmological implications. Pro-possible relationship to KREEP. Proceedings of the Lunar and Planetary ceedings of the Lunar Science Conference 8, 113–143.Science Conference 15. Journal of Geophysical Research 90, C365–383. Garcia, M. O. (1996). Petrography and olivine and glass chemistry of

Dobar, W. I. (1965). Simulated basalt and granite upwelling in vacuum. lavas from the Hawaii Scientific Drilling Project. Journal of GeophysicalIcarus 5, 399–405. Research 101, 11655–11713.

Donaldson, C. H., Usselman, T. M., Williams, R. J. & Lofgren, C. E. Gast, P. W. (1968). Trace element fractionation and the origin of(1975). Experimental modeling of the cooling history of Apollo 12 tholeiitic and alkaline magma types. Geochimica et Cosmochimica Actaolivine basalts. Proceedings of the Lunar Science Conference 6, 843–869. 32, 1057–1086.

Drake, M. J. & Weill, D. F. (1975). Partition of Sr, Ba, Y, Eu2+, Eu3+Ghiorso, M. S. (1994). Algorithms for the estimation of phase stability

and other REE between plagioclase feldspar and magmatic liquid: in heterogeneous thermodynamic systems. Geochimica et Cosmochimicaan experimental study. Geochimica et Cosmochimica Acta 39, 689–712. Acta 58, 5489–5501.

Drever, H. I. (1956). The geology of Ubekendt Ejland, West Greenland. Ghiorso, M. S. & Sack, R. O. (1995). Chemical mass transfer inPart III. The picritic sheets and dykes of the east coast. Meddelelser magmatic processes IV. A revised and internally consistent thermo-om Gronland 137(4), 39 pp. dynamic model for the interpolation and extrapolation of liquid–solid

Drever, H. I. & Johnston, R. (1957). Crystal growth of forsteritic olivine equilibria in magmatic systems at elevated temperatures and pres-in magmas and melts. Transactions of the Royal Society of Edinburgh 63, sures. Contributions to Mineralogy and Petrology 119, 197–212.289–315. Ghiorso, M. S., Hirschmann, M. M. & Sack, R. O. (1995). New

Eggins, S. M. (1992). Petrogenesis of Hawaiian tholeiites: 2, aspects of software models thermodynamics of magmatic systems. EOS Trans-dynamic melt segregation. Contributions to Mineralogy and Petrology 110,

actions, American Geophysical Union 75, 571–576.398–410. Ghosh, A. & McSween, H. J., Jr (1998). A thermal model for the

Elphic, R. C., Lawrence, D. J., Feldman, W. C., Barraclough, B. L., differentiation of asteroid 4 Vesta, based on radiogenic heating.Maurice, S., Binder, A. B. & Lucey, P. G. (1998). Lunar Fe and Ti

Icarus 134, 187–206.abundances: comparison of Lunar Prospector and Clementine data.

Giguere, T. A., Taylor, G. J., Hawke, B. R. & Lucey, P. G. (2000).Science 281, 1493–1496.

The titanium contents of lunar mare basalts. Meteoritics and PlanetaryEngel, A. E., Engel, C. G. & Havens, R. G. (1965). Chemical char-

Science 35, 193–200.acteristics of oceanic basalts and the upper mantle. Geological Society

Glickson, A. Y. (1976). Earliest Precambrian ultramafic–mafic volcanicof America Bulletin 76, 719–734.

rocks: ancient oceanic crust or relic terrestrial maria. Geology 4,Ernst, R. E. & Buchan, K. L. (1997). Giant radiating dyke swarms:201–205.their use in identifying pre-Mesozoic large igneous provinces and

Golombek, M. P., Cook, R. A., Economou, T., Folkmer, W. M.,mantle plumes. In: Mahoney, J. J. & Coffin, M. F. (eds) LargeHaldemann, A. F. C., Kallemeyr, P. H., Knudsen, J. M., Manning,

Igneous Provinces. Geophysical Monograph, American Geophysical Union 100,R. M., Moore, H. J., Parker, T. J., Rieder, R., Schofield, J. T.,297–333.Smith, P. H. & Vaughn, R. M. (1997). Overview of the MarsFalloon, T. J., Green, D. H., Danyushevsky, L. V. & Faul, U. H.Pathfinder mission and assessment of landing site predictions. Science(1999). Peridotite melting at 1·0 and 1·5 GPa: an experimental278, 1743–1748.evaluation of techniques using diamond aggregates and mineral

Goodrich, C. A. & Keil, K. (1987). Mare basalt and other clasts inmixes for determination of near-solidus melts. Journal of Petrology 40,Yamato lunar meteorites Y-791197, -82192 and -82193. Memoirs of1343–1375.the National Institute of Polar Research, Special Issue 46, 56–70.Feigenson, M. D., Patino, L. C. & Carr, M. J. (1996). Constraints on

Greeley, R. & Spudis, P. D. (1978). Mare volcanism in the Herigoniuspartial melting imposed by rare earth element variations in Maunaregion of the Moon. Proceedings of the Lunar and Planetary Science ConferenceKea basalts. Journal of Geophysical Research 101, 11815–11829.9, 3333–3349.Feldman, W. C., Barraclough, B. L., Maurice, S., Elphic, R. C.,

Greeley, R. & Womer, M. B. (1981). Mare basin filling on the Moon:Lawrence, D. J., Thomsen, D. R. & Binder, A. B. (1998a). Majorlaboratory simulations. Proceedings of the Lunar and Planetary Sciencecompositional units of the Moon: Lunar Prospector thermal andConference 12B, 651–663.fast neutrons. Science 281, 1489–1492.

Greeley, R., Lee, S. W., Crown, D. A. & Lancaster, N. (1990).Feldman, W. C., Maurice, S., Binder, A. B., Barraclough, B. L., Elphic,Observations of industrial sulfur flows: implications for Io. Icarus 84,R. C. & Lawrence, D. J. (1998b). Fluxes of fast and epithermal374–402.neutrons from Lunar Prospector: evidence for water ice at the lunar

Greeley, R., Kadel, S. D., Williams, D. A., Gaddis, L. R., Head, J.poles. Science 281, 1496–1501.W., McEwen, A. S., Murchie, S. L., Nagel, E., Neukum, G., Pieters,Ford, C. E., Biggar, G. M., Humphries, D. J., Wilson, G., Dixon, D.C. M., Sunshine, J. M., Wagner, R. & Belton, M. J. S. (1993).& O’Hara, M. J. (1972). Role of water in the evolution of the lunarGalileo mapping observations of lunar maria and related deposits.crust; an experimental study of sample 14310; an indication of lunarJournal of Geophysical Research 98, 17183–17205.calc-alkaline volcanism. Proceedings of the 3rd Lunar Science Conference

Greeley, R., Fagents, S. A., Harris, R. S., Kadel, S. D., Williams, D.3(1), 207–229.A. & Guest, J. E. (1998). Erosion by flowing lava: field evidence.Ford, C. E., O’Hara, M. J. & Spencer, P. N. (1977). The origin ofJournal of Geophysical Research 103, 27325–27345.lunar feldspathic liquids. Philosophical Transactions of the Royal Society of

Green, D. H. (1972). Archaean greenstone belts may include terrestrialLondon, Series A 285, 193–197.equivalents of lunar maria? Earth and Planetary Science Letters 15,Gaetani, G. A. & Grove, T. L. (1999). Wetting of mantle and olivine

by sulfide melt. Implications for Re/Os ratios in mantle peridotite 263–270.

1630


Green, D. H. (1975). Genesis of Archaean peridotitic magmas and Head, J. W. & Wilson, L. J. (1979). Alphonsus-type dark-halo craters:morphology, morphometry and eruption conditions. Proceedings of theconstraints on Archaean geothermal gradients and tectonics. Geology

3, 15–18. Lunar and Planetary Science Conference 10, 2861–2897.Head, J. W., Adams, J. B., McCord, T. B., Pieters, C. & Zisk, S.Green, D. H. & Ringwood, A. E. (1967). The genesis of basaltic

magmas. Contributions to Mineralogy and Petrology 15, 103–190. (1978). Regional stratigraphy and geologic history of Mare Crisium.In: Merrill, R. B. & Papike, J. J. (eds) Mare Crisium: the View from LunaGribble, C. D. & O’Hara, M. J. (1967). Interaction of basic magma

with pelitic materials. Nature 214, 1198–1201. 24. Geochimica et Cosmochimica Acta Supplement 9, 43–74.Heiken, G., Vaniman, D. & French, B. M. (eds) (1991). Lunar Sourcebook:Grieve, R. A. F., Stoffler, D. & Deutsch, A. (1991). The Sudbury

structure: controversial or misunderstood? Journal of Geophysical Re- a User’s Guide to the Moon. Cambridge: Cambridge University Press,736 pp.search 96, 22753–22764.

Griffiths, S. A., Basu, A., McKay, D. S. & Nace, G. (1981). Petrology Herbert, F., Drake, M. J. & Sonett, C. P. (1978). Geophysical andgeochemical evolution of the lunar magma ocean. Proceedings of theof Apollo 15 station 9A surface and drive tube soils. Proceedings of the

Lunar and Planetary Science Conference 12B, 475–484. Lunar and Planetary Science Conference 9, 249–262.Herzberg, C. & O’Hara, M. J. (1998). Phase equilibrium constraintsGrove, T. L. (1981). Compositional variations among Apollo 15 green

glass spheres. Proceedings of the Lunar and Planetary Science Conference 12B, on the origin of basalts, picrites and komatiites. Earth-Science Reviews

44, 39–79.935–948.Grove, T. L. & Bence, A. E. (1979). Crystallization kinetics in a Hildreth, W. (1979). The Bishop Tuff: evidence for the origin of

compositional zonation in silicic magma chambers. Geological Societymultiply saturated basalt magma: an experimental study of Luna 24ferrobasalt. Proceedings of the Lunar and Planetary Science Conference 10, of America, Special Papers 180, 43–74.

Hiroi, T., Buizel, R. P., Sunshine, J. M., Pieters, C. M. & Takeda, H.439–478.Grove, T. L. & Vaniman, D. T. (1978). Experimental petrology of (1995). Grain sizes and mineral compositions of surface regoliths of

Vesta-like asteroids. Icarus 115, 374–386.very low Ti (VLT) basalts. In: Merrill, R. B. & Papike, J. J. (eds)Mare Crisium: the View from Luna 24. Geochimica et Cosmochimica Acta Hirschman, M. M. & Stolper, E. M. (1995). A possible role for

garnet pyroxenite in the origin of the ‘garnet signature’ in MORB.Supplement 9, 445–471.Halliday, A. N. & Lee, D.-C. (1999). Tungsten isotopes and the early Contributions to Mineralogy and Petrology 124, 185–208.

Holzheid, A., Borisov, A. & Palme, H. (1994). The effect of oxygendevelopment of the Earth and Moon. Geochimica et Cosmochimica Acta

63, 4157–4179. fugacity and temperature on nickel, cobalt and molybdenum insilicate melts. Geochimica et Cosmochimica Acta 58, 1975–1981.Hamilton, W. (1970). Bushveld Complex—a product of impacts? Geo-

logical Society of South Africa, Special Publication 1, 367–374. Hooper, P. R. (1997). The Columbia River flood basalt province:current status. In: Mahoney, J. J. & Coffin, M. F. (eds) Large IgneousHapke, B. (1989). The surface of Io: a new model. Icarus 79, 56–74.

Harker, A. (1908). The geology of the Small Isles of Inverness-shire (Sheet 60). Provinces. Geophysical Monograph, American Geophysical Union 100, 1–27.Hoppa, G. V., Tufts, B. R., Greenberg, R. & Geissler, P. E. (1999).Memoirs, Geological Survey of Scotland. Glasgow: HMSO, 210 pp.

Harris, P. G. (1957). Zone refining and the origin of potassic basalts. Formation of cycloidal features on Europa. Science 285, 1899–1902.Horz, F. (1978). How thick are mare basalts? Proceedings of the Lunar andGeochimica et Cosmochimica Acta 12, 195–208.

Hart, S. R. & Allegre, C. J. (1980). Trace elements and magma genesis. Planetary Science Conference 9, 3311–3331.Horz, F., Grieve, R., Heiken, G., Spudis, P. & Binder, A. (1991). LunarIn: Hargraves, R. B. (ed.) Physics of Magmatic Processes. Princeton, NJ:

Princeton University Press, pp. 121–159. surface processes. In: Heiken, G., Vaniman, D. & French, B. M.(eds) Lunar Sourcebook: a User’s Guide to the Moon. Cambridge: CambridgeHart, S. R. & Davis, K. E. (1978). Nickel partitioning between olivine

and silicate melt. Earth and Planetary Science Letters 40, 203–219. University Press, pp. 61–120.Housley, R. M. (1978). Modeling lunar volcanic eruptions. ProceedingsHaskin, L. A. (1978). Trace element composition of Luna 24 Crisium

VLT basalt. In: Merrill, R. B. & Papike, J. J. (eds) Mare Crisium: the of the Lunar and Planetary Science Conference 9, 1473–1484.Howard, K. A., Head, J. W. & Swann, G. A. (1972). Geology ofView from Luna 24. Geochimica et Cosmochimica Acta Supplement 9, 593–611.

Haskin, L. & Korotev, R. L. (1977). Test of a model for trace element Hadley Rille. Proceedings of the Lunar Science Conference 3, 1–14.Howells, S. & O’Hara, M. J. (1978). Low solubility of alumina in enstatitepartition during closed system solidification of a silicate liquid.

Geochimica et Cosmochimica Acta 41, 921–939. and uncertainties in estimated geotherms. Philosophical Transactions of

the Royal Society of London, Series A 288, 471–486.Haskin, L. & Warren, P. (1991). Lunar chemistry. In: Heiken, G.,Vaniman, D. & French, B. M. (eds) Lunar Sourcebook: a User’s Guide Hu, H.-N. & Taylor, L. A. (1977). Lack of chemical fractionation in

major and minor elements during agglutinate formation. Proceedingsto the Moon. Cambridge: Cambridge University Press, pp. 357–474.Haskin, L. A., Jacobs, J. W., Brannon, J. C. & Haskin, M. A. (1977). of the Lunar Science Conference 8, 3645–3656.

Hubbard, N. J., Vilas, F. & Keith, J. E. (1978). From Serenity toCompositional dispersions in lunar and terrestrial basalts. Proceedings

of the Lunar Science Conference 8, 1731–1750. Langemak: a regional chemical setting for Mare Crisium. In: Merrill,R. B. & Papike, J. J. (eds) Mare Crisium: the View from Luna 24. GeochimicaHawke, B. R. & Bell, J. F. (1981). Remote sensing studies of lunar

dark-halo impact craters: preliminary results and implications for et Cosmochimica Acta Supplement 9, 13–32.Humphries, D. J., Biggar, G. M. & O’Hara, M. J. (1973). Phaseearly volcanism. Proceedings of the Lunar and Planetary Science Conference

12B, 485–508. equilibria and origin of Apollo 15 basalts. In: Chamberlain, J.W. &Watkins, C. (eds) The Apollo 15 Lunar Samples. Houston, TX: LunarHawkesworth, C. J., Lightfoot, P. C., Fedorenko, V. A., Blake, S.,

Naldrett, A. J., Doherty, W. & Gorbachev, N. S. (1995). Magma Science Institute, pp. 103–107.Hunter, R. H. & Sparks, R. S. J. (1987). The differentiation ofdifferentiation and mineralization in the Siberian continental flood

basalts. Lithos 34, 61–88. the Skaergaard intrusion. Contributions to Mineralogy and Petrology 95,451–461.Head, J. W. & Coffin, M. F. (1997). Large igneous provinces: a

planetary perspective. In: Mahoney, J. J. & Coffin, M. F. (eds) Large Hunter, R. H. & Sparks, R. S. J. (1990). The differentiation of theSkaergaard intrusion—Reply to A. R. McBirney and H. R. Naslund.Igneous Provinces. Geophysical Monograph, American Geophysical Union 100,

411–438. Contributions to Mineralogy and Petrology 104, 248–254.

1631


Huppert, H. E. & Sparks, R. S. J. (1980a). The fluid dynamics of a Kargel, J. J., Komatsu, G., Baker, V. R. & Strom, R. G. (1993). Thebasaltic magma chamber replenished by influx of hot, dense ultra- volcanology of Venera and VEGA landing sites and the geochemistrybasic magma. Contributions to Mineralogy and Petrology 75, 279–289. of Venus. Icarus 103, 253–275.

Huppert, H. E. & Sparks, R. S. J. (1980b). Restrictions on the com- Kaula, W. M. (1979). Thermal evolution of the earth and moonpositions of mid-ocean ridge basalts: a fluid dynamical investigation. growing by planetismal impacts. Journal of Geophysical Research 84,Nature 286, 64–68. 999–1008.

Huppert, H. H. & Sparks, R. S. J. (1985a). Cooling and contamination Keiffer, S. W., Lopes-Gautier, R., McEwen, A., Smythe, W., Keszthelyi,of mafic and ultramafic magmas during ascent through continental L. & Carlson, R. (2000). Prometheus: Io’s wandering plume. Sciencecrust. Earth and Planetary Science Letters 74, 371–386. 288, 1204–1208.

Huppert, H. H. & Sparks, R. S. J. (1985b). Komatiites I: Eruption and Keil, K. & Wilson, L. (1993). Explosive volcanism and the compositionflow. Journal of Petrology 26, 694–725. of cores of differentiated asteroids. Earth and Planetary Science Letters

Huppert, H. H., Sparks, R. S. J. & Turner, J. S. (1982). The effects of 117, 111–124.volatiles on mixing in calcalkaline magma systems. Nature 297, Keil, K., Prinz, M. & Bunch, T. E. (1972). Mineralogy, petrology and554–557. chemistry of some Apollo 12 samples. Proceedings of the Lunar Science

Huppert, H. H., Sparks, R. S. J., Turner, J. S. & Arndt, N. T. (1984). Conference 2, 319–341.The emplacement and cooling of komatiite lavas. Nature 309, 19–23. Kerr, A. C., Tarney, J., Marriner, G. F., Klaver, G. Th., Saunders,

Ida, S., Canup, R. M. & Stewart, G. R. (1997). Lunar accretion from A. D. & Thirlwall, M. F. (1996). The geochemistry and petrogenesisan impact generated disk. Nature 389, 353–357. of the late-Cretaceous picrites and basalts of Curacao, Netherlands

Irvine, T. N. (1977). Origin of chromitite layers in the Muskox intrusion Antilles: a remnant of an oceanic plateau. Contributions to Mineralogyand other layered intrusions: a new interpretation. Geology 5, 273–277. and Petrology 124, 29–43.

Irvine, T. N. (1980). Magmatic infiltration metasomatism, double- Kesson, S. E. & Lindsley, D. (1976). Mare basalt petrogenesis—adiffusive fractional crystallisation and adcumulus growth in the review of experimental studies. Reviews of Geophysics and Space PhysicsMuskox intrusion and other layered intrusions. In: Hargraves, R. B. 14, 361–373.(ed.) Physics of Magmatic Processes. Princeton, NJ: Princeton University Kesson, S. E. & Ringwood, A. E. (1977). Further limits on the bulkPress, pp. 325–383. composition of the Moon. Proceedings of the Lunar Science Conference 8,

Irvine, T. N. & Kushiro, I. (1976). Partitioning of Ni and Mg between 411–431.olivine and silicate liquids. Carnegie Institution of Washington, Yearbook

Keszthelyi, L. & McEwen, A. (1997). Magmatic differentiation of Io.75, 668–675.

Icarus 130, 437–448.Irvine, T. N., Andersen, J. C. Ø. & Brooks, C. K. (1998). Included

Kiefer, W. S. & Murray, B. C. (1987). The formation of Mercury’sblocks (and blocks within blocks) in the Skaergaard intrusion: geologic

smooth plains. Icarus 72, 477–491.relations and the origin of rhythmic modally graded layers. Geological

Kieffer, H. H., Jakosky, B. M., Snyder, C. W. & Matthews, M. S. (eds)Society of America Bulletin 110, 1398–1447.

(1992). Mars. Tucson, AZ: University of Arizona, 1498 pp. +Jackson, E. D. (1968). The character of the lower crust and upper6 maps.mantle beneath the Hawaiian Islands. Proceedings of the 23rd International

Koeberl, C. & Reimold, W. U. (1995). Early Archaean spherule bedsGeological Congress 1, 131–150.

in the Barberton Mountain Land, South Africa—no evidence forJakobsson, S. P., Jonsson, J. & Shido, F. (1978). Petrology of the westernimpact origin. Precambrian Research 74, 1–33.Reykjanes Peninsula, Iceland. Journal of Petrology 19, 669–705.

Konopliv, A. S., Binder, A. B., Hood, L. L., Kucinskas, A. B., Sjogren,Jamieson, B. G. (1970a). Differentiation of ascending basic magma.W. L. & Williams, J. G. (1998). Improved gravity field of the MoonGeological Journal 10, 141–156.from Lunar Prospector. Science 281, 1476–1480.Jamieson, B. G. (1970b). Phase relations in some tholeiitic lavas il-

Kopal, Z. (1977). Dynamical arguments which concern melting of thelustrated by the system R2O3–XO–YO–ZO2. Mineralogical MagazineMoon. Philosophical Transactions of the Royal Society of London, Series A37, 537–554.285, 561–568.Jamieson, B. G. & Clarke, D. B. (1970). Potassium and associated

Korotev, R. L. & Haskin, L. A. (1988). Europium mass balance inelements in tholeiitic basalts. Journal of Petrology 11, 183–204.polymict samples and implications for plutonic rocks of the lunarJohnson, K. T. M., Dick, H. J. B. & Shimizu, N. (1990). Melting incrust. Geochimica et Cosmochimica Acta 52, 1795–1813.the oceanic upper mantle: an ion-microprobe study of diopsides in

Kuppers, M. & Jockers, K. (1997). A multi-emission imaging study ofabyssal peridotites. Journal of Geophysical Research 95, 2661–2678.the Io plasma torus. Icarus 129, 48–71.Johnson, T. V. (1990). The Galilean satellites. In: Beatty, J. K. &

Lancaster, M. G., Guest, J. E. & Magee, K. P. (1995). Great lava flowChaikin, A. (eds) The New Solar System. Cambridge: Cambridgefields on Venus. Icarus 118, 69–86.University Press; Cambridge, MA: Sky, pp. 171–188.

Langmuir, C. H. (1989). Geochemical consequences of in situ crys-Jolliff, B. L., Rockow, K. M., Korotev, R. L. & Haskin, L. A. (1996).tallisation. Nature 340, 199–205.Lithologic distribution and geologic history of the Apollo 17 site:

Langmuir, C. H., Bender, J. F., Bence, A. E., Hanson, G. N. & Taylor,the record in soils and small rock particles from the highland massifs.S. R. (1977). Petrogenesis of basalts from the FAMOUS area: Mid-Meteoritics and Planetary Science 31, 116–145.Atlantic Ridge. Earth and Planetary Science Letters 36, 133–156.Jolliff, B. L., Gaddis, L. R., Ryder, G., Neal, C. R., Shearer, C. K.,

Lassiter, J. C. & DePaolo, D. J. (1997). Plume/lithosphere interactionElphic, R. C., Johnson, J. R., Keller, L. P., Korotev, R. L.,in the generation of continental and oceanic flood basalts: chemicalLawrence, D. J., Lucey, P. G., Papike, J. J., Pieters, C. M.,and isotopic constraints. In: Mahoney, J. J. & Coffin, M. F. (eds)Spudis, P. D. & Taylor, L. A. (2000). New views of the Moon:Large Igneous Provinces. Geophysical Monograph, American Geophysical Unionimproved understanding through data integration. EOS Transactions,

100, 335–355.American Geophysical Union 81(349), 354–355.Laul, J. C. & Papike, J. J. (1980). The Apollo 17 drill: chemistry of sizeKamenetsky, V. S., Sobolev, A. V., Joron, J.-L. & Semet, M. P. (1995).

fractions and the nature of the fused soil component. Proceedings ofPetrology and geochemistry of Cretaceous ultramafic volcanics fromEastern Kamchatka. Journal of Petrology 36, 637–662. the Lunar and Planetary Science Conference 11, 1395–1411.

1632


Laul, J. C., Vaniman, D. T. & Papike, J. J. (1978). Chemistry, mineralogy Lopes-Gautier, R., Doute, S., Smythe, W. D., Kamp, L. W., Carlson,R. W., Davies, A. G., Leader, F. E., McEwen, A. S., Geissler, P.and petrology of seven >1 mm fragments from Mare Crisium. In:

Merrill, R. B. & Papike, J. J. (eds) Mare Crisium: the View from Luna E., Keiffer, S. W., Keszthelyi, L., Barbinis, E., Mehlman, R., Segura,M., Shirley, J. & Soderblom, L. A. (2000). A close-up look at Io24. Geochimica et Cosmochimica Acta Supplement 9, 537–568.

Laul, J. C., Smith, M. R., Wanke, H., Jagoutz, E., Dreibus, G., Palme, from Galileo’s near-infrared mapping spectrometer. Science 288,1201–1204.H., Spettel, B., Burghele, A., Lipschutz, M. E. & Verkouteren, R.

M. (1986). Chemical systematics of the Shergotty meteorite and the Love, S. G. & Keil, K. (1995). Recognising Mercurian meteorites.Meteoritics 30, 269–278.composition of its parent body (Mars). Geochimica et Cosmochimica Acta

50, 909–926. Lowe, D. R. & Byerly, G. R. (1986). Early Archaean silicate spherulesof probable impact origin. Geology 14, 83–86.Lawrence, D. J., Feldman, W. C., Barraclough, B. L., Binder, A. B.,

Elphic, R. C., Maurice, S. & Thomsen, D. R. (1998). Global Lucey, P. G., Blewett, D. T. & Hawke, B. R. (1998). Mapping theFeO and TiO2 content of the lunar surface. Journal of Geophysicalelemental maps of the Moon: the Lunar Prospector gamma-ray

spectrometer. Science 281, 1484–1489. Research 103, 3679–3699.Ma, M.-S., Schmitt, R. A., Warner, R. D., Taylor, G. J. & Keil, K.Levin, B. J. (1979). On the core of the Moon. Proceedings of the Lunar

and Planetary Science Conference 10, 2321–2323. (1978). Genesis of Apollo 15 olivine normative mare basalts: traceelement correlations. Proceedings of the Lunar and Planetary Science Con-Light, M. (1999). Full Moon. London: Jonathan Cape.

Lightfoot, P. C., Naldrett, A. J., Gorbachev, N. S., Doherty, W. & ference 9, 523–533.Maaløe, S. (1982). Geochemical aspects of permeability controlledFedorenko, V. A. (1990). Geochemistry of the Siberian trap of the

Noril’sk area, USSR, with implications for the relative contributions partial melting and fractional crystallisation. Geochimica et Cosmochimica

Acta 46, 43–57.of crust and mantle to flood basalt magmatism. Contributions to

Mineralogy and Petrology 104, 631–644. Maaløe, S. (1999). Magma accumulation in Hawaiian plume sources.American Journal of Science 299, 139–156.Lightfoot, P. C., Hawkesworth, C. J., Hergt, J., Naldrett, A. J., Gor-

bachev, N. S., Fedorenko, V. A. & Doherty, W. (1993). Re- Maaløe, S. & Jakobsson, S. P. (1980). P–T phase relations of a primaryoceanite from the Reykjanes Peninsula, Iceland. Lithos 13, 237–246.mobilization of continental lithosphere by mantle plumes: major,

trace element and Sr-, Nd- and Pb-isotope evidence for picritic Mackin, J. H. (1969). Origin of the lunar maria. Geological Society of

America Bulletin 80, 735–748.and tholeiitic lavas of the Noril’sk district, Siberian trap, Russia.Contributions to Mineralogy and Petrology 114, 171–188. Marsh, J. S., Hooper, P. R., Rehacek, J., Duncan, R. A. & Duncan,

A. R. (1997). Stratigraphy and age of Karoo basalts of Lesotho andLightfoot, P. C., Hawkesworth, C. J., Olshefsky, K., Green, T., Doherty,W. & Keays, R. R. (1997). Geochemistry of Tertiary tholeiites and implications for correlations within the Karoo igneous province. In:

Mahoney, J. J. & Coffin, M. F. (eds) Large Igneous Provinces. Geophysicalpicrites from Qeqertarssuaq (Disko Island) and Nausuaq, WestGreenland with implications for the mineral potential of comagmatic Monograph, American Geophysical Union 100, 247–272.

Marvin, U. B. & Warren, P. H. (1980). A pristine eucrite-like gabbroinclusions. Contributions to Mineralogy and Petrology 128, 139–163.Lindstrom, M. M., Nielsen, R. L. & Drake, M. J. (1977). Petrology from Descartes and its exotic kindred. Proceedings of the Lunar and

Planetary Science Conference 11, 507–521.and geochemistry of lithic fragments separated from the Apollo 15deep-drill core. Proceedings of the Lunar Science Conference 8, 2869–2888. Masuda, A., Nakamura, N., Kurasawa, H. & Tanaka, T. (1972). Precise

determination of rare-earth elements in the Apollo 14 and 15Lindstrom, M. M., Lindstrom, D. J., Korotev, R. L. & Haskin, L.(1986). Lunar meteorite Yamato-791197: a polymict anorthositic samples. Proceedings of the Lunar Science Conference 3, 1307–1313.

McBirney, A. R. (1979). Effects of assimilation. In: Yoder, H. S.norite breccia. Memoirs of the National Institute of Polar Research, Special

Issue 41, 58–75. (ed.) The Evolution of the Igneous Rocks—Fiftieth Anniversary Perspectives.Princeton, NJ: Princeton University Press.Lissauer, J. J. (1997). It’s not easy to make the Moon. Nature 389,

327–328. McBirney, A. R. & Naslund, H. R. (1990). The differentiation of theSkaergaard intrusion. Contributions to Mineralogy and Petrology 104,Lofgren, G., Donaldson, C. H., Williams, R. J., Mullins, O., Jr &

Usselman, T. M. (1974). Experimentally reproduced textures and 235–240.McBirney, A. R. & Noyes, R. M. (1979). Crystallisation and layeringmineral chemistry of Apollo 15 quartz-normative basalts. Proceedings

of the Lunar Science Conference 5, 447–469. of the Skaergaard intrusion. Journal of Petrology 20, 487–554.McEwen, A. S., Simonelli, D. P., Senske, D. R., Klaasen, K. P.,Longhi, J. (1977). Magma oceanography: 2. Chemical evolution and

crustal formation. Proceedings of the Lunar Science Conference 8, 601–621. Keszthelyi, L., Johnson, T. V., Geissler, P. E., Carr, M. H. & Belton,M. J. S. (1997). High temperature hotspots on Io as seen by theLonghi, J. (1999). Phase equilibria constraints on angrite petrogenesis.

Geochimica et Cosmochimica Acta 63, 573–585. Galileo solid state imaging (SSI) experiment. Geophysical Research Letters

24, 2443–2446.Longhi, J. & Ashwal, L. D. (1985). Two stage models for lunarand terrestrial anorthosites: petrogenesis without a magma ocean. McEwen, A. S., Keszthelyi, L., Spencer, J. R., Schubert, G., Matson,

D. L., Lopes-Gautier, R., Klaasen, K. P., Johnson, T. V., Head, J.Proceedings of the Lunar and Planetary Science Conference 15. Journal of

Geophysical Research 90, C571–584. W., Geissler, P., Fagents, S., Davies, A. G., Carr, M. H., Brenemean,H. H. & Belton, M. J. S. (1998). High temperature silicate volcanismLonghi, J. & Boudreau, A. E. (1979). Complex igneous processes and

the formation of the primitive lunar crustal rocks. Proceedings of the on Jupiter’s moon, Io. Science 281, 87–91.McEwen, A. S., Belton, M. J. S., Breneman, H. H., Fagents, S. A.,Lunar and Planetary Science Conference 10, 2085–2105.

Longhi, J., Walker, D., Grove, T. L., Stolper, E. M. & Hays, J. F. Geissler, P., Greeley, R., Head, J. W., Hoppa, G., Jaeger, W. L.,Johnson, T. V., Keszthelyi, L., Klaasen, K. P., Lopes-Gautier, R.,(1974). The petrology of Apollo 17 mare basalts. Proceedings of the

Lunar Science Conference 5, 447–469. Magee, K. P., Milazzo, M. P., Moore, J. W., Pappalardo, R. T.,Phillips, C. B., Radebaugh, J., Schubert, G., Schuster, P., Simonelli,Lopes-Gautier, R., Davies, A. G., Carlson, R., Smythe, W. D., Kemp,

L., Soderblom, L., Leader, F. L. & Mehlman, R. (1997). Hot D. P., Sullivan, R., Thomas, P. V., Turtle, E. P. & Williams, D. A.(2000). Galileo at Io: results from high-resolution imaging. Sciencespots on Io: initial results from Galileo’s near infrared mapping

spectrometer. Geophysical Research Letters 24, 2439–2442. 288, 1193–1198.

1633


McKay, D. S., Heiken, G., Basu, A., Blandford, G., Simon, S., Reedy, Mitroff, I. I. (1974). The Subjective Side of Science: a Philosophical Inquiry into

the Psychology of the Apollo Moon Scientists. Amsterdam: Elsevier, 329R., French, B. M. & Papike, J. (1991). The lunar regolith. In: Heiken,G., Vaniman, D. & French, B. M. (eds) Lunar Sourcebook: a User’s pp.

Mittlefehldt, D. W. & Lindstrom, M. M. (1990). Geochemistry andGuide to the Moon. Cambridge: Cambridge University Press, pp.285–356. genesis of the angrites. Geochimica et Cosmochimica Acta 54, 3209–3218.

Mittlefehldt, D. W., McCoy, T. J., Goodrich, C. A. & Kracher, A.McKenzie, D. (1984). The generation and compaction of partial melts.Journal of Petrology 25, 713–765. (1998). Non-chondritic meteorites from asteroidal bodies. In: Papike,

J. J. (ed.) Planetary Materials. Mineralogical Society of America, Reviews inMcKenzie, D. (1985a). 230Th–238U disequilibrium and the meltingprocess beneath ridge axes. Earth and Planetary Science Letters 72, Mineralogy 36, 4.1–4.195.

Miyamoto, M. & Takeda, H. (1994). Evidence for excavation of deep149–157.McKenzie, D. (1985b). The extraction of magma from the crust and crustal material of a Vesta-like body from Ca compositional gradients

in pyroxene. Earth and Planetary Science Letters 122, 343–349.mantle. Earth and Planetary Science Letters 74, 81–91.McKenzie, D. & Nimmo, F. (1999). The generation of martian floods Moore, J., McEwen, A., Albin, E. & Greeley, R. (1986). Topographic

evidence for shield volcanism on Io. Icarus 67, 181–183.by the melting of ground ice above dykes. Nature 397, 231–233.McKenzie, D. & O’Nions, R. K. (1991). Partial melt distributions from Morris, R. V., Haskin, L. A., Biggar, G. M. & O’Hara, M. J. (1974).

Measurement of the effects of oxygen on the oxidation states ofinversion of rare earth element concentrations. Journal of Petrology 32,1021–1091. europium in silicate glasses. Geochimica et Cosmochimica Acta 38, 1447–

1459.McSween, H. Y. & Treiman, A. H. (1998). Martian meteorites. In:Papike, J. J. (ed.) Planetary Materials. Mineralogical Society of America, Morse, S. A. (1982). Adcumulus growth of anorthosites at the base of

the lunar crust. Proceedings of the Lunar and Planetary Science ConferenceReviews in Mineralogy 36, 6.1–6.53.McSween, H. Y., Murchie, S. L., Crisp, J. A., Bridges, N. T., Anderson, 13. Journal of Geophysical Research 87, A10–18.

Morse, S. A. (1986a). Thermal structure of crystallizing magma withR. C., Bell, J. F., Britt, D. T., Bruckner, J., Dreibus, G., Economou,T., Ghosh, A., Golombek, M. P., Greenwood, J. P., Johnson, J. R., two phase convection. Geological Magazine 123, 205–214.

Morse, S. A. (1986b). Convection and adcumulus growth. Journal ofMoore, H. J., Morris, R. V., Parker, T. J., Rieder, R., Singer, R.& Wanke, H. (1999). Chemical, multispectral, and textural con- Petrology 27, 1183–1214.

Morse, S. A. (1987). Origin of earliest planetary crust: role of com-straints on the composition and origin of rocks at the Mars Pathfinderlanding site. Journal of Geophysical Research 104, 8679–8715. positional convection. Earth and Planetary Science Letters 81, 118–126.

Morse, S. A. (1990). A discussion of Hunter and Sparks (ContribMelosh, H. J. (1989). Impact Cratering: a Geologic Process. Oxford Monographs

on Geology and Geophysics, 11. Oxford: Oxford University Press, 245 Mineral Petrol 95: 451–461). Contributions to Mineralogy and Petrology

104, 240–244.pp.Melson, W. G., Jarosewich, E., Switzer, G. & Thompson, G. (1972). Morse, S. A. (1993). Behaviour of a perched crystal layer in a magma

ocean. Journal of Geophysical Research 98, 5347–5353.Basaltic nuees ardentes of Ulawun Volcano, New Britain. Smithsonian

Contributions to the Earth Sciences 9, 15–32. Morse, S. A. & Nolan, K. M. (1985). Kiglapait geochemistry VII:yttrium and the rare earth elements. Geochimica et Cosmochimica ActaMetzger, A. E., Haines, E. L., Parker, R. E. & Radolinski, R. G.

(1977). Thorium concentrations in the lunar surface I: Regional 49, 1621–1644.Muir, I. D. & Tilley, C. E. (1964). Basalts from the northern part ofvalues and crustal content. Proceedings of the Lunar Science Conference 8,

949–999. the rift zone of the Mid-Atlantic ridge. Journal of Petrology 5, 409–434.Murase, T. & McBirney, A. R. (1973). Properties of some commonMeyer, C. (1978). Ion microprobe analyses of aluminous lunar glasses:

a test of the ‘rock type’ hypothesis. Proceedings of the Lunar and Planetary igneous rocks and their melts at high temperatures. Geological Society

of America Bulletin 84, 3563–3692.Science Conference 9, 1551–1570.Meyer, P. S., Dick, H. J. B. & Thompson, G. (1989). Cumulate gabbros Naldrett, A. J. (1989). Magmatic Sulfide Deposits. Oxford Monographs on

Geology and Geophysics, 14. Oxford: Oxford University Press, pp.from the Southwest Indian Ridge, 54 degrees S–7 degrees 16′E;implications for magmatic processes at a slow spreading ridge. 1–186.

Naldrett, A. J., Fedorenko, V. A., Asif, M., Shushen, L., Kunlov, V.Contributions to Mineralogy and Petrology 103, 44–63.Milholland, C. S. & Presnall, D. C. (1998). Liquidus phase relations E., Stekhin, A. I., Lightfoot, P. C. & Gorbachev, N. S. (1996).

Controls on the composition of Ni–Cu sulfide deposits as illustratedin the CaO–MgO–Al2O3–SiO2 system at 3·0 GPa: the aluminouspyroxene thermal divide and high pressure fractionation of picritic by those at Noril’sk, Siberia. Economic Geology 91, 751–773.

Nash, D. B., Carr, M. H., Gradie, J., Hunter, D. M. & Yoder, C. F.and komatiitic magmas. Journal of Petrology 39, 3–27.Minear, J. W. (1980). The lunar magma ocean: a transient lunar (1986). Io. In: Burns, J. A. & Mathews, M. S. (eds) Satellites. Tucson,

AZ: University of Arizona, pp. 629–688.phenomenon? Proceedings of the Lunar and Planetary Science Conference 11,1941–1955. Naughton, J. J., Derby, J. V. & Lewis, V. A. (1971). Vaporization from

heated lunar samples and the investigation of lunar erosion byMinear, J. W. & Fletcher, C. R. (1978). Crystallization of a lunarmagma ocean. Proceedings of the Lunar and Planetary Science Conference 9, volatilized alkalis. Proceedings of the Lunar Science Conference 2, 449–457.

Naughton, J. J., Hammond, D. A., Margolis, S. V. & Muenow, D. W.263–283.Minster, J. F. & Allegre, C. J. (1978). Systematic use of trace elements (1972). The nature and effect of the volatile cloud produced by

volcanic and impact events on the Moon as derived from a terrestrialin igneous processes: Part III: inverse problem of batch partialmelting in volcanic suites. Contributions to Mineralogy and Petrology 68, model. Proceedings of the Lunar Science Conference 3, 2015–2024.

Navon, O. & Stolper, E. M. (1987). Geochemical consequences of melt37–52.Minster, J.-F., Minster, J. B., Treuill, M. & Allegre, C. J. (1977). percolation: the upper mantle as a chromatographic column. Journal

of Geology 95, 285–307.Systematic use of trace elements in igneous processes: Part II.Fractional crystallisation processes in volcanic suites. Contributions to Neal, C. R., Taylor, L. A., Schmitt, R. A., Hughes, S. S. & Lindstrom,

M. (1989). High alumina (HA) and very high potassium (VHK)Mineralogy and Petrology 61, 49–77.

1634


basalt clasts from Apollo 14 breccias, part 2—whole rock geo- O’Hara, M. J. (1995a). Trace element geochemical effects of integratedmelt extraction and ‘shaped’ melting regimes. Journal of Petrology 36,chemistry: further evidence for combined assimilation and fractional

crystallization within the lunar crust. Proceedings of the Lunar and 1111–1132.O’Hara, M. J. (1995b). Imperfect melt separation, finite increment sizePlanetary Science Conference 19, 147–161.

Neal, C. R., Mahoney, J. J., Kroenke, L. W., Duncan, R. A. & and source region flow during fractional melting and the generationof reversed or subdued discrimination of incompatible elements.Petterson, M. G. (1997). The Ontong–Java plateau. In: Mahoney,

J. J. & Coffin, M. F. (eds) Large Igneous Provinces. Geophysical Monograph, Chemical Geology 121, 27–50.O’Hara, M. J. (1998). Volcanic plumbing and the space problem—American Geophysical Union 100, 183–216.

Neukum, G. (1977). Lunar cratering. Philosophical Transactions of the Royal thermal and geochemical consequences of large-scale assimilationin ocean island development. Journal of Petrology 39, 1077–1089.Society of London, Series A 285, 267–272.

Niu, Y. (1997). Mantle melting and melt extraction processes beneath O’Hara, M. J. (2000). Flood basalts and lunar petrogenesis. Journal of

Petrology 41, 1121–1125.ocean ridges: evidence from abyssal peridotites. Journal of Petrology

38, 1047–1074. O’Hara, M. J. & Biggar, G. M. (1969). Diopside–spinel equilibria,anorthite and forsterite reaction relationships in silica-poor liquidsNiu, Y. (1999). Comments on some misconceptions in igneous and

experimental petrology and methodology: a reply. Journal of Petrology in the system CaO–MgO–Al2O3–SiO2 at atmospheric pressure andtheir bearing on the genesis of melilitites and nephelinites. American40, 1195–1203.

Norman, M. D. (1994). Sudbury igneous complex: impact melt or Journal of Science (Schairer Volume) 267-A, 364–390.O’Hara, M. J. & Biggar, G. M. (1972). A point of phase equilibriumendogenous magma? Implications for lunar crustal evolution. In:

Dressler, B. O., Grieve, R. A. F. & Sharpton, V. L. (eds) Large interpretation in connection with lavas from the Apollo 12 site. Earth

and Planetary Science Letters 16, 388–390.Meteorite Impacts and Planetary Evolution. Geological Society of America,

Special Papers 293, 331–341. O’Hara, M. J. & Biggar, G. M. (1975). Evolution of mare basalts. In:Origins of Mare Basalts and their Implications for Lunar Evolution. LunarNorman, M., Coish, R. A. & Taylor, L. A. (1978). Glasses in the Luna

24 core and petrogenesis of ferrobasalts. In: Merrill, R. B. & Papike, Science Institute Contribution 234, 115–119.O’Hara, M. J. & Biggar, G. M. (1977). Comment on ‘Mare BasaltJ. J. (eds) Mare Crisium: the View from Luna 24. Geochimica et Cosmochimica

Acta Supplement 9, 281–289. Petrogenesis—a Review of Experimental Studies’ by S. E. Kessonand D. Lindsley. Reviews of Geophysics and Space Physics 15, 253–255.Norman, R. E. & Strong, D. (1975). The geology and geochemistry of

ophiolitic rocks exposed at Ming’s Bight, Newfoundland. Canadian O’Hara, M. J. & Fry, N. (1996a). The highly compatible trace elementparadox—fractional crystallization revisited. Journal of Petrology 37,Journal of Earth Science 12, 777–797.

Obata, M. (1980). The Ronda peridotite: garnet-, spinel- and pla- 859–890.O’Hara, M. J. & Fry, N. (1996b). Geochemical effects of small packetgioclase-lherzolite facies and the P–T trajectories of a high tem-

perature mantle intrusion. Journal of Petrology 21, 533–572. crystallization in large magma chambers—further resolution of thehighly compatible element paradox. Journal of Petrology 37, 891–925.O’Hara, M. J. (1965). Primary magmas and the origin of basalts. Scottish

Journal of Geology 1, 19–40. O’Hara, M. J. & Humphries, D. J. (1975). Analysis of compositionvariations in high titanium basalts and related materials. Lunar ScienceO’Hara, M. J. (1968a). The bearing of phase equilibria studies on the

origin and evolution of igneous rocks. Earth-Science Reviews 4, 69–133. VI. Houston, TX: Lunar Science Institute, pp. 616–618.O’Hara, M. J. & Humphries, D. J. (1977a). Gravitational separationO’Hara, M. J. (1968b). Are ocean floor basalts primary magmas? Nature

220, 683–686. of quenching crystals: a cause of chemical differentiation in lunarbasalts. Philosophical Transactions of the Royal Society of London, Series AO’Hara, M. J. (1969a). The origin of eclogite and ariegite nodules in

basalt. Geological Magazine 106, 322–330. 285, 177–192.O’Hara, M. J. & Humphries, D. J. (1977b). Problems of iron gain andO’Hara, M. J. (1969b). The atmospheric pressure equilibrium and

fractional crystallization of basalt-like mixtures in the MgSiO3-rich iron loss during experimentation on natural rocks: the experimentalcrystallization of five lunar basalts at low pressure. Philosophicalpart of the plane CaSiO3–MgSiO3–Al2O3 and the nature of thermal

divides. In: Progress in Experimental Petrology First Report NERC Supported Transactions of the Royal Society of London, Series A 286, 313–380.O’Hara, M. J. & Mathews, R. E. (1981). Geochemical evolution in anResearch Units in British Universities 1965–1968, pp. 129–152.

O’Hara, M. J. (1972). Volatilization losses from lunar lava 14310. advancing, periodically replenished, periodically tapped, con-tinuously fractionated magma chamber. Journal of the Geological Society,Nature 240, 95–96.

O’Hara, M. J. (1977). Geochemical evolution during fractional crys- London 138, 237–277.O’Hara, M. J. & Schairer, J. F. (1963). The join diopside–pyrope attallization of a periodically refilled magma chamber. Nature 266,

503–507. atmospheric pressure. Carnegie Institution of Washington, Yearbook 62,107–115.O’Hara, M. J. (1980). Non-linear nature of the unavoidable long-lived

isotopic, trace and major element contamination of a developing O’Hara, M. J. & Yoder, H. S., Jr (1963). Partial melting of the mantle.Carnegie Institution of Washington, Yearbook 62, 66–71.magma chamber. Philosophical Transactions of the Royal Society of London,

Series A 297, 215–227. O’Hara, M. J. & Yoder, H. S., Jr (1967). Formation and fractionationof basic magmas at high pressures. Scottish Journal of Geology 3, 67–117.O’Hara, M. J. (1985). Importance of the ‘shape’ of the melting regime

during partial melting of the mantle. Nature 314, 58–61. O’Hara, M. J., Biggar, G. M. & Richardson, S. W. (1970a). Ex-perimental petrology of lunar material: the nature of mascons, seasO’Hara, M. J. (1993). Trace element geochemical effects of imperfect

crystal–liquid separation. In: Prichard, H. M., Alabaster, T., Harris, and the lunar interior. Science 167, 605–607.O’Hara, M. J., Biggar, G. M., Richardson, S. W., Ford, C. E. &N. W. B. & Neary, C. R. (eds) Magmatic Processes and Plate Tectonics.

Geological Society, London, Special Publication 76, 39–59. Jamieson, B. G. (1970b). The nature of seas, mascons and the lunarinterior in the light of experimental studies. Proceedings of Apollo 11O’Hara, M. J. (1994). Superimposing magma chamber and melting

regime processes: how easy is it to (mis)interpret the liquid products? Lunar Science Conference. Geochimica et Cosmochimica Acta Supplement 1(1),695–710.Mineralogical Magazine 58A, 661–662.

1635


O’Hara, M. J., Richardson, S. W. & Wilson, G. (1971). Garnet- Plescia, J. B. (1994). Geology of the small Tharsis volcanoes: JovusTholus, Ulysses Patera, Biblis Patera, Mars. Icarus 111, 246–249.peridotite stability and occurrence in crust and mantle. Contributions

Popper, K. (1972). Conjectures and Refutations. London: Routledge andto Mineralogy and Petrology 32, 48–68.Keegan Paul.O’Hara, M. J., Biggar, G. M., Hill, P. G., Jeffries, B. & Humphries,

Prinz, M., Bunch, T. E. & Keil, K. (1971). Composition and origin ofD. J. (1974). Plagioclase saturation in lunar high-titanium basalt.lithic fragments and glasses in Apollo 11 lunar samples. ContributionsEarth and Planetary Science Letters 21, 253–268.to Mineralogy and Petrology 32, 211–230.O’Hara, M. J., Humphries, D. J. & Waterston, S. (1975a). Petrogenesis

Puchelt, H. & Emmermann, R. (1977). REE characteristics of oceanof mare basalts: implications for chemical, mineralogical and thermalfloor basalts from the MAR 37°N (Leg 37 DSDP). Contributions tomodels for the Moon. Proceedings of Sixth Lunar Science Conference 6(1),Mineralogy and Petrology 62, 43–52.1043–1055.

Quick, J. E., Sinigoi, S. & Mayer, A. (1994). Emplacement dynamicsO’Hara, M. J., Saunders, M. J. & Mercy, E. L. P. (1975b). Garnet-of a large mafic intrusion in the lower crust, Ivrea–Verbano zone,peridotite, primary ultrabasic magma and eclogite: interpretation ofnorthern Italy. Journal of Geophysical Research 99, 21559–21573.upper mantle processes in kimberlite. Physics and Chemistry of the Earth

Rammensee, W. & Wanke, H. (1977). On the partition coefficient of9, 571–604.tungsten between metal and silicate and its bearing on the origin ofO’Hara, M. J., Fry, N. & Prichard, H. (2001). Minor phases as tracethe Moon. Proceedings of the Lunar Science Conference 8, 399–409.element carriers in non-modal crystal–liquid processes: bearing on

Ransford, G. A. (1979). A comparison of two accretional heatingbehaviour of REE, U, Th and the PGE in igneous melts andmodels. Proceedings of the Lunar and Planetary Science Conference 10,residues. Journal of Petrology (in revision).1867–1879.

O’Keefe, J. D. & Ahrens, T. J. (1994). Impact-induced melting ofRava, B. & Hapke, B. (1987). An analysis of the Mariner 10 color ratio

planetary surfaces. In: Dressler, B. O., Grieve, R. A. F. & Sharpton,map of Mercury. Icarus 71, 397–429.

V. L. (eds) Large Meteorite Impacts and Planetary Evolution. Geological Society Reid, A. M. & Jakes, P. (1974). Luna 16 revisited: the case for aluminousof America, Special Papers 293, 103–109. basalts (abstract). In: Lunar Science V. Houston, TX: Lunar Science

Pai, S. I., Hsu, Y. & O’Keefe, J. A. (1978). Similar explosive eruptions Institute, pp. 627–629.of lunar and terrestrial volcanoes. Proceedings of the Lunar and Planetary Reid, A. M., Warner, J. F., Ridley, W. I., Johnston, D. A., Harmon,Science Conference 9, 1485–1508. R. S., Jakes, P. & Brown, R. W. (1972). The major element

Palme, H. & Wanke, H. (1977). Lunar differentiation processes as composition of lunar rocks as inferred from glass compositions incharacterized by trace element abundances. Philosophical Transactions the lunar soil. Proceedings of the Lunar Science Conference 3, 363–378.of the Royal Society of London, Series A 285, 199–205. Rhodes, J. M. (1977). Some compositional aspects of lunar regolith

Papike, J. J., Ryder, G. & Shearer, C. K. (1998). Lunar materials. In: evolution. Philosophical Transactions of the Royal Society of London, SeriesPapike, J. J. (ed.) Planetary Materials. Mineralogical Society of America, A 285, 293–301.Reviews in Mineralogy 36, 5.1–5.234. Rhodes, J. M. (1983). Homogeneity of lava flows: chemical data for

Pearce, J. A. (1987). An expert system for the tectonic characterization historic Mauna Loan eruptions. Proceedings of the Lunar and Planetary

of ancient volcanic rocks. Journal of Volcanology and Geothermal Research Science Conference 13. Journal of Geophysical Research 88, A869–A879.Rhodes, J. M. (1996). Geochemical stratigraphy of lava flows sampled32, 51–65.

by the Hawaii Scientific Drilling Project. Journal of Geophysical ResearchPearce, J. A. & Cann, J. R. (1973). Tectonic setting of basic volcanic101, 11729–11746.rocks determined using trace element analysis. Earth and Planetary

Rhodes, J. M. & Blanchard, D. P. (1980). Chemistry of Apollo 11 low-Science Letters 24, 419–426.K mare basalts. Proceedings of the Lunar and Planetary Science ConferencePeng, Z., Mahoney, J., Hooper, P., Harris, C. & Beane, J. (1994). A11, 49–66.role for lower continental crust in flood basalt genesis? Isotopic and

Rhodes, J. M. & Blanchard, D. P. (1981). Apollo 11 breccias and soils:incompatible element study of the lower six formations of thealuminous mare basalts or multi-component mixtures? Proceedings ofWestern Deccan Traps. Geochimica et Cosmochimica Acta 58, 267–288.the Lunar and Planetary Science Conference 12, 607–620.Peng, Z. X., Mahoney, J. J., Hooper, P. R., Macdougall, J. D. &

Rhodes, J. M., Blanchard, D. P., Dungan, M. A., Brannon, J. C. &Krishnamurthy, P. (1998). Basalts of the northeastern Deccan Traps,Rodgers, K. V. (1977). Chemistry of Apollo 12 basalts: magma typesIndia: isotopic and elemental geochemistry and relation to south-and fractionation processes. Proceedings of the Lunar Science Conference 8,western Deccan stratigraphy. Journal of Geophysical Research 103,1305–1338.29843–29865.

Rhodes, J. M., Dungan, M. A., Blanchard, D. P. & Long, P. E. (1979).Peterson, J. E. (1978). Volcanism in the Noachis–Hellas region of Mars,

Magma mixing at mid ocean ridges. evidence from basalts drilled2. Proceedings of the Lunar and Planetary Science Conference 9, 3411–3432.

near 22° N on the Mid Atlantic Ridge. Tectonophysics 55, 35–61.Pieters, C. M. (1978). Mare basalt types on the front side of the Moon. Rhodes, R. C. (1975). New evidence for impact origin of the Bushveld

Proceedings of the Lunar and Planetary Science Conference 9, 2825–2849. Complex, South Africa. Geology 3, 549–554.Pieters, C. M. (1986). Composition of the Lunar Highlands crust from Richards, M. A., Duncan, R. A. & Courtillot, V. (1989). Flood basalts

near-infrared spectroscopy. Reviews of Geophysics 24, 557–578. and hot spot tracks: plume heads and tails. Science 246, 103–107.Pieters, C. M., Fischer, E. M., Rode, O. & Basu, A. (1993a). Optical Rieder, R., Economou, T., Wanke, H., Turkevich, A., Crisp, J.,

effects of space weathering. Journal of Geophysical Research 98, 20817– Bruckner, J., Dreibus, G. & McSween, H. Y., Jr (1997). The chemical20824. composition of the Martian soil and rocks returned by the mobile

Pieters, C. M., Head, J. W., Sunshine, J. M., Fischer, E. M., Murchie, Alpha Proton X-Ray Spectrometer: preliminary results from the X-S. L., Belton, M., McEwen, A., Gaddis, L., Greeley, R., Neukum, ray mode. Science 278, 1771–1774.G., Jaumann, R. & Hoffmann, H. (1993b). Crustal diversity of the Righter, K. & Drake, M. J. (1996). Core formation in Earth’s Moon,Moon: compositional analyses of Galileo solid state imaging data. Mars and Vesta. Icarus 124, 513–529.Journal of Geophysical Research 98, 17127–17148. Righter, K. & Drake, M. J. (1997). A magma ocean on Vesta: core

Plescia, J. B. (1981). The Tempe igneous province of Mars and formation and petrogenesis of eucrites and diogenites. Meteoritics and

Planetary Science 32, 929–944.comparisons with the Snake River Plains of Idaho. Icarus 45, 586–601.

1636


Ringwood, A. E. (1956). Melting relationships of Ni–Mg olivines and Schubert, G. & Sandwell, D. T. (1995). Global survey of possiblesubduction sites on Venus. Icarus 117, 173–196.some geochemical implications. Geochimica et Cosmochimica Acta 10,

297–303. Schultz, P. H. (1976). Moon Morphology. Austin, TX: University of TexasPress, 626 pp.Ringwood, A. E. (1975). Composition and Petrology of the Earth’s Mantle.

New York: McGraw–Hill. Scoates, J. S. (2000). The plagioclase–magma density paradox re-examined and the crystallization of Proterozoic anorthosites. JournalRobinson, M. S., Hawke, B. R., Lucey, P. G. & Smith, G. A. (1992).

Mariner 10 multispectral images of the eastern limb and farside of of Petrology 41, 627–649.Scott, D. H. & Tanaka, K. L. (1981). Mars: a large highland volcanicthe Moon. Journal of Geophysical Research 97, 18265–18274.

Roedder, E. & Weiblen, P. W. (1978). Melt inclusions in Luna 24 soil province revealed by Viking images. Proceedings of the Lunar and

Planetary Science Conference 12B, 1449–1458.fragments. In: Merrill, R. B. & Papike, J. J. (eds) Mare Crisium: the

View from Luna 24. Geochimica et Cosmochimica Acta Supplement 9, 495–522. Sharma, M. (1997). Siberian traps. In: Mahoney, J. J. & Coffin, M. F.(eds) Large Igneous Provinces. Geophysical Monograph, American GeophysicalRoeder, P. L. & Emslie, R. F. (1970). Olivine–liquid equilibrium.

Contributions to Mineralogy and Petrology 29, 275–289. Union 100, 273–295.Shaw, D. M. (1970). Trace element fractionation during anatexis.Rubin, A. E. & Mittlefehldt, D. W. (1992). Classification of mafic clasts

from mesosiderites: implications for endogenous igneous processes. Geochimica et Cosmochimica Acta 34, 237–243.Shaw, H. R. (1980). The fracture mechanisms of magma transportGeochimica et Cosmochimica Acta 56, 827–840.

Rubin, A. E. & Mittlefehldt, D. W. (1993). Evolutionary history of the from the mantle to the surface. In: Hargraves, R. B. (ed.) Physics of

Magmatic Processes. Princeton, NJ: Princeton University Press, pp.mesosiderite asteroid: a chronologic and petrologic synthesis. Icarus

101, 201–212. 201–264.Shaw, H. R. & Swanson, D. S. (1970). Eruption and flow rates of floodRuzicka, A., Snyder, G. A. & Taylor, L. A. (1997). Vesta as the

howardite, eucrite and diogenite parent body: implications for the basalts. In: Gilmour, E. H. & Stradling, D. (eds) Second Columbia River

Basalt Symposium Proceedings. Cheney, WA: East Washington Statesize of a core and for large scale differentiation. Meteoritics and Planetary

Science 32, 825–840. College Press, pp. 271–299.Shearer, C. K. & Papike, J. J. (1999). Magmatic evolution of the Moon.Ryan, M. P., Koyanagi, R. Y. & Fiske, R. S. (1981). Modeling

the three-dimensional structure of macroscopic magma transport American Mineralogist 84, 1469–1494.Shearer, C. K., Papike, J. J. & Rietmeijer, F. J. M. (1998). The planetarysystems: application to Kilauea volcano, Hawaii. Journal of Geophysical

Research 86, 7111–7129. sample suite and environments of origin. In: Papike, J. J. (ed.)Planetary Materials. Mineralogical Society of America, Reviews in MineralogyRyder, G. (1994). Coincidence in time of the Imbrium basin impact

and Apollo 15 KREEP volcanic flows: the case for impact induced 36, 1.1–1.28.Shirey, S. B., Klewin, K. W., Berg, J. H. & Carlson, R. W. (1994).melting. In: Dressler, B. O., Grieve, R. A. F. & Sharpton, V. L.

(eds) Large Meteorite Impacts and Planetary Evolution. Geological Society of Temporal changes in the source of flood basalts: isotopic and traceelement evidence from the 1100 Ma old Keweenawan MamainseAmerica, Special Papers 293, 11–18.

Ryder, G. & Marvin, U. (1978). On the origin of Luna 24 basalts and Point formation, Ontario, Canada. Geochimica et Cosmochimica Acta

58, 4475–4490.soils. In: Merrill, R. B. & Papike, J. J. (eds) Mare Crisium: the View

from Luna 24. Geochimica et Cosmochimica Acta Supplement 9, 339–355. Shirley, D. N. (1983). A partially molten magma ocean. Proceedings of

the Lunar and Planetary Science Conference 13. Journal of Geophysical ResearchSagan, C. (1979). Sulfur flows on Io. Nature 280, 750–753.Sato, M. (1976). Oxygen fugacity and other thermochemical parameters 88, A519–A527.

Showman, A. P. & Malhotra, R. (1999). The Galilean satellites. Scienceof Apollo 17 high-Ti basalts and their implication on the reductionmechanism. Proceedings of the Lunar Science Conference 7, 1323–1344. 286, 77–84.

Sinigoi, S., Quick, J. E., Clemens-Knott, D., Mayer, A., Demarchi,Sato, M. (1978). Oxygen fugacity of basaltic magmas and the role ofgas forming elements. Geophysical Research Letters 5, 447–449. G., Mazzucchelli, M., Negrini, L. & Rivalenti, G. (1994). Chemical

evolution of a large mafic intrusion in the lower crust, Ivrea–VerbanoSato, M. (1979). The driving mechanism of lunar pyroclastic eruptionsinferred from the oxygen fugacity behavior of Apollo 17 orange zone, northern Italy. Journal of Geophysical Research 99, 21574–21590.

Smith, D. E., Zuber, M. T., Solomon, S. C., Phillips, R. J., Head, J.glass. Proceedings of the Lunar and Planetary Science Conference 10, 311–325.Sato, M., Hickling, N. L. & McLane, J. E. (1973). Oxygen fugacity W., Garvin, J. B., Banerdt, W. B., Muhleman, D. O., Pettingill, G.

H., Neumann, F. G., Lemoine, F. G., Abshire, J. B., Aharonson,values of Apollo 12, 14 and 15 lunar samples and reduced state oflunar magmas. Proceedings of the Lunar Science Conference 4, 1061–1079. A., Brown, C. D., Hauck, S. A., Ivanov, A. B., McGovern, P. J.,

Zwally, H. J. & Duxbury, T. C. (1999). The global topography ofScarrow, J. H. & Cox, K. G. (1995). Basalts generated by decompressiveadiabatic melting of a mantle plume: a case study from the Isle of Mars and implications for surface evolution. Science 284, 1495–1503.

Smith, R. L. (1979). Ash-flow magmatism. Geological Society of America,Skye, NW Scotland. Journal of Petrology 36, 3–22.Schaber, G. G. (1973). Lava flows in Mare Imbrium: geologic evaluation Special Papers 180, 5–27.

Smith, J. V. (1981). Progressive differentiation of a growing Moon.from Apollo photography. Proceedings of the Lunar Science Conference 4,73–92. Proceedings of the Lunar and Planetary Science Conference 12B, 979–990.

Snyder, G. A., Hall, C. M., Lee, D. C., Taylor, L. A. & Halliday, A.Schaber, G. G., Boyce, J. M. & Moore, H. J. (1976). The scarcity ofmappable flow lobes on the lunar maria: unique morphology of the N. (1996). Earliest high-Ti volcanism on the Moon: Ar40–Ar39,

Sm–Nd and Rb–Sr isotopic studies of group D basalts from theImbrium flows. Proceedings of the Lunar Science Conference 7, 2783–2800.Schenk, P. M., McEwen, A. S., Davies, A. G., Davenport, T., Joves, Apollo 11 landing site. Meteoritics and Planetary Science 31, 328–334.

Sobolev, A. V. & Shimizu, N. (1993). Ultra-depleted primary meltK. & Fessler, B. (1997). Geology and topography of Ra Patera, Io,in the Voyager era: prelude to eruption. Geophysical Research Letters included in an olivine from the mid-Atlantic ridge. Nature 363,

151–154.24, 2467–2470.Schreiber, H. D. (1977). Redox states of Ti, Zr, Hf, Cr, and Eu in Sobolev, A. V., Dmitriev, L. V., Barsukov, V. L., Nevsorov, V. N. &

Slutsky, A. B. (1980). The formation conditions of the high-mag-basaltic magmas: an experimental study. Proceedings of the Lunar Science

Conference 8, 1785–1807. nesium olivines from the monomineralic fraction of the Luna 24

1637


regolith. Proceedings of the Lunar and Planetary Science Conference 11, Storey, W. C. (1973). Volatilization studies on a terrestrial basalt andtheir applicability to volatilization from the lunar surface. Nature,105–116.

Solomon, S. C. & Longhi, J. (1977). Magma oceanography: 1. Thermal Physical Science 241, 154–157.Storey, W. C., Humphries, D. J. & O’Hara, M. J. (1974). Experimentalevolution. Proceedings of the Lunar Science Conference 8, 583–599.

Sparks, R. S. J., Huppert, H. E., Kerr, R. C., McKenzie, D. P. & petrology of sample 77135. Earth and Planetary Science Letters 23,435–438.Tait, S. R. (1985). Postcumulus processes in layered intrusions.

Geological Magazine 122, 555–568. Takahashi, K. & Masuda, A. (1978). Two lunar meteorites, Yamato-791197 and -82192: REE abundances and geochronological dating.Spencer, J. R. & Schneider, N. M. (1996). Io on the eve of the Galileo

mission. Annual Review of Earth and Planetary Sciences 24, 125–190. Memoirs of the National Institute of Polar Research, Special Issue 46, 71–88.Takeda, H., Mori, H., Ishii, T. & Miyamoto, M. (1981). Thermal andSpencer, J. R., Stansberry, J. A., Dumas, C., Vakil, D., Pregler, R.,

Hicks, M. & Hege, K. (1997a). A history of high-temperature Io impact histories of pyroxenes in lunar eucrite-like gabbros andeucrites. Proceedings of the Lunar and Planetary Science Conference 12B,volcanism: February 1995 to May 1997. Geophysical Research Letters

24, 2451–2454. 1297–1313.Taylor, G. J., Warner, R. D., Wentworth, S., Keil, K. & Sayeed, U.Spencer, J. R., Sartoretti, P., Ballester, G. E., McEwen, A., Clarke, J.

T. & McGrath, M. A. (1997b). The Pele plume (Io): observations (1978). Luna 24 lithologies: petrochemical relationships among lithicfragments, mineral fragments, and glasses. In: Merrill, R. B. &with the Hubble Space Telescope. Geophysical Research Letters 24,

2471–2574. Papike, J. J. (eds) Mare Crisium: the View from Luna 24. Geochimica et

Cosmochimica Acta Supplement 9, 303–320.Spencer, J. R., Jessup, K. L., McGrath, M. A., Ballester, G. E. & Yelle,R. (2000). Discovery of gaseous S2 in Io’s Pele plume. Science 288, Taylor, G. J., Warren, P., Ryder, G., Delano, J., Pieters, C. & Lofgren,

G. (1991). Lunar rocks. In: Heiken, G., Vaniman, D. & French, B.1208–1210.Spera, F. (1980). Aspects of magma transport. In: Hargraves, R. B. M. (eds) Lunar Sourcebook: a User’s Guide to the Moon. Cambridge:

Cambridge University Press, pp. 183–284.(ed.) Physics of Magmatic Processes. Princeton, NJ: Princeton UniversityPress, pp. 265–323. Taylor, S. R. (1975). Lunar Science: a Post-Apollo View. New York:

Pergamon, 372 pp.Sprague, A. L., Kozlowski, R. W. H., Witteborn, F. C., Cruikshank,D. P. & Wooden, D. H. (1994). Mercury: evidence for anorthosite Taylor, S. R. & Bence, A. E. (1975). Evolution of the lunar highland

crust. Proceedings of the Lunar Science Conference 6, 1121–1141.and basalt from mid-infrared (7·3–13·5 �) spectroscopy. Icas 109,156–167. Taylor, S. R. & Jakes, P. (1974). The geochemical evolution of the

Moon. Proceedings of the Lunar Science Conference 5, 1287–1305.Spudis, P. D. (1994). The large impact process inferred from thegeology of lunar multiring basins. In: Dressler, B. O., Grieve, R. A. Taylor, S. R., Gorton, M. P., Muir, P., Nance, W., Rudowski, R. &

Ware, N. (1973a). Composition of the Descartes region, lunarF. & Sharpton, V. L. (eds) Large Meteorite Impacts and Planetary Evolution.

Geological Society of America, Special Papers 293, 1–10. highlands. Geochimica et Cosmochimica Acta 37, 2665–2683.Taylor, S. R., Gorton, M. P., Muir, P., Nance, W., Rudowski, R. &Spudis, P. & Pieters, C. (1991). Global and regional data about the

Moon. In: Heiken, G., Vaniman, D. & French, B. M. (eds) Lunar Ware, N. (1973b). Lunar highlands composition: Apennine Front.Proceedings of the Lunar Science Conference 4, 1445–1459.Sourcebook: a User’s Guide to the Moon. Cambridge: Cambridge University

Press, pp. 595–632 and 11 plates. Tegner, C., Robins, B., Reginiussen, H. & Grundvig, S. (1999).Assimilation of crustal xenoliths in a basaltic magma chamber: SrSpudis, P. D., Swann, G. A. & Greeley, R. (1988). The formation of

Hadley Rille and implications for the geology of the Apollo 15 and Nd isotopic constraints from the Hasvik layered intrusion,Norway. Journal of Petrology 40, 363–380.region. Proceedings of the Lunar Science Conference 17, 243–254.

Staid, M. I., Pieters, C. M. & Head, J. W. (1996). Mare Tranquillitatis: Thompson, R. N. (1972). Melting behaviour of two Snake River lavasat pressures up to 35 kb. Carnegie Institution of Washington, Yearbook 71,basalt emplacement history and relation to lunar samples. Journal of

Geophysical Research—Planets 101, 23213–23228. 406–410.Thompson, R. N. (1987). Phase equilibria constraints on the genesisStansberry, J. A., Spencer, J. R., Howell, R. R., Dumas, C. & Vakil,

D. (1997). Violent silicate volcanism on Io in 1996. Geophysical Research and magmatic evolution of oceanic basalts. Earth-Science Reviews 24,161–210.Letters 24, 2455–2458.

Stanton, R. L. & Bell, J. D. (1969). Volcanic and associated rocks of Thompson, R. N., Morrison, A. M., Hendry, G. L. & Parry, S. J.(1984). An assessment of the relative roles of crust and mantle inthe New Georgia group, British Solomon Islands. Overseas Geology

and Mineral Resources 10, 113–145. magma genesis: an elemental approach. Philosophical Transactions of

the Royal Society, Series A 310, 549–590.Stoffler, D., Deutsch, A., Avermann, M., Bischoff, L., Brockmeyer, P.,Buhl, D., Lakomy, R. & Muller-Mohr, V. (1994). The formation of Tompkins, S. & Pieters, C. (1999). Mineralogy of the lunar crust: results

from Clementine. Meteoritics and Planetary Science 34, 25–41.the Sudbury structure, Canada: towards a unified impact model. In:Dressler, B. O., Grieve, R. A. F. & Sharpton, V. L. (eds) Large Tompkins, S., Pieters, C. M., Mustard, J. F., Pinet, P. & Chevrel, S.

D. (1994). Distribution of materials excavated by the lunar craterMeteorite Impacts and Planetary Evolution. Geological Society of America,

Special Papers 293, 303–318. Bulliardus and implications for the geologic history of the Nubiumregion. Icarus 110, 261–274.Stolper, E. M. (1977). Experimental petrology of eucritic meteorites.

Geochimica et Cosmochimica Acta 41, 587–611. Tonks, W. B. & Melosh, J. (1993). Magma ocean formation due togiant impacts. Journal of Geophysical Research 98, 5319–5455.Stolper, E. M. (1980). A phase diagram for mid-ocean ridge basalts:

preliminary results and implications for petrogenesis. Contributions to Turcotte, D. L. & Ahern, J. L. (1978). Magma production and migrationwithin the Moon. Proceedings of the Lunar and Planetary Science ConferenceMineralogy and Petrology 74, 13–20.

Stolper, E. & Walker, D. (1980). Melt density and the average com- 9, 307–318.Turner, F. J. & Verhoogen, J. (1951). Igneous and Metamorphic Petrology.position of basalt. Contributions to Mineralogy and Petrology 74, 7–12.

Stolper, E. M., McSween, H. Y. & Hays, J. F. (1979). A petrogenetic New York: McGraw–Hill.Turner, F. J. & Verhoogen, J. (1960). Igneous and Metamorphic Petrology,model of the relationships among achondritic meteorites. Geochimica

et Cosmochimica Acta 43, 589–602. 2nd edn. New York: McGraw–Hill.

1638


Turner, S., Hawkesworth, C., Gallagher, K., Stewart, K., Peate, D. & Wallace, P. J. & Carmichael, I. S. E. (1992). Sulfur in basaltic magmas.Geochimica et Cosmochimica Acta 56, 1863–1874.Mantovani, M. (1996). Mantle plumes, flood basalts and thermal

models for melt generation beneath continents: assessment of a Walter, M. J. (1999). Comments on ‘Mantle melting and melt extractionprocesses beneath ocean ridges: evidence from abyssal peridotites’conductive heating model and application to the Parana. Journal of

Geophysical Research 101, 11503–11517. by Yaoling Niu. Journal of Petrology 40, 1187–1193.Warren, P. & Kallemeyn, G. W. (1993). The ferroan anorthositic suite,Tuttle, O. F. & Bowen, N. L. (1958). The origin of granite in the light of

experimental studies in the system NaAlSi3O8–KalSi3O8–SiO2–H2O. the extent of primordial lunar melting, and the bulk composition ofthe Moon. Journal of Geophysical Research 98, 5445–5455.Geological Society of America, Memoirs 74, 153 pp.

Vaniman, D. T. & Papike, J. J. (1977a). The Apollo 17 drill core: Warren, P. H. (1997). Magnesium oxide–iron oxide mass balanceconstraints and a more detailed model for the relationship betweencharacterization of the mineral and lithic component (sections 70007,

70008, 70009). Proceedings of the Lunar Science Conference 8, 3128–3159. eucrites and diogenites. Meteoritics and Planetary Science 32, 945–963.Warren, P. H. & Wasson, J. T. (1979). Effects of pressure on theVaniman, D. T. & Papike, J. J. (1977b). The Apollo 17 drill core:

modal petrology and glass chemistry (sections 70007, 70008, 70009). crystallization of a ‘chondritic’ magma ocean and implications forbulk composition of the Moon. Proceedings of the Lunar and PlanetaryProceedings of the Lunar Science Conference 8, 3161–3193.

Vaniman, D. T., Labotka, T. C., Papike, J. J., Simon, S. B. & Laul, Science Conference 10, 2051–2083.Watts, A. B., ten Brink, U. S., Buhl, P. & Brocher, T. M. (1985).J. C. (1979). The Apollo 17 drill core: petrologic systematics and

the identification of a possible Tycho component. Proceedings of the A multichannel seismic study of lithospheric flexure across theHawaiian–Emperor seamount chain. Nature 315, 105–111.Lunar and Planetary Science Conference 10, 1185–1227.

Vaniman, D. T., Dietrich, J., Taylor, G. J. & Heiken, G. (1991). Weitz, C. M. & Basilevsky, A. T. (1993). Magellan observations of theVenera and Vega landing site regions. Journal of Geophysical ResearchExploration, samples and recent concepts of the Moon. In: Heiken,

G., Vaniman, D. & French, B. M. (eds) Lunar Sourcebook: a User’s 98, 17069–17097.West, W. D. (1958). The petrology and petrogenesis of forty-eight flowsGuide to the Moon. Cambridge: Cambridge University Press, pp. 5–26.

Viljoen, M. P. & Viljoen, R. P. (1969a). The geology and geochemistry of Deccan Trap penetrated by borings in western India. Transactions

of the National Institute of Sciences of India 4, 1–56.of the lower ultramafic unit of the Onverwacht group, and a proposednew class of igneous rocks. Geological Society of South Africa, Special Wetherill, G. W. (1985). Occurrence of giant impacts during the growth

of the terrestrial planets. Science 228, 877–879.Publication 2, 55–85.Viljoen, M. P. & Viljoen, R. P. (1969b). Evidence of the existence of White, R. S. & McKenzie, D. (1989). Magmatism at rift zones; the

generation of volcanic continental margins and flood basalts. Journala mobile peridotite magma. Geological Society of South Africa, Special

Publication 2, 87–112. of Geophysical Research 94, 7685–7729.White, R. S. & McKenzie, D. (1995). Mantle plumes and flood basalts.Vinogradov, A. P. (1971). Preliminary data on lunar ground brought

to Earth by automatic probe ‘Luna 16’. Proceedings of the Lunar Science Journal of Geophysical Research 100, 17543–17585.Wilkinson, J. F. G. (1982). The genesis of mid-ocean ridge basalt. Earth-Conference 2, 1–16.

Vogel, D. C., Keays, R. R., James, R. S. & Reeves, S. J. (1999). The Science Reviews 18, 1–57.Wilkinson, J. F. G. (1991). Mauna Loan and Kilauean tholeiitesgeochemistry and petrogenesis of the Agnew intrusion, Canada: a

product of S-undersaturated, high-Al and low-Ti tholeiitic magmas. with low ‘ferromagnesian fractionated’ 100Mg/(Mg+ Fe2+) ratios:primary liquids from the upper mantle. Journal of Petrology 32,Journal of Petrology 40, 423–450.

Voshage, H., Hofmann, A. W., Mazzucchelli, M., Rivalenti, G., Sinigoi, 863–907.Williams, D. A., Kerr, R. C. & Lesher, C. M. (1998). EmplacementS., Raczek, I. & Demarchi, G. (1990). Isotopic evidence for a hybrid

lower crust formed by magmatic underplating. Nature 347, 731–736. and erosion by Archaean komatiite lava flows at Kambalda. Journal

of Geophysical Research 103, 27533–27549.Wager, L. R. & Brown, G. M. (1968). Layered Igneous Rocks. Edinburgh:Oliver and Boyd. Williams, D. A., Kerr, R. C. & Lesher, M. (1999). Thermal and fluid

dynamics of komatiitic lavas associated with magmatic Fe–Walker, D. (1983). Lunar and terrestrial crust formation. Proceedings of

the Lunar and Planetary Science Conference 14. Journal of Geophysical Research Ni–Cu–(PGE) sulphide deposits. In: Keays, R. R., Lesher, C. M.,Lightfoot, P. C. & Farrow, C. E. G. (eds) Dynamic Processes in Magmatic88, B17–25.

Walker, D., Longhi, J. & Hays, J. F. (1975). Differentiation of a very Ore Deposits and their Application in Mineral Exploration. Geological Association

of Canada, Short Course 13, 367–412.thick magma body and implications for the source regions of marebasalts. Proceedings of the Lunar Science Conference 6, 1103–1120. Williams, G. E. (2000). Geological constraints on the Precambrian

history of the Earth’s rotation and the Moon’s orbit. Reviews ofWalker, D., Kirkpatrick, R. J., Longhi, J. & Hays, J. F. (1976a).Crystallization history of lunar picritic basalt sample 12002: phase- Geophysics 31, 37–59.

Wilson, L. & Keil, K. (1991). Consequences of explosive eruptions onequilibria and cooling-rate studies. Geological Society of America Bulletin

87, 646–656. small solar system bodies: the case of the missing basalts from theaubrite parent body. Earth and Planetary Science Letters 104, 505–512.Walker, D., Longhi, J., Kirkpatrick, R. J. & Hays, J. F. (1976b).

Differentiation of an Apollo 12 picrite magma. Proceedings of the Lunar Wilson, L. & Keil, K. (1997). The fate of pyroclasts produced inexplosive eruptions on the asteroid 4 Vesta. Meteoritics and PlanetaryScience Conference 7, 1365–1389.

Walker, D., Shibata, T. & De Long, S. E. (1979). Abyssal tholeiites Science 32, 813–823.Wilson, M. (1989). Igneous Petrogenesis. A Global Tectonic Approach. London:from the Oceanographer fracture zone II: phase equilibria and

mixing. Contributions to Mineralogy and Petrology 70, 111–125. Thompson, 466 pp.Wolfe, E. W., Neal, C., Banks, N. G. & Duggan, T. J. (1988). GeologicWalker, G. P. L. (1973). Lengths of lava flows. Philosophical Transactions

of the Royal Society of London, Series A 274, 107–118. observations and chronology of eruptive events. In: Wolfe, E. W.(ed.) The Pu’u ‘O’o Eruption of Kilauea Volcano, Hawai’i: Episodes 1 throughWalker, R. J. & Papike, J. J. (1981). The Apollo 15 regolith: chemical

modeling and mare/highland mixing. Proceedings of the Lunar and 20, January 3, 1983 through June 8, 1984. US Geological Survey, Professional

Papers 1463, 99 pp.Planetary Science Conference 12B, 509–517.

1639


Wooden, J. L., Czamanske, G. K., Fedorenko, V. A., Arndt, N. T., Wyllie, P. J., Cox, K. G. & Biggar, G. M. (1962). The habit of apatitein synthetic systems and igneous rocks. Journal of Petrology 3, 238–243.Chauvel, C., Bouse, R. M., King, B.-S. W., Knight, R. J. & Siems,

D. F. (1993). Isotopic and trace element constraints on mantle and Yakovlev, O. I. & Basilevsky, A. T. (1994). Experimental studies ofgeochemical aspects of impact cratering. In: Dressler, B. O., Grieve,crustal contributions to Siberian continental flood basalts, Noril’sk

area, Siberia. Geochimica et Cosmochimica Acta 57, 3677–3704. R. A. F. & Sharpton, V. L. (eds) Large Meteorite Impacts and Planetary

Evolution. Geological Society of America, Special Papers 293, 73–91.Wright, T. L. (1971). Chemistry of Kilauea and Mauna Loa lava inspace and time. US Geological Survey, Professional Papers 735, 40 pp. Yanai, K. (1991). Gabbroic meteorite Asuka-31: preliminary ex-

amination of a new type of lunar meteorite in the Japanese collectionWright, T. L. (1973). Magma mixing as illustrated by the 1959 eruption,Kilauea volcano, Hawaii. Geological Society of America Bulletin 84, of Antarctic meteorites. Proceedings of the Lunar and Planetary Science

Conference 21, 317–324.849–958.Wright, T. L. & Fiske, R. S. (1971). Origin of the differentiated and Yang, H.-J., Frey, F. A., Rhodes, J. M. & Garcia, M. O. (1996).

Evolution of Mauna Kea volcano: results from the initial phase ofhybrid lavas of Kilauea volcano, Hawaii. Journal of Petrology 12, 1–66.Wright, T. L. & Tilling, R. I. (1980). Chemical variation in Kilauea the Hawaii Scientific Drilling Project. Journal of Geophysical Research

101, 11747–11767.eruptions 1971–1974. American Journal of Science 280-A, 777–793.Wright, T. L., Swanson, D. A. & Duffield, W. A. (1975). Chemical Yoder, H. S. & Tilley, C. E. (1962). Origin of basalt magmas: an

experimental study of natural and synthetic rock systems. Journal ofcomposition of Kilauean east-rift lava 1968–1971. Journal of Petrology

16, 110–133. Petrology 3, 342–532.Young, A. T. (1984). No sulfur flows on Io. Icarus 58, 197–226.Wyllie, P. J. (1979). The petrogenesis and the physics of the earth. In:

Yoder, H. S. (ed.) The Evolution of the Igneous Rocks—Fiftieth Anniversary Zimbelman, J. R. (1998). Emplacement of long lava flows on planetarysurfaces. Journal of Geophysical Research 103, 27503–27516.Perspectives. Princeton, NJ: Princeton University Press, pp. 483–520.

1640


AP

PE

ND

ICE

S

Tab

leA

1:

Pha

ses

inex

peri

men

tson

Apo

llo

17

mar

eba

salts

and

oran

geso

il(1

21

)

1.A

to

xyg

enfu

gac

ity

abo

ut

hal

fa

log

un

itb

elo

wFe

–FeO

equ

ilib

riu

m,

clo

seto

the

intr

insi

co

xyg

enfu

gac

ity

of

7427

5

Ru

nT

(°C

)%

H2

log

fO2

Co

nta

iner

t(h

)70

017

7021

570

275

7027

571

055

7156

972

135

7423

574

255

7427

575

035

7507

5o

r.so

il

(atm

)6

1174

220

7.32

113

5268

·0−

10·4

7M

o1

5.36

313

2668

·0−

10·7

4M

o3

z

7.32

013

2568

·0−

11·2

4M

o18

5.36

412

8169

·5−

11·3

0M

o18

7.31

812

7971

·6−

11·2

4M

o5

z

5.36

512

5364

·0−

11·3

4M

o18

zz

5.36

612

2969

·5−

11·9

0M

o18

szsz

5.36

712

0270

·0−

12·2

5M

o18

szsz

5.40

511

7873

·0−

12·7

0M

o17

aio

szio

szis

zai

szai

osz

5.39

411

3970

·0−

13·0

5M

o17

cio

psz

cio

psz

cio

psz

cio

psz

cio

psz

oz

osz

z

osz

szsz

osz

sz

osz

aosz

aio

szai

osz

aio

szai

osz

aio

szz

aio

szio

sz

acio

psz

cio

psz

acio

psz

cio

psz

acio

psz

cip

szci

op

szci

osz

1641


Tab

leA

1:

cont

inue

d

2.A

to

xyg

enfu

gac

ity>

1lo

gu

nit

bel

ow

Fe–F

eOeq

uili

bri

um

,a

littl

eb

elo

wth

ein

trin

sic

oxy

gen

fug

acit

yo

f70

017

Ru

nT

(°C

)%

H2

log

fO2

Co

nta

iner

t(h

)70

017

7021

570

275

7027

571

055

7156

972

135

7423

574

255

7427

575

035

7507

5o

r.so

il

(atm

)6

1174

220

5.32

212

0780

·0−

12·7

4M

o19

aosz

5.31

811

7780

·0−

13·1

1M

o17

5.38

211

7279

·5−

13·1

4M

o17

aosz

osz

iosz

5.32

011

6380

·0−

13·2

8M

o18

aio

sz

5.38

111

5779

·7−

13·3

4M

o17

aio

szio

szio

sz

5.40

311

5680

·0−

13·3

8M

o17

aio

pz

aip

sz

5.31

911

5280

·0−

13·4

3M

o17

5.38

911

4778

·3−

13·3

9M

o18

aio

szio

szio

sz

5.39

111

4480

·0−

13·5

3M

o19

cio

pz

cio

pz

5.32

311

4080

·0−

13·5

9M

o17

aio

pz

5.38

011

3978

·5−

13·5

0M

o17

acio

pz

cio

psz

cio

pz

5.32

411

2980

·0−

13·7

3M

o16

cio

pz

5.37

911

2478

·1−

13·6

9M

o18

acio

pz

cio

pz

cip

z

5.37

811

1078

·6−

13·9

1M

o17

cip

zci

pz

cip

z

5.37

710

9478

·6−

14·1

4M

o22

cip

cip

zci

pz

iosz

aio

szao

szai

osz

iosz

iosz

aio

szai

osz

aio

szai

oz

aio

sziz

iosz

aio

szai

osz

aio

szac

iop

szci

op

zac

iosz

acio

psz

acio

psz

acio

psz

cio

pz

acip

zac

iop

zac

iop

z

cip

zci

op

zac

iop

z

cio

pci

op

1642


3.A

to

xyg

enfu

gac

itie

sw

ell

abo

ve,

at,

and

up

toh

alf

alo

gu

nit

bel

ow

,th

ein

trin

sic

oxy

gen

fug

acit

yo

f70

017

plu

sm

etal

licir

on

Ru

nT

(°C

)%

H2

log

fO2

Co

nta

iner

t(h

)70

017

7021

570

275

7027

571

055

7156

972

135

7423

574

255

7427

575

035

7507

5o

r.so

il

(atm

)6

1174

220

5.41

511

8187

·7−

13·6

1Fe

17ai

oz

ioz

aiz

aio

zai

osz

5.39

511

372·

0−

8·47

Ag

Pd

17ci

kop

szci

kop

szci

kpsz

cikp

szci

kop

sz

5.39

311

4082

·8−

13·7

6M

o1

acio

psz

acio

psz

cio

psz

aip

szac

iop

sz

5.40

811

4383

·7−

13·7

9Fe

17io

psz

iosz

iop

zio

psz

iop

sz

5.39

211

4385

·1−

13·8

8M

o2

acio

psz

acio

zac

iop

szac

iop

szac

iosz

5.40

411

4085

·3−

13·9

4Fe

17ai

op

szac

iop

zim

op

zim

op

zci

op

z

5.40

911

4386

·9−

14·0

3Fe

17ac

imo

psz

acim

op

zac

imo

pz

imp

zci

mo

psz

5.41

311

4387

·3−

14·0

6Fe

17ac

imo

pz

acim

op

zac

imo

pz

acip

zac

imo

pz

5.39

911

4187

·7−

14·1

4Fe

17ac

imo

pz

acim

op

zac

imo

pz

acim

op

zac

imo

pz

5.40

111

4089

·1−

14·2

6Fe

17ac

imo

pz

acim

op

zac

imp

zac

imp

zac

imo

pz

aim

oz

aim

oz

aio

zai

osz

aio

szai

zai

oz

ciko

psz

ciko

psz

ciko

psz

ciko

psz

ciko

psz

ciko

psz

ciko

psz

ciko

sz

acio

zci

op

szac

iop

szac

iop

szac

iop

szci

op

zac

iop

szac

iosz

ioz

iosz

iosz

iop

szio

psz

ioz

iosz

iosz

acio

zai

oz

aio

zac

iop

szac

ioz

cio

pz

acio

pz

cio

psz

cio

szio

szio

psz

cio

psz

cip

zci

op

sz

cim

oz

acim

oz

acim

oz

acim

op

szac

imo

psz

acim

op

sz

acio

zac

imo

zac

imo

zac

imo

pz

acim

op

szac

imp

zac

imo

pz

acim

oz

acim

op

zac

iosz

acim

op

szac

imo

pz

acim

pz

acim

op

zac

imo

sz

acim

op

zac

imo

pz

acm

op

zac

imo

pz

acim

op

zac

mp

zac

imo

pz

acim

op

sz

Ph

ases

:a,

arm

alco

lite;

c,cl

ino

pyr

oxe

ne;

i,ilm

enit

e;k,

pse

ud

ob

roo

kite

,ke

nn

edyi

te;

m,

iro

nm

etal

;o

,o

livin

e;p

,p

lag

iocl

ase;

s,sp

inel

;z,

gla

ss;

liqu

id.

Un

der

lined

,sm

all

amo

un

tso

nly

.

1643


Table A2: Phases in experiments on Apollo 17 valley soils

1. At oxygen fugacity about half a log unit below Fe–FeO equilibrium

Run T (°C) % H2 log f O2 Container t (h) 70181 71501 74241 74261 75061 75081

(atm)

7.320 1325 68·0 −10·81 Mo 18 z

7.318 1279 71·6 −11·24 Mo 5 z z z sz z sz

2. At oxygen fugacity >1 log unit below Fe–FeO equilibrium

Run T (°C) % H2 log f O2 Container t (h) 70181 71501 74241 74261 75061 75081

(atm)

5.322 1207 80·0 −12·74 Mo 19 osz osz osz osz osz aosz

5.318 1177 80·0 −13·11 Mo 17 opsz iosz

5.320 1163 80·0 −13·28 Mo 18 opsz aciopsz copsz opsz aopsz aopsz

5.389 1147 78·3 −13·39 Mo 18 aiopsz aciopsz

5.391 1144 80·0 −13·53 Mo 19 aciopz

5.323 1140 80·0 −13·59 Mo 17 ciopz ciopz ciopz iopz iopz ciopsz

5.324 1129 80·0 −13·73 Mo 16 ciopz ciopz ciopz ciopz ciopz ciopz

Phases: a, armalcolite; c, clinopyroxene; i, ilmenite; k, pseudobrookite, kennedyite; m, iron metal; o, olivine; p, plagioclase;s, spinel; z, glass; liquid. Underlined, small amounts only.

1644


Tab

leA

3:

Mic

ropr

obe

anal

yses

ofgl

asse

sfr

omex

peri

men

tal

char

ges

Ru

nS

amp

leS

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nO

Mg

OC

aON

a 2O

K2O

Mo

O2

mg

-no

.

5.41

53

{7001

7∗40

·62

11·4

89·

570·

3417

·47

0·26

7·71

11·0

10·

470·

040·

0144

·1

7021

540

·05

11·6

69·

280·

3517

·25

0·29

7·71

11·4

10·

410·

02n

il44

·4

7027

5,6

41·2

310

·80

10·5

10·

2817

·52

0·26

7·82

10·9

50·

41n

.d.

nil

44·4

7027

5,11∗

40·5

711

·43

10·3

80·

2917

·34

0·27

6·48

11·2

60·

44n

il0·

0144

·4

7105

5∗40

·63

11·0

59·

590·

3017

·29

0·28

7·76

10·9

60·

420·

020·

0144

·5

7156

9∗40

·31

11·8

88·

980·

3617

·57

0·28

7·74

11·2

90·

440·

050·

0244

·0

7213

5∗40

·24

11·7

69·

230·

3617

·44

0·28

7·66

11·2

30·

43n

il0·

0444

·0

7423

5∗40

·27

11·5

59·

240·

3517

·61

0·27

7·80

11·0

60·

400·

01n

il44

·2

7425

5∗40

·16

11·7

09·

960·

3617

·04

0·24

7·74

11·4

00·

410·

01n

il44

·8

7427

5∗40

·60

11·7

69·

320·

3317

·32

0·25

7·69

11·4

50·

430·

01n

il44

·2

7503

541

·05

10·7

69·

250·

2517

·33

0·29

6·40

11·8

20·

490·

09n

il39

·7

7507

5∗40

·50

11·4

79·

320·

3517

·61

0·27

7·73

11·1

20·

390·

070·

0144

·0

5.40

5170

215

39·0

112

·44

8·80

0·29

18·8

50·

257·

5411

·03

0·42

0·01

0·10

41·7

5.38

2270

215

39·5

811

·78

9·09

0·27

17·7

30·

267·

6211

·46

0·39

0·03

nil

43·4

5.41

5370

215

40·0

511

·66

9·28

0·35

17·2

50·

297·

7111

·41

0·41

0·02

nil

44·4

5.38

1270

215∗

41·2

510

·63

9·58

0·23

16·6

40·

237·

1212

·06

0·46

0·05

0·06

43·3

5.39

5370

215

42·9

58·

349·

71n

il19

·03

0·30

6·27

11·3

70·

600·

100·

0137

·0

5.39

4170

215∗

41·8

99·

339·

980·

1617

·74

0·28

6·43

12·2

00·

470·

090·

0339

·3

5.38

9270

215∗

41·4

010

·15

9·64

0·20

17·2

20·

266·

7112

·10

0·46

0·03

0·02

41·0

5.38

0270

215

42·3

28·

9610

·13

0·21

17·5

30·

256·

2811

·76

0·54

0·06

nil

39·0

5.39

3370

215∗

42·5

09·

4210

·14

0·16

16·7

80·

266·

4412

·64

0·47

0·07

nil

40·7

5.39

2370

215∗

41·7

89·

889·

780·

2115

·95

0·27

6·85

12·3

00·

480·

03n

il43

·4

5.40

8370

215∗

39·9

49·

549·

610·

1720

·66

0·23

5·88

12·3

40·

46n

iln

il33

·7

5.40

4370

215∗

43·6

49·

499·

660·

1918

·54

0·41

†6·

2713

·02

0·48

0·05

0·02

37·7

5.40

9370

215∗

42·6

49·

6610

·18

0·19

16·4

80·

256·

4812

·23

0·44

0·02

nil

41·3

5.41

3370

215∗

42·6

39·

7710

·17

0·18

16·0

80·

296·

5812

·04

0·44

0·02

nil

42·2

5.39

9370

215∗

43·0

99·

2210

·49

0·19

15·9

40·

61†

6·45

11·6

20·

540·

04n

il41

·9

5.40

1370

215

49·0

28·

5310

·81

0·17

13·5

20·

96†

6·21

12·3

20·

620·

130·

0645

·1

5.37

7270

215

44·8

68·

749·

930·

0817

·37

0·28

5·38

11·2

40·

670·

030·

0335

·6

∗Oliv

ine

anal

ysis

bel

ow

.†R

un

gai

ned

Mn

Ofr

om

imp

ure

iro

nfo

il.A

llo

xid

ized

iro

nre

po

rted

asFe

O.

Su

per

scri

pts

1–3

refe

rto

sub

sect

ion

so

fTa

ble

A1

wh

ere

exp

erim

enta

lco

nd

itio

ns

and

coex

isti

ng

ph

ases

are

reco

rded

.

1645


Tab

leA

4:

Mic

ropr

obe

anal

yses

ofol

ivin

esfr

omex

peri

men

tal

char

ges

Ru

nS

amp

leS

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nO

Mg

OC

aON

a 2O

K2O

Mo

O2

mg

-no

.

5.41

53

{7001

737

·39

0·39

0·08

0·25

24·1

70·

3037

·05

0·59

0·03

nil

nil

73·2

7027

5,11

38·1

80·

200·

080·

2323

·87

0·28

37·6

30·

390·

03n

iln

il73

·8

7105

538

·54

0·38

0·08

0·23

23·7

90·

2938

·06

0·40

0·02

nil

nil

74·1

7156

938

·15

0·31

0·07

0·26

24·1

60·

3237

·41

0·32

0·04

nil

0·01

73·4

7213

538

·23

0·21

0·08

0·21

24·1

80·

2937

·40

0·36

0·04

nil

nil

73·4

7423

538

·10

0·30

0·07

0·26

24·1

80·

2837

·49

0·41

0·02

nil

0·01

73·5

7425

538

·37

0·33

0·09

0·23

23·2

90·

2737

·69

0·38

0·02

nil

0·01

74·3

7427

538

·69

0·34

0·09

0·25

23·4

80·

1738

·47

0·40

0·03

nil

0·06

74·5

7507

538

·20

0·32

0·06

0·26

23·5

10·

2537

·64

0·35

0·03

0·01

nil

74·1

5.38

1270

215

38·0

70·

320·

100·

1024

·74

0·21

36·4

70·

460·

08n

il0·

0572

·5

5.39

4170

215

37·1

60·

550·

070·

1828

·27

0·34

33·0

10·

470·

02n

il0·

0367

·6

5.38

9270

215

37·6

90·

350·

080·

1726

·81

0·33

34·5

10·

420·

01n

iln

.d.

69·7

5.39

3370

215

37·6

20·

460·

100·

1227

·39

0·33

33·9

10·

620·

02n

il0·

0168

·9

5.39

2370

215

37·6

60·

330·

080·

1427

·13

0·28

33·9

30·

520·

050·

010·

0269

·1

5.40

8370

215

36·3

90·

340·

090·

1431

·19

0·36

31·3

10·

430·

02n

iln

il64

·2

5.40

4370

215

37·1

00·

570·

120·

1729

·12

0·45∗

32·9

70·

470·

01n

iln

.d.

66·9

5.40

9370

215

37·7

70·

400·

090·

1726

·06

0·34

34·9

40·

420·

02n

iln

il70

·6

5.41

3370

215

37·6

10·

290·

070·

1626

·73

0·33

34·9

00·

380·

05n

iln

il70

·4

5.39

9370

215

37·8

30·

440·

070·

1825

·75

0·66∗

34·5

90·

44n

il0·

040·

0270

·6

5.37

9270

215

36·8

50·

480·

120·

1232

·14

0·41

30·2

40·

490·

01n

il0·

0562

·7

∗Ru

ng

ain

edM

nO

fro

mim

pu

reir

on

foil.

Su

per

scri

pts

1–3

refe

rto

sub

sect

ion

so

fTa

ble

A1

wh

ere

exp

erim

enta

lco

nd

itio

ns

and

coex

isti

ng

ph

ases

are

reco

rded

.

1646


Tab

leA

5:

Mic

ropr

obe

anal

yses

ofsp

inel

sfr

omex

peri

men

tal

char

ges

Ru

nS

amp

leS

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nO

Mg

OC

aON

a 2O

K2O

Mo

O2

5.41

53

{7001

70·

0922

·57

7·76

24·5

335

·49

0·36

8·77

0·23

0·02

nil

0·01

7105

50·

1223

·33

7·23

23·1

735

·24

0·41

8·72

0·28

0·03

0·06

0·06

7156

90·

0723

·68

6·77

24·3

536

·04

0·37

8·68

0·16

0·05

nil

0·03

7425

50·

6422

·91

8·11

23·8

534

·93

0·34

9·19

0·41

0·01

nil

0·01

7427

50·

2023

·05

7·54

23·9

735

·26

0·35

8·93

0·38

0·05

nil

nil

7507

50·

7722

·24

7·32

23·2

135

·78

0·37

8·58

0·51

0·06

0·02

0·01

5.40

5170

215

0·24

23·1

06·

5520

·21

37·7

00·

358·

300·

230·

020·

011·

47

5.38

2270

215

0·33

23·3

36·

5521

·22

37·2

20·

398·

430·

420·

020·

020·

50

5.38

1270

215

0·20

23·8

86·

8823

·09

36·5

10·

407·

970·

170·

020·

020·

41

5.39

5370

215

0·22

19·7

35·

059·

4555

·75

0·38

5·95

0·39

0·03

0·01

nil

5.39

4170

215

0·44

24·1

26·

8619

·86

38·9

20·

376·

830·

660·

02n

il1·

89

5.38

9270

215

0·56

21·3

77·

0121

·84

37·8

20·

387·

590·

450·

010·

010·

64

5.40

8370

215

0·07

28·5

05·

7114

·40

44·7

30·

356·

110·

220·

03n

il0·

01

All

oxi

diz

edir

on

rep

ort

edas

FeO

.S

up

ersc

rip

ts1–

3re

fer

tosu

bse

ctio

ns

of

Tab

leA

1w

her

eex

per

imen

tal

con

dit

ion

san

dco

exis

tin

gp

has

esar

ere

cord

ed.

1647


Tab

leA

6:

Mic

ropr

obe

anal

yses

ofar

mal

colite

san

dke

nned

yite

from

expe

rim

enta

lch

arge

s

Ru

nS

amp

leS

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nO

Mg

OC

aON

a 2O

K2O

Mo

O2

5.41

53

{7001

70·

1174

·67

2·06

2·20

13·7

00·

037·

550·

330·

04n

iln

il

7027

5,6

0·10

75·3

61·

912·

2313

·66

0·14

7·60

0·28

0·03

nil

0·03

7027

5,11

0·09

74·1

42·

062·

0014

·51

0·13

6·87

0·25

0·03

nil

nil

7105

50·

1175

·07

2·20

2·19

13·8

60·

097·

610·

270·

03n

il0·

01

7156

90·

1175

·34

1·93

2·26

13·6

30·

117·

590·

350·

03n

il0·

01

7213

50·

0774

·70

2·01

2·16

14·0

80·

097·

670·

280·

03n

iln

il

7423

50·

0774

·90

1·94

2·25

13·7

20·

047·

620·

280·

04n

iln

il

7425

50·

3575

·02

1·97

2·24

13·3

50·

117·

780·

280·

03n

il0·

01

7427

50·

1074

·80

1·97

2·19

13·4

60·

097·

790·

290·

03n

iln

il

7507

50·

1174

·53

1·96

2·22

13·7

10·

097·

670·

370·

030·

01n

il

5.39

12

{7001

70·

0774

·32

2·03

2·30

13·3

6n

.d.

8·05

n.d

.n

.d.

n.d

.n

.d.

7423

50·

1574

·30

1·93

2·05

14·8

2n

.d.

7·57

n.d

.n

.d.

n.d

.n

.d.

7425

50·

1474

·73

1·93

1·99

14·9

2n

.d.

7·61

n.d

.n

.d.

n.d

.n

.d.

7507

50·

1674

·17

2·01

2·11

14·7

7n

.d.

7·53

n.d

.n

.d.

n.d

.n

.d.

5.39

5370

215

0·42

63·4

61·

991·

0724

·43

0·13

6·33

0·49

0·01

0·03

0·01

5.39

2370

215

0·17

73·8

81·

972·

0313

·63

0·12

7·88

0·45

0·01

0·02

nil

5.41

3370

215

0·09

74·5

32·

051·

6914

·65

0·13

7·25

0·28

0·03

nil

nil

5.39

9370

215

0·11

73·5

31·

971·

5614

·62

0·22∗

7·07

0·38

0·01

nil

nil

5.40

1370

215

0·11

75·5

82·

031·

5614

·00

0·29∗

7·49

0·43

0·01

0·02

nil

∗Ru

ng

ain

edM

nO

fro

mim

pu

reir

on

foil.

All

oxi

diz

edir

on

rep

ort

edas

FeO

.S

up

ersc

rip

ts1–

3re

fer

tosu

bse

ctio

ns

of

Tab

leA

1w

her

eex

per

imen

tal

con

dit

ion

san

dco

exis

tin

gp

has

esar

ere

cord

ed.

1648


Tab

leA

7:

Mic

ropr

obe

anal

yses

ofilm

enites

from

expe

rim

enta

lch

arge

s

Ru

nS

amp

leS

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nO

Mg

OC

aON

a 2O

K2O

Mo

O2

5.39

12

{7001

70·

0856

·65

0·53

2·60

31·2

0n

.d.

8·48

n.d

.n

.d.

n.d

.n

.d.

7027

5,6

0·02

56·4

90·

492·

3832

·83

n.d

.7·

50n

.d.

n.d

.n

.d.

n.d

.

7027

5,11

0·06

56·4

00·

532·

2932

·85

n.d

.7·

73n

.d.

n.d

.n

.d.

n.d

.

7156

90·

1156

·60

0·53

2·52

32·1

5n

.d.

8·10

n.d

.n

.d.

n.d

.n

.d.

7423

50·

1555

·50

0·51

2·46

32·2

7n

.d.

8·10

n.d

.n

.d.

n.d

.n

.d.

7425

50·

0756

·86

0·50

2·52

32·3

9n

.d.

8·08

n.d

.n

.d.

n.d

.n

.d.

7503

5n

.d.

56·4

70·

52n

.d.

32·2

7n

.d.

n.d

.n

.d.

n.d

.n

.d.

n.d

.

7507

50·

0656

·85

0·49

2·51

32·0

3n

.d.

8·21

n.d

.n

.d.

n.d

.n

.d.

5.40

5170

215

0·02

56·0

50·

512·

3532

·37

0·31

8·50

0·24

nil

0·02

0·24

5.38

2270

215

0·03

55·7

00·

532·

6331

·84

0·29

8·58

0·37

0·03

nil

0·11

5.38

1270

215

0·03

55·8

70·

512·

5832

·10

0·35

8·57

0·25

0·03

nil

0·17

5.39

5370

215

0·03

50·9

50·

591·

3139

·40

0·35

6·73

0·27

0·01

0·01

nil

5.39

4170

215

0·05

55·1

90·

431·

8534

·55

0·35

7·14

0·30

0·02

0·07

0·32

5.38

0270

215

0·06

55·6

40·

461·

9134

·71

0·36

7·22

0·34

0·02

nil

0·15

5.39

3370

215

0·20

55·2

40·

502·

2033

·84

0·35

7·87

0·47

0·05

0·03

0·07

5.39

2370

215

0·09

55·6

00·

622·

3231

·65

0·36

8·40

0·32

0·01

nil

n.d

.

5.40

8370

215

0·04

55·3

50·

481·

4935

·74

0·31

6·41

0·34

0·01

nil

nil

5.40

4370

215

0·02

55·5

60·

471·

9634

·29

0·55∗

7·11

0·26

nil

0·03

nil

5.40

9370

215

0·01

56·6

40·

501·

9332

·85

0·36

7·71

0·21

0·02

nil

nil

5.41

3370

215

0·03

56·5

10·

532·

1032

·53

0·47

7·87

0·25

0·02

nil

nil

5.39

9370

215

0·06

54·4

51·

001·

8032

·71

0·86∗

7·68

0·30

0·02

0·03

nil

5.40

1370

215

0·08

55·3

60·

472·

0631

·12

1·22∗

7·74

0·58

0·02

0·02

nil

5.37

9270

215

0·06

54·7

20·

401·

2636

·14

0·40

6·00

0·31

0·03

0·03

0·13

5.37

8270

215

0·06

54·8

70·

331·

1236

·60

0·42

5·35

0·33

0·02

0·03

0·13

5.37

7270

215

0·06

53·8

50·

330·

9039

·69

0·47

4·44

0·40

0·02

0·02

0·10

∗Ru

ng

ain

edM

nO

fro

mim

pu

reir

on

foil.

All

oxi

diz

edir

on

rep

ort

edas

FeO

.S

up

ersc

rip

ts1–

3re

fer

tosu

bse

ctio

ns

of

Tab

leA

1w

her

eex

per

imen

tal

con

dit

ion

san

dco

exis

tin

gp

has

esar

ere

cord

ed.

1649


Tab

leA

8:

Mic

ropr

obe

anal

yses

ofcl

inop

yrox

enes

from

expe

rim

enta

lch

arge

s

Ru

nS

amp

leS

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nO

Mg

OC

aON

a 2O

K2O

Mo

O2

5.39

4170

215

51·2

21·

771·

870·

5110

·53

0·21

17·1

216

·38

0·06

0·01

0·05

5.40

9370

215

48·0

24·

064·

660·

858·

100·

2214

·18

19·5

60·

11n

il0·

01

5.41

3370

215

49·0

13·

024·

070·

6610

·41

0·35

16·2

215

·48

0·11

nil

0·02

5.39

9370

215

48·6

13·

494·

130·

788·

990·

2613

·99

17·7

10·

10n

il0·

01

5.37

9270

215

49·0

63·

633·

870·

4514

·80

0·27

14·3

714

·03

0·17

0·02

nil

5.37

8270

215

50·2

92·

542·

910·

3713

·99

0·26

13·6

915

·26

0·15

0·03

nil

All

oxi

diz

edir

on

rep

ort

edas

FeO

.S

up

ersc

rip

ts1–

3re

fer

tosu

bse

ctio

ns

of

Tab

leA

1w

her

eex

per

imen

tal

con

dit

ion

san

dco

exis

tin

gp

has

esar

ere

cord

ed.

1650


Tab

leA

9:

Mic

ropr

obe

anal

yses

ofpl

agio

clas

esfr

omex

peri

men

tal

char

ges

Ru

nS

amp

leS

iO2

TiO

2A

l 2O

3C

r 2O

3Fe

OM

nO

Mg

OC

aON

a 2O

K2O

Mo

O2

5.39

5370

215

46·6

20·

4831

·52

nil

1·28

0·01

0·40

17·2

71·

310·

030·

03

5.39

4170

215

46·8

80·

9531

·25

0·03

1·29

0·12

0·47

17·7

81·

030·

040·

10

5.38

0270

215

46·8

70·

4932

·09

0·02

1·09

0·03

0·41

17·8

01·

310·

030·

03

5.40

4370

215

46·2

10·

2433

·36

nil

0·60

nil

0·31

18·3

01·

060·

01n

il

5.41

3370

215

47·4

60·

2431

·98

nil

0·57

0·01

0·40

17·2

71·

310·

03n

il

All

oxi

diz

edir

on

rep

ort

edas

FeO

.Pla

gio

clas

esar

eA

n89

·0±

1.S

up

ersc

rip

ts1–

3re

fer

tosu

bse

ctio

ns

of

Tab

leA

1w

her

eex

per

imen

talc

on

dit

ion

san

dco

exis

tin

gp

has

esar

ere

cord

ed.

1651

Documents

Flood Basalts, Basalt Floods or Topless Bushvelds? Lunar