Introduction

The variation of temperature with depth in theEarthrsquos crust and mantle cannot be directly mea-sured and must be inferred One of the more directindications of interior temperatures is provided bythe eruption temperatures of volcanoes and thematching of those temperatures with knowledge ofthe depth of origin and magmasource relation-

ship In the lsquoPlate Tectonicsrsquo model of the modernEarth it is commonly inferred that Mid-OceanRidge (MOR) volcanism is an expression of con-vective upwelling from relatively shallow mantledepths To a first approximation MOR volcanismis characterized by globally similar primary orparental magma compositions with the most mag-nesian quenched glass observed in all youngocean basins having eruption temperatures around

Eur J Mineral2001 13 437-451

Primary magmas and mantle temperatures

DAVID H GREEN1) TREVOR J FALLOON12) STEPHEN M EGGINS1) and GREGORY M YAXLEY1)

1) Research School of Earth Sciences Australian National University Canberra ACT Australia 02002) School of Earth Sciences University of Tasmania Hobart Tasmania Australia 7005

Abstract The composition of olivine phenocrysts in Hawaiian tholeiitic picrites and in Mid-Ocean Ridge picritesvary up to Mg

913 and Mg921 respectively The compositions and liquidus temperatures of the magmas crystallizing

the most magnesian phenocrysts can be estimated and we find that anhydrous liquidus temperatures (at 1 bar pres-sure) of Hawaiian tholeiitic picrites average 1365degC for E-MOR picrites average 1355degC and for N-MOR picritesaverage 1335degC Water contents of the magmas decrease in the order Hawaiian picrites E-MOR picrites to N-MORpicrites and consideration of liquidus depression by these water contents leads to the conclusion that magma tem-peratures for all types were approximately 1325degC at ~ 1 bar The data from parental or primary magmas suggeststhat the temperature contrast between lsquoHot-Spotrsquo and MOR magmas is pound 20degC Application of information from par-tial melting studies of lherzolites and liquidus studies of the Hot-Spot and MOR picrites leads to the conclusion thatboth lsquoHot-Spotrsquo and MOR primary basalts are derived from mantle with potential temperature Tp ~ 1430degC Insofaras primitive magmas may be used to infer the potential temperature of their sources there is no evidence for a tem-perature contrast of D Tp = 100-250degC between lsquoHot-Spotrsquo or lsquoDeep Mantle Plumersquo sources and ambient (MORsource) asthenospheric mantle

Although magma temperatures are similar the residual mantle compositions for Hawaiian picrites are refractoryharzburgites more refractory (including CrCr+Al ratio) than the lherzolite to harzburgite residue from MOR picriteextraction It is argued that the buoyancy plume and geophysically anomalous mantle beneath the Hawaiian Arch isdue to compositional and not temperature contrasts in the upper mantle The four-component mixing identified in theHawaiian source is attributed to interaction between old subducted lithospheric slabs buoyant or suspended in theupper mantle and surrounding ambient mantle at Tp = 1430degC

Key-words mantle melting mantle potential temperature plumes primary magmas

0935-1221010013-0437 $ 375atilde 2001 E Schweizerbartrsquosche Verlagsbuchhandlung D-70176 Stuttgart

DOI 1011270935-122120010013-0437

This paper was presented at the EMPG VIIImeeting in Bergamo Italy (April 2000)

E-mail directorrsesanueduau

DH Green TJ Falloon SM Eggins GM Yaxley

1200degC-1250degC This global first-order similarityled to the concept of a generalized mantle poten-tial temperature (Tp) with Tp raquo 1280degC (McKenzieamp Bickle 1988) The crust and lithosphere areinterpreted as a thin boundary layer over theasthenosphere which is inferred to have a poten-tial temperature of 1280degC and adiabatic tempera-ture distribution with depth The asthenospherecomposition is inferred to be a reasonably well-mixed and slightly residual lherzolite evolvedfrom the silicate component of the primitive Earthby separation of the continental and oceanic crustand lithosphere over ~ 45 billion years

Although MOR basalts are interpreted as sam-pling a modern well-mixed mantle source(asthenospheric mantle) a second relatively dis-tinctive volcanism lsquoisland chainrsquo or lsquohot spotrsquo vol-canism is widely interpreted as sampling adifferent source or sources The lsquohot spotrsquo sourcesare relatively fixed with respect to plate-move-ments and are fixed or show very slow movementrelative to one another For the best-studied exam-ples of lsquohot-spot volcanismrsquo such as the Hawaiian-Emperor Island chain Iceland Reacuteunion theAzores etc the individual volcanoes sit on top ofa broad topographic swell of ~ 1 km elevationabove the ocean floor The presence of the broadtopographic high (eg the Hawaiian Swell orArch) in isostatic equilibrium leads to the infer-ence of a buoyancy plume or lower density col-umn as the cause of the hot-spots and the source ofhot-spot volcanism

The buoyancy plume is normally attributed toa temperature difference with DT ~ 200ndash250degC(rarely to 500degC or down to 100degC) between thenormal mantle and upwelling plume (McKenzieamp Bickle 1988) In much of the literature onmantle plumes particularly on plumes from apostulated thermal boundary layer at the core-mantle boundary there is explicit reference totemperature difference between upwelling plumeand ambient mantle of at least 200degC (Davies1999) A number of authors have suggested thatthere is evidence from hot-spots for the presenceof high-temperatu re picritic ie olivine -richmagmas in contradistinction to the olivine tholei-ites considered to be characteristic of mid-oceanridges [using the McKenzie amp Bickle (1988)analysis]

The theme of this paper is to critically exam-ine the evidence for DT ~ 100ndash250degC betweenplume sources as exemplified by Hawaii in par-ticular and MORB sources for both N-MORBand E-MORB by examining the evidence fromprimitive or primary magmas themselves

Some first principles

The Earthrsquos mantle is dominated by peridotiteand peridotite is the probable source for most pri-mary mantle-derived magmas The minerals ofthe mantle source region are all FeMg solid solu-tions and melting with increasing temperature isthus a continuous process with predictable FeMgpartitioning between crystalline phases and liquidA similar conclusion applies to other substitutionsand coupled substitutions in olivine pyroxenesgarnet spinel or plagioclase In the simple case ofFeMg partitioning between olivine (as the majorphase) and liquid the effect of increasing pressuremoves the melting loop to higher temperature buthas very little effect on the partitioning For a par-ticular liquid the liquidus olivine is known andconversely for a particular mantle containingdefined olivine composition the FeMg ratio (butnot the FeO and MgO content) of the liquid isknown (Roedder amp Emslie 1970)

Because the mantle source is multiphase melt-ing also has the character of eutectic melting iemelting of the multiphase assemblage begins attemperatures below the solidus temperature of anyone phase alone The temperature for beginning ofmelting the lsquopseudo-eutecticrsquo increases withincreasing pressure but the olivine solidus temper-ature increases more slowly with increasing pres-sure than the pyroxene solidus As a consequencethe position of the lsquoeutectic-likersquo melt movestowards more olivine-rich compositions withincreasing pressure (Green amp Ringwood 1967Falloon amp Green 1988)

The subduction process at convergent marginscarries crust particularly oceanic crust and coollithosphere back into the upper mantle and sub-ducted crustal and lithospheric components under-go a sequence of metamorphic reactions includingdehydration reactions towards higher pressureand higher temperature assemblages The averagecrust of ocean basins is basaltic in compositionand reacts to quartz eclogite or coesite eclogite inthe pressure range of 1 to 5 GPa (~ 30 to 170 km)Within widely accepted models of plate tectonicsincluding the concept of deep mantle plumesthere is the expectation or hypothesis that meltingof subducted oceanic crust will occur under condi-tions of eclogite stability Thus eclogite meltingwithin the subducted slab is one process which isinvoked to explain transfer of crustal includingocean-floor sediments geochemical signaturesfrom subducted slab to the overlying mantlewedge ie to the source region for volcanics char-acteristic of island arcs and back-arc basins

438

DH Green TJ Falloon SM Eggins GM Yaxley

1200degC-1250degC This global first-order similarityled to the concept of a generalized mantle poten-tial temperature (Tp) with Tp raquo 1280degC (McKenzieamp Bickle 1988) The crust and lithosphere areinterpreted as a thin boundary layer over theasthenosphere which is inferred to have a poten-tial temperature of 1280degC and adiabatic tempera-ture distribution with depth The asthenospherecomposition is inferred to be a reasonably well-mixed and slightly residual lherzolite evolvedfrom the silicate component of the primitive Earthby separation of the continental and oceanic crustand lithosphere over ~ 45 billion years

Although MOR basalts are interpreted as sam-pling a modern well-mixed mantle source(asthenospheric mantle) a second relatively dis-tinctive volcanism lsquoisland chainrsquo or lsquohot spotrsquo vol-canism is widely interpreted as sampling adifferent source or sources The lsquohot spotrsquo sourcesare relatively fixed with respect to plate-move-ments and are fixed or show very slow movementrelative to one another For the best-studied exam-ples of lsquohot-spot volcanismrsquo such as the Hawaiian-Emperor Island chain Iceland Reacuteunion theAzores etc the individual volcanoes sit on top ofa broad topographic swell of ~ 1 km elevationabove the ocean floor The presence of the broadtopographic high (eg the Hawaiian Swell orArch) in isostatic equilibrium leads to the infer-ence of a buoyancy plume or lower density col-umn as the cause of the hot-spots and the source ofhot-spot volcanism

The buoyancy plume is normally attributed toa temperature difference with DT ~ 200ndash250degC(rarely to 500degC or down to 100degC) between thenormal mantle and upwelling plume (McKenzieamp Bickle 1988) In much of the literature onmantle plumes particularly on plumes from apostulated thermal boundary layer at the core-mantle boundary there is explicit reference totemperature difference between upwelling plumeand ambient mantle of at least 200degC (Davies1999) A number of authors have suggested thatthere is evidence from hot-spots for the presenceof high-temperatu re picritic ie olivine -richmagmas in contradistinction to the olivine tholei-ites considered to be characteristic of mid-oceanridges [using the McKenzie amp Bickle (1988)analysis]

The theme of this paper is to critically exam-ine the evidence for DT ~ 100ndash250degC betweenplume sources as exemplified by Hawaii in par-ticular and MORB sources for both N-MORBand E-MORB by examining the evidence fromprimitive or primary magmas themselves

Some first principles

The Earthrsquos mantle is dominated by peridotiteand peridotite is the probable source for most pri-mary mantle-derived magmas The minerals ofthe mantle source region are all FeMg solid solu-tions and melting with increasing temperature isthus a continuous process with predictable FeMgpartitioning between crystalline phases and liquidA similar conclusion applies to other substitutionsand coupled substitutions in olivine pyroxenesgarnet spinel or plagioclase In the simple case ofFeMg partitioning between olivine (as the majorphase) and liquid the effect of increasing pressuremoves the melting loop to higher temperature buthas very little effect on the partitioning For a par-ticular liquid the liquidus olivine is known andconversely for a particular mantle containingdefined olivine composition the FeMg ratio (butnot the FeO and MgO content) of the liquid isknown (Roedder amp Emslie 1970)

Because the mantle source is multiphase melt-ing also has the character of eutectic melting iemelting of the multiphase assemblage begins attemperatures below the solidus temperature of anyone phase alone The temperature for beginning ofmelting the lsquopseudo-eutecticrsquo increases withincreasing pressure but the olivine solidus temper-ature increases more slowly with increasing pres-sure than the pyroxene solidus As a consequencethe position of the lsquoeutectic-likersquo melt movestowards more olivine-rich compositions withincreasing pressure (Green amp Ringwood 1967Falloon amp Green 1988)

The subduction process at convergent marginscarries crust particularly oceanic crust and coollithosphere back into the upper mantle and sub-ducted crustal and lithospheric components under-go a sequence of metamorphic reactions includingdehydration reactions towards higher pressureand higher temperature assemblages The averagecrust of ocean basins is basaltic in compositionand reacts to quartz eclogite or coesite eclogite inthe pressure range of 1 to 5 GPa (~ 30 to 170 km)Within widely accepted models of plate tectonicsincluding the concept of deep mantle plumesthere is the expectation or hypothesis that meltingof subducted oceanic crust will occur under condi-tions of eclogite stability Thus eclogite meltingwithin the subducted slab is one process which isinvoked to explain transfer of crustal includingocean-floor sediments geochemical signaturesfrom subducted slab to the overlying mantlewedge ie to the source region for volcanics char-acteristic of island arcs and back-arc basins

438

Melting of basaltic composition components isalso invoked within the lsquodeep mantle plumersquohypothesis including concepts of accumulationand reheating of subducted crustlithosphere slabswithin a core-mantle boundary layer Ascent ofinhomogeneous plumes from such a boundarylayer is inferred to lead to melting during ascentincluding at shallower depths preferential melt-ing of eclogite components (Davies 1999)Finally detachment of subducted slabs of crustand lithosphere may occur followingcontinentcontinent collision or migration oftrenchessubduction zones Such suspended slabsof inhomogeneous crust lithosphere and enclos-ing ambient mantle may over time reach lsquonormalrsquomantle temperatures mdash and in doing so undergomelting in lithologies with lower solidus tempera-tures than normal mantle In a later section wereturn to discussion of melting in an inhomoge-neous mantle source comprised of asthenosphericmantle refractory lithosphere and subductedoceanic crust

The arguments and deductions of this paperare based on major element and refractory minorelements and on phase equilibria constraints Acorollary is that relative abundances of incompati-ble elements particularly incompatible trace ele-ments may provide information on the meltingprocess and conditions but like radiogenic iso-topes are better indicators of the precursor pro-cesses and pre-melting history affecting the sourceregion

Temperatures of primary magmas

A Hawaiian tholeiitic magmas

The main cone-building phase of Hawaiianvolcanoes is dominated by olivine tholeiite mag-mas ie hypersthene-normative magmas whichvary from tholeiitic picrites with gt 30 modalolivine (~ 25-20 normative olivine) to olivine-phyric quartz tholeiites (lt 5 modal olivine ~ 5 normative quartz) A recent paper by Norman ampGarcia (1999) presented data on picritic magmasfrom six volcanoes or eruptive centres Norman ampGarcia showed that for each volcano the olivinecompositions plotted in a distinctive field in a CaOvs Mg = 100Mg(Mg+Fe2+) diagram ie the cal-cium content of olivine correlated with the magmacomposition such that Loihi represented the highCa-end (with 025 to 040 wt CaO in olivine) Inaddition the most magnesian olivines were sup3 Mg

88in all cases and reached Mg

915 for Mauna Loa

Norman amp Garcia inferred that the magnesianolivines were phenocrysts and not xenocrystsfrom disrupted lithospheric xenoliths on the basisof the CaO vs Mg correlations [implying equilib-rium between host magma and olivine and con-trasting with the low CaO content (005 to 01 CaO) in olivine of spinel lherzolite xenoliths] Theauthors inferred that the primary or parental mag-mas were picritic with 15-16 MgO in the man-tle-derived liquids

It is possible to use the data on Hawaiianpicrites to evaluate the most primitive magmas foreach volcano and to determine their temperaturesof eruption There is now a large body of experi-mental data on liquidus temperatures of basalticmagmas at 1 atm and at high pressure If werestrict ourselves to basaltic liquids with less than25 Na2O (ie largely tholeiitic liquids) the for-mulation of a relationship between liquidus tem-perature and the major element composition ofliquid [the lsquoFordrsquo geothermometer in Ford et al(1983)] allows the estimation of anhydrous liq-uidus temperature for tholeiitic liquids directlyfrom their major element content (Fig 1 TFord

Thermometer vs TExperimental) From the FeMg parti-tioning relationship between olivine and liquid(Roedder amp Emslie 1970) it is also possible toestimate the liquidus olivine composition for aparticular bulk-rock (picrite) or glass composition

In Table 1 we have listed each of the picritecompositions for which olivine phenocryst com-positions are provided by Norman amp Garcia(1999) In column 2 the MgO content of the bulkrock is given and column 3 lists the composition of

Primary magmas and mantle temperatures 439

Fig 1 Comparison of calculated liquidus temperaturevs experimental temperature using the LiquidusTemperature vs Composition relationships of Ford etal (1983) Data are from experiments for liquids with pound30 Na2O and combine Fig 7a and 7b of Falloon ampDanyushevsky (2000)

DH Green TJ Falloon SM Eggins GM Yaxley

the liquidus olivine for that bulk composition Incolumn 4 the most magnesian olivine phenocrystactually observed in the rock is listed In the Loihiexamples the extremely magnesian bulk composi-tions have liquidus olivine composition of Mg =936 to 953 but the most magnesian olivineobserved is Mg

88 (LO-02-2) or Mg885 (LO-01-

04F LO-02-04C) Olivine of this composition isincrementally subtracted from the bulk to obtain anew composition (the inferred primary magma)which has liquidus olivine matching the observedmost magnesian phenocryst This calculationassumes an oxidation state of QFM + 05 log unitsand incrementally extracts equilibrium olivine in001 increments using the PETROLOG pro-gram (Danyushevsky 1998) In column 5 theMgO of the inferred primary magma is listed andusing the Ford geothermometer the liquidus tem-perature at 1 atm is calculated for the three Loihiexamples (1329degC 1347degC 1358degC) It should be

noted that if these particular picrites had actuallyevolved from a primary picrite with more magne-sian liquidus olivine (eg Mg

90) then the calcula-tion should have used Mg

90 rather than Mg88 and

proceeded with olivine extraction only to reach thecomposition crystallizing Mg

90 Thus for the Loihicase primary liquids may have been richer inMgO (and with higher liquidus temperatures ~1380degC) For the Kilauean picritic glass (Clague etal 1991) the liquidus olivine for the glass com-position (147 MgO) is Mg

896 but the mostmagnesian phenocryst is Mg

907 In this caseolivine of composition defined by the KD

FeMgOlliq

relationship is added until a composition with liq-uidus olivine Mg

907 is reached This liquid has166 MgO and an anhydrous liquidus tempera-ture of 1381degC Calculations for Hualalai MaunaLoa Kohala and Koolau give primary liquids with144-184 MgO and anhydrous liquidus temper-atures from 1351degC to 1410degC

440

Table 1 Compositional data from Hawaiian picrites as given by Norman amp Garcia (1999) with their specimen num-bers

The anhydrous liquidus temperatures average1365degC but Hawaiian tholeiitic magmas havewater contents of 03-08 H2O (Garcia et al1989 Michael 1995) and this has a significanteffect on liquidus temperatures Apart from thepicritic glass from Kilauea (Clague et al 1991)we do not have analyzed water contents of thepicrites of Table 1 Correlations between H2O andother incompatible elements including the degreeof LREE enrichment have been noted(Danyushevsky et al 2000 Michael1995) and toa first approximation we have taken the H2O-con-tent of primitive Hawaiian picrites to be 05 wtThis water content would give a liquidus depres-sion of ~ 50degC (Fig 2) Taking the average anhy-drous liquidus of 1365degC (Table 1) we infereruption temperatures of 1315degC for Hawaiianparental picrites

To summarise the mineralogy and bulk com-position of Hawaiian tholeiitic picrites providesevidence for primary magmas of around 16 MgO [135-184 MgO] with eruption tempera-tures of ~ 1315degC These are all magmas withcharacteristic LREE enrichment (column 7 Table1) In a later section we consider the melting con-ditions for these magmas

B Mid Ocean Ridge Basalts

On the basis of experimental studies of liq-uidus phases of the most magnesian MORB glass-es and of the melting behaviour of a model MORBpyrolite we have argued that the parental or pri-mary magmas in the mid-ocean ridge setting arepicritic tholeiites with sup3 13-14 MgO This argu-ment can now be supported by data available onglass and olivine phenocryst compositions indredged picritesolivine tholeiites In Table 2 wepresent an analysis of the temperatures of MORBmagmas of N-MORB type (ie LaYb lt 1 column7) and E-MORB type (LaYb gt1) Because of therapid quenching under pressure (sea-floor) ofthese magmas some of the glass compositions arevery magnesian (column 1) The liquidus olivinefor these glasses is listed in column 2 assuming anoxidation state of QFM ndash 05 log units (Ballhaus etal 1991) However in each case there are olivinephenocrysts of more magnesian compositionfrom Mg

915 to Mg921 For each of these glass

compositions liquidus olivine may be addedincrementally (using 001 wt increment of equi-librium olivine and PETROLOG programDanyuschevsky et al 2000) until the modified

Primary magmas and mantle temperatures 441

Fig 2 Role of water-content in silicate melts in depressing liquidus temperature Heavy curve is from Falloon ampDanyushevsky (2000) compiled from literature data in which water content in glass is determined directly Lightercurve is for olivine-rich compositions which do not readily quench to glass and in which data are obtained by brack-eting liquidus temperatures for known added water contents Olivine-rich basanite data from Green (1973) (soliddots based on liquidi for particular water contents) and unpublished data (open circles mid-point of temperatureinterval between liquid and olivine + liquid experiments for 1 45 7 H2O) Peridotitic komatiite data fromGreen et al (1975) Most of the data plotted in Falloon amp Danyushevsky (2000) are derived from more polymerized(ie normative olivine + hypersthene or hypersthene + quartz) liquids than the olivine-rich basanite or peridotitickomatiite and it is possible that water has a greater effect on liquidus depression in olivine-rich liquids

DH Green TJ Falloon SM Eggins GM Yaxley

melt composition has liquidus olivine matchingthe observed phenocryst of maximum forsteritecontent The resultant compositions (MgO-con-tent) are listed in column 4 (130-155 MgO)and their anhydrous liquidus temperatures in col-umn 5 (1319degC to 1362degC) In MORB water con-tents and K2O contents are closely correlated(Danyushevsky et al 2000) The effect of water inlowering liquidus temperatures is 0-15degC in N-MORB as measured water contents (and K2O con-tents) are pound 01 E-MORB have higher K2O andH2O contents (Danyushevsky et al 2000) and theeffect of dissolved water is to lower liquidus tem-peratures by ~ 30degC For the data set of Table 2the average anhydrous liquidus temperature is1345degC and considering a small effect of dis-solved water the inferred eruption temperature forthe most primitive MORB magmas which arepicrites is around 1325degC

Comparison of lsquoHot-Spotrsquo vs lsquoMORBrsquoprimary magma temperatures

From the data of Tables 1 and 2 and the pre-ceding discussion we infer that the temperaturesof eruption of primary magmas at the HawaiianlsquoHot-Spotrsquo (cone-building phase) are the samewithin ~ 20degC as those of primary magmas inmid-ocean-ridge settings including both N-

MORB and E-MORB types The slightly higheranhydrous liquidus temperatures of the lsquoHot-Spotrsquomagmas are reduced by higher volatile contents(particularly H2O) in lsquoHot-Spotrsquo magmas relativeto E-MORB and particularly to N-MORB

Compositional differences mdash MORB vsHot-Spot magmas

If the evidence from the primitive magmasthemselves argues against a significant (gt 20degCapproximately) temperature difference betweenthe product of MORB petrogenesis and lsquoHot-Spotrsquopetrogenesis then it is important to evaluate thenature and causes of compositional differencesbetween the two settings Relative to MORBlsquoHot-Spotrsquo basalts have long been equated withhigher incompatible element contents with LREEenrichment and HREE depletion and with differ-ences in isotopic ratios from Pb-isotopes to noblegas isotopes Some isotopic differences imply sep-aration between MORB sources and lsquoHot-Spotrsquosources for periods in excess of 1 billion years(Norman amp Garcia 1999 and references therein)The contrast between MORB and lsquoHot-Spotrsquo orOIB (Ocean Island Basalt) geochemistry has beenmade more complex by the recognition of E-MORB (Enriched MORB) geochemistry at mid-ocean ridges and by recognition that OIB

442

Table 2 Compositional data from Mid-Ocean Ridge picrites in which Mg-rich glass and olivine phe-nocryst compositions are available

geochemical characteristics are evident inintraplate basalts not linked to lsquoHot-Spotrsquo traces

As noted previously it is the differences inmajor-element compositions and in refractory-ele-

ment abundances and ratios of magmas which areuseful in terms of constraining the nature and com-position of residual phases We thus examine themajor-element compositions of primitive basalts

Primary magmas and mantle temperatures 443

Fig 3 Comparison of compositions of primitive MOR picrites of Table 2 and primitive lsquoHot Spotrsquo picrites of Table1 using projections into the normative lsquobasalt tetrahedronrsquo [Ol+Di+Qz+(Jd+CaTs+Lc)] Inserts show selected areasof the tetrahedral faces Projections are from(a) Olivine on to the surface Di+(Jd+CaTs+Lc)+Qz and (b) from Diopside on to the surface Ol+(Jd+CaTs+Lc)+QzThe MOR picrites form a tight cluster in both projections noting that the olivine-addition calculations from theMORB glasses gives a field elongated along the lherzolite melting trend at 15 reg 2 GPa and along a similar trend(15-2 GPa) in the projection from olivine (Fig4)The Hawaiian picrites show excellent clustering particularly in the projection from olivine and their trend and posi-tion clearly define a harzburgitic melting trend (Fig 3a and Fig 4)

DH Green TJ Falloon SM Eggins GM Yaxley444

Fig 4 Picrite compositions (Fig 3) compared with melting trends for Hawaiian Pyrolite and MORB Pyrolite (Greenamp Falloon 1998 and references therein) The MOR Picrites lie between the 15 to 2 GPa melting trends with lher-zolite to harzburgite residue (cusp in Fig 4a marking clinopyroxene disappearance from residue) Hawaiian Picrites(Loihi Hawaii) lie on the harzburgite residue trend (4a) or in the harzburgite residue field (4b) for Hawaiian Pyrolitewith Loihi and Hawaii possibly derived from a similar source plotting near the Hawaiian Pyrolite composition butLoihi representing a lower degree of partial melting Koolau picrites clearly require a different source and residuewhich from Fig 4a must have a lower CaAl and may have lower CaNa ratio than MORB Pyrolite or HawaiianPyrolite In Fig 4a and 4b we have plotted (open star) the composition of extremely refractory harzburgite fromPapuan Ultramafic Belt (PUM harzburgite) as illustrative of a potentially buoyant subducted slabwedge composi-tion (Olivine is Mg

92-3 Spinel is Cr80-90) We have also plotted the composition of a rhyodacite melt derived from

coesite eclogite at 35 GPa 1250degC and 1300degC (Yaxley amp Green 1998) Finally we have plotted the field of highMg basanitic and nephelinitic intraplate basalts which are host to spinel or spinel + garnet lherzolite xenoliths Thesecompositions are small melt fractions (lt 5 melt) from peridotite (C+H+O) in the incipient melt regime in equi-librium with Ol+Opx+Cpx+Ga Source mantle peridotites for Hawaiian Hot Spot magmas can be derived by refer-tilizing refractory mantle by addition of rhyodacite (eclogite melt) and basaniteolivine nephelinite(peridotitendash(C+H+O) incipient melt) with or without asthenospheric mantle (MORB Pyrolite)

DH Green TJ Falloon SM Eggins GM Yaxley444

Fig 4 Picrite compositions (Fig 3) compared with melting trends for Hawaiian Pyrolite and MORB Pyrolite (Greenamp Falloon 1998 and references therein) The MOR Picrites lie between the 15 to 2 GPa melting trends with lher-zolite to harzburgite residue (cusp in Fig 4a marking clinopyroxene disappearance from residue) Hawaiian Picrites(Loihi Hawaii) lie on the harzburgite residue trend (4a) or in the harzburgite residue field (4b) for Hawaiian Pyrolitewith Loihi and Hawaii possibly derived from a similar source plotting near the Hawaiian Pyrolite composition butLoihi representing a lower degree of partial melting Koolau picrites clearly require a different source and residuewhich from Fig 4a must have a lower CaAl and may have lower CaNa ratio than MORB Pyrolite or HawaiianPyrolite In Fig 4a and 4b we have plotted (open star) the composition of extremely refractory harzburgite fromPapuan Ultramafic Belt (PUM harzburgite) as illustrative of a potentially buoyant subducted slabwedge composi-tion (Olivine is Mg

92-3 Spinel is Cr80-90) We have also plotted the composition of a rhyodacite melt derived from

coesite eclogite at 35 GPa 1250degC and 1300degC (Yaxley amp Green 1998) Finally we have plotted the field of highMg basanitic and nephelinitic intraplate basalts which are host to spinel or spinel + garnet lherzolite xenoliths Thesecompositions are small melt fractions (lt 5 melt) from peridotite (C+H+O) in the incipient melt regime in equi-librium with Ol+Opx+Cpx+Ga Source mantle peridotites for Hawaiian Hot Spot magmas can be derived by refer-tilizing refractory mantle by addition of rhyodacite (eclogite melt) and basaniteolivine nephelinite(peridotitendash(C+H+O) incipient melt) with or without asthenospheric mantle (MORB Pyrolite)

for compatibility with the results of high-pressuremelting studies For this purpose the calculatedprimary magma compositions are projected intothe basalt tetrahedron (Yoder amp Tilley 1962) usinga normative projection (Fig 3 Green amp Falloon1998)

The MORB glasses of Table 2 when projectedfrom diopside on to the base of the basalt tetrahe-dron [Ol+(Jd+Cats+Lc)+Qz] define a small fieldwithin the larger field of MORB glasses with Mg

gt 68 defined by Falloon amp Green (1988) The cal-culation of primary liquids based on the most mag-nesian olivine phenocrysts moves this field tohigher normative olivine compositions againlying within the field of primary MOR picritesdefined by Falloon amp Green (1987 1988) In theprojection from olivine the glasses and estimatedprimary magmas are coincident and define a smallfield near the 15-20 normative diopside contourWhen the experimentally defined partial meltingtrends for MORB pyrolite are plotted on the nor-mative tetrahedron and from mass balance calcu-lations it is clear that these inferred primarymagma compositions are consistent with deriva-tion from the MORB pyrolite composition by ~20-25 melting and magma segregation at 15-20 GPa leaving a clinopyroxene-bearing harzbur-gite residue

With higher incompatible element contents inlsquoHot-Spotrsquo or OIB basalts and noting their relativeHREE-depletion and LREE enrichment it is com-monly inferred that these magmas derive fromdeeper melting processes with compositionsreflecting equilibrium with garnet as a residualphase In terms of plotting in the normative basalttetrahedron higher-pressure liquids in equilibriumwith garnet lherzolite will lie to the silica-under-saturated side of the Ol+Di+Plag plane and furthertowards the olivine apex in the projection fromdiopside As shown in Fig 4 this is the field ofmantle xenolith-bearing intra-plate basalt (alkaliolivine basalts olivine-rich basanites to olivinenephelinites and olivine melilitites) and experi-mental studies of the liquidus phases of selectedhigh-Mg basalts with added H2O plusmn CO2 haveestablished conditions for genesis of such melts bylow degrees of partial melting at pressures sup3 2 GPa(eg Frey et al 1978)

It is therefore somewhat unexpected that theinferred primary magmas for the Hawaiian picritesplot to the silica-rich side of the Ol+Di+Plag planeof the normative tetrahedron It is particularlynotable that each volcano forms a tight cluster inthe projection from olivine but a scatter largely inthe direction of olivine addition or subtraction in

the projection from diopside This scatter is with-in the harzburgite-residue field (ie close to theOl-Qz-Di face of the tetrahedron rather than in thelherzolite-residue field which lies across the Ol-Plag-Di plane In the projection from olivine thecompositions from each volcano consistentlydefine trends which we can identify from partialmelting studies as lsquoharzburgite-residue meltingtrendsrsquo In the projection from olivine this trend isalmost perpendicular to the lsquolherzolite meltingtrendrsquo which controls melt composition withincreasing degree of melting up to the eliminationof Ca-clinopyroxene The dispersion alongolivine-control lines in the projection from diop-side suggests that the olivine-addition and subtrac-tion methods applied to the picrites is capable ofimprovement eg for Loihi samples the mostmagnesian olivine observed at Mg

885 might havebeen re-equilibrated and a more magnesian sourceMg

90-91 required The plotting position in the pro-jection from olivine is however lsquoblindrsquo to thisuncertainty

In Fig 4 the primary picrites from Hawaii arecompared with the melting trends from Hawaiianpyrolite and from MORB pyrolite Loihi and thefour volcanoes from the main island Hawaii ploton or very close to a harzburgite-residue meltingtrend with Loihi representing a smaller degree ofmelting and possibly lying at the clinopyroxene-out cusp On the other hand Koolau picrites areconsistent with a harzburgite -residue meltingtrend but for a source lherzolite with a lower CaAlratio than MORB pyrolite or Hawaiian pyrolite(evident from the plotting position in the olivineprojection) In the projection from diopside thereis a suggestion that melt-segregation may haveoccurred at 10-15 GPa rather than 15-20 GPainferred from the plotting positions of the Loihiand Hawaii (main island) data

Finally experimental studies of the liquidusphases of primitive N-MORB glasses (particularlyDSDP 3-18) and of olivine-enriched derivativesestablished saturation in olivine followed byclinopyroxene to ~ 18 GPa and olivine+orthopy-roxene followed by clinopyroxene at 20 GPa(Green et al 1979 Green amp Falloon 1998)These liquids are thus consistent with lherzoliteresidues In contrast experimental study ofKilauean tholeiitic picrite showed olivine fol-lowed by orthopyroxene at 1-15 GPa andorthopyroxene alone as a liquidus phase at 18GPa (Green amp Ringwood 1967 Green 1970Eggins 1992) Trace-element geochemists haveused the relative LREE enrichment and HREEdepletion of Hawaiian tholeiites to argue for gar-

Primary magmas and mantle temperatures 445

DH Green TJ Falloon SM Eggins GM Yaxley

net as a residual phase in melt extraction requiringboth a very small degree of partial melting and ahigh pressure of melt segregation However boththe direct experimental studies and the fact thatprimitive Hawaiian tholeiitic picrites lie in thefield for harzburgite not lherzolite and particular-ly not garnet lherzolite residues establish thatgarnet is not a residual phase buffering the com-position of primary Hawaiian picrites

To summarize the major-element compositionof parental or primary MOR picrites (N- and E-MORB) show that the MOR picrites have lherzo-lite to harzburgite residues However the parentalor primary Hawaiian picrites have harzburgiteresidues with Loihi magmas indicating a some-what lower degree of melting at and slightlybeyond clinopyroxene-out in the peridotite melt-ing interval Mauna Loa Kilauea Hualalai andKohala all indicate more refractory harzburgiteresidues but a similar source composition toLoihi while the major-element composition ofKoolau picrites indicates a distinctive source ofrelatively higher AlCa ratio

Further evidence of refractory characterof Hawaiian sourceresidue relative to

MORB sourceresidue

It has previously been noted that the CaO con-tent (CaMgSiO4 solid solution) of olivine phe-nocrysts in Hawaiian magmas increases in the orderKoolau lt Hawaii (Kilauea Mauna Loa HualalaiKohala) lt Loihi (Norman amp Garcia 1999) Thisparameter reflects the normative diopside content ofthe magmas (Fig 3 and 4) Norman amp Garcia (1999Fig 3) also showed that in a plot of Cr vs Mg ofspinel phenocrysts and spinels included in olivinethe Cr of Hawaiian spinels is 65-75 contrastingwith MORB spinels at Cr ~ 10-60 and approachingthe spinel in high Ca boninites (Cr = 70-80) andlow-Ca boninites (Cr = 80-90) In direct meltingstudies of spinel lherzolite (Jaques amp Green 1980)residual spinel increases from Cr = 30 to Cr = 80with increasing degree of melting at 1 and 15 GPaThus the higher Cr of liquidus spinels for Hawaiianpicrites relative to MOR picrites is strong support-ing evidence that the source peridotite for Hawaiianpicrites was a more refractory composition in termsof CrAl ratio than the source lherzolite for MORpicrites Alternatively if source peridotites weresimilar in composition then the Hawaiian picritesrequire a much higher degree of partial melting aconclusion inconsistent with their relative incom-patible element contents

In addition to the difference in CrAl Ballhauset al (1991) and Norman amp Garcia (1999) demon-strate that the Hawaiian spinel compositions indi-cate fO2 conditions near QFM + 05 log unitsslightly more oxidized than spinels of MORB(Ballhaus et al 1991)

Thermal vs compositional anomaly forHawaiian lsquoplumersquo

One of the major arguments for the existenceof a deep-seated thermal plume beneath Hawaiihas been the observation that the Hawaiian Islandchain lies on the crest of a broad topographic swellelevated approximately 1 km above the surround-ing ocean floor It has been inferred that the topo-graphic high is a consequence of lower meandensity in the upwelling column (a buoyancyplume) and that the lower density is due to highertemperatures (White amp McKenzie 1989) A tem-perature difference of ~ 200degC is required to pro-duce a density difference Dr = 002 A similardensity difference is produced by a compositionaldifference in olivine from Mg

89 to Mg91

If the composition of average asthenosphericmantle is close to the MORB pyrolite estimatebased on natural lherzolites with 3-4 CaOAl2O3 Mg raquo 89 then the density (at room TP) ofa garnet lherzolite assemblage is ro = 335-336Mg

Ol = 895 After extraction of ~ 20 MORpicrite to form oceanic crust and lithosphere theresidual lherzolite in the lithosphere has ~ 15-2 CaO Al2O3 and the density for garnet lherzolite isro = 333 Mg

Ol = 90 We also know that in theconvergent margin environment magmas such asback-arc basin tholeiites high-Ca and low-Caboninites are formed and the latter in particularleave extremely refractory harzburgite with mag-nesian olivine Mg

925 low Al2O3 low CaOorthopyroxene (Mg

925) and chromian spinel (Cr

= 80-90) Such harzburgites are characteristic ofophiolitic ultramafics of Western Pacific type asillustrated in Papua New Guinea New CaledoniaPhilippines etc Garnet does not form in suchrefractory harzburgite at high pressure (below heenstatite to majorite reaction) and ro = 330 Mg

Ol= 925 Thus in the subduction of oceanic crustand lithosphere including elements of the overly-ing mantle wedge bodies of refractory lherzoliteto harzburgite may be returned to the mantle withboth a temperature contrast (cooler) and composi-tional contrast (Dr = 002 to 006) The buoyancyof such slabs relative to asthenospheric mantlewill be determined by compositional factors and

446

their approach to temperature equilibrium withsurrounding mantle Such slabs may becomedetached and suspended in the upper mantle

We suggest that subduction of cool lithospherein the slab-wedge environment of Western Pacifictype has left residues of inhomogeneous harzbur-gitic to lherzolitic mantle which become neutral(suspended) or buoyant within the upper mantleCompositional buoyancy implying suspendedslabs of lithospheric mantle at 150 to 500 kmdepth is proposed as an alternative cause of theHawaiian Swell

Melting of old neutrally buoyantsubducted slabs

If detached slabs of refractory lithosphere andformer oceanic crust become suspended in theupper mantle then partial melting may be initiatedin several ways

a) Conductive heating will given time erasethe temperature contrast between slab and ambient

mantle Compositions with lower solidus tempera-tures will melt preferentially and such partialmelts may migrate causing compositional andtemperature redistribution

b) A redox contrast between subducted slab(IW + 3 to 4 log units) and asthenospheric or deep-er mantle (IW + 1 log unit) is a potential focus formelting (Green et al 1987) If normal astheno-sphere has small water and carbon contents [asinferred from measurable CampH contents ofMORB] then at = IW + 1 log unit a potentialmobile fluid phase is dominated by (CH4 + H2O)(Green et al 1987) at temperatures near the peri-dotite solidus and pressures sup3 3 GPa If such amobile fluid phase encounters a volume of mantlewith higher fO2 then oxygen is extracted from themantle volume carbon is precipitated and watercontent and activity in the fluid phase is increasedSince the solidus of lherzolite harzburgite oreclogite is sharply depressed as fH2O in a fluidincreases the redox front becomes a locus forincipient melting in the peridotitendash(C+H+O) sys-tem

Primary magmas and mantle temperatures 447

Fig 5 Projection from Clinopyroxene (Jadeite+Diopside) on to plane Olivine + Garnet + Coesite in the high pres-sure normative tetrahedron (Yaxley amp Green 1998) The figure illustrates extraction of picritic melts lying near thelherzolite minimum melt for the composition marked lsquoMantlersquo Residues approach the Olivine + Orthopyroxene joinOn the coesite-normative side of the figure the composition of average Oceanic Crust is plottedIn an inhomogeneous mantle of recycled lithosphere (mantle + residue) = Oceanic Crust which is being re-heated oris part of a Mantle Plume melting first begins at the (coesite+omphacite+garnet) minimum (~ 1220degC at 35 GPa)and the melt is rhyodacitic in composition Extraction of such melts drives the residue composition on to the garnet+ pyroxene thermal divide and migration of melt into and reaction with or crystallization within surrounding peri-dotite moves the bulk compositions of the lsquoMantlersquo and lsquoResiduersquo in the direction illustrated

DH Green TJ Falloon SM Eggins GM Yaxley

c) As the refractory slab becomes warmer byconductive heating its density will decrease and itmay become positively buoyant If the volume islarge then a diapir or plume may begin to ascendmdash partial melting will occur by adiabatic decom-pression and will occur selectively according tothe chemical inhomogeneity of the plume ordiapir

The key melting relationships of a mixed man-tle region at depths of ~ 120 km (35 GPa) areillustrated in Fig 5 in which compositions are pre-sented in terms of high-pressure normative com-ponents reflecting high-pressure mineralogy ieolivine orthopyroxene clinopyroxene (includingjadeite solid solution) garnet and coesite Thepyroxene-garnet plane divides silica-normativecompositions (basaltic andesitic dacitic composi-tions) from olivine -normative compositions(olivine-rich picrites olivine nephelinites andperidotites) Referring to Fig 5 at 35 GPa anaverage oceanic crust composition has subsolidusmineralogy of garnet + clinopyroxene (omphacite)+ coesite At 1250degC this composition consists ofdacitic to rhyodacitic melt with residual garnet +omphacite The melt is strongly reactive towardsenclosing peridotite and will react and freeze at ornear the eclogiteperidotite contact The enclosingperidotite is then locally enriched in orthopyrox-ene garnet and clinopyroxene mdash refertilized lher-zolite with distinctive low CaAl and TiAl andrelatively high NaCa (Compositional vectorsindicating extraction of rhyodacitic melt fromoceanic crust and addition to pyrolitic mantle or toresidual harzburgitic mantle mdash after basaltpicriteextraction mdash are shown in Fig 5 and may beinferred from the plotted positions of eclogitemelts pyrolite and harzburgite on Fig 4)

With increasing temperature the melt compo-sition in the eclogite component is constrained tolie on the garnet + clinopyroxene plusmn orthopyroxenecotectic (Fig 5) but cannot mix across the pyrox-ene + garnet plane This plane acts as a thermaldivide with residual garnet + clinopyroxene plusmnorthopyroxene from the original eclogite becom-ing progressively more Mg-rich and jadeite-poorrespectively as the dacitic to andesitic melt leavesthe eclogite to react with and freeze within theenclosing peridotite This type of enrichment orrefertilisation of refractory harzburgite or lherzo-lite is suggested as the mechanism for generatingthe distinctive Koolau source identified previous-ly

If the temperature reaches ~ 1420degC then melt-ing occurs at the olivine + pyroxene + garnet min-imum in the peridotite Melts are picritic

nephelinites controlled by the coexistence ofolivine + orthopyroxene + clinopyroxene + garnetThis type of refertilisation by migration of incipi-ent melt of picritic nephelinite character mayoccur in an intraplate setting and initiate the buoy-ancy by partial melting and detachment of a diapiror plume from the larger source (slab) or its mar-gins

In an evolving regime of temperature increaseat high pressure or in adiabatic decompression atTp ~ 1450degC the differential melting behaviourbetween coesite eclogite and enclosing harzbur-gite or lherzolite will lead towards homogeniza-tion of mineral compositions in compatible andincompatible elements (but not in strongly refrac-tory components such as Cr2O3 NiO) Howeverthe original heterogeneity of the subducted ocean-ic crust refractory lithosphere and enclosing man-tle will remain evident in modal variations eg anoriginal coesite eclogite with Mg ~ 65 may final-ly become a garnet (Mg83) + diopside (Mg91) lensor layer contrasting with surrounding garnet(Mg83) lherzolite or harzburgite in which garnet isCr-rich and diopside rich in Na + Cr

If during adiabatic upwelling of a plume ordiapir of inhomogeneous recycled mantle ofharzburgite to garnet pyroxenite composition thegarnet lherzolite solidus is exceeded and meltingincreases with decompressionupwelling then thecontinuing presence of melt trapped within theplume or diapir will tend to homogenize phasecompositions but not residual phase proportionsWith increasing melting at decreasing pressuremelts are constrained to lie on the garnet +clinopyroxene + orthopyroxene + olivine cotecticfor low degrees of melting at high pressure adjust-ing to the (Cr+Al) spinel + clinopyroxene +orthopyroxene + olivine (lherzolite) cotectic athigher degrees of melting at lower pressure andthen to the spinel (Cr-rich) + orthopyroxene +olivine (harzburgite residue) for even higherdegrees of melting The positions of these cotec-tics in the normative basalt tetrahedron have beendefined by experimental studies and reflect bothbulk composition and PT control (Fig 4)

In relation to the preceding discussion it is rel-evant to note recent work on the HoromanPeridotite in Hokkaido Japan Takazawa et al(2000) have inferred a complex P T time historywhich is summarized in Fig 6 This history is ofprimary crustlithosphere formation followed bysubduction and modification by melting [includ-ing both selective melting of eclogite (slab andwedge environment) and incipient melting in gar-net-lherzolite regime (intraplate setting)] This

448

DH Green TJ Falloon SM Eggins GM Yaxley

c) As the refractory slab becomes warmer byconductive heating its density will decrease and itmay become positively buoyant If the volume islarge then a diapir or plume may begin to ascendmdash partial melting will occur by adiabatic decom-pression and will occur selectively according tothe chemical inhomogeneity of the plume ordiapir

The key melting relationships of a mixed man-tle region at depths of ~ 120 km (35 GPa) areillustrated in Fig 5 in which compositions are pre-sented in terms of high-pressure normative com-ponents reflecting high-pressure mineralogy ieolivine orthopyroxene clinopyroxene (includingjadeite solid solution) garnet and coesite Thepyroxene-garnet plane divides silica-normativecompositions (basaltic andesitic dacitic composi-tions) from olivine -normative compositions(olivine-rich picrites olivine nephelinites andperidotites) Referring to Fig 5 at 35 GPa anaverage oceanic crust composition has subsolidusmineralogy of garnet + clinopyroxene (omphacite)+ coesite At 1250degC this composition consists ofdacitic to rhyodacitic melt with residual garnet +omphacite The melt is strongly reactive towardsenclosing peridotite and will react and freeze at ornear the eclogiteperidotite contact The enclosingperidotite is then locally enriched in orthopyrox-ene garnet and clinopyroxene mdash refertilized lher-zolite with distinctive low CaAl and TiAl andrelatively high NaCa (Compositional vectorsindicating extraction of rhyodacitic melt fromoceanic crust and addition to pyrolitic mantle or toresidual harzburgitic mantle mdash after basaltpicriteextraction mdash are shown in Fig 5 and may beinferred from the plotted positions of eclogitemelts pyrolite and harzburgite on Fig 4)

With increasing temperature the melt compo-sition in the eclogite component is constrained tolie on the garnet + clinopyroxene plusmn orthopyroxenecotectic (Fig 5) but cannot mix across the pyrox-ene + garnet plane This plane acts as a thermaldivide with residual garnet + clinopyroxene plusmnorthopyroxene from the original eclogite becom-ing progressively more Mg-rich and jadeite-poorrespectively as the dacitic to andesitic melt leavesthe eclogite to react with and freeze within theenclosing peridotite This type of enrichment orrefertilisation of refractory harzburgite or lherzo-lite is suggested as the mechanism for generatingthe distinctive Koolau source identified previous-ly

If the temperature reaches ~ 1420degC then melt-ing occurs at the olivine + pyroxene + garnet min-imum in the peridotite Melts are picritic

nephelinites controlled by the coexistence ofolivine + orthopyroxene + clinopyroxene + garnetThis type of refertilisation by migration of incipi-ent melt of picritic nephelinite character mayoccur in an intraplate setting and initiate the buoy-ancy by partial melting and detachment of a diapiror plume from the larger source (slab) or its mar-gins

In an evolving regime of temperature increaseat high pressure or in adiabatic decompression atTp ~ 1450degC the differential melting behaviourbetween coesite eclogite and enclosing harzbur-gite or lherzolite will lead towards homogeniza-tion of mineral compositions in compatible andincompatible elements (but not in strongly refrac-tory components such as Cr2O3 NiO) Howeverthe original heterogeneity of the subducted ocean-ic crust refractory lithosphere and enclosing man-tle will remain evident in modal variations eg anoriginal coesite eclogite with Mg ~ 65 may final-ly become a garnet (Mg83) + diopside (Mg91) lensor layer contrasting with surrounding garnet(Mg83) lherzolite or harzburgite in which garnet isCr-rich and diopside rich in Na + Cr

If during adiabatic upwelling of a plume ordiapir of inhomogeneous recycled mantle ofharzburgite to garnet pyroxenite composition thegarnet lherzolite solidus is exceeded and meltingincreases with decompressionupwelling then thecontinuing presence of melt trapped within theplume or diapir will tend to homogenize phasecompositions but not residual phase proportionsWith increasing melting at decreasing pressuremelts are constrained to lie on the garnet +clinopyroxene + orthopyroxene + olivine cotecticfor low degrees of melting at high pressure adjust-ing to the (Cr+Al) spinel + clinopyroxene +orthopyroxene + olivine (lherzolite) cotectic athigher degrees of melting at lower pressure andthen to the spinel (Cr-rich) + orthopyroxene +olivine (harzburgite residue) for even higherdegrees of melting The positions of these cotec-tics in the normative basalt tetrahedron have beendefined by experimental studies and reflect bothbulk composition and PT control (Fig 4)

In relation to the preceding discussion it is rel-evant to note recent work on the HoromanPeridotite in Hokkaido Japan Takazawa et al(2000) have inferred a complex P T time historywhich is summarized in Fig 6 This history is ofprimary crustlithosphere formation followed bysubduction and modification by melting [includ-ing both selective melting of eclogite (slab andwedge environment) and incipient melting in gar-net-lherzolite regime (intraplate setting)] This

448

evolution from refractory mantle to re-enrichedinhomogeneous mantle is followed by upwellingand incipient melting in the plagioclase lherzolitestability field The sequence of events proposedfor Horoman Peridotite has similarities with themodel proposed here for the Hawaiian source (ref-erence also Green et al 1987 Fig 6 Green ampFalloon 1998 Fig 712)

Conclusions

Examination of bulk compositions glass com-positions and olivine phenocrysts from Hawaiianpicrites and from Mid-Ocean Ridge picrites (N-MORB and E-MORB) demonstrates that the liq-uidus temperatures of primary or parental magmasare approximately 1320degC in both settings Thereis no evidence from the magmas themselves for ahigh temperature (DT = 100-200degC) deep-seatedmantle plume beneath Hawaii as picritic magmasnot olivine tholeiite magmas are parental or pri-mary magmas of both the Hawaiian lsquoHot-Spotrsquoand Mid-Ocean Ridges

The primary picrites of both settings havemajor-element compositions consistent with equi-librium partial melting of lherzolite and magmasegregation at 15-20 GPa and approximately1430-1450degC The modern mantle has a potentialtemperature near 1430degC and this is a controllingfactor in Mid-Ocean Ridge lsquoHot-Spotrsquo Back-Arcand Island Arc magmatism

Although we argue for similar mantle temper-atures controlling Hawaiian (lsquoHot-Spotrsquo) volcan-ism and Mid-Ocean Ridge volcanism and forsimilar depths of magma segregation there isclear evidence for compositional differencebetween the primary magmas and thus betweentheir sources Hawaiian picrites have highervolatile (CO2+H2O) contents higher incompati-ble element contents (LILE) higher LREE andlower HREE than MOR picrites Converselythey have higher CrAl ratios in liquidus spinelsand the most magnesian liquidus olivines are sim-ilar in both cases We have demonstrated thatHawaiian picrites leave a more refractoryharzburgite residue in contrast to a lherzolitic toharzburgitic residue for MORB picrites

Primary magmas and mantle temperatures 449

Fig 6 The possible Pressure-Temperature-time history ofHoroman Peridotite as inferredby Takazawa et al (2000)Figure modified from Takazawaet al (2000) Fig17 as an illus-tration of a peridotite with a longhistory from primary upwellingthrough subduction and refertil-ization to uplift and reactions ina low pressure and high temper-ature emplacement

DH Green TJ Falloon SM Eggins GM Yaxley

The major element trace element and isotopicfingerprints of Hawaiian tholeiitic magmas appearto require a four-component mixing

a) Residual refractory harzburgite (MgOl ~ 91-92 CrSp ~ 70)

b) Local refertilization of residual harzburgitein a subduction environment by partial melting ofsubducted oceanic crust giving pyroxene-richhaloes and garnet pyroxenite residues from initialcoesite eclogite

c) Migration of an incipient melt characteris-tic of the peridotitendash(C+H+O) system at P gt 25GPa T ~ 1250-1450degC within the garnet stabilityfield and at PT conditions between the peri-dotitendash(C+H+O) solidus and the (C+H)-free peri-dotite solidus

d) Addition of (CH4+H2O) fluid phase at fO2 raquoIW + 1 log unit the fluid phase being derived fromdeeper levels on the mantle and probably carryinga noble-gas signature from deeper mantle

The time intervals and tectonic setting for pro-cesses a) to d) can be explored by trace elementand isotopic abundances The initial stage (a) maybe formation of lithosphere in a mid-ocean ridgeenvironment or may reflect several melt extractionevents culminating in boninite extraction in a con-vergent marginmantle wedge setting Stage (b) isenvisaged as a consequence of prograde metamor-phism and tectonic interleaving of subductedoceanic crust underlying oceanic lithosphere andoverlying mantle wedge lithosphere Stage (c) isenvisaged as an intraplate setting in whichdetached and neutrally buoyant lsquoold-subductedslabsrsquo approach ambient temperature over timeand begin to melt at the peridotitendash(C+H+O)solidus Stage (d) is continuous over time if man-tle degassing is pervasive although a (CH4+H2O)fluid is mobile at fO2 pound IW + 1 but initiates melt-ing at the peridotitendash(C+H+O) solidus if itencounters oxidized mantle such as old subductedslab In this case carbon is deposited and H2O-richfluid initiates melting (Green et al 1987 Green ampFalloon 1998)

The model proposed suggests that theHawaiian Swell or Arch is a response to density-contrast in or below the asthenosphere The densi-ty contrast is interpreted as a consequence of arelict more refractory but inhomogeneousharzburgitic volume of mantle (noting that Dro =001 is equivalent to DT ~ 100degC) Green et al(1987) and Green amp Falloon (1998) inferred thatsuch compositional lsquoplumesrsquo or anomalous mantlecould form from old subducted slabsAlternatively delamination of refractory deepcontinental lithosphere may provide the composi-

tional anomaly Trace element and isotope geo-chemists have consistently advocated the mixingof several components in the source for lsquoHot-Spotrsquobasalts representing reservoirs evolved indepen-dently over times up to 1-2 billion years (cfNorman amp Garcia 1999 and references therein)In terms of the lsquodeep mantle plumersquo model thisconcept of geochemically mixed source has beenincorporated by accumulating subducted oceaniccrust and lithosphere at the coremantle boundaryreheating this material as a thermal boundary layerand ascent as thermal plumes from the coreman-tle boundary (Davies 1999)

In this paper we detect no temperature differ-ence between magmatism at the type lsquoHot-Spotrsquo orlsquoPlume tailrsquo expression ie Hawaii and Mid-Ocean Ridge settings We argue that the cause ofisland chain volcanism is a compositional buoyan-cy anomaly at depths below or in the astheno-sphere Upwelling mantle plumes or diapirsfeeding individual Hawaiian volcanoes aresourced in the interface between old subductedslabs or delaminated continental lithosphere andambient upper mantle (MORB source) probablyat depths of 200-300 km with incipient melting[peridotitendash(C+H+O)] giving place to major melt-ing during adiabatic upwelling from these depthsThe lsquoHawaiian Plumersquo is inferred to be a thermaland partial melting anomaly only at relativelyshallow depths particularly in penetrating into theoceanic lithosphere but to have its origins in along-lived compositional anomaly at or nearambient mantle temperatures within the astheno-sphere of the Upper Mantle

References

Ballhaus C Berry RF Green DH (1991) High pres-sure experimental calibration of the olivine-orthopy-roxene-spinel oxygen geobarometer implications forthe oxidation state of the upper mantle ContribMineral Petrol 107 27-40

Clague DA Weber WS Dixon EJ (1991) Picriticglasses from Hawaii Nature 353 420

Davies GF (1999) ldquoDynamic Earth Plates Plumes andMantle Convectionrdquo Cambridge University PressCambridge 460 pp

Danyushevsky LV (1998) The effect of small amounts ofwater on fractionation of mid-ocean ridge basaltsAbst EOS 79 No 17 S375

Danyushevsky LV Eggins SM Falloon TJ ChristeDM (2000) H2O geochemistry in depleted to moder-ately enriched mid-ocean ridge magmas part 1 incom-patible behaviour implications for mantle storage andorigin of regional variations J Petrol 41 1329-1364

450

DH Green TJ Falloon SM Eggins GM Yaxley

The major element trace element and isotopicfingerprints of Hawaiian tholeiitic magmas appearto require a four-component mixing

a) Residual refractory harzburgite (MgOl ~ 91-92 CrSp ~ 70)

b) Local refertilization of residual harzburgitein a subduction environment by partial melting ofsubducted oceanic crust giving pyroxene-richhaloes and garnet pyroxenite residues from initialcoesite eclogite

c) Migration of an incipient melt characteris-tic of the peridotitendash(C+H+O) system at P gt 25GPa T ~ 1250-1450degC within the garnet stabilityfield and at PT conditions between the peri-dotitendash(C+H+O) solidus and the (C+H)-free peri-dotite solidus

d) Addition of (CH4+H2O) fluid phase at fO2 raquoIW + 1 log unit the fluid phase being derived fromdeeper levels on the mantle and probably carryinga noble-gas signature from deeper mantle

The time intervals and tectonic setting for pro-cesses a) to d) can be explored by trace elementand isotopic abundances The initial stage (a) maybe formation of lithosphere in a mid-ocean ridgeenvironment or may reflect several melt extractionevents culminating in boninite extraction in a con-vergent marginmantle wedge setting Stage (b) isenvisaged as a consequence of prograde metamor-phism and tectonic interleaving of subductedoceanic crust underlying oceanic lithosphere andoverlying mantle wedge lithosphere Stage (c) isenvisaged as an intraplate setting in whichdetached and neutrally buoyant lsquoold-subductedslabsrsquo approach ambient temperature over timeand begin to melt at the peridotitendash(C+H+O)solidus Stage (d) is continuous over time if man-tle degassing is pervasive although a (CH4+H2O)fluid is mobile at fO2 pound IW + 1 but initiates melt-ing at the peridotitendash(C+H+O) solidus if itencounters oxidized mantle such as old subductedslab In this case carbon is deposited and H2O-richfluid initiates melting (Green et al 1987 Green ampFalloon 1998)

The model proposed suggests that theHawaiian Swell or Arch is a response to density-contrast in or below the asthenosphere The densi-ty contrast is interpreted as a consequence of arelict more refractory but inhomogeneousharzburgitic volume of mantle (noting that Dro =001 is equivalent to DT ~ 100degC) Green et al(1987) and Green amp Falloon (1998) inferred thatsuch compositional lsquoplumesrsquo or anomalous mantlecould form from old subducted slabsAlternatively delamination of refractory deepcontinental lithosphere may provide the composi-

tional anomaly Trace element and isotope geo-chemists have consistently advocated the mixingof several components in the source for lsquoHot-Spotrsquobasalts representing reservoirs evolved indepen-dently over times up to 1-2 billion years (cfNorman amp Garcia 1999 and references therein)In terms of the lsquodeep mantle plumersquo model thisconcept of geochemically mixed source has beenincorporated by accumulating subducted oceaniccrust and lithosphere at the coremantle boundaryreheating this material as a thermal boundary layerand ascent as thermal plumes from the coreman-tle boundary (Davies 1999)

In this paper we detect no temperature differ-ence between magmatism at the type lsquoHot-Spotrsquo orlsquoPlume tailrsquo expression ie Hawaii and Mid-Ocean Ridge settings We argue that the cause ofisland chain volcanism is a compositional buoyan-cy anomaly at depths below or in the astheno-sphere Upwelling mantle plumes or diapirsfeeding individual Hawaiian volcanoes aresourced in the interface between old subductedslabs or delaminated continental lithosphere andambient upper mantle (MORB source) probablyat depths of 200-300 km with incipient melting[peridotitendash(C+H+O)] giving place to major melt-ing during adiabatic upwelling from these depthsThe lsquoHawaiian Plumersquo is inferred to be a thermaland partial melting anomaly only at relativelyshallow depths particularly in penetrating into theoceanic lithosphere but to have its origins in along-lived compositional anomaly at or nearambient mantle temperatures within the astheno-sphere of the Upper Mantle

References

Ballhaus C Berry RF Green DH (1991) High pres-sure experimental calibration of the olivine-orthopy-roxene-spinel oxygen geobarometer implications forthe oxidation state of the upper mantle ContribMineral Petrol 107 27-40

Clague DA Weber WS Dixon EJ (1991) Picriticglasses from Hawaii Nature 353 420

Davies GF (1999) ldquoDynamic Earth Plates Plumes andMantle Convectionrdquo Cambridge University PressCambridge 460 pp

Danyushevsky LV (1998) The effect of small amounts ofwater on fractionation of mid-ocean ridge basaltsAbst EOS 79 No 17 S375

Danyushevsky LV Eggins SM Falloon TJ ChristeDM (2000) H2O geochemistry in depleted to moder-ately enriched mid-ocean ridge magmas part 1 incom-patible behaviour implications for mantle storage andorigin of regional variations J Petrol 41 1329-1364

450

Dmitriev LV Sobolev AV Uchanov AVMalysheva TV Melson WG (1984) Primary dif-ferences in oxygen fugacity and depth of melting inthe mantle source regions for oceanic basalts EarthPlanet Sci Lett 70 303-310

Eggins SM (1992) Petrogenesis of Hawaiian tholei-ites 1 phase equilibria constraints ContribMineral Petrol 110 387-397

Falloon TJ amp Danyushevsky LV (2000) Melting ofrefractory mantle at 15 2 and 25 GPA under anhy-drous and H2O-undersaturated conditionsImplications for high-Ca boninites and the influenceof subduction components on mantle melting JPetrol 41 257-283

Falloon TJ amp Green DH (1987) Anhydrous partialmelting of MORB pyrolite and other peridotite com-positions at 10 kbar and implications for the origin ofprimitive MORB glasses Miner Petrol 37 181-219

ndash ndash (1988) Anhydrous partial melting of peridotitefrom 8 to 35 kbar and petrogenesis of MORB JPetrol Special Lithosphere Issue 379-414

Ford CE Russell JA Craven Fisk MR (1983)Olivine-liquid equilibria temperature pressure andcomposition dependence of the crystalliquid cationpartition coefficients for Mg Fe2+ Ca and Mn JPetrol 24 256-265

Frey FA Green DH Roy SD (1978) Integratedmodels of basalt petrogenesis - a study of quartztholeiites to olivine melilitites from southeasternAustralia utilizing geochemical and experimentalpetrological data J Petrol 19 463-513

Garcia MO Muenow DW Aggrey KE OrsquoNeil JR(1989) Major element volatile and stable isotopegeochemistry of Hawaiian submarine tholeiiticglasses J Geophys Res 94 10525-10538

Green DH (1970) The origin of basaltic andnephelinitic magmas Bennett Lecture University ofLeicester Trans Leicester Lit Philos Soc 44 28-54

ndash (1973) Conditions of melting of basanite magma fromgarnet peridotite Earth Planet Sci Lett 17 456-465

Green DH amp Falloon TJ (1998) Pyrolite ARingwood concept and its current expression InldquoThe Earthrsquos Mantle Composition Structure andEvolutionrdquo (INS Jackson Ed) CambridgeUniversity Press 311-380

Green DH amp Ringwood AE (1967) The genesis ofbasaltic magmas Contrib Mineral Petrol 15 103-190

Green DH Nicholls IA Viljoen R Viljoen M(1975) Experimental demonstration of the exis-tence of peridotitic liquids in earliest Archaean mag-matism Geology 3 15-18

Green DH Hibberson WO Jaques AL (1979)Petrogenesis of mid-ocean ridge basalts In ldquoTheEarth Its Origin Structure and Evolutionrdquo (MWMcElhinny Ed) Academic Press London 265-290

Green DH Falloon TJ Taylor WR (1987) Mantle-derived magmas - roles of variable source peridotiteand variable C-H-O fluid compositions In

ldquoMagmatic Processes and PhysicochemicalPrinciplesrdquo (BO Mysen Ed) Geochem SocSpec Publ 12 139-154

Jaques AL amp Green DH (1980) Anhydrous meltingof peridotite at 0-15 kb pressure and the genesis oftholeiitic basalts Contrib Mineral Petrol 73 287-310

Kamenetsky V (1996) Methodology for the study ofmelt inclusions in Cr-spinel and implications forparental melts of MORB from FAMOUS areaEarth Planet Sci Lett 70 303-310

McKenzie D amp Bickle MJ (1988) The volume andcomposition of melt generated by extension of thelithosphere J Petrol 29 625-679

McNeill AW amp Danyushevsky LV (1996)Composition and crystallization temperatures of pri-mary melts from Hole 896A basalts evidence frommelt inclusion studies In Alt JC Kinoshita HStokking LB and Michael PJ (eds) ldquoProc ODPSci Resultsrdquo 148 College Station TX (OceanDrilling Program) 21-35

Michael P (1995 )Regionally distinctive sources ofdepleted MORB Evidence from trace elements andH2O Earth Planet Sci Lett 131 301-320

Norman MD amp Garcia MO (1999) Primitive mag-mas and source characteristics of the Hawaiianplume petrology and geochemistry of shieldpicrites Earth Planet Sci Lett 168 27-44

Perfit MR Fornari DJ Ridley WI Kirk PDCasey J Kastens KA Reynolds JR EdwardsM Desonie D Shuster R Paradis S (1996)Recent vocanism in the Siqueiros transform faultpicritic basalts and implications for MORB magmagenesis Earth Planet Sci Lett 141 91-108

Roedder PL amp Emslie RF (1970) Olivine-liquidequilibrium Contrib Mineral Petrol 29 275-289

Takazawa E Frey FA Shimizu N Obtata M (2000)Whole rock compositional variations in an upper-mantle peridotite (Horoman Hokkaido Japan) Arethey consistent with a partial melting processGeochim Cosmochim Acta 64 695-716

White RW amp McKenzie DP (1989) Magmatism atrift zones the general of volcanic continental mar-gins and flood basalts J Geophys Res 94 7685-7729

Yaxley GM amp Green DH (1998) Reactions betweeneclogite and peridotite mantle refertilisation by sub-duction of oceanic crust Schweiz Mineral PetrogrMitt 78 243-255

Yoder HS amp Tilley CE (1962) Origin of basalt mag-mas an experimental study of natural and syntheticrock systems J Petrol 3 342-532

Received 15 August 2000Modified version received 16 January 2001Accepted 31 January 2001

Primary magmas and mantle temperatures 451