Phase transformations in the mantle

EARTH AND PLANETARY SCIENCE LETTERS 5 (1969) 401~12. NORTH-HOLLAND PUS; ISHING COMP.. AMSTE[~.DAM

PHASE T R A N S F O R M A T I O N S IN THE MANTLE

A.E.RINGWOOD Department of Geophysics and Geochemistry,

Australian National University, Canberra, A.C.T.

Received 5 Jamlary 1969

']l'his paper presents a rel¢iew of the constitution of the mantle between d,epths of 200 and 1200 km in tile ligh't of recent experimental investigations on high pressure phase trarlsformations in silicate minerals and on ~heir germana'le ~.rtaingaes. Phase trar~sformations occurring at depths to 600 km can be studied directly using apparatus which dt:velops >200 kb at elevated temperatures. These studies indicate th.at the rapid increase of ~ismie velocity around ~50-450 km is caused mainly by the transformation of pyroxenes into a new type of garnet structure and the transforrnagion of olivirte co the spinel (or related) structure. Current understanding of the constitution of the mantle belo'.~ 500 km rests heavily upon interpretatinn of phase tran!;formations in ~;ermanate analogue systems and on shock wave investigations. These suggest thalt around 600-700 kin, garnets and spinals transform to rtew phases possessing ilmenite, pl:rovskite and strontium plurabate structures with densilies ~nd elastic propertie:; resembling those of isoehemield mixed oxides (MgO + SiO2 (as stishovite) + AI203 + CaO, etc.). At greater depths, a further set of transformatiollls to new phases appear:; likely, causing the "zero-pressure density" of the inanile to at- tain values about 5% higher I~han an isoehemieal rob:tare of oxides. The phase transf,3rmations which have been directly observed or interred provide a quantitative explanation of the seismic velocity distribution between 30t~ and 800 kin. From a study of the elastic properties of the mantle it is concluded that the average FeO/(FeO + MgO) ratio of mantle silicates is probably approxinmtuly u~niform at 0. I I (molecular) throughout the entire mantle.

I. I[ntroduction

The mantle may be divided into three major regions on the basis of tile seismic velocity-depth distributions (fig. I). in the upper mantle, ex,~ending down to about 35(1 F.xn, velocity gradients are g.enerally small, except possibly in the vicinity of the Iow-veloeity zone. In the transition zone betweet~ 350 and 1050 kin, velocity gradients are high on the average. Recent observations [1-.7] ~ndieate that most of the velocity increase irt this region is concentrated in ~wo or three narrow zones around 400, 650 and (perhaps) 1050 kin. Below 1050 km and extending to the' core at 2900 km lies the lower mantle, where velocity gradients are relatively sraall and unifoml. This distribution of velocities clearly reflects a corresponding variation of other important physical properties and must ultimately be expltained in terms of the nature and properties of the mineral phases which are ~;table in the various regions of the mantle.

The first clue to the nature of the transition ;:one arose from the elasaieal studies on tile density o~" the mantle by Bullen [;3] who demonstrated that tills region is chemically inhomogeneous. The nature of the inhomogeneity was investigated by Birch [9 113] using an equation 6f state based upon finite strain theory. Birch concluded that the properties of the upper mantle 'were consistent with this region bting composed of familiar minerals such as olivines, l:yr- oxene!* and garnets. However, the elastic properties of tile lower mantle were very different and re- sembled those possessed by relatively closely pat ked oxides such as corundum, periclase, rutile and spinel. Birch proposed that the transition zone was characterized by a series of major phase transl:ormatinns resulting from the instability of olivine.;, pyroxeztes and garnets at high pressure. He argued that thes,~ minerals transformed into a new assemblage ot" :iose- packe~l polymorphs, that the transformations we re complete by about 100O kin, and that between l~)O0

402 A.E.RINGWOOD

, ; 1 0 - - J " " " "

[ ~"t f z o . E

UPPER MAN'fLE

0

J' a I ~ 1 n I ~__1 I I O 200 400 600 Eoa 1000 i ] :ao 14oo

DEPTH K,~

Fig. I. Distribution of seismic P wave veJocities i0 the outer 200 km of the mantle according to .leffreys [63] and Ar-

,:hambeau, Fiinn and Lambert 15't. The Archambeau et at. disttibutmn is for the Bilby NE profile in the USA. Several othe~ groups have recently reporced generally similar velo.

,:ity distributions in other regic.,ns [ 1-7].

and 2900 kin, no further major transformations oc- c.urred, this region being essentially homogeneous. Earlier, Jeffreys [ I ! ] and ]Bernal [I 2] had also suggested that a phase change of olivine to the spinel structure occurred at a depth of about 400 km and was responsible for the "20 degree discontinuity".

Since Birth's hypothesis was based upon an equation of state which was not universally accepted [ 13- ! 5] experimental verification was essential. It would be necessary to demonstrate that olivines, pyroxenes and garnets were indeed unstable at high pressttre, and would transform to new polymorphs at P, T conditions equivalent to those between 400 and 1000 kin, and further, that the elastic properties and densities of these new polymorphs were capable of explaining quantitatively the inferred properties of the lower mantle. The difficulty about testing the hypothesis experimentally was that until i 963, available static hig h pressure-temperature apparatus was incapable of reproducing the P, T coxJ- ditions in the earth at depths greater than about 300 k.m. Accordingly, in the preceding period it was necessary to employ indirect experimental methods based

on thermodynamics, companLttve crystal chemistry and particularly, upon the study of germanate isotypes of silicates. This latter technique arose from the fortunate circums~:ance that germanate.'; were often found to display the same kinds of p!tase transformations as the corresponding silicates, but at much lower pressures [ 16]. Accordingly, the study of pitas~." transformations in germanates at relative]Ly low pr~;ssures served as a good indicator'of the probable behat,iour of silicates at pressures beyond the experimentally attainable rang:e. Furthermore, the thermodynamic investigation of , solid-solution equilibria betw,,~en a low pressure silicate end-member (e.g. Mg2Si04 forsterite) and a geln'- manate possessing a denser structure (e.g. Ni2GeO4 spinel) provided sufficient information to calculate the pressure r~,quired for the silicate to transform to the dense structure, in this wa), I calculated in I956 1117] that forsteritc should tratlsform to the spinel structure at a pressure of [75 ± 55 kb at 1500°C and that Mg2SiO4 spinel would be I I ± 3% denser than the olivine. This prediction, which was substantially verified ten years later [18], w..t.~: the first quantitative evidence supporting the Bernal and Birch hypothesis. A third indirect method was 1o study the solid solution of forsterite in a spinel structure [19-21 ] over the available pressure range and to extrapolate the observed phase boundary over l.o ghe pure forsterite composition, in this way, a pressure of 130 -+ 20 kb t;ar the olivine-spinel transtbrrnation at 600°C was obtained.

The results of this indirect phase of investigation which covered the period 1956-1963 may be sum- marized as follows [ 16]. AI~ of the germanate oil- vines and pyroxenes which wer,~ stuclied in the pressure range 0 - 9 0 kb were found to be unstable at high pressures and transformed to denser phases, sug- gesting strongly that the corresponding silicates would transform similarly at higher pressures. Whenever quantitative estimates of the pressures required for the transformation of upper ra~.ntle sdieates were made by the methods described, above, the transformation pressures were found to be within the pressure range in the transition zon,. ~. During this period, 1 found that the silicate olivine.,; Fe2SiO4 [22], Ni2SiO4 [23] and Co2~qiO 4 [24] could be transformed to spinel structures at 20-70 kb and that the spinels were about 10% denser than the' corresponding divines. Fayalite (F.~2SiO4)is a significant cam-

PHASE TRANSFORMATIONS IN THE MANTt.E 403

ponl~nt of mantle olivine. These results further supported the probability that :magnesia-rich olivines wouhi transform at higher pressure. An important disoovery was made by Stishov and Papaya [25] who showed that at abuut I00 kb [261] (1600°C) silica could be transformed to the:: rutil~, structure. The new po~ymorph "stishovite" po:;sessed a density of 4.28 g cm "3. The demonstration that the co-ordination of silicon (like germanium) coald change from 4 to 6 under pressure greatly exte:Etded the range of possible trans.formation structures for silicates.

These results provided powerful qualitative support for Birch's hypothesis. It was posgible to con, struc't a model series of transformations in mantle silicates based upon the above indirect evidence which yielded densities and elastic properties in good agreement with those inferred for the lower mant:le [27-29] . Nevertheless, there remained a strong i:a- centive to construct apparalus which could develop pressures and temperatures equivalent to those irl the transdtion zone so that the predicted new silicate po[y- morphs could be synthesized directly and their properties measured, leading to all understanding of the fine structure of the mantle: ill tiffs region. Also, iLhe possibility of discovery of completely new and un- predicted phases remained. The indirect raethods used do not predict with certainty, and are inapplic- able in cases - e.g. coesite, where a silical:e does ~'tot have a germanate isotype.

In 1966, an apparatus capable of developing pressu res above 200 kb simultaneously with high temperatures wits developed at the Australian National Uni- versity [ 18, 30]. This is eq~jivalent to a depth of 600 kin. Using this apparatus, many new phase transformations of importance in the mantle have been rills- covered both in silicates and germanates. These are revie,~'ed in the fol~owing pages. Other laboratories have also reported the operation of appalratus ca-. pable o,f pressures in this neighbourhood [31,32,69]. Another comparatively recent development has been the application of shock wave techniques capabl~.~ of developing transient pressures in the megabar i:atlge to materials of geophysical interest [33]. This technique has revealed a number of phase transformations in silicates and has provided valuable information on the equations of state of the high pressure phase,', [34].

2. Review of phase transformations relevant to con- stltution of transition zone

2. I. Olivine.sp#rel transformation Olivine is probably the most abundant minera[ in

tI:e upper mantle and it is of crucial importance io establish its high-pressure behaviour. Olivine-spinel polymorphism was discovered by Goldschmidt [35] in Mg2GeO4. By analogy, Jeffreys [I l] and Bernal [12] suggested that Mg2SiO4 might also transform to a spinel at high pressure, and that this could be of con:fiderable geophysical significance. This view was s~apported by the discovery that the olivines Fe2SiO4, C'o2SiO4 and Ni2SiO4 transformed to spinel pol~.- morphs at pressures in the range 20 70 kilobars [22-24] . Recent investigations in my laboratory have also shown that the olivines MgMnGeO4, CoMnGeO4 and FeMnGeO4 transtbrm to spinel structures at modest pressures. The application of indirect melhods. based upon the study of germanate-si[icate equilit~ria led to the con~:lusion that pare Mg2SiO4 olivine should transform to the spinel structure between 100 and 200 kb [17, 19--21.36-381.

In 1966, three laboratories reported progress in syn- tlaesizing spinel solid solutions along the join Fe2SiO4. Mg28iO4. Ringwood and Major [181 described the synthesis at 170 kb of a complete series of solid solution,~ firom pure Fe2SiO 4 spinel to a spinel containing 80 m,~l % of Mg2SiO4 (fig. 2). These were the first syntheses of spinels with Mg/~e ratios close to those in tile earth's mantle, and thus aonfirmed, to a considerable degree, the predictions which had been made earlier. Sclal and Carrison [39] and Akimoto and Fujisawa [37] reported the synthesis of spinals containing up tq~ 35% of Mg2SiO4. These workers [37, 38] published aa investigation of phase boundaries in tile system Fe2SiO4.. Mg2SiO4 in the pressure range 40-95 kb and '~00- 1200°C. Extrapolation of phase boundaries suggested a pressure of about t40 kb (1000°C) for ~he olivine- spinel transition in pure Mg2SiO 4.

~n compositions between 80 and 100% Mg~SiO4, Ringwood an~t Major [18] were unable to synthesize true spinals. These o!.Mnes invariably transt'or~ned above 150 kb into a phase of low symmetry ~,hich pos,,~essed a complex X-ray diffraction pattern related to that of spinel but with many more refle.xin~ls. The transformation often went to completion. This phase,

404 A.E.RINGWOOD

,~ A I" _ . j

i ..... _ O 20 40 6,0 80 100 Mg,S iO. ' ~ O L Fe,SiO~

Fig. 2. Lattice parameters of Mg2SiO4 -Fe2SiO4 spinel solid solutions transformed ;it 1'70 kb and 1000°C from .synthetic olivine solid solutions of known composition. Data from

Ringwood and Major [401.

which we called fl-Mg2Si04, was about 8% denser than olivine [85]. Shortly after publication of our results [18. 86]. Akimoto and Ida [31 ] claimed to have synthesized a true Mg2Si04 ~pinel. Although a partial transformation of olivine was undoubtedly achieved, the evidence which they published is in- !atfficient at present to establish that the new phase produced was Mg2SiO4 spinel rather than ~-Mg2SiOa. in nine runs, the highest degree of transformation observed was 50% and only 6 reflexions attributable co a new phase were ob:;erved. Four eli' these, includ- ing the two strongest, agreed well with those o f ~ Mg2Si04 whilst one of the others may have been an olivine reilexion. Moreover the rq:lativi~ intensities of the refiexions did not agree well with those expected for Mg2Si04 spinel [40]. Finally, the refractive fin- dex (I .70 -+ 0.01) of A.kimoto and Ida's phase agreed with that of ~-Mg2SiOa (1.702 ± 0.005) [86] but was.significantly less than that expected for Mg2Si04 spinel (1.72) [18]. A further cLaiim to have synthesized Mg2SiO 4 spinel in 1966 WalS marie by Kawal, Endoh and Sakata [87]. This claim can be discounted. The X-ray data reported do not resemble those of Mg2Si04 spinel. The phase synthesized by these workers was in all probability a hydrated magnesium- silicate [87].

Ringwood and Major [40] have completed art extensive irtvestigation of phase relations in the system L~Ig2SiO4-Fe2Si04 over the pressure range 50-200 kb at 10'Od°C (fig. 3!~. Results at pressures below 95 kb are in reasonable agreement with those of Akimo- ~o and Fujisawa [381. A notable feature is the large field of ~-Mg2Si04 which occurs in l~lg-rich composi-

l i i " l T - i i • 20(

r. ,B~

let m . BETA

:JaOLIVINE (~) • t~ I 16C-~ SPINEL ( Y )

PHASE • • g - t a + Y

.a: al n'+,0 • ~J~t 14C -D ~ +B + 1" gl

S P I N E L ( ~ ' )

t2c souo SNS " ~ - ' ~-~

• I . • ' D i i ,/ool / , .o :oo t

• . ~ " OU'clrqE In) "] . - " SOLID SOLUTIONS -~

I o o 4 I I I I I . I I I |

20 40 60 80 100 MOL PERCENT Mg,SiO,

6C

4C

F%SiO,

Fig. 3. Phase diagram at 1000*C for lhe system Mg2SJ.D 4- Fe2SiO 4 based upon the results of Ringwood and Major i401. Starting materials were synthetic (M~:Fe)2SiO 4 olivines of

known co m po~;ition.

tions. Initially, we suspected that this phase was a metastahie quench product [18]. However all of the more recent extensive experimental results [40] are consistent with the assumption that it is thermn- dynamically stable in its synth,esis field. The steep boundaries between the spinel mid 13 fields (fig. 3) are indicative of similar densities, although at low pressure.,; the spinel is more than 2% denser than the ~-phase. Evidently the latter has a higher compres- sibility than spinel, and at high pressure, the densities become more nearly equal. The phase relationships shown on fig. 3 also imply that the free energy difference between the spinel and/~-phase is very small and it is possible that the extent of the ~-Mg2SiO4 field could be strongly influenced by temperature or by solid solution formation with other components, e.g. MgAI204. This complicates the application of the experimq.~ntal results to the mantle.

PHASE TRANSFORMATIONS IN THE MANTLE 405

If it i~; assumed that the present type of phase diagram is applicable in the mantle, then olivine of co,m- position Mg89Fell (sect. 3..1)would first ~ransform partially to spinel with incneasi~ag depth. On entering the (~ 4~) field, the spinel wou~ld transform to l/-Mg2SiO4, and at a pressulre about 5 t:b higher, all the olivine would have transformed to the &phase. Hinderer, the details of this. transformation may well change at higher temperature arid in the presence of other components, it is possible that the ~-phase may yet transform to spinel at higher pressures depend;ling upon the effect of these variables upon the (')'+~) field boundaries. Regardless of the mineralogical details of the transformation, the geoplaysical results are likely to be similar because of the probable similarity in physical properties, of spinel and B-Mg2SiO4. Olivine is finally demonstrated to transform to a spinel-like phase in the mantle. A'11000°C, fig. 3 shows that the mid-point o:F' the transfon~aation would be at t 18 kb (357 kin) with a transition width of 13 kb, (40 kin). If the temperature at the transition is, say, about 1600°C, and the gradient dP/dTof the transition is similar to that of the olivine-spinel tram', formation in fayalite (33 bars/°C) [88] the mid-point of the transformation would occur at 415 km which is consistent with observation (fig. 1).

2.2. Further transformations of spinels In (MgFe)2SiO4 spinels, the silicol~ ~Jtoms are tetra-

hedraliy coordinated. Further transfor~la~ions to phases characterized by octahedral coo~'dination are to be expected at higher pressures.

Pdngwood and Reid [45] have inves'!igated the transformations of a large number of Sl:dnals at h:igh pressure in an attempt to clarify the systematic corstal chemical and thermodynamic fa,:.tors govern- ing spinel transformations. Three princi]gal classes of transformations were observed. The first type in- volved transformaUon into single A2 BO4-type phases. The spinels Mn2SnO4, Mn2TiOa,, Zn2SnO4, Zn2Ti04 and FeMnGeO4 transformed to a struct~.lre related ,~o that of strontium phimbate. The olivine, IVln2GeO4 also transforms to the strontium plumbate structure [41 ]. This structure is often formed bet~een end- members possessing rock-salt and rudle ,. tructures, and is considered possible for (MgFe)2Si~34 [411 . Both cations are octahedrally coordinated and the densities of this class of structures are e~lual to those

of isochemical (An rocksalt + BO2 rutile) mixtures. Ca2GeO4 olivine transforms to the K2NiF4 stt'ucture at high pressures [42]. This structure, whie z" is about 5% denser than the isochemical mixed o;, ~es, might also be attained by some spinels under high pn~ssures.

A~ternatively, a spinel may partially disproportionate into a mixture of an ilmenite phase plus a tocksalt phase [28]. The density of this assemblage is close to that of isochemical mixed oxides. This type of transformation has been observed in Mg2TiO4. Fe2 r io4 and Co2TiO4 [43] and in MgZnTiO4 [45] and it is considered possible that Mg2SiO4 'spinel' may transform analogously into a mixture of' MgSiO 3 iln~enite + MgO [ 16, 28]. A third type of transformaticn for (MgFe)2SiO4 spinel which has beer~ suggested I tO. 44, 72, 78, 84] is complete dissociation into a mixture of oxides, MgO + Fen + sin2 (Stishovite). Ring- wood and Reid 145] found that the spinels Mg2SnO4, Co2SnO4, FeAI204, NiA1204 and COAI204, indeed dissociated in this manner into their simple oxicle components. This type of transformation appears to be restricted to compounds which possess relativety small free energies of formation from constituent oxides at zero pressure. Because of the rather large free energy of formation of Mg2SiO 4 from the oxides it is not considered likely that Mg2SiO4 will transform in this fashion [ 16, 45]. It is considered more probable that Mg2SiO4 will either transform to the Sr2PbO4 structure or disproportionate to MgO + MgSiO3 (ilmedite). The observed substantial solid solubility of Mg 2" and Si 4+ in the Sr2PbO4 nlodification of Mn2GeO4 at high pressures [451 support the view that Mg2SiO4 may ultimately transform to the gr2PbO4 structure.

2.3. Transformations in pyroxe~es After the olivines, the pyroxenes are probably the

next most abundant class of minerals in the upper manltle. Pyroxenes are observed to display several modes of transformation at high pressure. The first is a partial disproportionation:

2 ABO3 (pyroxene) ~ A2BO4 (spinel) + BO2 (rl~ .de).

The pyroxenes FeSiO3, FeGeO3, CoSiO3, CoGe03 and (MgNi)GeO 3 all transform in this manner [46-.-48]. The increase in density is about 15%. The transt\~rmation laas been studied in the system FeSiO3.-MgSiO3 [49, 50]. These results suggest that enstatite wotdd transform to a mixture of Mg2SiO4 spinel + stishovite

40b A.F.RINGWOOD

at 200-3~0 kb, providing no other ~ransformations intervened. Alternatively, pyroxenes may transform to the llmenite structure. This has been observed in MgGeO3 [4.6] and MnGeO 3 [46]. In tiffs structure, both cations are octahedrally co-ordinated, and the density increase is about 15-18%. Studies of the pyroxene-ilmenite transformatinn in the system MgGeO3-MgSiO3 suggest that MgSi03 could also transfom~ to the ilmenite structure between 200 and 300 kb [49, 50]. This inference is supported by shock-wave data [34].

A third mode of transformation of pyroxenes and pyroxgnoids is to tile garnet structure. This was first observed in CaGeO3 (wolla~tonite str.) which transforms under pressure to a garnet w'hh tile formula Ca3(CaGe)Ge3Ol2 [46]. Analogous behaviour is shown by CdGeO 3 [46]. More recently, it has been found that MnSiO 3 transforms to a garnet Mn 5(MnSi)Si3012 at about 100 kb [51 ]. In this structure, the octahedraI sites are shared by Mn 2 and Si 4 ions which are presumably ordered. Thus, one quarter of the silicon atoms change from 4 to 6 coordination in this transformatJion. The compositions MgSiO3 7% A1203, FeSiO3 10% A1203, CaSiO3 10% A1203 and CaMgSi206 10% AI203 were a/so found to transform to garnet structures at about I00 kb [5L 52]. There appears to be a continuous solid solution at high pressure from ordi- nary pyrope garnet Mg3AI2Si3012 extendhlg at ~east 60% oS ~he way towards a garnet, Mg3(MgSi)Si3Ot2. Ca-r~ch and Fe-rieh pyroxenes behave similarly. It is possibk~ that at pressures above 200 kb, pure MgSiO3 couId crystallize as a garnet. This type of transformation is probably of considerable importance in the mantle because the presence of AI203 permits the pyroxenes to transform to the garnet structure at much lower pressures than are required for the alternative pyroxene transformations discussed above. The effective increase in density of the pyroxene component as it transforms to the garnet structure is close to 10%.

2.4. Transformations in garnets Garnet is probably an important constituent of

the upper mantle and the occurrence ~ff the transfor- matinns discussed abole makes i,t likely that garnet is even more important belov'¢ 350 kin. Since most of the silicon atoms in garnets are te trahedrally ce-ordi-

rtated it can be expected that garnets will transform at higher pressures to denser phases characterized by octahedral silicon.

One such transformation which appears likely is for Mg-rich garnets to transform to the ilmenite structure. This has been observed in the systera Mg3A12Ge3OI2-Mg3AI2Si3OI2 at high pressure [53]. The density increase is 8% amt lhe pressure required for transformation of pure pyrope is probably betx,,een 200 and 300 kb [53]. Calcium-dch garnets transform in another manner. CaGeO3 and CdGeO3 garnets were found to transform to parovst:ile structures I51 ]. This cgass of structures is commonly formed between end-members possessing rocksalt and rutile structures and is about 7% denser than an isoehemical oxide mixture on the average. CaGeO3 perovskite was found to dassolve more than 35 mol.% of CaSiO3 at 170 kb [5 I, 81 ]. It appears highly probable that CaSi03 will ultimately transform to a peruvskite structure with a density about 7.5% higher t]han a nffxture of CoO + stishovite. Germaniura grossularite Ca3AI2Ge]OI2 apparently dissociates at pressu:re into CaGeO3 (perovskite) + A1203 [81].

It is possible that ag still hilgl~er pressures than are required for the ilmehite tram;formation, the MgSiO3 component of the ilmenite type solid solution might also transform to a perovskite s~:ructure. This would liberate AI203 which will probably combine with MgO to form the high pressure MgAI204 phase indicated by shock wave data [33, 34]. This phase, which is about 8.5% denser than an isochemical mixture of MgO + A1203, may possess the calcium ferrite structure [34].

2.5. Transformations in aluminosilicates and alumino. germanates

At 35 kb, AI2GeO5 displays a kyanite structure. At still higher pressures it dissociat,~d into a mixture ok" .~J203 + GeO2 (futile) [45]° This suggests that AI2SiO 5 may behave likewise.

At relatively low pressures~ Ge and Si a[bite, anor- thite and nepheline transform to familiar phases such as jadeite, quartz, grossularite, kyanite and NaAIO2 [54-56] . However, at higher pressures most feldspars transform to the hollandite stru:ture. This tra:lsfor- mation has been found in KAISJ308 [57, 58], KAIGe308 [59], NaAIGe308 [58], RbAIGe308 [58], Ba0.sAISi3Og and Sr0.sAISi308 [60]. In addition,

PHASE TRANSFORMATIONS IN THE MANTLE 407

:;hock wave evidence strongly indicates that NaAISi308 and andesine ultimately transform to this structure [34]. The hollandite structure is about 50% denser than feldspar. It is related to the rutile structure and alt~ of the silicon and Iduminium atoms are oatahedral- ly coordinated. It is probable that in the lower mantle po;tassium occurs in this structure. However, the hollandite form of albite is probably not stable in a silica- undersaturated environment [58], and it is more pro- baboo that sodium occurs in a high pressure NItAISiO4 phase possessing the calcium ferrite structure [56]. This structure is d~splayed by the ldgh pressurd." modi- Iication of NaAIGeO4. The density of NaAISiO4 in this sltrueture wou]td be 3.9 g era-3. The silicon al~d aluminium atoms are octahedraliy co-ordinated whilst the sodium atoms are in 8-fold coordination. Germa- nium jadeite NaAIGe206 disproportionates into NaAIGeO4 (calcium ferrite type) and Gee2 (futile) at high pressure [56], and it is anticipated that common jadeite will likewise diaproportionate at higher pressures in to NaAISiO4 (calcium ferrite) plus stishovite.

3. The eo~nstitution of the mantle

3.1. Chemical composition It has been argued elsewhere that the primary

chemical composition of the upper mantle corm- ,.:ponds to a mixture of about 3 parts of peridotite to l part basalt [ 16, 61]. This composition (pyre- kite) is given in table 1. The relative abundances of ~he prineip~d metallic (except Fe) components of pyrolfite, Mg, Si, AI, Ca, are similar to those m chon- dritic meteorites [61] and in the solar photosphere [62]. Accordingly, it is reasonable to assume that to a first approximation, these abundances are applicable to the entire mantle. The FeffFe + Mg) ratio in pyrolite is O.l l molecular. We assume, initially, that this ratio, is also applicable to.the entire mantle. W~., now investigate whether the phase transforma- tiotls discussed in the pre:ious section occurring in material of py;rolite composition (table, l) are capable of explaining quantitatively the available geoph,,s- ical information on the mantle. The mean atomic: weight of pyro~ite is 21.34.

Table l Pyn~>lite model * composP:ion (wt%) I151.

SiO2 452;7 AI203 3.$7 Fe203 0.¢6 Cr203 OA 3 FeO 8,47 MgO 37.8 l CaO 3.11 Na20 0.5~

• Simplified,

3,2. Seismk" velocity depth distributions The seismic velocity distributions of Jef,¥eys [631

(fig. 1) and Gutenburg I64] were based up, m deliber- ately smoothed travel t~me data and hence '.he derived velocity distributions between 400 and 1000 km were also smooth. My earlier efforts to explain this velocity distribution in terms of phase translbrmations attempted to use the smearing out effects associated with solid solution formation [16, 27.28]. See also reg'. [83]. This assumption was consistent with information then available, however, more recent experimental data [38, 40, 521 .render this less plausible as a quantitative exphmation, although :~ome degree of smealing due to solid solution is inevitable. Fortunately, the need to rely entirely upon this explanation appears to have dis- appeared. Recent studies of velocity-depth distributions [1-7] show that the velocity increase in the transition zone is not smoo~h, but is concentrated in t,~o or perhaps three regions ("discontinuities") around 400 kin, 650 km and 1050 km (fig. I). The increase at 1050 km is not firmly est:~blished, and may be spread among a number of smaller jumps in this region. The r,~vised velocity dist dbutions are indeed more readily e~ plained in terms of phase transformations [! 6, 52, 65 -.67] than is the smoother Jeffreys" distribution.

3.3. M.in'eralogy as ~, function of depth We now attempt to set up a model which d,picts

the most probable variation of mineralogy with depth for the pyrolite composition (table I ) based u[~on the experimer~tal data p~reviously reported. It is empha- sized that we are dealing with a model subject in future changes as new and mlproved experimental dat:iL become available. The mineralogy down to 600 km is

408 A.E.RINGWOOD

base d direcIly upon the results of high pressure ~;xper- iments which reproduce the ~.T conditions down to this depth. Below 600 Rra, the model is based largely upon indirect evidence such as germanate analogue wstems, combined with shock wave data.

] 50-350 km (approx.) This region is homogeneous ~md would be com-

posed of [68] : wt%

Olivine (Mgk%).ggiO~. '37 Orthopyrox,.'ue (Mgl?e)SiO3 ~17 Omphaeitie clinupyroxene (CaM~IFe)2Si206-

NaA~;i206 s. sn, ]12 Pyrop~ garnet (MgFeCa) 3(AICr) 2Si3012 1,4

Zero pressure density (d0) of this assemblage is 3 38 gcm -3. ~qnerals are characterized by 4-fold co-ordinated Si and by 6 nod 8-fold co-ordinated Mg~, Fe :and Ca.

350-450 km "'discontinuity" The principal tlansfonr~ztions are (i) pyroxen~!~-

garnet transformation: The pyroxene component: of the upper man:tie forms solid solution with pyrope garnet characterized bY paJrtial oetahedral eoordinatio,I of silicon, i.e. MiAI2Si3012- Ma(MSi)SiaOI2 solid solutions where M = Mg, F-% Ca. This transformation probably occurs at a smaller depth than the olivine. "'spinel" transformation. Partial overlap is possible because of solid solution effects. It is possible thalt in the luture, improved seismic techniques m~ty resolve these two transformationffJin the mantl~,,. (ii) Olivine- "spinel" transformation: As discussed in sect. 2a, the details of this ttansforraatian are likely to be complex, and ~nvolce transformation of olivine both to tile true spinel and to the fl-Mg2SiO4 phases. Nevertheless, because of the similarity in physical properties and transition parameters, and also because of the possible structural relationships between the two polymorphs, it is convenient for the present to treat them together as "'spinel". For a tempegture of about 1600~C, the mid-poi:at of the olivinf:-"spinel" trartsformation will occur close to 415 ',¢m with a transition width of 40 km (section 2.1).

450-600 km Homogeneous region composed of:

wt% (MgFe)2SiO 4 "spinel" 57 Garnet solid solution 39 Jadeite NaAISi206 4

Zero pressure density of this assemblage: 3.66 g c m -3. Silicon coordination: mainly 4-fold with some 6-fold.

650-700 km "discontinuity" Principal transformations are:

(i) "Spinel" transforms to Sr2PbO4 structure or alternatively disproportinnates to an ilmenite type phase + MgO.

(ii) Pyrupe-rich component efgarnet transforms to ilmenite structure.

(iii) Calcinm-rich component of garnet transforms to perovskite structure.

(iv) Jadeite disproportionates to yield calcium ferrite structure.

It is unlikely that these transformations occur at exactly the same depth and it appears probable that this "'discontinuity" may be smeared out over a wider interval than is indicated in fi:g. 1.

700-1050 kra Homogeneous region composed of:

w ~ llmenitc solid solution

(MgFe)SiO 3 - (AICrFe)2CJ 3 36 Strontium plumbate strncrare

(MgFe) 2Si04 55 (alternatively (MgFo)SiO 3 ilmenite + (MgFe)O)

Perovskite CaSiO3 6,5 Calcium fertite NaAISiO 4 2.5

gzro pressure density 3.99. This would be increased to ~4.03 by partial solid solution of (FeMn)SiO3 in perovskite structure. "Tfis regiol~ characterized by 6-fold coordination ofMg, Fe 'and Si. Mean zero- pressure density is almost identical to mean density o f an isochemieal mixture of cemponent o~ides (SiO2 as stishovite).

Below 1050 km Fig. 1 shows a single diseontinuity here but it may

PHASE TRAHSFDRMAT|ONS IN THE MANTLE z~09

be spread into a series of smaller jumps [82!t. This may be interpreted in ~elTns of further transforrv.ations into phases which are denser than an isochemicai oxide mixture, and characterized by Mg coordinations high~.'r than 6. Siupport for the existence of such phases is given by germanate anatogue studies and by" shock wave inw.'stigations [34ii. Possible transformations would he to an assemblage of (MgF¢)SiO3 perovskite [16, 28] + (CaFeMn)SiO3 perovskite [51] + Mg(A1CrFe)204 calcium ferrite [34] + (MgFe)2SiO4 (K2NiF4 type)[42] + NaAISiO4 [56] (calcium ferrite). Other possibilit,~es exist. The zero.pre~ure density of such an assemblage wouM be about 7% higher than that of the raixed ox;dq.~s.

3.4. The lower mantle The increases in seismic velocities at the 40,0 and

650 km discontinuRies are explained qu~mtita~:iveiy by the mineral assemblages proposed. Using the calculated zero-pressure densities, corrected for self compression by the B1rch-Murnaghan equation l i e ] , and for thermal expansion [10, 66] and applying the empirical Birch velocity-density ralationskip Vp ~ 3.16 p [70], the velocity jump at the 400 km discontinuity is 0.75 km/sec compared to obsemed seismic estimates of 0.65 [4] and 0.9 km/sec [I5]. Using the same approach, the velocity jump at ~.he 650 km "discontinuity" is 1.0 l~n/sec compared with seismic values of 0.9 [4} and 1.1 [5] km/sec. Thus it appears that phase transformations in a pyro3ite composition provide a satisfactory explanation of physical properties down to about 1000 k~.

The nature of the phase transformations responsible for increasing the de:nsity of pyrtdite up ~o that of the equivalent mixed i:;ochemical nxides appears to be fairly well understood. The occurrence of further phase transformations leading to still denser states and also of possible variations in Fe/Mg ratio is considerably more speculative and rely heavily on studies of analogue compounds. Shock wave data provide valuable infi)rmation on this question..

Fig. 4 shows the estimated range of densities permissible for the lower mantle [29, 44] on the assumption that the upper mantle is of pyrolffe composition. On the same diagram, ~Lhe densities of three simplified model lower mantle compositions based upon shock wave data [33, 71] are given. The first is dunite, corrected to a mean atomic weight of

2] .34. Studies of dunites under shock compression indicate that they are shocked to a phase or phase assemblage possessing properties closely resembling those of a mixture of MgO, Fee and sin2 (stishovite) [33, 34, 72]. Thus, the phase to which the dnnite t~'ansforms under shock is presumably characterized by octahedrally coordinated Si, Mg and Fe. The secom! model composition is the estimated density of a m~ig~ture of MgO, Fee and sin2 stishovite (33 = 21.34~'p in the pyrolite proportions. The third model is an AI203 -Fe203 solid solution with 33 = 21.34. The densities were obtained from shock data [34, 71 ] and from elastic constants derived from ultrasonic mca~ulements [73]. Justification for the model rests upon ~he 6 - 6 coordination in corundum solid solu.. tions, close resemblance of these solutions to the probable (MgFe)SiO3-AI203 ilmenite solid solution which, is believed to occur as an important phase below 650 km, and the systematic velocity-density- compres:dhility relationships existing among MgO- AI2G3--SiO2 and other oxides [70, 74, 75].

It caJr~ be seen that the densities of all of these mod,i:ls rail substantially below the observed density distribution in the lower mantle. Temperature correc- tions in t]he cases of the dunite and AI203- Fe203 would only increase the discrepancies. Accordingly, it appea~s highly probable that the inwer mantle is denser than pyroiite (Fe/(Fe + Mg) = 0.11) occurring in a phase assemblage characterized by 6-fold coordination o~ Si, Mg and Fe. This conclusion parallels one reached by other investigators [29, 66, 76-78].

The higher density of the lower mantle might be caused by an increased Fe/Mg ratio in this region, by phase changes leading to denser structures with Mg and Fe ig~ a coordination higher than 6, or by a com- bination ,ff these factots. It is useful to compare the elastic ral:ios $ (k/p, where k = hulk modulus, p = density) which are obtained directly from the ob- se~'vcd seismic velocities with the ¢~ values estimated for the mode] lower mantle composition given above. Since increasing Fe/Mg values and phase transform;t- tions to denser structures affect $ in opposite direc- tions, this comparimn ..: )rides a power fu[ constraint upon alternative choices.

The comparison is given in fig. 5. It is seen that tile experimental SE values for dunite 133} and for the MgO-F¢O-SiO2 [79} mixture are smaller than the observed seismic ~b S values at similar pressures.

410 A.E.RINGWOOD

DEPTH K M

1500 2 0 0 0 2 5 0 0 J i

o.

4.4 I I I I I f I l I -- ( O0 6 0 0 8 0 0 IOOO 1200 I ~ 0 0

PRESSURE K I L O S A R S

Fig. 4. Comparison of density disltrlbotion in lower man fie [ 29 44] (consistent with pyrol[te upper mantle) with denslties of some rondel mantle materials derived principally from shock wave data [ 33, 71 I. Density-pressure relationships for dunlte arm (AIFe)20 3 are along the respectiveHugoniot.~; and the estimated temperatures [38, 79] are shown. In the case, of the MgO-I:eO-.SiO2 mix, the Hugon~ot densities have been eon!ected to allow for the probable higher temperatures in the lowe]: mantle. The indicated tempera-

tu]res are notional.

DEPTH KM

1500 2000 2,500 120 i i I

.I(,..

'~E ~ 110 i " " ~ '~' "''

:= 100 ~)" ...., , ~ . /

o ~,.,"

o , o , , , > ,

80 ~," / /

70 - - - I I I I I I I _ I I

400 600 800 tO00 1200 1400 P~ESS~RE KILOBARS

Fig. 5. Comparison of observed ~,~ values in lower mantle [291 with 0E values of some model mantle materials deriw!d from shock wave data [ 33, 7 I, 84J. DE values obt~dned along H ugoniots have been corrected [ i 0, 29] to allow for the effects of the probable

higher temperatures in tee lower mantle. The indicated temperatures are notional.

PHASE TRANSFORMATION IN THE MANTLE

Art increase in Fe/Mg ratio would further decrease eE and thus increase the discrepancy. On the other hand, a phase change would increase ¢'E, thereby reducing the discrepancy. I f we introduce a phase change sufficient to achieve agreement with the densities in the lower :mantle (fig. 4) the corresponding increase in, ¢ can be estimated from the seismic equation of s~ate [80] (Pl/P2) = (~I/¢2) n where 2 < n < 3. The calculated ¢~E "~alues so obtained are in good agreement with the observed seismic ¢PS. In the ease of the A1203 -Fe203 model composition, the eE values are somewhat higher than the observed ~S. However, il' the Fe203/AI203 ratio is increased sufficiently t~ bring the densities into agreement making an appra- priate temperature correction (fig. 4) ti~.a ~E values are then substantially smaller than tire observed ¢ values, so that a phase change also becomes necessary in order to resolve the discrepancy.

Thus the model dunite and MgO-FeO-SiO2 sys- lems with .,~ = 21.34 as in pyrolite are capable of satisfying both the observed density distributions and seismic ¢S distribution in the lower mantle if they tranfform to a structural state characterized by Mg-Fe coordinations higher than 6, and do not require lhe lower mantle to have a higher Fe/Mg ratio than the pyrolite upper mantle. The AI203- Fe203 model requires both a small increase in Fe/Mg r~tio, and transformation to a denser phase: than ilrm;nite or corundum, in order to aclfieve compatibility.

All thlree models therefore imply that phase changes ~n the mantle ultimately lead to densities about 5% higher than are appropriate for a pyrolite mineral assemblage characterized by octahedral co- ordinatiol~ of Mg, Fe, Si, AI and Cr. These transfo!r- mations e.g. to MgSiO3 (perovskite), MgAI204 (calcium ferrite), Mg2SiO4 (K2 NiF4 structure) and also perhaps to other structures as yet unknown [81] :may be respnnsible for the additional sharp increases in seismic velocities below 1000 kin, iudicated by recen~ studies [5, 82]. Only one of the three models, perhaps the least relevant one, implied also that the lower mantle had a (slightly) ~igher mean atomic weight than a pyrolite upper mantle. These results are at variance with some recent studies by other workers [66, 77, 78, 88], who argue that the FeO/(FeO + MgO) ratio in the lower mantle is much liigher than in the upper mantle. The geophysical and

geoahemical implications of this issue are far-reaching and will be discussed in a subsequent paper.

Acknowledgements

The first draft of this paper was written whilst I was an NSF Senior Post Doctoral Fellow at the California Institute of Technology. The support and hospitality extended by the NSF and Caltech are gratefully acknowledged. I wish also to acknowledge the valu~ of helpful discussions with Dr. D.L. Amlerson. The ¢--P relationships for MgO, SiO2 and AI203 were kindly made available to me by Dr. T.J.Ahrens whc. calculated them from shock-wave data [331 . The able assistance and collaboration of my technician Mr. A. Ma~;or in much of the experimental work described here is also gratefully acknowledged.

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Phase transformations in the mantle