American Mineralogist, Volume 79, pages 497-503, 1994

Melting of pyrope,MgrAlrSirOrr, at7-16 GPa J. Zn.lNc* Departmentof Earth and EnvironmentalSciences, The City University of New York, New York 10036,U.S.A.

C. Hnnznpnc Department of Geological Sciences,Rutgers University, New Brunswick, New Jersey08903, U.S.A. and Center for High PressureResearchand Department of Earth and SpaceSciences,The State University of New York at Stony Brook, Stony Brook, New York 11794,U.S.A.

Ansrucr Melting experiments on pyrope have been performed from 7 to l6 GPa using the multianvil press.The results show that the melting curve of pyrope is unusual relative to those for pyroxene and forsterite becauseeither it is linear in T-P spaceor it has a break in the slopeat around l0 GPa. At 3-10 GPa the melting curve appearsto be normal in that it can be calculatedfrom reasonablethermodynamic and elastic parametersusing the method of minimizing the Gibbs free energyof melting. However, at pressures> 10 GPa, the observedmelting temperaturesare higher than predicted. This can be explained either by cation disordering in crystalline pyrope or by a stiffening of liquid MgrAlrSirO,, associated with pressure-induced coordinationchangesinvolving Al3*. In either case,pyrope is stabilized relative to liquid, and the slope dZldP of the melting curve is higher than that for enstatiteand forsterite at pressuresin excessof l0 GPa. The effectofpressure is, therefore, to increasethe thermal stability of pyrope at the expenseof olivine and to place garnet on the liquidus for komatiite, peridotite, and chondrite compositions.

Ir.rrnonucrroN The liquidus phasefor komatite, peridotite,and chondrite compositions changesfrom olivine to garnet at high pressures(Herzberg, 1983; Takahashi, 1986; Ohtani et al., I 986; Ito and Takahashi,1987;-Herzberget al., I 990; Zhangand Herzberg,1993),which increasesopportunities for garnet fractionation to occur in nature. Garnet fractionation has been demonstratedto be important in understandingthe geochemistryof some komatiites (Herzberg, 1992), and it may have occurred during an early diflerentiation event in the Earth (Ohtani and Sawamoto, 1987;Herzbergand Gasparik,l99l). The melting temperaturesof pyrope, MgrAlrSirO,r, at high pressureare thereforeimportant for understandingthe stability of garnet in multicomponent systems. Early studieshave shown that pyrope melts incongruently to liquid + spinel or liquid + aluminous enstatite up to 3.5 GPa (Boyd and England, 1962),and it melts congruentlyat higherpressures(Boyd and England,1962; Ohtani et al., 1981;Irifune and Ohtani, 1986).Of particular interest in these studies is the slope of the pyrope melting curve, which was reported to changefrom about 82 "C/GPa at 3.5 GPa to 0 'ClGPa at pressuresabove l0 GPa (Irifune and Ohtani, 1986). This differs from the melting curve of forsterite,which maintains a positive * Present address: Center for High PressureResearch, Department of Earth and SpaceSciences,The State University of New York at Stony Brook, Stony Brook, New York I 1794,U.S.A.

0003-{04xl94l0506-0497$02.00

slopeat all pressures(Ohtaniand Kumazawa,1981;Presnall and Walter, 1993). The effect of pressureinferred from theseend-membersystemsis to increasethe stability of olivine at the expenseof garnet,just the opposite of what is observedin multicomponentsystems.A major objectiveofthis work is to resolvethis contradictionby reinvestigatingthe melting curve of pyrope. Exprnrprnx'rAt,

METHoDS AND RESULTS

All experimentswerecarriedout usingthe 2000-t splitspheremultianvil apparatus(USSA-2000)locatedat Stony Brook. A detailed description of the press,the sample assembly,and the techniqueshasbeengiven in numerous papers(Gasparik,1989,1990;Herzberget al., 1990;Liebermann and Wang, 1992) arrd is not repeatedhere. Two typesof startingmaterialwereused.One wascrystalline pyrope synthesizedfrom a stoichiometricmixture of MgO, Al,O., and SiO, at 5 GPa and 1300"C for 5 h; optical examination and X-ray diffraction showed that the recovered starting material consisted largely of pyrope but containedminor amounts of unreactedoxides. The second type of starting material was a pyrope-free mixture of oxides,and this was usedto examinehow the experimental results might be afected by the nature of the startingmaterial. The startingmaterialswere loadedinto Re containers, and assembliesof 10 mm were used. These were then fired at 1000 "C in an Ar atmospherefor I h prior to the experiment to expel all HrO. The time to reach the target temperaturewas l5-30 min, and experimentdurations

49'.l

498

ZHANG AND HERZBERG: MELTING OF PYROPE

Trale 1. Experimentalresultson the meltingof pyrope P(GPa)

70 8.0 8.0

en on 9.0 v5 1 00 10.0 11.5 115 1 30 13.0 130 13.0 14.5 145 1 45 1 60 16.0 16.0 1 60

f fC)

1980 1980 2010 2040 2035 2070 2060 2110 2050 2150 2190 2100 2200 2190 2300 2280 2300 2365 2200 2300 2380 2s00

t (min) J.5

3.0 3.0 4.O J.C

4.O 40 30 3.0 3.0 4.O 0.5 1.0 3.0 3.0 1.0 5h 40 3.0 3.0 1.0 30

Starting materials

Expt products

mixture mixture mixture mixture mixture mixture mixture pyrope pyrope pyrope mixture pyrope pyrope pyrope pyrope pyrope mixture mixture pyrope pyrope pyrope pyrope

Py.,L-Py Py,L Py,L Py Py Py Py' L Py Py Py Py Py Py Py,L Py Py Py Py Py Py Py,L

'Py : pyrope. '. L: liquidquenchedto glass + extremelyfine-grained ( 10 GPa and 2000 'C, dependent.Another sourceofextra entropy in pyrope is an arbitrarily selectedtemperature.Rigden et al. (1988) from positional disorder of the small Mg cation at high also observedthis kind ofbehavior from shock-waveextemperaturesif thermal expansionis accommodatedby perimentson molten Ano,uDiouo,a compositionthat cona substantialincreasein the sizeofthe largedodecahedral tains 15.4olo AlrOr. The volume in their experimentswas site. In any case,it is concludedthat cation disordering observed to changegradually over the pressureinterval in pyrope at temperaturesabove 2100 'C is a possible from I atm to 25 GPa, but negligiblyat higherpressures. way of explainingour experimentaldata and may explain Rigden et al. (1988) interpretedtheir resultsto indicatea why the melting curve has the break in slopeat l0 GPa gradualtransformationof Al and Si from dominantly tet(Fig. a). rahedralto octahedralcoordinationin the range I atm to We concludethis analysiswith an examinationof pos- 25 GPa; at higher pressuresthey suggestedthat these sible changesin the properties of pyrope liquid. The most transformationsmay be essentiallycomplete,and there obvious considerationinvolves possibleAl3+ coordina- may be a similarity in the compressibilitiesof the liquid tion changesin the liquid from fourfold to sixfold coor- and an increasein the melting slopeof MgSiO. perovskite dination at around l0 GPa, where there may be a break at around 60 GPa, but this break has not been verified in the melting slope.The evidencefor coordination in- by Zen andBoehler(1993).This interpretation,however, creasesin Al3* at about this pressurewas from'z?Alsolid- is ambiguous becauseP-V breaks were not observed in state NMR measurementson silicateglasses(Ohtani et shock-wave experiments for molten anorthite (36.60/o al., 1985;Xue et a1.,1991),moleculardynamicssimu- AlrOr: Rigdenet al., 1989)and for molten komatiite(8.20lo lations (Angell et al., 1982, 1987),and the observationof AlrO.: Miller et al., l99l) over a comparablepressure garnetquenchedfrom liquid pyropeat pressures>7 GPa range. Furthermore, the melting curve of jadeite (Na(Irifune and Ohtani, 1986). The essentialquestion inAlSi,O6) from2.4 to 16.5GPa showsno I-Pbreaks (Fig.

502

ZHANG AND HERZBERG: MELTING OF PYROPE 160

f = 2000'C

-g o E 14o o

E C) o E 12o 5 o 100 o

5

10

20

15

Pressure GPa Fig. 5. PossibleP- Zrelationsof liquid MgrAl,SirO,, at 2000 "C derived from the melting of ordered pyrope along the melting curve in Fig. 4. The solid curve was calculatedfrom the elastic and thermodynamic parameterslisted in Table 2, with K,, raised from 6.5 to 10.2at pressuresin the range 10-16 GPa. The broken curvewascalculatedwith K;: 6.5 (Fig. 3). Errorsstemming from uncertainties in temperature, enthalpy, and entropy of melting and from the molar volume of crystalline pyrope are +4 cm3/mol.

6) that could be attributed to structural changes that are known to occur in albite glass (Ohtani et al., 1985). The evidence is therefore somewhat contradictory, but the bulk of it indicates that disordering in pyrope may be more important than structural changes in liquid MgrAlrSirO,r. Spectroscopic studies on pyrope synthesized at around

2700 Py

2200

o f 6

o

CL

E

1700

Solid

1200 0

Acxxowr,nocMENTs This researchwas partially supportedby grants from the National ScienceFoundation to Claude Herzberg(EAR-89 I 6836 and EAR-9 I I 7 l 84). The high-pressureexperimentsreported in this paper were performed in the Stony Brook High PressureLaboratory, which is jointly supportedby the National ScienceFoundation (EAR-8917563) and the State University of New York at Stony Brook. This laboratory is now a part of the NSF Scienceand Technology Center for High PressureResearch(EAR8920239)establishedat Stony Brook in conjunction with Princeton University and the Geophysical Laboratory of the Carnegie Institution of Washington Thanks are extended to J. Bass, G. Harlow, G. Miller, E. Ohtani, and L. Stixrude for their helpful discussionsand comments. Special thanks are extendedto Tibor Gasparik for technical assistance.This is Mineral PhysicsInstitute Publication no. 78.

Rnrnnrxcns crrED

Liquid

o o

2500 'C could be important in constraining these possibilities. Figure 6 summarizesthe melting curve of pyrope in relation to the melting curves of enstatite(Boyd et al., 1964;Presnalland Gasparik, 1990),forsterite(Davis and England,1964; Ohtani and Kumazawa,l98l; Presnall and Walter, 1993), and jadeite (Litvin and Gasparik, 1993).The very high melting temperaturesobservedfor pyrope at around l5 GPa show that pressurestabilizes garnetat the expenseof forsterite,in agreementwith phase equilibrium studies in multicomponent systems(Takahashi,I 986;Ohtaniet al., I 986;Ito and Takahashi,1987; Herzberget al., 1990; Zhang and Herzberg, 1993). We emphasizethat the experimentalmethod utilized in this study is identical to that used for the enstatite and forHerzsterite fusion curves (Presnalland Gasparik, 19901, berg et al., 1990; Presnalland Walter, 1993),and it is also identical to the method usedin calibratingthe solidus curve in peridotite systems(Herzberget al., 1990). Therefore,the resultspresentedhere cannot be an artifact of the experimentalmethod.

6

12

18

PressureGPa Fig. 6. The melting curve of pyrope compared with those for enstatite (Presnall and Gasparik, 1990), forsterite (Presnall and Walter, 1993),and jadeite (Litvin and Gasparik, 1993).

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Mev 10, 1993 Meruscnrm REcEIvED Meu-rscnrrr AcCEPTED Jnm;env 19, 1994