EARTH AND PLANETARY SCIENCE LETTERS 5 (I968) 76-78, NORTH-HOLLAND PUBLISHING COMP,, AMSTERDAM

H I G H - P R E S S U R E T R A N S F O R M A T I O N S IN P Y R O X E N E S 11

A.E.RiNGWOOD and Alan MAJOR Deparrmenir of Geophysics and Geochemisto,. Australian National University

Received 12 September 1968

"l'wo years ago, we presented I 11 some preliminary cesulls upon phase equilibria at high pressures in the pl/roxene ~ystems FeSiO3-MgSiO 3 and MgGeO 3- MgSi'O 3. We have since obtained additional experimen- tal data which permit, for the first time, the construc- tion of preliminary high-pressure phase diagrams for these sysSems. Because of the importance of pyroxenes and their high pressure transformation products in the man~le, and the probability that comprehensive studies of the above systems will not be completed for a con- siderab[e period, it was decided to publish the current rcsullls as a supplement to the previous paper.

High pressure experiments were carried out using a Bddgman anvil device equipped with international heater [21. The pressure as determined in an experi- ment includes a correction based upon the use of the coesite-stishovite transformation as a calibration point:. In the previous paper I l l this pressure wa,,; taken as I lO kb at 1000oc. In the present paper, based upon the revised s~andard calibrations of Jeffrey et ai, [3] the pressure of this transformation is taken as 95 kb. Pressures quoted are believed accurate to about 10 per- cent between 70 kb and 150 kb. Above 150 kb, errors may well be higher. Other details of the experiments were described in the previous paper [ l ].

Starting materials consisted ofpytoxenes of known composition, homogeneous glasses, and oxide mixtures. The ]:atter consisted of previously synthesized FeO.MgO solid solutions intimately mixed with silicic acid m appropriate proportions. Further det~ails are given in captions to figs. 1 and 2. Samples were subjected to desired pressures at about 1000°C for periods of 3 - 5 rain and quenched urider pressure by terminating the power supply to the heater. After relaxation of pres- sure and recovery of samples, the phases produced were identified by optical and X-ray diffraction techniques.

In the system FeSi'O3-MgSiO 3 (filb 1) at pressures above 150 kb, the compositions of spinels in the three phase field (spinel + stishovi~e + clinopyroxene) were obtained from their accurately measured lattice parameters, using the lattice paramel'.¢r-composition

200 ' ,~ ~-SPINEL S SNS/

.

<~ I A°,~, +~,~o,,,~ l E3 ,htO L °/+ CLINOPYROXENE /

| / ~ CL.tNO YROXENE: J / / / ~ S. SOLUTIONS

t0O ~ I S S A N C E RESUL.TS~IO00o¢ L___-L...---..L 1 I C 20 40 60 BO tO0

~'eSiO~ MOL % MgSiO3 Eig. 1. Pha~:~ observed in the system l.'e$iO3-MgSiO 3 at pressures up Lo 200 kb .arid at approximately IO00°C. Sym- bols as follows: 0 Complgte transfo?nlafion to (spinel ÷ stlshovitc), 0 boundary determined by spinel composition obtained from lattice pi~-rameter. ~J Experimental tun in 3 phase field ill whicfi composition of spinel was determined. °1,2.3 inferred points !m b~undary of pyroxene stability field. Starting compos~iions: Pyroxenes, FSI00, FS83 MSI7; Glasses, FSso MSs0, FS30 FST0:, Oxide mixtures, FS25 MS7.";, Á:St5 MS185, ES = FeSiO 3, MS = MgSiO 3.

HIGH-PRESS'URE'rRANSFOt~MATIONS IN PYROXENES II 77

relationship for (MgFe)2SiO 4 spinel sol id solutions previously obtained by us [4,5]. This information served to delineate the boundaries of ~!~e (spinel + stishovite) field as shown. It is seen that a well de- fiued and continuous rel~ationship between pressme and composition of spinals in the three phase field exists. Because of the accuracy with which the spinel compositions may be obtained from their lattice parameters, this system may serVe as a useful pres- sure c~ibration standard in the future, and is alsa of value immediately for interlaboratory comparisons of pressures claimed to have been obtained from several different types of ultra-high pressure de'does now functioning in different laboratories.

The boundary (fig. 1) separating the presumed sta~ bility fields of single phase clinopyroxene solid solu- tions from the three phase field (spinel + stishovite + clinopyroxene) is subject to appreciable uncertainty because of kinetic difficulties and lack of equilibrium. Each of the numbered points represents an individual starting material upon which at least 10 runs over a wide range of pressur~ have been carried otzt. The degree of transformation into (spinel + stishovile) varied erratically from run to run and was not simply related to the pressure applied. The points represent our judgment of the most probable position of the phase boundary ou the basis ofresuRs obtained. This pro- blem notwithstanding, the gross phase relationships within the system appear to be clearly defined. A simple extrapolation of the phase boundaries suggests that pure MgSiO 3 would transform to a mixture of Mg2SiO 4 "spinel" plus stishovite at about 250-300 kb, providing that no other phase transformations inter- veued. The "spinel" phase would presumably be the ~Mg2SiO 2 phase, probably a distorted spinel, dis- covered previously by us {4,5 ].

Results on tile system MgGeO3-MgSiO 3 require little discussion additionM to that giver~ in the previous paper. Starting materials were synthetic pyroxenes (betweeu MGt00 and MG20 MS80 ) and homogeneous #asses between MG2s MS7$ and MSlo 0. Fields of ilmenite, (ilmenite + pyroxene), and pyroxene solid solutions appear to be clearly define¢,, A simple extra- polatio~ of the phase boundaries ~aggests that art ilmeuite, modification of MgSiC 3 might also become stable between 250 and 300 kb. It is not possible in the light of present data to determine whether pure MgSiO 3 will ultimately transform to a (~Mg2SiO 4 +

~60

~40

03 r,- ,ff rn ~00 0 ,_l ,#

80

ILl ,1" ;D 60 O~ 0") ILl

~ 4o

o

1 ~YROXE.E / / , . s,s. /

: . °

,o/,o S,%s

/

FtECONNAISSANCE

RESULTS

T ~ 1000 "C _ _ _ _ L _ _ _ l . . . . . ~ t

20 40 60 80

20

o 0 100

M'gGe05 M O L % MgSi03

Fig. 2. Phase~ I~re~nt in the system MgGeO3-MgSiO 3 at pres- sures up to 170 kl~, and at approximately I000°C. Symbols as follows • Ilom¢*geneous ih'ncnite-type solid solutions, 4~ two phase field of iitnenitc -~ pyroKene solid salutions, Opyroxenc ~olid solutions. Starting materials: see text.

srishovite) mixture or to an ilmenite type phase. Still another possibility exists: MgSiO 3 may transform in- stead to a vamet structure Mg3(MgSi)Si3O12 I 1,6] Interpretation by Anderson e'. al. [71 of the properties of a new plhase of bronzite obtained under shock wave conditions 181 suggests that the bronzite trans- formed to an ilmenite structure above about 300 kb. However it is not certain whether this phase is thermodynamically stable relative to the (,BMg2SiO 4 + stishovite) and garnet alternatives. Clearly, MgSiO 3 pyroxene ;s much more stable at high pressures than was previously anticipated 19] and it appears likely

78 A.E.RINGWOOD and A.MAJOR

that the conclusion drawn by Sclar et al. [10] on the basis of their experimental investigations, that elino- enstatite transforms to a rnixture of forsterJte plus st'sbovite at about 1 ] 5 kb , 500-800°C, is '~ong.

REFERENCES

[1] A.E.Ringwood and A.Major, Earth Planet. SoL Letters ] (1966) 35L

[2] A.E.Ringwood and A.Major, Phys. Earth Planet. Int. 1 (1968) 164.

[3] R.N.Jeffet2t,, J.D.Ba~:ett, H.B.Van:~leet and H.T.Hall, J. Appl. Phys. 37 (1966) 3172.

[4] A.E.Rin~vood and A.Major, Earth Planet, Sel. Letters I (1966) 241.

[5] A.E.Ri:ngwood and A.M~jor, unpubH~ed obser~'atioris, 1968.

~6] A.E.Rirlgwood,, EaCh Planet. Sci. Letters 2 (19~7) 2:1,5. [7] D.L.Anderson, T.Al~rens and A.E.Ringwood, ire prepa-

ration (1968), [P,] R.G.Mt:Queen. S,P.Marsh and J.N,Fritz, J. Geephys.

Res. 72 (1967) 4999. [9] A.E.Ringwood and M.Seabrook, J. Geophys. tJ,es. 68,

(1963) 4601. [ 10] C.B.Sclar. LC.Carrison Hid C.M.Schwartz. J. Geophys.

Res. 69 (196,U 325.