The system MgO-FeO-SiO 2 at high pressures and temperatures — phase equilibria and elastic properties

Abstract

Advances, during the Upper Mantle Project, in investigations on phase equilibria and elastic properties of the mantle minerals in the MgO-FeO-SiO 2 system are reviewed. The experimental procedure for a phase equilibrium study is described. Some advantages and disadvantages of various high-pressure, high-temperature apparatus are discussed. Necessities of establishing a pressure calibration method at high temperatures are also pointed out. A brief description on a method, recently developed for the measurements of ultrasonic wave velocities of very small samples, is given. Olivine-spinel solid solution equilibria in the system Mg 2SiO 4-Fe 2SiO 4 have been studied at several laboratories over the pressure range 40–200 kbar at 800 and 1,000°C. Ringwood and Major first discovered a peculiar mode of the high-pressure transformation in compositions close to pure Mg 2SiO 4, resulting from the formation of β phase. Crystal structure of the β phase was clarified as modified spinel structure through a series of studies on the high-pressure transformation of Co 2SiO 4 and Mn 2GeO 4. The appearance of the modified spinel phase in the Mg 2SiO 4-Fe 2SiO 4 system was confirmed in further investigations at the author's laboratory. The isothermal section of the phase diagram for the Mg 2SiO 4-Fe 2SiO 4 system was constructed at 800 and 1,000°C on the basis of these recent experimental results. At 800°C a continuous series of spinel solid solutions was synthesizable from Fe 2SiO 4 to (Mg 0.9Fe 0.1) 2SiO 4. At 1,000°C, however, all the attempts to synthesize a true spinel phase of (Mg 0.9Fe 0.1) 2SiO 4 were unsuccessful up to about 140 kbar and coexistence of the modified spinel phase with true spinel was usually identified. This suggests that the stability field of the β(Mg, Fe) 2 SiO 4 is highly temperature dependent. A remarkable expansion of the β(Mg, Fe) 2SiO 4 region is expected at the higher temperatures. Based on the cell parameters of βMg 2SiO 4 and the extrapolated value for γMg 2SiO 4 (true spinel), the density increase associated with forsterite- βMg 2 SiO 4 transformation, and forsterite — γMg 2 SiO 4 transformation was calculated to be 7.9% and 10.8% respectively. Experimental data on the phase equilibria of the MgSiO 3-FeSiO 3 system are presented. A highpressure disproportionation of clino-pyroxene solid solutions into stishovite plus spinel solid solutions was found. It was established that the disproportionation curve for clino-ferrosilite (FeSiO 3) was well represented by the boundary curve for the coesite-stishovite transformation. A preliminary phase diagram for the MgSiO 3-FeSiO 3 system was constructed at 800 and 1,000°C from the data by Ringwood and Major and by the author's laboratory. Experimental data on the high-pressure phase transformations of SiO 2 are summarized. The boundary curve for the coesite-stishovite transformation was determined over the temperature range 550–1,200°C in the pressure range 83–101 kbar by means of a tetrahedral anvil press. The transition curve was fitted by the linear relation P( kbar) = 67 + 0.028 T(° C). This determination was found to be in reasonable agreement with the previous data. A stoichiometric compound, Fe 1,000O, was synthesized at high-pressures above 40 kbar at 775°C by a reaction between wüstite, Fe 0.950O, and metallic iron. The cell dimension of Fe 1,000O was determined to be 4.323 ± 0.001 A ̊ . Compressional- and shear-wave velocities of the synthetic (Mg, Fe) 2 SiO 4 olivine, Fe 2SiO 4 spinel, (Mg, Fe)SiO 3 orthopyroxene, eoesite, stishovite and Fe 0.98O were measured by means of the ultra-sonic pulse transmission method. Results are represented on Birch's diagram, where the wave velocities are plotted as a function of density. It was found that the wave velocities of these ferromagnesian silicates and oxide decrease linearly with the increase of the FeO/(FeO+MgO) ratio, and that the isomorphic lines of (Mg, Fe) 2 SiO 4 olivine, (Mg, Fe)SiO 3 orthopyroxene and (Mg, Fe)O magnesiowüstite are approximately parallel to each other. The Compressional- and shear-wave velocities of the true spinel phase of Mg 2 SiO 4 were estimated to be 10.0 km/sec and 5.7 km/sec respectively. Compressional- and shear-wave velocities of stishovite were determined to be 11.0 km/sec and 5.55 km/sec. The bulk modulus of stishovite was calculated from these values to be 3.43 Mbar. Compressional-wave velocity of the three polymorphs of silica, αquartz, coesite and stishovite, was found to increase regularly along Birch's mean atomic weight line of M = 21 .