Phase relations in the system MgSiO3–Al2O3 up to 52 GPa and 2000 K

Abstract

Phase relations in the system MgSiO3–Al2O3 have been determined for pressures from 15 to 52GPa and at a temperature of 2000K from in situ synchrotron X-ray diffraction measurements obtained using sintered diamond anvils in a multi-anvil apparatus. A wide two-phase region of garnet and corundum is found to exist up to 27GPa; at higher pressures, a phase assemblage of aluminous bridgmanite and corundum is stabilized. The solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum are highly dependent on pressure; the Al2O3 content in bridgmanite increases from 12mol% at 27GPa to 29mol% at 52GPa, forming bridgmanite with the pyrope composition (25mol% Al2O3) at about 45GPa. In contrast, the MgSiO3 content in corundum decreases from 24mol% at 27GPa to 9mol% at 52GPa. The minimum pressure where the pyropic bridgmanite is formed is significantly higher than the result of the earlier study (∼37GPa) on the pyrope composition. These results suggest the Al2O3 content in bridgmanite and the MgSiO3 content in corundum are good pressure indicators in the mixed-phase region at pressures above 30GPa, where virtually no pressure references are available for quench experiments. The new phase diagram in the system MgSiO3–Al2O3 confirms that bridgmanite is the dominant host mineral for Al2O3 in the pyrolitic or peridotitic compositions under the pressure and temperature conditions of the Earth’s lower mantle.