Phase Relations in the System MgSiO₃-Al₂O₃ up to 2300 K at Lower Mantle Pressures

Phase relations in the system MgSiO₃-Al₂O₃ were investigated at pressures of 27–45 GPa and temperatures of 1700, 2000, and 2300 K using sintered diamond and tungsten carbide anvils in a multianvil apparatus. The bulk compositions in the MgSiO₃-Al₂O₃ binary system crystallize a phase assemblage of pyrope and corundum at pressures below 27 GPa and an assemblage of bridgmanite and corundum at pressures above 27 GPa regardless of temperatures. The solubility of Al₂O₃ in bridgmanite and that of MgSiO₃ in corundum increases significantly with increasing temperature. The solubility of Al₂O₃ in bridgmanite increases from 6.7 mol % at 1700 K to 21.8 mol % at 2500 K under a constant pressure of 27 GPa. Bridgmanite becomes more aluminous with increasing pressure from 27 to 45 GPa at a given temperature. The MgSiO₃ content in corundum increases with increasing pressure at pressure lower than 27 GPa, while it decreases at pressure higher than 27 GPa. Our results suggest that bridgmanite can incorporate a considerably higher Al₂O₃ content than that of the pyrope composition (25 mol % Al₂O₃). The present study further suggests that the entire Al₂O₃ component is accommodated into bridgmanite in the pyrolite lower mantle. However, Al₂O₃ cannot be fully accommodated into bridgmanite in the coldest parts of subducted slabs in the shallow part of the lower mantle, and therefore, additional phases such as MgAl₂O₄ with calcium ferrite-type structure are necessary to host the excess Al₂O₃.