Limited intersolubility of Ca and Mg between bridgmanite and CaSiO3-perovskite in the lower mantle

Abstract

Bridgmanite and CaSiO3-perovskite, respectively, are the first and third most abundant minerals in the Earth's lower mantle. The intersolubility of Ca and Mg between these two minerals is still under debate. Some studies indicated limited intersolubility, while others suggested a complete dissolution of CaSiO3-perovskite into bridgmanite at lower mantle conditions. This controversy leads to different explanations of the physical properties of the lower mantle such as density, elasticity, and viscosity, accordingly modifying our understanding of the nature and dynamics of the Earth’s interior. Therefore, it is essential to determine the intersolubility of bridgmanite and CaSiO3-perovskite.The intersolubility of Ca and Mg between bridgmanite and CaSiO3-perovskite can be affected by the temperature, pressure, and bulk compositions of the system. Previous studies showed an increase in intersolubility with temperature, but reported no robust pressure and composition dependence. Because FeAlO3 is the second dominant component in bridgmanite, investigating the effect of FeAlO3 content and pressure dependence is essential to clarify the phase relation of the Earth’s lower mantle.This study determined the pressure and FeAlO3 dependence on the intersolubility of bridgmanite and CaSiO3-perovskite at pressures of 27 to 40 GPa at a constant temperature of 2300 K using a multi-anvil press. Two compositions of MgSiO3:CaSiO3 = 1:1 and MgSiO3:FeAlO3:CaSiO3 = 3:2:5 were examined. SEM images clearly show the existence of two phases in all products. TEM-EDS analyses indicate a decrease in the CaSiO3 content of bridgmanite in both systems with increasing pressure. The FeAlO3 component only slightly enhances the CaSiO3 content of bridgmanite: adding the FeAlO3 component increases the CaSiO3 content of bridgmanite from 0.04(2) to 0.3(2) mol.% at 40 GPa. The MgSiO3 content of CaSiO3-perovskite decreases with increasing pressure from 1.8(7) to 0.06(8) mol.% in the FeAlO3-free system, and from 1.2(5) mol.% mol to an unmeasurably small value in the FeAlO3-bearing system. We conclude that, even with the presence of a FeAlO3 component in bridgmanite, the intersolubility is limited and remains nearly constant at different pressures. Thus, bridgmanite and CaSiO3-perovskite should coexist in the lower mantle, and CaSiO3-perovskite is the host mineral of large lithophile elements such as Ca in the lower mantle.