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

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.