Solid solution behaviour of CaSiO and MgSiO perovskites.

Using density functional simulations within the generalized gradient approximation and projector-augmented wave method together with thermodynamic modelling, the reciprocal solubilities of MgSiO and CaSiO perovskites were calculated for pressures and temperatures of the Earth's lower mantle from 25 to 100 GPa and 0 to 6,000 K, respectively. The solubility of Ca in MgSiO at conditions along a mantle adiabat is found to be less than 0.02 atoms per formula unit. The solubility of Mg in CaSiO is even lower, and most important, the extent of solid solution decreases with pressure. To dissolve CaSiO perovskite completely in MgSiO perovskite, a solubility of 7.8 or 2.3 mol% would be necessary for a fertile pyrolytic or depleted harzburgitic mantle, respectively. Thus, for any reasonable geotherm, two separate perovskites will be present in fertile mantle, suggesting that Ca-perovskite will be residual to low degree melting throughout the entire mantle. At the solidus, CaSiO perovskite might completely dissolve in MgSiO perovskite only in a depleted mantle with <1.25 wt% CaO. These implications may be modified if Ca solubility in MgSiO is increased by other major mantle constituents such as Fe and Al. [ABSTRACT FROM AUTHOR]