The role of iron on the solid solution of CaSiO3 and MgSiO3 perovskites in the lower mantle

The solid solution of CaSiO3 and MgSiO3 perovskites is an important control on the properties of the lower mantle but the effect of one of the largest defective elements (iron) on this solution is largely unknown. Using density functional theory (DFT), ferrous iron's influence on the reciprocal solubility of MgSiO3 and CaSiO3 perovskite (forming a single mixed perovskite phase) was calculated under pressures and temperatures of 25 - 125 GPa and 0 - 3000 K, respectively. Except in iron-rich conditions, iron preferentially partitions into the mixed perovskite phase over bridgmanite. This is a small effect (KD~0.25-1), however, when compared to the partitioning of ferrous iron to ferropericlase which rules out phase mixing as a mechanism for creating iron-rich regions. Iron increases the miscibility of the two perovskite phases/reduces the temperature at which the two phases mix but this effect is highly nonlinear. We find that for pyrolytic mantle (Ca%=12.5 where Ca%=Ca/ (Ca + Mg)) a perovskite iron concentration of ~13% leads to the lowest mixing temperature/highest miscibility. With this composition, 1% iron in the pyrolytic solution would lead to mixing at ~120 GPa along the geothermal gradient and 6.25% iron leads to mixing at ~115 GPa. At high iron concentrations Fe starts to impair miscibility, with 25% iron leading to mixing at ~120 GPa. Thus, in normal pyrolytic mantle iron should induce a small amount of mixing near the D'' layer. Extremely iron rich parts of the lower mantle such as potentially at the CMB and in ULVZs are, however, not a likely source of phase mixed perovskites due to the nonlinear effect of ferrous iron on phase mixing.