Incompatibility of iron in post-perovskite and the stability of basal magma oceans in super-Earths

Post-perovskite is expected to dominate much of the solid mantles of rocky exoplanets, yet iron partitioning between post-perovskite and silicate melt, which controls the compositional evolution and buoyancy of crystallizing magma oceans, is unconstrained at these pressures. We use first-principles molecular dynamics and thermodynamic integration to compute the Fe--Mg distribution coefficient K D between post-perovskite and (Mg,Fe)SiO 3 liquid at 150--600 GPa and 6000--10000 K. Iron is strongly incompatible in post-perovskite and becomes increasingly so with pressure. Combining K D with equations of state, we find that iron enrichment of residual liquid reverses the solid--liquid density contrast, causing post-perovskite to become buoyant. Basal magma oceans are therefore gravitationally stable in super-Earth exoplanets up to 4 M.