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arXiv · 2609.13527

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

Francis Dragulet·Lars Stixrude
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Abstract

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$_{\oplus}$.

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Francis Dragulet, Lars Stixrude. 2026-09-11. Incompatibility of iron in post-perovskite and the stability of basal magma oceans in super-Earths. https://arxiv.org/abs/2609.13527

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