Massive Rocky Planets May Suppress Deep Melting

Abstract Melting properties of silicates are critical to understanding the thermal evolution of rocky planetary interiors. In the deep mantles of massive rocky planets, post‐post‐spinel is potentially one of the most abundant mineral phases, yet its melting behavior remains unconstrained. Here, we determine the melting curve of post‐post‐spinel using thermodynamic integration based on ab initio calculations. We find that the melting temperature of post‐post‐spinel exceeds that of post‐perovskite by at least 400 K at 500 GPa. Furthermore, the volume and the entropy of melting suggest only weak pressure dependence. Comparisons with the geotherms of rocky planets of various sizes and potential temperatures indicate that the refractory nature of post‐post‐spinel may confine the deep mantles of massive rocky planets to the solid state, except under the conditions of extremely high potential temperatures or high impurity contents, such as iron and volatiles.