Planetary differentiation in the aftermath of giant impacts

The compositions of the cores and mantles of Earth-like planets are largely set by metal-silicate equilibration at high pressures and temperatures. Recently, the analytic equations of state (ANEOS) models for (Mg,Fe)₂SiO₄ minerals, forsterite and olivine, and an iron-alloy (Fe₈₅Si₁₅) were updated to improve the calculated temperatures in impact simulations [1,2]. These improved models were used to calculate the temperatures within the Earth-like planets in the aftermath of giant impacts. We have conducted several representative giant impact simulations. After the impact, the temperature of the upper mantle lies above the solidus and reaches supercritical conditions. In addition, both the differential rotation between the core and mantle and the addition of highly shocked projectile core results in supercritical temperatures at the core-mantle boundary. In the supercritical regions, we expect metal and silicates to be miscible. In most accretionary giant impacts, the majority of incoming metal merges quickly with the target's core. However, in very energetic cases, some metal, up to 10% of Earth's core mass, is dissolved within the silicate fluid of the mantle. As the planet cools, that metal will precipitate out of the supercritical fluid and rain into the growing core, equilibrating with the cooling magma ocean. The temperatures of this process lie beyond the current range of metal-silicate partitioning experiments used to predict the composition of the core during planet accretion, but preliminary calculations using a metal-silicate partitioning parameterization extrapolated to these high temperatures indicate that the post-equilibration silicate is enriched in FeO when compared with the ambient mantle. Our work suggests that the presence of a dense, iron-enriched layer at the base of the mantle is an expected outcome of the giant impact stage of accretion.

[1] Stewart, S.T. New forsterite ANEOS model in press https://arxiv.org/abs/1910.04687 and DOI: 10.5281/zenodo.3478631

[2] Stewart, S.T. New iron-alloy ANEOS model DOI: 10.5281/zenodo.3866550