High-temperature miscibility of iron and rock during terrestrial planet formation

TL;DR

This paper investigates the high-temperature mixing behavior of iron and rock during planet formation, revealing a new differentiation process that occurs at lower temperatures than previously thought, affecting core-mantle composition.

Contribution

It provides the first thermodynamic determination of the Fe-MgO solvus closure temperature at high pressures, indicating a new mode of planetary core-mantle differentiation after giant impacts.

Findings

Mixing of MgO and iron occurs at temperatures around 4000 K at low pressure.

High-temperature equilibration can deliver Mg-rich material into the early planetary core.

The solvus closure temperature has a weak positive pressure dependence.

Abstract

The accretion of a terrestrial body and differentiation of its silicate/oxide mantle from iron core provide abundant energy for heating its interior to temperatures much higher than the present day Earth. The consequences of differentiation on the structure and composition of planets are typically addressed considering only the interaction of molten iron with an immiscible `rocky' phase. We demonstrate that mixing in a representative system of liquid or solid MgO and liquid iron to a single homogeneous liquid occurs at sufficiently low temperature to be present in the aftermath of a giant impact. Applying the thermodynamic integration technique to density functional theory molecular dynamics simulations, we determine the solvus closure temperature for the Fe-MgO system for pressures up to 400 GPa. Solvus closure occurs at $\sim$4000 K at low pressure, and has a weak positive pressure dependence, such that its gradient with respect to depth is less steep than an adiabatic temperature profile. This predicts a new mode of core-mantle differentiation following the most energetic giant impacts, with exsolution of iron from the mixture beginning in the outer layers of the planet. We demonstrate that high-temperature equilibration results in delivery of nominally insoluble Mg-rich material to the early core. Since MgO is the least soluble major mantle component in iron at low temperatures, these results may represent an upper bound on temperature for mixing in terrestrial planets.