MgO Miscibility in Liquid Iron

TL;DR

This study uses density functional theory simulations to investigate the phase behavior of MgO in liquid iron under extreme conditions, revealing a high critical temperature for complete miscibility relevant to planetary interiors.

Contribution

It provides the first detailed phase diagram of MgO-Fe liquid coexistence, showing MgO acts as a component with symmetric regular solution behavior at high pressures and temperatures.

Findings

Critical temperature for miscibility: 7000 K at 68 GPa.

MgO behaves as a component with indistinguishable Mg and O in phases.

MgO exsolution rate is too small to drive a planetary dynamo.

Abstract

We explore phase equilbria on the MgO-Fe join as a prototype of lithophile-core interaction in terrestrial planets. Our simulations, based on density functional theory, are based on a two-phase method: fluids of initially pure MgO and Fe compositions are allowed to establish a dynamic equilbrium across a near-planar interface. Methods for analyzing the composition and other properties of the two coexisting phases show that MgO behaves as a component, with indistinguishable Mg and O concentrations in Fe-rich and oxide-rich phases. The phase diagram is well described as that of a symmetric regular solution, a picture confirmed by independent one-phase determinations of the enthalpy, entropy, and volume of mixing. The critical temperature, above which there is complete miscibility across the MgO-Fe join is 7000 K at 68 GPa, and 9000 K and 172 GPa. The rate of MgO exsolution from the Fe-rich liquid on cooling is similar to that found in previous experimental studies, and is too small to drive a dynamo.