Thermodynamic Properties of Liquid Metal Using First-Principles Molecular Dynamics: Implications for the Compositional and Thermal State of the Earth's Outer Core

Abstract

The Earth's outer core consists of Fe-Ni metallic liquid, with minor quantities of light elements such as Si, S, O, C, and H. Accurate equations of state of candidate liquids are essential to translate seismic observations into constraints on the outer core's compositional and thermal state. However, the experimental database is sparse and incomplete. We apply first-principles molecular dynamics simulations to determine thermal properties of liquid metal alloys at P–T conditions spanning the outer core. We calibrate a rigorous form of pressure shift with valid high- and low-P limits to correct for ab initio density overestimates. Corrected results are consistent with both diamond anvil cell and shock wave experiments on Fe. We simulate liquid iron binaries (Fe-O, Fe-Si, Fe-S, Fe-Ni, Fe-C, and Fe-H) and some ternary and higher-order alloys to construct an accurate multicomponent mixing model. We use the Adams-Williamson equation to obtain self-consistent outer core velocity-density-pressure-temperature-depth profiles for given compositions and inner-core boundary temperatures and find a posterior distribution of model parameters that fit seismic radial velocity models via Markov Chain Monte Carlo Bayesian inference. For seismic models that assume a homogeneous and adiabatic outer core (PREM and EPOC-V), we identify parameter ranges that provide good fits across the entire radius of the outer core, with or without hydrogen. In contrast, body-wave models that lack an a priori constraint (ak135 and ek137) cannot be fitted within their stated precision near the top and bottom of the outer core; these seismic models require an inhomogeneous or non-adiabatic outer core.