Non-negligible Oxygen in the Earth's Inner Core: The importance of high temperatures

The core of the Earth must have some light elements which are small in concentration but could have dramatic effects on the behavior of the core. Oxygen is one such element. It has long been concluded based on experiments and theoretical calculations (Alfè, Gillan, and Price 2002) that the inner core partitions negligible amounts of O from an enriched outer core and thus its possible effects can be ignored. An oxygen-rich outer and oxygen-poor inner cores has also been proposed as a way to explain various seismic data (Badro, Côté, and Brodholt 2014). The discovery of Fe-O superionic alloys (He et al. 2022) calls these conclusions into questions as the state of O is substantially different at high vs low temperatures which could affect extrapolations of experimental results to high temperatures.

Focusing on the most thermally stable superionic alloys in the inner core, our study systematically investigates the partitioning behaviour of oxygen between solid inner and liquid outer core by an advanced combination of ab initio molecular dynamics (AIMD) simulations and the two-phase thermodynamics (2PT) model (Lin, Blanco, and Goddard 2003). We conclude that while O remains favoured in the liquid state under core conditions non-negligible amounts of O enter the inner core and thus its possible presence cannot be ignored. With realistic concentrations of O in the outer core we produce a density contrast between liquid and solid oxygen that is in the range of that observed at the inner core boundary (ICB) thus showing the importance of obtaining accurate partitioning values and their effect on seismic structure.

This study provides a new and reliable approach to the thermodynamic properties of the superionic state and a new theoretical basis for understanding the internal structure of the Earth's core, contributing to understanding of the complexity of the Earth's interior and providing useful insights into future directions of research in the field of Earth sciences. It also shows the stark difference between high and low temperature structures and how accurate temperatures need to be considered when looking at core structures.

 

Alfè, D., M. J. Gillan, and G. D. Price. 2002. 'Ab initio chemical potentials of solid and liquid solutions and the chemistry of the Earth’s core', The Journal of Chemical Physics, 116: 7127-36.

Badro, James, Alexander S. Côté, and John P. Brodholt. 2014. 'A seismologically consistent compositional model of Earth’s core', Proceedings of the National Academy of Sciences, 111: 7542-45.

He, Yu, Shichuan Sun, Duck Young Kim, Bo Gyu Jang, Heping Li, and Ho-kwang Mao. 2022. 'Superionic iron alloys and their seismic velocities in Earth’s inner core', Nature, 602: 258-62.

Lin, Shiang-Tai, Mario Blanco, and William A. Goddard. 2003. 'The two-phase model for calculating thermodynamic properties of liquids from molecular dynamics: Validation for the phase diagram of Lennard-Jones fluids', The Journal of Chemical Physics, 119: 11792-805.