[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-P02] Estimation of the Earth's outer core composition based on the elastic properties of liquid Fe-Ni-O-S quaternary alloys

Chihiro Matsushita¹, *Taku Tsuchiya¹ (1.Geodynamics Research Center, Ehime University)

Keywords:ab initio molecular dynamics, high-pressure thermoelasticity, outer core chemistry

The density of Earth's outer core is known to have a distinct deficit of ~10 wt% compared to pure iron, which is thought to be due to the presence of light elements (e.g. Birch, 1964). Currently, sulfur, carbon, oxygen, silicon, and hydrogen are considered as their candidates, but the detail light element composition of the outer core remains uncover as a long term controversial issue in deep Earth science. Thermoelasticity of iron-light element liquid alloys is key to resolve this problem (e.g. Ichikawa and Tsuchiya, 2020). In this study as a first step to the comprehensive understanding of the elasticity of iron-nickel-light elements quaternary liquid alloys, we pick up O and S and conduct and investigate density (ρ) and P-wave velocity (V_P) of Fe-Ni-O-S liquids with varying the O/S ratios by means of the ab initio molecular dynamics (AIMD) simulation method. Following Ichikawa and Tsuchiya (2020), light element fractions are optimized to reproduce the data of ρ and V_P to the PREM (Dziewonski, Anderson, 1981) values simultaneously as much as possible. Results show that with increasing the O content the pressure values at the constant volume continuously decrease, but nevertheless the ρ and V_P can be constant by tuning the inner-outer core boundary (ICB) temperature and maybe by the O/S ratio. This indicates that each O and S fraction cannot be constrained uniquely from the ρ and V_P only, but the variation of the best-fit O/S ratio is deterministic. The Fe-Ni-O-S liquid alloy can be considered one of likely compositions of the Earth's outer core. The present results would be helpful to constrain their O and S fraction.