Equation of state and thermodynamic properties of liquid Fe-O in the Earth’s outer core

Abstract

Equation of state (EoS) plays a crucial role in the prediction of the composition of the outer core. Here, we calculated pressure (P)-volume (V)-temperature (T) data of liquid iron-oxygen alloys (Fe-X wt.% O, X = 0, 2.8, 6.1, and 9.9) under the outer core conditions (∼136–330 GPa, 4000–6000 K) by first-principles molecular dynamics simulations. We established an EoS for liquid Fe-O alloys with parameters including P, T, V, and O concentrations. Consequently, thermodynamic properties of liquid Fe-O alloys such as density (ρ), thermal expansion coefficient, isothermal and adiabatic bulk modulus, and sound velocity (V_P) are calculated. To constrain the O content, we predicted the ρ-P and V_P-P profiles along the geotherm and compared them with data from the Preliminary Reference Earth Model (PREM). We conclude that the adiabatic T profile as a function of depth affects the prediction of O content dramatically. With several anchored T_ICB, the composition of Fe-6.1 wt.% O matches the PREM data with an acceptable range of error. But strictly speaking, the distribution in the outer core is probably uneven. In such case, we state that the O content in the outer core cannot be higher than approximately 6.1 wt.%.

Keywords

Equation of state / First-principles molecular dynamics / Liquid iron-oxygen alloys / Thermodynamic properties

CRediT authorship contribution statement

Miaoxu Xie: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Writing – original draft. Jie Fu: Conceptualization, Methodology, Supervision, Writing – review & editing. Anatoly B. Belonoshko: Conceptualization, Writing – review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This study was financially supported by the National Natural Science Foundation of China (NSFC) (Grant No. 11804175), the Program for Science and Technology Innovation Team in Zhejiang (Grant No. 2021R01004), the Natural Science Foundation of Ningbo (Grant No. 2021J099) and the K.C. Wong Magna Foundation in Ningbo University. Anatoly B Belonoshko appreciates the financial support from Nanjing University. Graphic abstract was drawn by Figdraw.

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