Structure and Non-Ideal Mixing of Fe-Ni-S Liquid at High Temperature and Pressure and Its Implication for the Earth's Outer Core Composition

Document Type

Article

Publication Date

11-1-2024

Abstract

The effect of light elements (LEs) such as sulfur on the physical properties of liquid iron-nickel alloy under the earth's outer core conditions is critical for understanding the core composition and dynamics. First-principles molecular dynamics simulations were employed to model Fe-Ni-S liquid with S concentrations in the range of (0–25) atomic percent (at%) at 4050 K and (0–33.33) at% at 5530 K and pressures relevant to the core-mantle boundary (CMB) and inner core boundary (ICB), respectively. The thermodynamic mixing properties of Fe-Ni-S liquid were calculated, showing that the excess volume for Fe-Ni-S alloys deviates negatively from ideal mixing by −0.33% at 12.5 at% S at the CMB and −0.35% at 17 at% S at the ICB. Similarly, the excess enthalpy negatively deviated from the ideal mixing by −3.4 kJ/mole and −13 kJ/mole at the similar S concentrations at CMB and ICB, respectively, indicating non-ideal mixing throughout the outer core. Similar behaviors are observed for isothermal bulk modulus (KT) and seismic velocity. The short- and intermediate-range structures were analyzed and used to explain the non-ideal mixing behaviors. The results suggest that extrapolations using ideal mixing underestimates the sound velocity by ∼0.14 km/s near CMB and ∼0.10 km/s near ICB, which is significant for constraining the core composition. If S is the only LE, the density at 10–12 wt% S matches the preliminary reference earth model (PREM). The seismic velocity at 12–15 wt% S matches PREM. These results suggest the presence of other LEs in the outer core.

Publication Source (Journal or Book title)

Journal of Geophysical Research Solid Earth

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