Helium and Argon Partitioning Between Liquid Iron and Silicate Melt at High Pressure

Abstract

Whether Earth’s core is a significant repository of noble gases is an open question because their partitioning between liquid metal and silicate melt during core formation is poorly known. Here we calculated the He and Ar partition coefficients (D) between liquid Fe and MgSiO3 melt from 20 to 135 GPa at 5,000 K using ab initio molecular dynamics combined with thermodynamic integration. Our simulations show that DHe does not change significantly with pressure, while DAr increases strongly with increasing pressure. Furthermore, we found that the metal/silicate partitioning behaviors of He and Ar are controlled by their atomic size and the charge density properties of the host compositions. These results indicate that Earth’s core is a plausible reservoir for 3He and could possibly account for the high 3He/4He ratios in the OIBs that associated with deep‐rooted plumes, but the core is unlikely to be a significant source for 36Ar.

The Helium Elemental and Isotopic Compositions of the Earth's Core Based on Ab Initio Simulations

“…Therefore the variation of in Figure 2a is mainly controlled by T rather than P . A recent computational study (Xiong et al., 2021) also suggested a negligible P effect on . The T effect can be seen in Figure 2b: increases approximately by three orders of magnitude from 3,000 to 5,000 K for all metallic compositions of this study.…”

“…The T effect can be seen in Figure 2b: m /s E D increases approximately by three orders of magnitude from 3000 to 5000 K for all metallic compositions of this study. A similar T effect was also reported in a previous DFT-MD study (Xiong et al, 2018) which has shown potassium iron-silicate partition coefficient varies by three orders of magnitude from 3000 to 5000 K. Given the strong T effect and that the P-T conditions in our study follow the temperature profile of a magma ocean (Figure S1), our results may be more relevant to represent metal-silicate fractionation of He during planetary accretion and differentiation than the results of DFT-MD simulations from Xiong et al (2021) at 5000 K and experiments from Bouhifd, Jephcoat, et al (2013) over a limited P and T range.…”

“…The r Δ E P V term is negligible as the partial molar volumes of He in metallic and silicate melts are indistinguishable within uncertainties (Figure S5). Therefore the variation of m /s E D in Figure 2a is mainly controlled by T rather than P. A recent computational study (Xiong et al, 2021) also suggested a negligible P effect on m /s E D . The T effect can be seen in Figure 2b: m /s E D increases approximately by three orders of magnitude from 3,000 to 5,000 K for all metallic compositions of this study.…”

“…Recently, Xiong et al. (2021) computed He partition coefficients between pure Fe and MgSiO 3 melts using molecular dynamics (MD) simulations based on density functional theory (DFT). Their results support the core to be a host for primordial He.…”

“…Previous results from experiments byBouhifd, Jephcoat, et al (2013) andMatsuda et al (1993), and simulations by Y Zhang and Yin (2012). andXiong et al (2021) are shown as small symbols with border colors representing temperatures/ pressures.…”