Partitioning of noble gases (He, Ne, Ar, Kr, Xe) during Earth’s core segregation: A possible core reservoir for primordial noble gases

Abstract

Noble gas isotopes in plume-influenced oceanic-island basalts (OIBs) clearly indicate the existence of distinct reservoirs somewhere in the deep Earth that are characterized by their more primitive noble gas isotopic signatures. As one of the largest differentiated layers on Earth, whether the Earth’s liquid outer core can preserve ‘primordial’ noble gases is fundamental for better understanding the Earth’s formation and the mantle evolution. Since Earth’s core is believed to have formed via liquid iron segregated from magma ocean during the planetary accretion stage, how much noble gases could have been incorporated into Earth’s core depends on the partitioning of noble gases between liquid metal and molten silicates. In this study, we perform thermodynamic integration (TI) calculations based on first-principles molecular dynamics (FPMD) simulation to investigate the partition coefficients of noble gases (NG: He, Ne, Ar, Kr, Xe) between liquid Fe and MgSiO3 melt. The simulations have been performed by adopting conditions that the pressures are from 10 ∼ 135 GPa and the temperatures from 2300 ∼ 5000 K to fully account for possible core-mantle equilibrium conditions. By analyzing the temperature and pressure dependence of the results, we suggest that the metal-silicate partitioning of noble gases (DNG) merely depends on the temperature, with extremely low DNG < 0.001 at temperatures < 2500 K (20 GPa), but increases to different degrees at high temperatures. The DNG tends to be more affected by temperature as the noble gas atomic mass increases, with the least temperature effect on the partition coefficient of He (DHe), which increases from ∼ 10−3 at 2300 K (10 GPa) to ∼ 10−1 (5000 K, 135 GPa) near the core-mantle boundary (CMB), and DNe = 10−6 ∼ 10−3, DAr = 10−6 ∼ 10−2, DKr = 10−7 ∼ 10−1 and DXe = 10−5 ∼ 100 from 2300 to 5000 K, respectively. Analysis of electronic interactions indicates that noble gases have positive chemical bonding with Fe, Mg, and Si atoms, and the bonding strengths increase with pressure as well as increasing noble gas atomic mass. Based on our calculated partition coefficients, we suggest that Earth’s core may have preserved significant amounts of noble gases considering the volatile-rich environment from which the core segregated.