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

Abstract

We use density functional theory-based molecular dynamics simulations to predict the partitioning behavior of helium (He) between coexisting metal and silicate melts at conditions of the magma ocean and the current core–mantle boundary. Helium strongly favors silicate over metal at low pressures and temperatures (10 GPa and 3000 K) but it becomes approximately two orders of magnitude more compatible with metal at greater pressures and temperatures (50 GPa and 4000 K) expected in a deep magma ocean. We further examine He partitioning behavior for varying metal compositions (pure Fe, Fe-S, and Fe-O alloys) and find that oxygen enhances He incorporation into the core by one to two orders of magnitude. The He elemental and isotopic compositions of the Earth's core are estimated to be ∼4.2 ng/g and ∼140 atmospheric 3He/4He ratio assuming the core containing some amounts of oxygen as required to explain the core density deficit. Our results suggest that the core may play a key role as a reservoir for the He signature recorded in ocean island basalts with distinctively high 3He/4He ratios.