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

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. Plain Language Summary The Earth's core has been suggested to be a long‐term host for the isotope 3He that must be preserved from the formation of the Earth more than 4 billion years ago. This 3He isotope is shown to be present in ocean island basalts (for example Hawaii or Iceland) in an amount that is untypical for other basalts that form the ocean floor. Using advanced quantum mechanical modeling, we show that, although He favors silicate over metallic melts at high pressure and temperature, conditions under which Earth's core formed, the core possibly maintains a high primordial 3He content relative to 4He. Therefore, the core may play a significant role as a deep‐rooted source enriched in 3He for the ocean island basalts. Key Points We perform ab initio calculations showing that helium is lithophile but becomes increasingly compatible with metal at deep mantle conditions The core has a low helium elemental abundance (∼4.2 ng/g) but possibly maintains a high primordial 3He/4He ratio (∼140 times atmospheric) The core potentially plays a key role in serving as a reservoir for He sampled by ocean island basalts with high 3He/4He ratios