The magma ocean was a huge helium reservoir in the early Earth

Abstract

Helium from mantle-derived rocks bears in its isotopic ratios the traces of Earth differentiation, degassing, and long term geodynamic evolution. The low 4He/3He ratio of rocks whose origins are in the deep mantle, like ocean island basalts (OIBs), reflect potentially concealed geochemical reservoirs rich in primordial 3He and isolated from the surface. Here we search for the geological repository of primordial He lasting from the core formation. For this, we determine the chemical behaviour of He in the magma ocean (MO) during core formation. We perform ab initio molecular dynamics simulations at temperatures and pressures along the MO adiabat. Our results show that primordial He was largely trapped in MO. At the top, under the hot dense early atmosphere, He remained mainly in the MO and degassed only later, at low atmospheric pressure. At the bottom, He partitions preferentially into the MO rather than the liquid core. The origin of the OIBs reflects a large contribution and contamination from mantle sources and no contribution from the top of the outer core, depleted in primordial He. We suggest the search for the He reservoirs should be done at the base of the solid mantle.

Topics & Concepts