Primordial Helium‐3 Exchange Between Earth's Core and Mantle

Volatiles from the solar nebula are known to be present in Earth's deep mantle. The core also may contain solar nebula‐derived volatiles, but in unknown amounts. Here we use calculations of volatile ingassing and degassing to estimate the abundance of primordial 3He now in the core and track the rate of 3He exchange between the core and mantle through Earth history. We apply an ingassing model that includes a silicate magma ocean and an iron‐rich proto‐core coupled to a nebular atmosphere of solar composition to calculate the amounts of 3He acquired by the mantle and core during accretion and core formation. Using experimentally determined partitioning between core‐forming metals and silicate magma, we find that dissolution from the nebular atmosphere deposits one or more petagrams of 3He into the proto‐core. Following accretion, 3He exchange depends on the convective history of the coupled core‐mantle system. We combine determinations of the present‐day surface 3He flux with estimates of the present‐day mantle 3He abundance, mantle and core heat fluxes, and our ingassed 3He abundances in a convective degassing model. According to this model, the mantle 3He abundance is evolving toward a statistical steady state, in which surface losses are compensated by enrichments from the core. Plain Language Summary Each year, about 2 kg of the rare gas helium‐3 escapes from Earth's interior, mostly along the mid‐ocean ridge system. Helium‐3 is primordial, created shortly after the Big Bang and acquired from the solar nebula as the Earth formed. Geochemical evidence indicates the Earth has deep reservoirs of helium‐3, but their locations and abundances remain uncertain. Our models of volatile exchange during Earth's formation and evolution implicate the metallic core as a leaky reservoir that supplies the rest of the Earth with helium‐3. Our results also suggest that other volatiles may be leaking from the core into the mantle. Key Points The mantle and core acquired petagrams of helium‐3 from the solar nebula The core is a major helium‐3 reservoir in the Earth Helium‐3 leaks from the core to the mantle