Mantle 182W-3He heterogeneities explained by long-term core-mantle interaction

Ocean island basalts feature lower ratios of radiogenic W and He isotopes to their stable counterparts than the background mantle. Traditionally, mantle 3He/4He heterogeneities have been attributed to an undegassed reservoir in the lower mantle that has retained primordial 3He. However, such a reservoir would also be characterized by high 182W/184W, instead of the observed low 182W/184W, thus requiring some additional mechanism to explain the observed anti-correlation between 182W/184W and 3He/4He in ocean island basalts. A single mechanism that can explain mantle heterogeneities in both W and He isotopes is diffusive isotopic exchange across the core-mantle boundary. Indeed, Earth's core is expected to harbor an abundance of W with low 182W/184W and potentially large amounts of He with high 3He/4He. We model the long-term isotopic evolution of mantle domains subject to diffusive exchange of W and He with the core. After applying geodynamical calculations to predict the composition of upwellings expressed at Earth's surface, we find that persistent exchange of W and He across the core-mantle boundary can quantitatively reproduce the present-day mantle 182W-3He heterogeneities. No undegassed mantle reservoir, nor any other early fractionation or accretion processes, are required, bearing significance on early Earth conditions such as the depth extent of magma oceans.