B2 - Stable isotope fractionation of S, Te and Pd and the roles of core formation and late accretion on siderophile volatile elements in the Earth

Research areas: Geochemistry, cosmochemistry, petrology

Summary

In current formation models of the Earth, the budgets of the siderophile volatile elements (SVE) S, Se, and Te and the highly siderophile elements (HSE) in the silicate Earth are assumed to predominantly derive from the late accretion of material that had a broadly chondritic bulk composition. But, some HSEs, most notably Pd, appear to display less siderophile behaviour at the very high temperatures and pressures that prevailed during Earth’s core formation. Thus, a late veneer is not needed to explain the elevated Pd abundances in the Earth’s mantle. However, the abundances of other HSEs (Ir, Os) cannot be accounted for by equilibrium partitioning during core formation, even under very high temperatures and pressures. These contrasting results can be reconciled if the HSE abundances in the Earth’s mantle reflect not only the late veneer but also a residual signature of core formation. This would also be consistent with the elevated Pd/Ir and Pd/Os of the Earth’s mantle, although these non-chondritic ratios may likewise be a distinctive feature of the late-accreted material.

Similar to the HSEs, the budget of the SVEs S, Se, and Te in the silicate Earth are commonly thought to derive almost entirely from the late veneer. This view has been challenged, however, by recent data on the S isotopic composition of ocean ridge basalts derived from the Earth’s mantle. These data suggest that the S isotopic composition in the Earth’s mantle and in chondrites is different, which was interpreted to reflect mass-dependent fractionation of S isotopes during metal–silicate segregation. This difference would imply that about half of the S in the Earth’s mantle might be left over from metal–silicate segregation during core formation, meaning that only the remaining S was delivered by the late veneer. This conclusion, however, contradicts available data on the metal–silicate partitioning of S, Se, and Te and on the chondritic abundance ratios of S, Se, and Te in mantle rocks, which suggest that >90% of the budget of S and essentially the complete budget of Se and Te were delivered by a late veneer.

Therefore, in light of these discrepancies, the objectives of this project are twofold: first, we aim to study the stable isotope composition of S, Te, and Pd in samples derived from the terrestrial mantle, as well as the stable isotope composition of Te and Pd in chondritic bulk rocks. Second, we aim to experimentally calibrate the stable isotope fractionation imparted by metal–silicate equilibration during core formation by performing isotope fractionation experiments at a range of relevant conditions.

Overall, this study will contribute critical data to help resolve the discrepancy between the isotopic and concentration data of S, and it will also help to assess whether the elevated Pd abundances in the Earth’s mantle can or cannot be a residual signature of core formation. As such, by determining how core formation and the late veneer affected the budget of HSEs and SVEs in the Earth’s mantle, this study will help us quantitatively constrain the relative roles of these crucial events in Earth’s history.