[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-05] Core-mantle partitioning and the bulk Earth abundances of hydrogen and carbon: Implications for their origins and delivery timing

*Kei Hirose^1,4, Yutaro Tsutsumi¹, Naoya Sakamoto², Shuhei Mita¹, Shunpei Yokoo¹, Han Hsu³, Hisayoshi Yurimoto² (1.The University of Tokyo, 2.Hokkaido University, 3.National Central University, 4.Institute of Science Tokyo)

Hydrogen and carbon are important volatile elements on our planet, and their sources and the timing of the delivery to the growing Earth are of great interest. We determined the metal/silicate partition coefficients of hydrogen and carbon, D_H and D_C, simultaneously under typical conditions of Earth’s core formation. Our experiments demonstrate that both D_H and D_C diminish in the presence of carbon and hydrogen, respectively, indicating their strong interactions. With these partitioning data, we investigated core concentrations and the bulk Earth abundances of hydrogen and carbon based on core formation scenarios that are compatible with the core mass fraction and the bulk silicate Earth composition. The results demonstrate >95% of the bulk Earth budgets of both hydrogen and carbon may be present in the core, markedly different from those using the D_H and D_C individually determined in earlier experiments. Our modeling also suggests that water and carbon were delivered to the Earth after 50% accretion by building blocks containing ~1–3 wt% H₂O and ~0.2–0.6 wt% C on average. Such H₂O and C contents are not consistent with the pure enstatite chondrite origin of the Earth but support the involvement of some carbonaceous chondrite-type materials that provided about half of the Earth’s water and carbon budgets.