Metal-silicate partitioning behaviors of Ga and Ge at high pressures and implications for the Earth’s volatile accretion

Abstract

Gallium (Ga) and germanium (Ge) are moderately siderophile and volatile elements whose metal-silicate partitioning behaviors are valuable to understand both core-formation and volatile accretion processes. In this study, we performed metal-silicate partitioning experiments at pressures of 22–70 GPa and temperatures of 3728–4740 K, using laser-heated diamond anvil cells, to explore the effects of pressure, temperature, and metal composition on Ga and Ge partitioning. Thermodynamic modeling using our experimental data and those from the literature reveals that the metal affinities of both Ga and Ge decrease as pressure and temperature increase, with Ga being less siderophile than Ge. Our fitting results confirm that the presence of S and Si in metal can reduce the siderophility of both Ga and Ge, consistent with previous findings at relatively low pressures and temperatures. Our results also demonstrate that O in metal has opposing effects on the metal-silicate partitioning of Ga and Ge. It increases the metal affinity of Ga, contrary to previous thought, but decreases that of Ge. Incorporating these partitioning behaviors, we performed multi-stage core formation modeling to search for accretion scenarios and factors that can reproduce the bulk silicate Earth abundances of Ga, Ge, and S. Our results suggest that Ga and Ge were likely accreted throughout the entire stages of Earth’s accretion rather than accreted solely in the late stage for the final 10–20 % of Earth’s mass growth.