Constraining compositional proxies for Earth's accretion and core formation through high pressure and high temperature Zn and S metal-silicate partitioning

Zinc is a moderately volatile and slightly siderophile element, and therefore provides information into the timing and conditions of volatile element delivery to Earth as well as the redistribution of these elements during planetary differentiation. Specifically, due to its similar volatility with S, it has been assumed that the Earth and its source material maintain the same relative abundances of these elements, and therefore the same S/Zn ratio. In this study, we have conducted Zn metal-silicate partitioning experiments at pressures up to 81 GPa and temperatures up to 4100 K in diamond anvil cells, for two distinct silicate compositions (one pyrolitic, one basaltic) and varying S contents in order to characterize Zn metal-silicate partitioning as a function of these variables. These results have been input into Earth formation models where various parametric controls have been evaluated–namely source material, impactor size and volatile delivery–to determine plausible sets of conditions that can generate present-day bulk silicate Earth (BSE) Zn and S abundances. Modelling results indicate that to arrive at present-day BSE contents for Zn and S, and with core S contents of ∼2 wt% or less, the Earth likely accreted heterogeneously – initially from a volatile-depleted source material compositionally akin to the metal and silicate chondrules of CH chondrites, with later delivery of more volatile-rich material.