Elemental partitioning and isotopic fractionation of Zn between metal and silicate and geochemical estimation of the S content of the Earth's core

Abstract

Zinc metal-silicate fractionation provides experimental access to the conditions of core formation and Zn has been used to estimate the S contents of the Earth's core and of the bulk Earth, assuming that they share similar volatility and that Zn was not partitioned into the Earth's core. We have conducted a suite of partitioning experiments to characterize Zn metal-silicate elemental and isotopic fractionation as a function of time, temperature, and composition. Experiments were conducted at temperatures from 1473-2273K, with run durations from 5-240 minutes for four starting materials. Chemical and isotopic equilibrium is achieved within 10 minutes. Zinc metal-silicate isotopic fractionation displays no resolvable dependence on temperature, composition, or oxygen fugacity. Thus, the Zn isotopic composition of silicate phases can be used as a proxy for bulk telluric bodies. Results from this study and literature data were used to parameterize Zn metal-silicate partitioning as a function of temperature, pressure, and redox state. Using this parameterization and viable formation conditions, we have estimated a range of Zn contents in the cores of iron meteorite parent bodies (i.e. iron meteorites) of ~0.1-150 ppm, in good agreement with natural observations. We have calculated the first geochemical estimates for the Zn contents of the Earth's core and of the bulk Earth, at 242 +/-107 ppm and 114 +/-34 ppm (respectively), that consider the slightly siderophile behavior of Zn and are therefore significantly higher than previous estimates. Assuming similar volatility for S and Zn, a chondritic S/Zn ratio, and considering our new estimates, we have calculated a geochemical upper bound for the S content of the Earth's core of 6.3 +/-1.9 wt%. This indicates that S may be a major contributor to the density deficit of the Earth's core or that the S/Zn ratio for the Earth is non-chondritic.