Earth’s deep magma ocean never reached sulfide saturation

The chondritic abundances of highly siderophile elements in the Earth’s mantle have been proposed to result from the combination of pervasive exsolution and segregation of sulfide from the magma ocean, along with late accretion. Therefore, evaluating the sulfur storage capacity (i.e. sulfur solubility) of the deep magma ocean is key to understanding the late stages of Earth’s formation. Here, we have investigated the effects of high pressure and high temperature on the solubility of sulfur in the deep primordial magma ocean by melting and equilibrating a pyrolytic glass with pure FeS in a laser-heated diamond anvil cell. Chemical analysis of the run products show that significant amounts of sulfur can be stored deep in the magma ocean (∼1 wt. %). We developed a thermodynamic parameterisation predicting the solubility of sulfur at sulfide saturation (SCSS) depending on pressure, temperature and silicate composition. We show that SCSS always exceeds the amount of sulfur present in the mantle due to metal-silicate partitioning, consistent with previous work. Consequently, it appears that Earth’s magma ocean likely never reached sulfur saturation, thereby preventing the segregation of a sulfide-rich melt, also referred as the “Hadean matte”.