Volatile elements are critical to life and planetary evolution, and their delivery to Earth has profound implications for planetary habitability. They may have been accreted during Earth's main growth (Rubie et al., 2015), or via the late veneer, after core formation (Albarede, 2009). Determining the dominant process is key to understanding Earth's volatile inventory.

Sulfur (S) is a key tracer for volatile delivery due to its dual nature as both a siderophile and volatile element, being sensitive to planetary differentiation processes such as core segregation and partial volatilization during magma ocean phases. Earth's basalts have variable 34S/32S sulfur isotope ratio signatures, with estimates for mantle sources that are between -1.3±0.3‰ and -0.7±0.1‰ (in δ34S) (Labidi and Cartigny, 2016), deviating from that of chondrites (0.04±0.31‰) (Gao and Thiemens, 1993a, 1993b). It is unclear whether the signature from basalts reflect a core/mantle equilibration event, sulfur evaporation from a magma ocean, or geodynamic evolution of Earth's mantle. Experimental data have suggested that metal-silicate isotopic fractionations are resolvable but remain small, leaving most of the isotopic shift unexplained (Labidi et al., 2016). However, the previous experiments rely on volatile-saturated conditions, which could inhibit isotopic fractionation. 

We conducted new piston-cylinder experiments on metal-silicate segregation using an experimental set up that does not induce volatile saturation. Additionally, we apply a triple-isotope technique combined with time series in order to thoroughly assess sulfur isotopic equilibrium. In experiments lasting up to 12 hours, we observe a small but resolvable equilibrium isotope fractionation between metal and silicate, meaning that incomplete core equilibration may account for at least a fraction of the isotopic shift observed between terrestrial basalts and chondrites.