Experimental determination of equilibrium sulfur isotope fractionation factors in the gas-silicate melt-sulfide liquid system

Sulfur, despite its minor presence (∼ 250 µg.g −1 ) in the bulk silicate Earth, can exhibit high solubility in melts, up to 1.5 wt. %, depending on the melt’s oxidation state. Sulfur is also integral to the formation of economically significant metal ores. This study reports high-temperature equilibrium sulfur isotope fractionation between melt, sulfide, and gas phases using a dynamic 1-atm gas-mixing furnace. Equilibrium experiments conducted on a basaltic system at temperatures of 1200, 1300, and 1400 °C indicate that equilibrium sulfur isotope fractionation between a sulfide liquid and SO 2 gas is a linear function of oxygen concentration in the sulfide phase. The fractionation factor is expected to be close to 0 ‰, for a pure FeS sulfide liquid. The experiments also reveal an isotopic fractionation of S from – 0.49 ± 0.97 up to + 5.30 ± 0.59 ‰ between the silicate melt and sulfide liquid, with the silicate melt being enriched in 34 S. This variation was best modelled by combining compositional elements of sulfide liquid (oxygen and sulfur) and silicate melt (MgO and Na 2 O). Our results show a new mechanism of sulfur isotope fractionation between silicate melt and sulfide liquid that results in isotopically heavier melts without sulfate due to the presence of oxygen in the sulfide liquid. Applying our model to Mid-Oceanic Ridge Basalts (MORBs) shows zero S isotope fractionation is expected when the composition of the sulfide liquid is between Fe 0.92 S and Fe 0.86 SO 0.05 . This study shows that measurements of oxygen concentration in natural sulfides are required to accurately interpret sulfur isotope compositions during magma evolution.