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. We focus on a basaltic system formed by mantle melting, offering a representative insight into the behaviour of mantle-derived sulfur. Equilibrium experiments conducted at temperatures of 1200, 1300, and 1400 °C indicate that equilibrium sulfur isotope fractionation between a sulfide liquid and SO2 gas is a linear function of oxygen concentration in the sulfide phase. For a pure FeS sulfide liquid, the fractionation is expected to be close to 0 ‰. They also reveal a S isotopic fractionation from –0.49 ± 0.97 up to +5.30 ± 0.59 ‰ between the melt and sulfide liquid, with the melt being enriched in 34S. In our experiments, the factors affecting the melt-sulfide liquid fractionation are the concentrations of MgO and Na2O in the melt, as well as oxygen and sulfur in the sulfide liquid. By applying our model to MORB basalts, which are known to exhibit zero S isotope fractionation with a sulfide liquid, we determine that the composition of the sulfide liquid ranges between Fe0.92S and Fe0.86SO0.05. This study shows that measurements of oxygen concentration in natural sulfides are required to accurately interpret sulfur isotope composition during magma evolution.