Magmatic sulfide segregation generates heavy sulfur isotope signatures in global arc systems

How sulfur is transferred from subducting slabs to the mantle and returned to the surface is central to understanding sulfur cycling in subduction zones. Arc magmas and volcanic gasses are commonly enriched in 34 S relative to the mantle, a signature widely attributed to slab-derived sulfur inputs. Sulfide saturation and segregation are widespread during magma generation and differentiation, yet their effects on sulfur isotope fractionation remain poorly constrained. Here we use high-pressure experiments at 0.7–1.5 GPa and 1050–1400 °C to quantify sulfur isotope fractionation between sulfide liquid, monosulfide-solid-solution (MSS), and silicate melts over oxygen fugacities ranging from ∼FMQ−2 to FMQ+1.5. We find that sulfide–silicate melt fractionation is strongly controlled by temperature and oxygen fugacity. Sulfide liquid–silicate melt fractionation (∆ 34 S Sul‒Sil ) ranges from −1.8 to +0.07‰, whereas MSS–silicate melt fractionation (∆ 34 S MSS‒Sil ) ranges from −2.8 to −0.03‰, with the largest fractionation occurring under oxidized conditions. Fractionation approaches zero at high temperatures and reduced conditions typical of mid-ocean ridge basalts, consistent with the near-mantle δ 34 S values of natural MORBs. In contrast, under oxidized arc conditions, sulfide segregation preferentially removes isotopically light sulfur and progressively enriches residual melts in 34 S. Applying our experimentally determined fractionation factors to arc magma differentiation shows that residual melt δ 34 S can increase by up to ∼8‰, demonstrating that sulfide segregation can make a substantial contribution to the positive δ 34 S signatures observed in arc magmas, volcanic gasses, and continental crust. Our results demonstrate that magmatic sulfide segregation can substantially modify primary sulfur isotope signatures and therefore needs to be considered when using sulfur isotopes to quantify slab-derived sulfur fluxes. This mechanism can generate elevated δ 34 S in arc systems without necessarily requiring isotopically heavy sulfur inputs from subducting slabs.