The sulfur content and isotopic composition of the subarc mantle

Sulfur plays a critical role in modulating redox cycling on Earth. Yet, sulfur’s behavior during subduction and in mantle redox reactions is debated. We analyzed 34 S/ 32 S in mafic arc melt inclusions from contrasting subduction zones and modeled slab-mantle interaction to investigate the subduction zone sulfur cycle. We find that degassing may enrich or deplete the melt strongly in 34 S as a function of melt redox state. After correction for this effect, arc magmas have a substantially narrow range of δ 34 S values (+3 ± 2‰), higher than the ambient upper mantle (−1‰). Slab-derived sulfur is oxidized, evidenced by a concurrent increase in mantle Fe 3+ and sulfur contents, and contributes up to 86% of the mantle wedge’s sulfur budget. Arc magma δ 34 S values reflect a common slab source for sulfur: the oceanic crust. Subduction zones act as a “filter” for oxidative power and 34 S, effectively returning these to the surface over geological timescales.