Isotopic Compositions of Sulfides in Exhumed High-Pressure Terranes: Implications for Sulfur Cycling in Subduction Zones

Subduction is a key component of Earth's long-term sulfur cycle; however, the mechanisms that drive sulfur from subducting slabs remain elusive. Isotopes are a sensitive indicator of the speciation of sulfur in fluids, sulfide dissolution-precipitation reactions, and inferring fluid sources. To investigate these processes, we report delta S-34 values determined by secondary ion mass spectroscopy in sulfides from a global suite of exhumed high-pressure rocks. Sulfides are classified into two petrogenetic groups: (1) metamorphic, which represent closed-system (re)crystallization from protolith-inherited sulfur, and (2) metasomatic, which formed during open system processes, such as an influx of oxidized sulfur. The delta S-34 values for metamorphic sulfides tend to reflect their precursor compositions: -4.3 to +13.5 for metabasic rocks, and -32.4 to -11.0 for metasediments. Metasomatic sulfides exhibit a range of delta S-34 from -21.7 to +13.9 parts per thousand. We suggest that sluggish sulfur self-diffusion prevents isotopic fractionation during sulfide breakdown and that slab fluids inherit the isotopic composition of their source. We estimate a composition of -11 parts per thousand to +8 parts per thousand for slab fluids, a significantly smaller range than observed for metasomatic sulfides. Large fractionations during metasomatic sulfide precipitation from sulfate-bearing fluids, and an evolving fluid composition during reactive transport may account for the entire similar to 36 parts per thousand range of metasomatic sulfide compositions. Thus, we suggest that sulfates are likely the dominant sulfur species in slab-derived fluids. Plain Language Summary Sulfur is one of the key ingredients for life and drives many biochemical and geochemical reactions in Earth systems. The exchange of sulfur between Earth's exterior and interior during subduction is an important long-term component of the global sulfur cycle. In our study, we use stable isotopes of sulfur as a tracer of sulfur loss and migration from subducting oceanic plates. We demonstrate the utility of sulfur isotopes as a tracer by identifying potential sources of sulfur in the subducting plate. We suggest that the isotopic composition is unaffected by the dissolution of sulfur-bearing minerals and infer that the large compositional range of sulfides formed from fluids expelled from the subducting plate reflect the speciation of sulfur in the fluid. This study represents the first global overview of sulfur isotopes in subducted metamorphic rocks. These data may be compared with sulfur isotope measurements in volcanic arcs overlying subduction zones to trace sulfur from the subducting plate through the overriding plate.