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Abstract:
Sulfur is transferred into the overlying mantle wedge during subduction of oceanic plates, with important ramifications for magmatic processes in volcanic arcs. Many studies have shown that arc magmas are more oxidized and enriched in 34S than mid-ocean ridge basalts, which has been linked to the transfer of slab-derived volatiles, such as sulfate, into the overlying mantle wedge and into arc magmas. However, the transfer mechanisms and the oxidation states of slab fluids is still under debate, and particularly the role of sulfur is currently widely discussed. Here we present bulk rock and in situ sulfur isotope results from metasomatic, eclogitic metagabbros from the Voltri Massif in Italy that are in contact with serpentinites (Schwarzenbach et al., 2024). Previous petrological work of this contact showed that metasomatism by fluid-mediated mass transfer of Mg from the serpentinite into the metagabbro caused formation of actinolite-chlorite schists and metagabbro rich in epidote and Na- and Na-Ca amphiboles along the contact (Codillo et al., 2022). Abundant euhedral to subhedral pyrite in the metasomatized metagabbro show distinct correlations between in situ sulfur isotope analyses and sharp Co and Ni growth zones documenting multiple generations of pyrite formation. In particular, a trend of increasing δ34S values from core to rim in pyrite associated with inclusions of distinct high pressure silicate minerals documents the input of 34S-enriched sulfur during metasomatism of the eclogitic metagabbros concurrent to subduction metamorphism. Using thermodynamic modeling our study shows that the infiltrating fluids equilibrated with the serpentinite before entering the metagabbro and that these fluids were HS--bearing. Infiltration of these HS--bearing and 34S-enriched fluids led to redox reactions in the Fe-Ti metagabbro involving sulfur, iron, and likely carbon, and the formation of euhedral pyrite with δ34S values of up to 12.5‰ in the metasomatized metagabbro. We argue that this process of 34S-enriched sulfur mobilization from serpentinites is pervasive along the plate interface in subduction zones and infer that melting of such metasomatic material can explain the 34S-enriched signatures observed in arc magmas.
