Sub-arc mantle oxidation is often attributed to sulfates from seawater transported in aqueous fluids derived from slab dehydration, but sulfur speciation and origin(s) in high-pressure fluids remain weakly constrained. Here, we use the four sulfur stable isotopes (³²S, ³³S, ³⁴S, ³⁶S) to decipher processes, e.g. mixing of two end-member reservoirs and open-system reaction occurring during sulfur devolatilization, and deduce the nature and origin of sulfur species in aqueous slab-derived fluids. We analyzed high pressure metamorphic rocks from the Monviso massif Lower Shear Zone (LSZ; Western Alps), recording evidence of an interface-parallel, channelized fluid flow near peak burial metamorphic conditions (∼550 °C and 2.7 GPa). We sampled variably metasomatized Fe-Ti metagabbros, hybrid (talc) schists and serpentinites containing sulfides supposed to preserve the aqueous fluid composition, as well as other alpine metagabbro, metabasalt and serpentinites from greenschist to eclogite-facies devoid of the studied metasomatic overprint. The millimeter to centimeter-sized sulfide crystals found in abundance within LSZ lithologies are mainly pyrites containing inclusions of the peak burial mineral assemblage (e.g. garnet, omphacite, rutile) indicating their crystallization at peak conditions or during incipient exhumation. The sulfides in the non-metasomatic metagabbros and metabasalts cover a narrow range in δ'³⁴S of 0.89 ± 0.63 ‰ and Δ'³³S of 0.017 ± 0.01 (n = 6) that could be inherited from hydrothermal sulfides formed on the seafloor. In contrast, the LSZ sulfides from the Fe-Ti metagabbros, serpentinites and hybrid schists (n = 24) are enriched in ³⁴S and ³³S (bulk δ'³⁴S from 4.91 to 21.32 ‰ and Δ'³³S from 0.02 to 0.06 ‰) compared to the sulfides from the non-metasomatic rocks. Mixing between reduced sulfur species cannot explain all the data without requiring an end-member composition as high as δ³⁴S ∼ 22 ‰ and Δ³³S ∼ 0.06 ‰, i.e. values rarely recorded by sulfides in the literature. Instead, we propose that this ³³S-³⁴S-enrichment results from open-system sulfate reduction to sulfides in aqueous fluids at 550°C, with an initial δ³⁴S ∼ 12 ± 3 ‰ and Δ³³S ∼ 0.025 ± 0.010 ‰. Because this signature is different from seawater sulfate, we suggest that sulfate was produced by hydrothermal sulfide oxidation during antigorite breakdown at 650°C, in agreement with Mg, Ni and Cr enrichments in the metasomatized HP mafic lithologies. This model is consistent with relatively oxidized ³⁴S-enriched arc lavas and can potentially explain recycled sulfur with positive δ³⁴S signature recorded by ocean island basalts.