Subduction zone sulfur mobilization and redistribution by intraslab fluid–rock interaction

Abstract

Subduction zones mediate the sulfur exchange between Earth’s interior and surface, and play a critical role in the long-term global sulfur cycle. Dehydration of the subducted slab releases aqueous fluids, which in turn interact with the surrounding wall-rocks during reactive flow through the slab. Fluids are regarded as the key agent for sulfur transfer within the slab and subsequently into the mantle wedge. However, the mechanisms underlying sulfur mobilization and redistribution during fluid–rock interaction are not fully understood. Here, we investigate three high-pressure blueschist/eclogite–selvage–vein systems from the Southwestern Tianshan metamorphic belt in northwestern China to explore sulfur behavior during fluid–rock interaction along channelized fluid pathways and within their alteration halos. Petrologic observations on mineral parageneses and δ34S compositions of sulfides reveal two different scenarios of sulfur mobilization and redistribution during fluid–rock interaction at HP conditions. Fe-rich and medium-fS2 (near the pyrite–pyrrhotite buffer) fluids favor pyrrhotite stability over pyrite and can trigger the pyrite–to–pyrrhotite transition in the reaction halo. In this case the bulk-rock sulfur budget may not vary but the sulfur isotope exchange is still effective, producing a δ34S mixing trend in the metasomatic zones. In contrast, Fe-poor and high-fS2 fluids cause conversion of pyrrhotite to pyrite, significant sulfur addition, and sulfur isotope exchange in alteration selvages. Medium-fS2 fluids can transport sulfur and thus are effective in transferring sulfur out of the slab, whereas high-fS2 fluids cause sulfur sequestration along fluid pathways and considerably reduce long-distance sulfur transfer. Multiple fluid infiltration events with different fluid sources result in several pyrite overgrowth sequences, recorded by contrasting Co–Ni element distributions and in-situ δ34S zoning of pyrite. The heaviest δ34S value (+25‰) thus far reported in HP rocks has been found in vein pyrite, suggesting that a seawater sulfate-derived δ34S signature can be transported to great depths in a subduction zone, even though the timing and mechanism of sulfate–to–sulfide reduction in the subduction zone remain unconstrained. Our study provides natural evidence for fluid-mediated sulfide transformation, sulfur transport/storage, and sulfur isotope exchange during fluid–rock interaction, and illuminates the role these processes play in sulfur release from the subducting slab and sulfur subduction into the deep mantle.