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Abstract:
The release and transport of volatiles, including sulfur-bearing species, by subduction related dehydration fluids are some of the key mechanisms of the deep sulfur cycle and link the surface with the crust and mantle sulfur budgets. However, parameters like the speciation of sulfur and the prevailing redox conditions during dehydration-related and fluid-mediated transport are still highly debated. To gain a deeper understanding of sulfur transport, distribution and speciation driven by different fluid migration processes at the blueschist-eclogite facies transition, we investigate samples from the New Caledonian Pouébo eclogite mélange (Taetz et al., 2016). These samples comprise two generations of garnet-quartz-bearing veins with adjacent omphacite-rich reaction halos in a blueschist metabasalt matrix. The veins are interpreted as both internal dehydration veins and external transport veins formed during prograde to peak metamorphism of the oceanic slab (Taetz et al., 2016). Therefore, these samples offer an ideal opportunity to link sulfur transport and speciation with different fluid migration processes at HP/LT subduction conditions.
Using the sulfur mineral distribution within and at varying distance to the veins, combined with in situ sulfur isotope and sulfide trace element compositions, three generations of pyrite formation and two main stages of sulfur mobilization at peak metamorphic conditions are inferred. The first stage of sulfur mobilization is linked to the formation of dehydration veins at low fO2 conditions, by the breakdown of water-bearing phases. At this stage, the primary wall rock pyrite (δ34S = -3.4‰ to -35.7‰) is partially leached from the wall rock and reprecipitates as fine-grained pyrite aggregates with δ34S values averaging at -12.4‰ in and along the newly forming, small-scale quartz-garnet-bearing dehydration veins. The second stage of sulfur mobilization takes place during the infiltration of an external fluid and formation of the transport vein. Due to a significant decrease of sulfide minerals towards the transport vein and the occurrence of Fe-oxide decomposition rims around the selvage pyrites, we infer an oxidizing character of the external fluid. This causes the development of a redox gradient between more oxidizing vein and more reducing matrix during selvage formation and enables sulfur mobilization and oxidation in the selvage area balanced by the reduction of omphacite Fe3+. The absence of sulfur-bearing minerals in the transport vein itself indicates, furthermore, that dissolved sulfate was removed from the investigated vein system by the passing external fluid.
Based on the studied vein systems, we imply that in subduction zones at the blueschist-eclogite facies transition fluid-mediated sulfur mobilization and speciation is mainly controlled by the fluid-rock ratio. While internally-buffered rock dehydration fluids carry sulfur in its reduced form, fluid-dominated transport veins may carry sulfur mostly in its oxidized form.
