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Abstract: The concentrations and isotope compositions of carbon and sulfur in Ligurian serpentinites having different metamorphic histories were analyzed in order to investigate the behavior of sulfur and carbon during seafloor serpentinization and during subduction metamorphism. Serpentinites associated with gabbros and with seafloor hydrothermal sulfide deposits in northern Apennine ophiolites that exhibit only slight effects of low-grade metamorphism have high sulfur contents (190–1440ppm total sulfur) and δ34STotal S values of up to 9.8‰ through interaction with hydrothermal fluids in high-temperature ultramafic-hosted hydrothermal systems on the seafloor. Fluid circulation was driven by mafic intrusions and the hydrothermal fluids contained a mixture of basaltic and seawater-derived sulfur. The serpentinites contain 250–480ppm total carbon having δ13CTotal C values of −9.4 to −6.0‰, reflecting mixtures of seawater carbonate (~0‰) and reduced (organic) carbon having δ13C≈−25‰ and derived from multiple sources. Entrainment of cold seawater into the subsurface beneath seafloor hydrothermal sulfide deposits maintained locally low temperatures and enabled microbial reduction of seawater sulfate. Combined with previous work documenting low-temperature serpentinization on the seafloor away from such gabbro-driven high-temperature hydrothermal activity, these results provide an integrated view of serpentinization processes in tectonically extended oceanic basement exposed in oceanic core complexes. High-P–T antigorite serpentinites in the Voltri massif (Ligurian Alps) have sulfur and carbon contents and isotope compositions similar to those of the Apennine ophiolites, indicating that the protoliths underwent similar hydrothermal processes on the seafloor. Recrystallization of chrysotile and lizardite to antigorite and minor partial dehydration to olivine during subduction metamorphism result in no significant changes in the contents or isotope compositions of sulfur or carbon in the serpentinites. We estimate that the uptake of carbon and sulfur by serpentinites in oceanic basement is comparable to or greater than that per unit volume of mafic oceanic crust. The high P–T stability of serpentine can, however, make serpentinites important for transport of carbon and sulfur to greater depths in subduction zones. [Copyright &y& Elsevier]

Abstract: We use the concentrations and isotope compositions of water, carbon, and sulfur in serpentinites and their dehydration products to trace the cycling of volatiles during subduction. Antigorite serpentinites from the Cerro del Almirez complex, Spain, contain 9–12wt.% H2O and 910±730ppm sulfur, and have bulk δ18O values of 8.6±0.4‰, δD=−54±5‰, and δ34S=5.0‰, consistent with serpentinization at temperatures of ~200°C by seawater hydrothermal fluids in a seafloor setting. The serpentinites were dehydrated to chlorite–harzburgite (olivine+orthopyroxene+chlorite) at 700°C and 1.6–1.9GPa during subduction metamorphism, resulting in loss of water, and sulfur. The chlorite–harzburgites contain 5.7±1.9wt.% H2O, and have bulk δ18O=8.0±0.9‰, and δD=−77±11‰. The rocks contain 650±620ppm sulfur having δ34S=1.2‰. Dehydration of serpentinite resulted in loss of 5wt.% H2O having δ18O=8–10‰ and δD=−27 to −65‰, and loss of 260ppm sulfur as sulfate, having δ34S=14.5‰. The contents and δ13C of total carbon in the two rock types overlap, with a broad trend of decreasing carbon contents and δ13C from ~1300 to 200ppm and −9.6 to −20.2‰. This reflects mixing between reduced carbon in the rocks (210ppm, δ13C≈−26‰) and seawater-derived carbonate (δ13C≈−1‰). Our results indicate: 1) Serpentinized oceanic peridotites carry significant amounts of isotopically fractionated water, carbon and sulfur into subduction zones; 2) Subduction of serpentinites to high P and T results in loss of water, and sulfur, which can induce melting and contribute to 18O, D, and 34S enrichments and oxidation of the sub-arc mantle wedge; and 3) Isotopically fractionated water, carbon, and sulfur in serpentinite dehydration products are recycled deeper into the mantle where they can contribute to isotope heterogeneities and may be significant for volatile budgets of the deep Earth. [Copyright &y& Elsevier]