Uptake of carbon and sulfur during seafloor serpentinization and the effects of subduction metamorphism in Ligurian peridotites

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 (190–1440 ppm 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 250–480 ppm 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.