Sulfur geochemistry of peridotite-hosted hydrothermal systems: Comparing the Ligurian ophiolites with oceanic serpentinites

Serpentinization processes at slow- and ultraslow-spreading ridges control the exchange of various elements between seawater and the oceanic lithosphere and play a major role in marine geochemical cycles. We use opaque mineral assemblages and sulfur isotope geochemistry to reconstruct variations in fluid fluxes, redox conditions and microbial activity prevailing during serpentinization and carbonate precipitation of serpentinites and ophicalcites from an ophiolite sequence in the Northern Apennines (Italy). We then compare our results with calcite-veined serpentinites from the Iberian Margin formed during the opening of the North Atlantic, and with serpentinites sampled at the active peridotite-hosted Lost City hydrothermal field on the Atlantis Massif near the Mid-Atlantic Ridge. The sulfide and oxide mineral assemblage of the serpentinites from the Northern Apennines is dominated by pyrite, pentlandite, millerite, siegenite, magnetite, and hematite, reflecting relatively oxidizing conditions, while the sulfur geochemistry reveals a dominance of sulfide sulfur over sulfate sulfur. δ34S values of sulfide and sulfate indicate that microbial sulfate reduction, leaching, and oxidation are the main processes that affected the sulfur isotope signature of these serpentinites. The opaque mineralogy in the serpentinites from the Northern Apennines is similar to that of the Iberian Margin and the southern wall of the Atlantis Massif, and generally represents the late stages of serpentinization, where intense fluid circulation leads to fairly oxidizing conditions and alkaline fluids lead to carbonate precipitation. However, the mineral assemblages also indicate strong fluctuations in oxygen fugacity, likely caused by volume expansion during serpentinization and tectonic activity along a mid-ocean ridge. Additionally, local enrichment in 34S in sulfides suggests that fluids interacted at depth with gabbros and subsequently circulated along shear zones, similar to observations along detachment fault surfaces at the Atlantis Massif. Our results, therefore, give further evidence that the ophicalcites and serpentinites from the Northern Apennines formed in a tectonic setting similar to the Atlantis Massif. We estimate a global annual uptake of seawater sulfate of 5.6 to 12 x 1011 g S by serpentinization of ultramafic rocks exposed to seawater on the ocean floor. We suggest that the thermal structure of the exposed mantle rocks and, in turn, the efficiency of microbes to reduce seawater sulfate to sulfide is a major factor controlling the storage of sulfur in serpentinites. Thus, cycling of sulfur in peridotite-hosted hydrothermal systems during the Jurassic was probably very similar to the processes observed today in systems along the Mid-Atlantic Ridge.