Oceanic detachment faults focus very large volumes of black smoker fluids

Abstract: It is generally assumed that the seawater-derived fluids that feed black smoker vent fields on the seafloor are discharged vertically from depths of around 1-3 km. New oxygen and strontium isotope data is presented which show that fluids at black smoker temperatures of 300-400°C were focused along a low angle detachment fault at 15°45'N near the mid-Atlantic Ridge. Isotopic alteration is the most extreme ever reported from oceanic rocks altered at similar temperatures, indicating intensely focused fluid flow both in recharge and discharge parts of the hydrothermal system. REE mobility in the fault rocks demonstrates isotopic alteration by evolved hydrothermal fluids, not conductively heated seawater. The fault zone protolith was predominantly ultramafic but also included mafic rocks, with metasomatic alteration to talc-tremolite-chlorite schists resulting mainly from chemical exchange between these lithologies during fluid flow. Fluids in equilibrium with this assemblage would be similar to ultramafic-hosted black smoker fluids. These data will be compared with results from IODP Site 1309, drilled in a very similar setting at 30 °N. A new model is presented in which hydrothermal circulation around detachment faults evolves from basalt-hosted (TAG-type) to footwall ultramafic-hosted (Rainbow-type) to low temperature ultramafic-hosted (Lost City type). Key features of our model are the intrusion of gabbro bodies immediately below the detachment to provide a heat source for circulation, and focusing of fluid flow into the detachment fault to allow venting away from the neovolcanic axis. ------------------------------------------------------------------------ S351minaires G351osciences Marines ------------------------------------------------------------------------