High-Salinity Seawater-Derived Fluids and Lower-Crust Hydrothermal Mineralization at the Vema Lithospheric Section, Central Atlantic

We reconstructed a magmatic-hydrothermal transition in gabbros spatially associated with mantle peridotites from the Vema Lithospheric Section. The presence of titanian pargasite suggests the reaction of an early augite-plagioclase assemblage with a highly evolved residual melt and/or high-temperature magmatic fluid. The overprinting hydrothermal event occurs as fracture patterns with cracks filled by Cl-rich ferropargasite and olivine (Fe(0.96)Mg(1.0)4SiO(4)) locally formed by reaction between igneous orthopyroxene and magnetite. Calculations based on the olivine-forming reaction and fluid inclusions suggest cooling of the lower-crust basic bodies down to 580-610 degrees C and the ingress of fluids with salinity of 20-22% (NaCl eq.) under lithostatic pressure. The reducing nature of the fluids (logfO(2) = -19.6 to -18.8) results from relatively low-temperature (< 315 degrees C) interaction of the fluids with peridotites prior to being heated up to about 600 degrees C in the gabbro zone.