High-salinity seawater-derived fluids and lower-crust hydrothermal mineralization at the Vema Lithospheric Section, Central Atlantic

Abstract 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 1.04 SiO 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 °C and the ingress of fluids with salinity of 20-22% (NaCl eq.) under lithostatic pressure. The reducing nature of the fluids (log f O 2 = -19.6 to -18.8) results from relatively low-temperature (