Signatures of residual melts, magmatic and seawater-derived fluids in oceanic lower-crust gabbro from the vema lithospheric section, central Atlantic

We present new data on mineral assemblages, mineral chemistry (major and trace element compositions), and fluid inclusions to reconstruct the magmatic to hydrothermal history of gabbroic rocks dredged in association with serpentinized mantle peridotites from the tectonically exposed Vema lithospheric section (central Atlantic). Textural relations and mineral chemistry of the samples indicate a two-stage magmatic evolution, from an early coarse-grained pyroxene-plagioclase cumulate to overprinted assemblages, including interstitial Fe-Ti oxides and titanian hornblende. Titanian hornblende in the gabbros is a product of interaction of crystal aggregates with an evolved residual melt or with a late-magmatic aqueous fluid at 800-900°C. The different contribution of residual melt and magmatic aqueous fluid is inferred from the behaviour of incompatible elements and plagioclase composition. One gabbro sample shows features indicative of a later stage hydrothermal event: formation of Fe- and Cl-rich hornblende sealing fractures in plagioclase, olivine coronas at orthopyroxene-magnetite grain boundaries, plagioclase-hosted high-salinity fluid inclusions associated with local enrichment of plagioclase in anorthite and Sr, and local augite to diopside re-equilibration. These features record the interaction of gabbro with seawater-derived fluid at temperature up to about 600°C. The high-salinity inclusions and reducing hydrothermal reactions in the gabbro indicate that the fluid phase characteristics may have been acquired through a preceding lower-temperature (<500°C) interaction of seawater with mantle peridotite (serpentinization) followed by infiltration of the modified aqueous solution into the higher-temperature (about 600°C) gabbroic zone in a geodynamic environment of a slow-spreading ridge segment end. Compositional features of the hydrothermal fluid were: salinity 20-22 wt % (NaCl eq.), Ca/(Ca+Na) ≤ 0.12, and log fO2 = -19.6 to -18.8.