Hydrothermal Alteration of the Oceanic Crust

Abstract

Recycling of the oceanic crust is potentially a dominant process in the formation of OIB mantle sources (Hofmann et al. 1978; 1986) and is a significant contribution to the flux of material between the continent and the depleted mantle (Albarede and Michard, 1986). The subducted oceanic crust is expected to comprise widely different end-members: continent-derived sediments of which the abyssal clays are an important fraction, metalliferous ridge-flank sediments, and the magmatic layers (basalts, dolerites, gabbros,…). First, these magmatic layers interact at high temperature with the overlying seawater in the hydrothermal systems recently discovered along most ridge segments around the world (black smokers). SiO₂ geothermobarometry of the vent solutions suggests that the last major chemical exchanges between hydrothermal solutions and the plutonic part of the oceanic crust take place 0.5–3.5 km below the seafloor, a depth that recent seismic data correlate with the top of an axial magma chamber. In addition, seawater reacts at low temperature with the uppermost basaltic flows and dikes, giving rise to the alteration most commonly studied on dredge samples and DSDP cores.