Transfer of continental Mg, S, O and U to the mantle through hydrothermal alteration of the oceanic crust

Abstract

The chemistry of high-temperature vent fluids and deposits from the East Pacific Rise and other submarine hydrothermal fluids, the conditions prevailing during the hydrothermal process, and the calculation of fluxes associated with cycling of seawater through mid-ocean ridges are reviewed. Due to conductive cooling and low-temperature processes at the ridge flanks, only the hydrothermal flux of elements which are stripped out from seawater (Mg, S, O, U) is known with a reasonable degree of confidence. A simple first-order model, in which these elements are transferred from the continental crust to the mantle through hydrothermal exchange at ridge crests, allows us to estimate the rate at which this transfer takes place. Mg should be 2–5 times less concentrated in the present-day crust than 4 Ga ago. Uptake of seawater sulfate during the hydrothermal process has to be qualitative and, through subsequent reduction, irreversible, as it is the only process which accounts for the high oxidation state of Fe in deep levels of the oceanic crust. A time constant of 100 Ma for continental sulfate removal indicates that S has to be massively reinjected into the continental crust by arc magmatism. Consequences for the secular evolution of sulfur isotopes in seawater are discussed. Progressive injection of oxidized basalt into the mantle explains why the terrestrial mantle is more oxidized than its extraterrestrial equivalents. U of the oceanic crust is increased by 20% during the hydrothermal process but loss of continental U through submarine hydrothermal alteration is negligible. However, hydrothermal leaching of Pb from the oceanic crust results in the subduction of oceanic layers with either high μ (altered basalts) and low μ (hydrothermal and abyssal sediments) which may later contribute to the genesis of oceanic basalts with distinctive isotopic properties.