Alteration Of Upper Oceanic Crust Research Articles

Hydrothermal alteration of upper oceanic crust formed at a fast-spreading ridge: mineral, chemical, and isotopic evidence from ODP Site 801

ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10–20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5–100 °C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (<100 °C) hydrothermal fluids. Iron oxyhydroxides subsequently formed in alteration halos along fractures where seawater circulated, and saponite and pyrite developed in the host rock and in zones of restricted seawater flow under more reducing conditions. Chemical changes include variably elevated K, Rb, Cs, and H 2O; local increases in Fe T, Ba, Th, and U; and local losses of Mg and Ni. Secondary carbonate veins have 87Sr/ 86Sr=0.706337–0.707046, and a negative correlation with δ 18O results from seawater–basalt interaction. Carbonates could have formed at any time since the formation of Site 801 crust. Variable δ 13C values (−11.2‰ to 2.9‰) reflect the incorporation of oxidized organic carbon from intercalated sediments and changes in the δ 13C of seawater over time. Compared to other oceanic basements, a major difference at Site 801 is the presence of two hydrothermal silica–iron deposits that formed from low-temperature hydrothermal fluids at the spreading axis. Basalts associated with these horizons are intensely altered (60–100%) to phyllosilicates, calcite, K-feldspar, and titanite; and exhibit large increases in K, Rb, Cs, Ba, H 2O, and CO 2, and losses of Fe T, Mn, Mg, Ca, Na, and Sr. These effects may be common in crust formed at fast-spreading rates, but are not ubiquitous. A second important difference is that the abundance of brown oxidation halos along fractures at Site 801 is an order of magnitude less than at some other sites (2% vs. 20–30%). Relatively smooth basement topography (<100 m) and high sedimentation rate (8 m/Ma) probably restricted the access of oxygenated seawater. Basement lithostratigraphy and early low-temperature hydrothermal alteration and mineral precipitation in fractures at the spreading axis controlled permeability and limited later flow of oxygenated seawater to restricted depth intervals.

Subduction fluxes of water, carbon dioxide, chlorine, and potassium

The alteration of upper oceanic crust entails growth of hydrous minerals and loss of macroporosity, with associated large‐scale fluxes of H2O, CO2, Cl−, and K2O between seawater and crust. This age‐dependent alteration can be quantified by combining a conceptual alteration model with observed age‐dependent changes in crustal geophysical properties at DSDP/ODP sites, permitting estimation of crustal concentrations of H2O, CO2, Cl−, and K2O, given crustal age. Surprisingly, low‐temperature alteration causes no net change in total water; pore water loss is nearly identical to bound water gain. Net change in total crustal K2O is also smaller than expected; the obvious low‐temperature enrichment is partly offset by earlier high‐temperature depletion, and most crustal K2O is primary rather than secondary. I calculate crustal concentrations of H2O, CO2, Cl−, and K2O for 41 modern subduction zones, thereby determining their modern mass fluxes both for individual subduction zones and globally. This data set is complemented by published flux determinations for subducting sediments at 26 of these subduction zones. Global mass fluxes among oceans, oceanic crust, continental crust, and mantle are calculated for H2O, Cl−, and K2O. Except for the present major imbalance between sedimentation and sediment subduction, most fluxes appear to be at or near steady state. I estimate that half to two thirds of subducted crustal water is later refluxed at the prism toe; most of the remaining water escapes at subarc depths, triggering partial melting. The flux of subducted volatiles, however, does not appear to correlate with either rate of arc magma generation or magnitude of interplate earthquakes.