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 H 2 O, CO 2 , Cl − , and K 2 O 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 H 2 O, CO 2 , Cl − , and K 2 O, 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 K 2 O is also smaller than expected; the obvious low‐temperature enrichment is partly offset by earlier high‐temperature depletion, and most crustal K 2 O is primary rather than secondary. I calculate crustal concentrations of H 2 O, CO 2 , Cl − , and K 2 O 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 H 2 O, Cl − , and K 2 O. 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.