Alteration of oceanic crust and geologic cycling of chlorine and water

We report new estimates of transport rates for H₂O and Cl between the mantle and surface reservoirs. Our estimates take into consideration alteration of oceanic crust, especially that of plutonic rocks, and possible subduction of sediments. The effect of (hydrothermal) alteration on the Cl budget seems to be negligible, but the effect on the H₂O budget is significant. Altered oceanic crust (excluding sediments) contains about 10 times as much H₂O as the unaltered crust, and its subduction may result in a net transport of H₂O to the upper mantle in subduction zones. However, the rate of expulsion of H₂O from the mantle by subduction-zone magmatism is comparable to the amount released by ridge magmatism, and is only about 10% of the amount subducted. Therefore, about 90% of the subducted H₂O must be returned to the mantle or returned to the crust by other processes. In addition, subduction of oceanic sediments to mantle depths will result in 1. (1) a further increase in the return rate of H₂O to the mantle reservoir, 2. (2) possible net transfer of Cl to the mantle, depending on the rate of pore water expulsion.