Mantle wedge water contents estimated from seismic velocities in partially serpentinized peridotites

Water carried into subduction zones with the down‐going plate and subsequently released by dehydration reactions at depth affects the composition of the mantle wedge, triggers partial melting and affects subduction zone seismicity. Partially serpentinized peridotite may be a significant reservoir for water in the subducted plate, the mantle wedge and the overriding plate. Here we develop a model that relates the degree of serpentinization and water content of partially serpentinized peridotites to their seismic P‐wave velocities. In partially‐serpentinized ultramafic rocks, a 1% decrease in P‐wave velocity corresponds to a 2.4% increase in serpentine content, and a 0.3% (0.18 moles/m3) increase in H2O content (up to a maximum of 13%). Where there is evidence of serpentinization, mantle serpentine content is typically ∼15%, corresponding to 4–5 wt% H2O (6–10 moles/m3).