For the first time, large quantities of water are identified throughout the length of a subducting plate, and an efficient pathway is defined for injecting water into the deep (>600 km) Earth, by the incorporation of water into the mantle of an oceanic plate as serpentine. Water is thought to facilitate plate tectonics, but the quantities of water involved, and thus its effect, are poorly known. Taking advantage of an anomalous seismic arrival recorded in the Tonga-Fiji subduction zone, the presence, volume, water content, and flux of serpentine and water associated with the subducting oceanic mantle at Tonga is quantified at 2 × 108 Tg/Ma H2O, many times larger than previous inferences based on surface observations. Water fluxes of this magnitude into the deep Earth will substantially increase the role of water in hypotheses of plate tectonic movements, mantle plumes, and flood basalt activity.

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