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It is widely proposed that the oceanic mantle is hydrated by outer rise normal faults, and carries large amounts of water to the deep mantle. However, the extent of oceanic mantle hydration is poorly constrained by existing observations, and is a major source of uncertainty in determining the total water delivered to the mantle. Full waveform modeling of dispersed P-wave arrivals from events deep within the Wadati-Benioff zone of northern Japan shows that hydrated fault zone structures are present at intermediate depths. Analysis of the P-wave coda associated with events 5-35 km below the top of the slab gives an overall indication of the bulk hydration of the subducting oceanic mantle, and can be explained by a 40-km-thick layer that is 17%-31% serpentinized. This suggests that the top of the oceanic mantle is 2.0- 3.5 wt% hydrated, subducting 170-318 Tg/m.y. of water per meter of arc beneath northern Japan. This order-of-magnitude increase in the estimated H 2 O fl ux in this arc implies that over the age of the Earth, the equivalent of as many as 3.5 present-day oceans of water could be subducted along the Kuril and Izu-Bonin arcs alone. These results offer the fi rst direct measure of the lower lithosphere hydration at intermediate depths, and suggest that regassing of the mantle is more vigorous than has previously been proposed.

Order of magnitude increase in subducted H2O due to hydrated normal faults within the Wadati-Benioff zone