The role of water in connecting past and future episodes of subduction

We investigate a weak, but persistent low S-velocity anomaly in the upper mantle beneath the US East Coast. This anomaly sits above the high-velocity lower-mantle S-velocity structure generally interpreted as relatively cool, subducted Farallon lithosphere. We argue that the most likely explanation for the lowered S-velocities is elevated water content, with hydrogen incorporated in defects and oxygen at regular lattice sites in the crystal structure of major mantle minerals olivine, wadsleyite, and ringwoodite. The subducted Farallon lithosphere is a potential source for this water while the lithosphere at the Atlantic North American margin is a likely recipient of the water. This water may be the vital element needed to allow the margin lithosphere to break and initiate subduction of the Atlantic lithosphere. In a broader geodynamic context we propose that a deep water cycle may be responsible for the longevity of plate tectonics in general and the Wilson Cycle in particular.