SEISMIC EVIDENCE FOR A PIPELINE OF WATER INTO THE DEEP MANTLE

Evidence from seismic attenuation tomography is presented to show that water is likely traveling with subducted lithosphere into the lower mantle in some regions of Earth. The first-ever whole-mantle 3-D model of shear-wave attenuation shows regions of extremely high seismic attenuation overlying the subducted lithosphere in the lower mantle beneath eastern Asia and western North America. The seismic model was constructed using more than 90,000 seismic shear wave phases taken from the IRIS global database of digital broadband seismograms from 1990 onward. The 3-D attenuation model strongly correlates with patterns of known whole-mantle convection: the low-attenuation anomalies correlate with long-standing regions of paleosubduction, and high-attenuation anomalies correspond to the asthenosphere, spreading ridges, and the Pacific and African megaplumes in the lower mantle. However, the largest high-attenuation anomalies are found in the depth of 700-1300 km beneath eastern Asia and North America, and these anomalies have attenuation values at the same level as the asthenosphere. There is a slight low-velocity anomaly associated with these features, and given this combination, the best explanation is the pumping of water into the lower mantle via subducted ocean lithosphere. Post-serpentinite hydrous phases like hydrous phase D have been found to be stable to depths as great as 1300 km if the temperature stays low enough, which occurs for these fast-subducting slabs. We propose that not all of the water in subducting lithosphere dehydrates in the upper mantle (generating arc magma), but that a small amount remains in the subducted ocean crust and doesn't dehydrate until reaching the top part of the lower mantle.