Dehydration Melting at the Top of the Lower MantleBrandon Schmandt1 , Steven D. Jacobsen2 , Thorsten W. Becker3 , Zhenxian Liu4 , Kenneth G. Dueker5 1 Department of Earth and Planetary Science, University of New Mexico, Albuquerque, NM, USA. 2 Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL, USA. 3 Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA. 4 Geophysical Laboratory, Carnegie Institution of Washington, Washington DC, USA. 5 Department of Geology and Geophysics. University of Wyoming, Laramie, WY, USA. ABSTRACT The high water storage capacity of minerals in Earth's mantle transition zone (410- to 660-kilometer depth) implies the possibility of a deep H2O reservoir, which could cause dehydration melting of vertically flowing mantle. We examined the effects of downwelling from the transition zone into the lower mantle with highpressure laboratory experiments, numerical modeling, and seismic P-to-S conversions recorded by a dense seismic array in North America. In experiments, the transition of hydrous ringwoodite to perovskite and (Mg,Fe)O produces intergranular melt. Detections of abrupt decreases in seismic velocity where downwelling mantle is inferred are consistent with partial melt below 660 kilometers. These results suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H2O in the transition zone.