Dehydration Melting Below the Undersaturated Transition Zone

A reflector 70–130 km below the base of the transition zone beneath Tibet is observed in receiver functions and underside seismic reflections, at depths consistent with the transition of garnet to bridgmanite. Contrast in water storage capacity between the minerals of the Earth's transition zone and lower mantle suggests the possibility for dehydration melting at the top of the lower mantle. First-principles calculations combined with laboratory synthesis experiments constrain the mantle water capacity across the base of the transition zone and into the top of the lower mantle. We interpret the observed seismic signal as consistent with 3–4 vol % hydrous melt resulting from dehydration melting in the garnet to bridgmanite transition. Should seismic signals evident in downwelling region result from water contents representative of upper mantle water globally, this constrains the water stored in nominally anhydrous minerals in the mantle to <30% the mass of the surface oceans.