Dehydration Melting Below the Undersaturated Transition Zone and Implications for Mantle Water Content

Abstract:

The dynamic mantle circulates material between the upper and lower mantle, including any water that may exist at those depths. The 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. In receiver functions and underside seismic reflections, we observe a reflector 70-130 km below the base of the transition zone beneath Tibet, depths consistent with the transition of garnet to bridgmanite. 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.