Mantle transition zone water triggers lithospheric weakening and spreading

Water (hydrogen) is crucial to the geochemical and dynamic evolution of Earth by modulating a variety of Earth's interior properties and processes, which govern differentiation, plate tectonics, and volcanism. Water addition is also expected to promote thinning and removal of lithosphere roots, as water can markedly decrease the viscosity of the lithospheric mantle. While the source of deep water is controversial, recent evidence suggests that a possible reservoir for volatiles is the mantle transition zone (MTZ) that can be sampled at the mid-ocean rift system. Geophysical and geochemical evidence collectively suggests that volcanism in the South China Sea (SCS) resulted from the interaction of the subducting slabs with the MTZ, making it an ideal location to help solve this puzzle. We present data of glasses and melt inclusions from SCS samples with high H2O/Ce (similar to 248-649, considerably higher than global mid-ocean ridge basalt values). These data, combined with increased depletion in highly incompatible elements and low oxygen fugacity in the SCS, are consistent with the presence of subducted serpentinized mantle in the MTZ, which was later recycled during mantle upwelling and triggered lithospheric thinning and rifting. Water originating from the MTZ was likely responsible for the initial hydration of the upper mantle during early Earth's history, potentially facilitating the development and maintenance of plate tectonics.