Hydrous melting in the deep mantle

Volatiles in the oceanic plate subduct into the mantle which greatly affects the dynamics in the Earth's interior. High-pressure and high-temperature experiments show that hydrous magnesium-rich silicates are important water carrier to the deep mantle. However, the devolatilization process which potentially could cause the formation of magma in the deep mantle (>300 km) remains enigmatic. Seismic tomography shows low velocities are both ponding at the top and bottom of the mantle transition zone (MTZ), respectively. It's pervasively attributed to dehydration melting by considering the large water capacity contrast between the MTZ and upper/lower mantle. Moreover, the globally distributed mid-mantle seismic reflectors at the depth of 800-1300 km probably result from heterogeneous mantle composition which mixed with basaltic rocks or dehydration of superhydrous phase B and subsequent melting. Yet those speculations from geophysical observations remain to be verified. Therefore, high-resolution petrological-thermomechanical models are designed to investigate the processes of volatile recycling, deep metasomatization and melting. Our preliminary results show that the cold serpentinized slab is capable of carrying water down to the lower mantle. There is no magma generated at the top of 410 km if the MTZ is originally dry.The dehydrated water cumulates at the bottom of the MTZ which cause undetectable amount of magma due to the endothermic boundary from ringwoodite to perovskite that cools down the lower mantle. Whereas magma could be generated by return flow which releases heat in the dry lower mantle.