The transportation and circulation of water in the Earth is global from the surface of the Earth to the lower mantle and the core. Water is trapped in hydrous minerals in subducting slabs, and is transported into the deep mantle. One of the most important water reservoirs is the mantle transition zone. Measurements of the diffusion of hydrogen in high-pressure minerals together with seismic and electrical conductivity measurements indicate that water is concentrated locally in the mantle transition zone beneath the subduction zone. The contrast in water solubility between upper mantle and transition zone minerals suggests that there is dehydration melting, and dense hydrous magma might exist in a stable state at the base of the upper mantle, which is consistent with seismic observations suggesting a low-velocity region at this depth.
