Hydrogen diffusivity in synthesized polycrystalline wadsleyite and ringwoodite, the major constituent minerals in the mantle transition zone, was measured by Fourier Transform Infra-Red (FTIR) Spectroscopy. Hydrogen diffusion rate in wadsleyite is roughly comparable to the average diffusion rate of hydrogen in olivine. The hydrogen diffusivity in ringwoodite is also measured and found that it is comparable to that of wadsleyite. Water transported to the transition zone by hydrous phases is dehydrated at the top of the lower mantle. The dehydrated water moves upwards and is reacted and absorved by the overlying ringwoodite phase in the slab of the transition zone depth. Superhydrous phase B can be formed as the reaction product. This mechanism is effective to concentrate water in the slab at the transition zone depths; even if the amount of water transported by the slab (with the subducting speed of 10 cm /year) is very small around 0.05 wt.%, absorption of the dehydrated water by overlying ringwoodite crystals can build the hydrous zone containing 1 wt.% water with a thickness of 250 km in the slab at the transition zone depths by continuous subduction during 50 Ma. The transition zone surrounding the slab may also be hydrated but the region may be limited to less than 1 km in width due to a relatively slow diffusion of hydrogen in wadsleyite and ringwoodite.
