Stability of hydrous silicate in the deep mantle and mantle dynamics

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The stability field of phase B in the MgO-SiO_2-H_2O system at high pressure and high temperature was studied by using the multiple arivil high pressure apparatus. The results of experiments are summarized as follows; (1) phase B melts incongruently at 20 GPa into spinel + periclase + liquid at 1500ﾟC. (2) Stable assembrage changes to modified spinel + periclase + liquid, and periclase + liquid with increasing temperature at 20 GPa. (3) Phase B decomposes into periclase + spinel + H_2O at around 22 GPa below 1500ﾟC. (4) phase B is stable at around 14 GPa below 1400ﾟC, whereas an unknown new phase is stable at higher temperatures at 14 GPa. The chemical analysis of the new phase indicates that the composition of the phase is richer in MgO than that of phase B. This suggests that phase B decomposes into the new phase and stishovite above 1400ﾟC at 4 GPa.These experimental results indicate that phase B is stable at pressures from 14 GPa to 22 GPa below 1400ﾟC. Thus water transported into the deep mantle by the down going slab is trapped in the crystal lattice of phase B in the transition zone of the upper mantle, whereas it decomposes into a mixture of periclase + spinel + H_2O at the base of the transition zone.