In this chapter we summarize recent results on the influence of water on major phase transformations in the Earth's mantle with implications for seismic discontinuity structure and mantle dynamics. The experimental data are based on quench multianvil and in situ X-ray diffraction studies. Differences in water solubility between olivine and wadsleyite, and between ringwoodite and Mg-perovskite + ferropericlase may displace the phase transition boundaries, which are responsible for the 410- and 660-km discontinuities, respectively. The results show that water expands the stability field of wadsleyite to lower pressures, which is consistent with broadening of the 410-km discontinuity in some regions of the mantle. A significant shift of the wadsleyite-ringwoodite phase transition to higher pressure caused by water may also be responsible for depth variations or absence of the 520-km discontinuity. Study of the post-spinel transformation in hydrous pyrolite indicates that the phase boundary also shifts to higher pressures. Displacement of this boundary with ~2 wt.% H2O corresponds to about 15 km at 1473 K. Thus, presence of water could account for half of the observed 30-40 km depressions at the 660-km discontinuity in subduction zones at this temperature. Study of the post-garnet transformation in anhydrous and hydrous MORB show that this phase boundary shifts to the lower pressures by ~2 GPa with the addition of 2-5 wt.% water. This observation demonstrates that the density crossover between peridotite and basaltic components near 660 km might be absent under hydrous conditions, inhibiting the separation of these components at the 660-km discontinuity.
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