Water is transported by subducting slabs into the transition zone and lower mantle. Important water carriers to the deep mantle may be serpentine, chlorite, phase A, and superhydrous phase B in peridotite; zoisite, lawsonite, and phengite in basalt; and topaz-OH and phase Egg in sediments. Phase D, stable in peridotite, and the δ-AlOOH phase in the sedimentary component may transport water to at least 1200–1500 km depth. Phase relations in hydrous peridotite show that the phase boundaries of olivine to wadsleyite and ringwoodite to Mg-perovskite + ferropericlase phase transitions shift to lower and higher pressures, respectively. Thus, elevation of the...