Results of high-pressure experiments on samples of hydrated mantle rocks show that the serpentine mineral antigorite is stable to similar to 720 degrees C at 2 gigapascals, to similar to 690 degrees C at 3 gigapascals, and to similar to 620 degrees C at 5 gigapascals. The breakdown of antigorite to forsterite plus enstatite under these conditions produces 13 percent H2O by weight to depths of 150 to 200 kilometers in subduction zones. This H2O is in an ideal position for ascent into the hotter, overlying mantle where it dan cause partial melting in the source region for calc-alkaline magmas at a depth of 100 to 130 kilometers and a temperature of similar to 1300 degrees C. The breakdown of antigorite in hydrated mantle produces an order of magnitude more H2O than does the dehydration of altered oceanic crust.