Deformation microstructures of olivine and chlorite in chlorite peridotites from Almklovdalen in the Western Gneiss Region, southwest Norway, and implications for seismic anisotropy

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