WATER IN NOMINALLY ANHYDROUS LOWER CRUSTAL MINERALS AS A FLUX FOR ANATEXIS

The most commonly cited mechanisms for partial melting of lower and middle crustal granitoids are 1) eutectic melting, induced by heating of the crust, commonly by underplating or invasion of crustal rocks by mantle-derived mafic melts and 2) dehydration melting, involving the breakdown of a hydrous mineral in a metamorphic reaction that produces hydrous melt and one or more peritectic minerals. Water enhances eutectic melting by lowering the liquidus temperature of the parent rock. However, many rocks in the deep crust may be relatively dry, so that eutectic temperatures are high. Water in nominally anhydrous minerals might contribute to melting and may help to explain melting that occurs at temperatures lower than those of the dry solidus. Only 500 ppm water in minerals that make up large volumes of crustal rocks (alkali feldspar, plagioclase feldspar, quartz) would lower the dry solidus of granite by ~24^oC at 1 GPa and, because of the small bulk distribution coefficient likely for water in these minerals, would produce a near-solidus melt with X_H20 = 0.248, corresponding to 8.3 weight percent water in the partial melt. Trace amounts of structural water can serve to weaken nominally anhydrous minerals, making them susceptible to deformation by dislocation creep. During deformational events, activation of water in the structures of nominally anhydrous minerals may enhance mineral deformation, and in turn, dynamic recrysatallization may lead to liberation of structural water and initiation of anatexis.