Rock-water systems, with special reference to the water-deficient region

The four types of subsolidus assemblages in silicate-water systems are: Type I, water-absent; assemblage of anhydrous minerals with no vapor phase. Type II, water-deficient and vapor-absent; assemblage of minerals including hydrous minerals, but with no vapor phase. Type III, water-deficient and vapor-present; an assemblage of minerals, with or without hydrous minerals, with a vapor phase; there is insufficient water present to saturate the liquid when the crystalline assemblage is completely melted at the existing pressure. Type IV, water-excess; an assemblage of minerals and vapor with at least enough water to saturate the liquid when the crystalline assemblage is completely melted at the existing pressure. Water-deficient systems of types II and III are significant for magma generation and crystallization; there are few experimental results. Water-deficient phase relationships can be estimated by interpolation between available experimental data for systems of types I and IV. Mantle magmas are generated in type I, II, or III systems. The first liquid produced in type III systems is H_20-saturated, and the first liquid in type II systems is H_2O-undersaturated. It appears that the normal product of partial fusion of many continental crustal rocks is a H_2O-undersaturated granite liquid in a crystal mush, and that liquids of intermediate composition are not generated because temperatures required are too high. Phase relationships are depicted for granodiorite in a pressure-temperature- water diagram