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The phase rule predicts hydrous phases to be stable in the absence of a coexisting hydrous fluid. Experiments in a synthetic system (CaO-Al₂O₃-SiO₂-H₂O ) show that, as a function of bulk water content, either (A) one hydrous phase (zoisite or lawsonite) may coexist with three anhydrous phases (grossular + kyanite + coesite), (B) two hydrous phases (lawsonite + zoisite) may coexist with two anhydrous phases (grossular+coesite), or (C) one hydrous phase (lawsonite) may coexist with two anhydrous phases (grossular + coesite) and fluid (water). Water-undersaturated reactions such as the transformation from assemblage (A) to (B) are important in synthetic and natural systems. An example is the pressure stability of zoisite: the fluid absent reaction zoisite = lawsonite + grossular + kyanite + coesite was experimentally determined to occur at 56 kbar (700 ^oC) to 67 kbar (1000 ^oC).

The results from the synthetic system can be applied to the natural system. It can be shown that any natural, fluid undersaturated system will achieve fluid saturation during metamorphism. An initially water undersaturated basalt will form hydrous phases during metamorphism and undergo a series of metamorphic reactions involving hydrous phases. The reactions must result in a "maximum preservation of water" and will finally saturate the bulk rock in water thus causing the release of water during prograde metamorphism. For a given bulk composition, the conditions at which the so called "water saturation plane" is reached depend mainly on initial water contents.