H2O in metamorphism and unexpected behaviour in the preservation of metamorphic mineral assemblages

The preservation of mineral assemblages that were fluid-present during thei r prograde history is primarily related to the consumption of the fluid by growth of more hydrous minerals as the retrograde history begins. The range of behaviour relating to the preservation of mineral assemblages is examin ed using calculated phase diagrams for fluid-saturated conditions, contoure d for the H2O content of the mineral assemblage. At equilibrium, as a miner al assemblage crosses contours of decreasing H2O content along a pressure-t emperature path, it dehydrates, the fluid being lost from the rock. If the assemblage crosses contours of increasing H2O content, the mineral assembla ge starts to rehydrate using any fluid on its grain boundaries. When the ro ck has consumed its fluid, the resulting mineral assemblage is that preserv ed in the rock. Conditions relating to the preservation of mineral assembla ges are discussed, and examples of the consequences of different pressure-t emperature paths on preservation in a metapelitic and a metabasic rock comp osition are considered on phase diagrams calculated with THERMOCALC.