Water deficiency as a factor in metamorphism of rocks

Mineral equilibria in magnesial rocks undergoing metamorphosis in closed systems at different H₂O content but at constant pressures and temperatures may be interpreted as follows: a) if H₂O is excessive, as in pelitic rocks, the metamorphic zonation (an orderly sequence of dehydrations) correctly represents changes in the temperature during the progressive metamorphism; b) if H₂O is deficient, as in the ortho-rocks, the metamorphic zonation may not always be representative of the temperature; however, when H₂O itself is zoned in the rock and the metamorphism is isothermal, the metamorphic zonation may become entirely analogous to the zonation in the presence of excessive H₂O; c) presence of metastable associations and rapid alternations of mineral parageneses, as in certain magmatic rocks, represent variations in H₂O content of the rocks rather than different facies of the metamorphism.