DEHYDRATION-MELTING REACTIONS AND THE FORMATION OF HYDROUS PHASES BY INCONGRUENT REACTION: AN ALTERNATIVE WAY TO VIEW THE CRYSTALLIZATION OF HYDROUS MAGMAS

The crystallization of hydrous magmas can be modeled as a series of incongruent reactions between hydrous melt and anhydrous minerals yielding a hydrous mineral assemblage. These reactions can buffer the water content of hydrous magmas at values below saturation, conceivably to the completion of crystallization, thus contradicting the maxim that "all magmas go to water saturation". Even if the buffering effect does not persist throughout the crystallization history, water contents of magmas in which these reactions have occurred will remain lower than magmas in which Rayleigh fractionation has occurred. Many resorption and reaction features can be attributed to reactive crystallization. These include, for example, the obvious reaction relationship between amphibole and pyroxene seen in many plutonic rocks and the partial resorption of early-crystallized plagioclase. Reactive crystallization will strongly influence the composition of late magmatic liquids (e.g. aplites and pegmatites) and melt inclusion compositions in late crystallizing phases.