Using Incongruent Equilibrium Hydration Reactions to Model Latter‐Stage Crystallization in Plutons: Examples from the Bell Island Tonalite, Alaska

Models using hydration crystallization reactions (the reverse of dehydration melting reactions such as amph+qtz=px+melt) for the Bell Island pluton define incongruent equilibrium crystallization paths from hydrous+melt+pyroxene+Fe-Ti oxides+calcic andesine (30%-50% solid) to a solid tonalite consisting mostly of hornblende, lower temperature end of Bowen's discontinuous reaction series and apply it to natural samples. Hydration crystallization provides an alternative to crystal fractionation for explaining variations in pluton chemistry, especially the compositions of late plutonic melts. Another characteristic of hydration crystallization is that the reactions have the potential to buffer the water content of the melt during crystallization. Two closed-system models, representing different sets of starting conditions and phases, are considered, based on least squares, mass-balance calculations of reactions and constrained by the petrography of the rocks. Model 1 starts with an average modified Bell Island leucotonalite melt coexisting with two pyroxenes, two Fe-Ti oxides, and plagioclase at the beginning of hydration crystallization. The starting assemblage of model 2 omits orthopyroxene and magnetite, includes amphibole, and uses a calculated melt composition. Both models generally predict, via different series of hydration crystallization reactions, the observed subsolidus mode. Model 2, however, is preferred based on petrographic observations of the Bell Island rocks, specifically the lack of magnetite and orthopyroxene, as well as certain textural features.