THE ROLE OF H2O IN PRODUCING LARGE CRYSTALS IN HIGHLY UNDERCOOLED GRANITE MELTS

Granite pegmatites in the continental crust are characterized by their large crystal sizes. There has been a shift in viewing pegmatites as products of very slow cooling of granite melts to viewing them as products of crystal growth in undercooled liquids. With this change there has been a renewed debate about the role of H₂O in the petrogenesis of pegmatites. On the basis of data on nucleation of minerals and new viscosity models for hydrous granite melts, it is argued that H₂O is an essential component in the petrogenesis of granitic pegmatites. H₂O is key to reducing the viscosity of granitic melts, which enhances their transport within the crust. It also dramatically reduces the glass transition temperature, which permits crystallization of melts at hundreds of degrees below the equilibrium solidus. Crystallization of melts below the thermodynamic solidus is demonstrated by fluid inclusion studies and other geothermometers. Newly acquired and published experimental data show that because H₂O drastically reduces the nucleation rates of silicate minerals, the minerals may not be able to nucleate in a cooling melt until it is substantially undercooled. In a cooling intrusion, nucleation starts at its highly undercooled margins, followed by inward crystal growth towards its slower-cooling, hotter core. Delay in nucleation may be caused by competition for crystallization by several minerals in the near-eutectic melts and by the very different structures of minerals and highly hydrated melts. Once a mineral nucleates, however, it may grow rapidly to a size that is determined by the distance between the site of nucleation and the point in the magma at which the temperature is approximately that of the mineral’s liquidus, assuming components necessary for mineral growth are available along the growth path. The unidirectional textures and internal zoning that typify many pegmatites are determined by internal temperature gradients. In contrast to rhyolites that degas prior to or during eruptions, granitic pegmatites are apparently able to retain H₂O during most of their crystallization histories within the confinement of their wall rocks.