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Contrib Mineral Petrol (2010) 160:313325
DOI 10.1007/s00410-009-0479-1
ORIGINAL PAPER
The role of H2O in rapid emplacement and crystallizationof granite pegmatites: resolving the paradox of large crystals in highly undercooled melts
Peter I. Nabelek Alan G. Whittington
Mona-Liza C. Sirbescu
Received: 1 July 2009 / Accepted: 5 December 2009 / Published online: 25 December 2009 Springer-Verlag 2009
Abstract Granite pegmatite sheets in the continental crust are characterized by very large crystals. 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 shift there has been a renewed debate about the role of H2O in the
petrogenesis of pegmatites. Based on data on nucleation of minerals and new viscosity models for hydrous granite melts, it is argued that H2O is the essential component in the petrogenesis of granite pegmatites. H2O is key to reducing the viscosity of granite 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 thermodynamic solidus, which has been demonstrated by uid inclusion studies and other geothermometers. Published experimental data show that because H2O drastically reduces the nucleation rates of silicate minerals, the minerals may not be able to nucleate until melt is substantially undercooled. In a rapidly 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
the 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 minerals liquidus, assuming components necessary for mineral growth are available along the growth path. Granite pegmatites are apparently able to retain H2O
during most of their crystallization histories within the connement of their wall rocks. Pegmatitic texture is a consequence of delayed nucleation and rapid growth at large undercooling, both of which are facilitated by high H2O (Li, B, F and...