The role of H2O in rapid emplacement and crystallization of granite pegmatites: resolving the paradox of large crystals in highly undercooled melts |
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| Authors: | Peter I Nabelek Alan G Whittington Mona-Liza C Sirbescu |
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| Institution: | (1) Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA;(2) Department of Geology, Central Michigan University, 314 Brooks Hall, Mount Pleasant, MI 48859, USA |
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| 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 fluid 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 mineral’s 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 confinement 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 P) contents in granite pegmatite melts. Without retention of H2O the conditions for pegmatitic textural growth may be difficult to achieve. Loss of H2O due to decompression and venting leads to microcrystalline texture and potentially glass during rapid cooling as seen in
rhyolites. In contrast, slow cooling within a large magma chamber promotes continuous exsolution of H2O from crystallizing magma, growth of equant crystals, and final solidification at the thermodynamic solidus. These are the
characteristics of normal granites that distinguish them from pegmatites. |
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