Controls of Fluxes and Kinetics on Crystallization of Granitic Liquids at Very Low Temperatures

A hallmark of many granite pegmatites is their disequilibrium features such as large variations in crystal size, bladed crystal shapes and strong mineralogical and chemical zoning across dikes. These features suggest very rapid crystallization of undercooled granite liquids. Multiple lines of evidence, including trapping temperatures of coexisting high and low density fluid inclusions, lack of tartan twinning in K-feldspar, ternary feldspar thermometry, and oxygen isotope equilibration temperatures, show that at least portions of some pegmatite dikes crystallize below 400°C (Sirbescu and Nabelek, 2003; Geology). Published data show that high concentrations of fluxing components, especially H₂O, that occur in granite melts strongly depress glass transition temperatures (Tg's), which allows the melts to remain in the liquid state even when undercooled below the equilibrium solidus. For example, for a water-saturated melt of an intermediate alkali feldspar composition, Tg is ~400°C, compared to Tg of ~800°C for a dry melt. Tg of B₂O₃ is 284° C and the addition of only a small amount of alkalis to SiO₂ reduces Tg by >800°C. Fluxed silicate liquids have also lower viscosities and enhanced diffusivities compared to unfluxed liquids. These properties permit rapid growth of few large crystals in undercooled silicate liquids prior to the onset of large viscosity increases associated with glass transition. Fractional crystallization of granite dikes leads to elevated concentrations of fluxing components in residual liquids. A change from aplitic “line-rock” texture to pegmatite texture that occurs in many sheeted granite dikes may represent a transition to highly fluxed conditions that promoted rapid growth of large crystals in the undercooled residual liquids.