Kinetics of Quenching of Hydrous Feldspathic Melts: Quantification Using Synthetic Fluid Inclusions

microthermometric analysis of fluid inclusions preserved during the isobaric quenching of H-2O-saturated, vesicular silicate melts provides a method for the determination of the glass transition temperature of hydrous silicate melts at high pressure. The method is based on the principle that the contraction of inclusion cavities during quenching is rate-limited by the volume relaxation of the melt. Viscous relaxation of the melt ceases during cooling at the glass transition temperature. Bulk densities of the fluid inclusions whose volumes are frozen at the glass transition preserve a record of the trapping event, i.e., the glass transition temperature. Liquid-vapor homogenization temperatures [T(H(L-V)] of the trapped inclusions are measured using a microscope heating-stage assembly. Bulk densities of H2O present in the inclusions at T(H(L-V)) and P(saturation) are determined from literature values as are the P-T trajectories of the corresponding isochores. The intersection of an isochore with the experimental pressure during the quench yields the glass transition temperature for that particular glass composition and quench rate. The method has been applied to seven compositions on the join albite-orthoclase. H2O-saturated melts along this join have been rapidly and isobarically quenched at 2000 bars. The total solubilities of H2O range from 5.12 to 6.03 +/- 0.15 wt%. The glass transition temperatures of the H2O-saturated melts range from 525 to 412-degrees-C. The compositional dependence of the glass transition is strongly nonlinear. Melts of intermediate composition exhibit a significantly lower glass transition than either end-member. The deviation from additivity reaches a maximum of 70-degrees-C at Ab50Or50 (molar basis). The information on T(g) can be combined with data for the properties of the quenched glasses to obtain liquid properties at hydrothermal conditions, for example, the viscosity and the thermal expansivity of the wet melts. The quantification of trapping temperatures for fluid inclusions in silicate melts also has potential applications in the study of the kinetics of melt degassing.