KINETICS OF QUENCHING OF HYDROUS FELDSPATHIC MELTS - QUANTIFICATION USING SYNTHETIC FLUID INCLUSIONS

A microthermometric analysis of fluid inclusions preserved during the isobaric quenching of H-2O-saturated, vesicular silicate melts provide s a method for the determination of the glass transition temperature o f 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 relaxati on of the melt ceases during cooling at the glass transition temperatu re. Bulk densities of the fluid inclusions whose volumes are frozen at the glass transition preserve a record of the trapping event, i.e., t he glass transition temperature. Liquid-vapor homogenization temperatu res [T(H(L-V)] of the trapped inclusions are measured using a microsco pe heating-stage assembly. Bulk densities of H2O present in the inclus ions at T(H(L-V)) and P(saturation) are determined from literature val ues as are the P-T trajectories of the corresponding isochores. The in tersection of an isochore with the experimental pressure during the qu ench yields the glass transition temperature for that particular glass composition and quench rate. The method has been applied to seven com positions on the join albite-orthoclase. H2O-saturated melts along thi s join have been rapidly and isobarically quenched at 2000 bars. The t otal solubilities of H2O range from 5.12 to 6.03 +/- 0.15 wt%. The gla ss transition temperatures of the H2O-saturated melts range from 525 t o 412-degrees-C. The compositional dependence of the glass transition is strongly nonlinear. Melts of intermediate composition exhibit a sig nificantly lower glass transition than either end-member. The deviatio n 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 pro perties of the quenched glasses to obtain liquid properties at hydroth ermal conditions, for example, the viscosity and the thermal expansivi ty of the wet melts. The quantification of trapping temperatures for f luid inclusions in silicate melts also has potential applications in t he study of the kinetics of melt degassing.