Evolution of bubble size distribution during the gradual degassing of granitic melt: Experimental data

The vesiculation of granitic melt at a steady pressure release from 1000 to 500 bar at T = 750 degreesC over a period of approximately one day was exp erimentally studied. The size of fluid bubbles in glasses was measured in a series of samples obtained by sequential quenching in the course of severa l experiments. It was established that the volume of released fluid phase l ags behind the equilibrium value. Depending on the content and size of air bubbles in the starting glass, the volume of the released phase at 500 atm either approaches the equilibrium value (27 vol %) or remains twice as low (13 vol %). The integral bubble volume during degassing does not substantia lly vary, remaining at a level of 3-6 x 10(6) per mm(3). Some increase in t he bubble content in the volume unit of glass may be interpreted as a resul t of heterogeneous nucleation at a rate of 1-3 bubbles per mm(3) per second . The distribution of bubble sizes in the samples that completed the series of release runs varied from exponential to unimodal with a maximum dependi ng on the content and dimensions of air microbubbles in the starting glass. The local SIMS analysis of the glasses after degassing showed water conten ts equal to the initial value or even somewhat higher. This is probably cau sed by secondary hydration at a temperature below T-g.