Abstract:
The vesiculation of granitic melt at a steady pressure release from 1000 to 500 bar at T = 750°C over a period of approximately one day was experimentally studied. The size of fluid bubbles in glasses was measured in a series of samples obtained by sequential quenching in the course of several experiments. It was established that the volume of released fluid phase lags 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 substantially vary, remaining at a level of 3-6 × 106 per mm3. Some increase in the bubble content in the volume unit of glass may be interpreted as a result of heterogeneous nucleation at a rate of 1-3 bubbles per mm3 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 depending on the content and dimensions of air microbubbles in the starting glass. The local SIMS analysis of the glasses after degassing showed water contents equal to the initial value or even somewhat higher. This is probably caused by secondary hydration at a temperature below Tg.