Homogeneous bubble nucleation in H2O- and H2O-CO2-bearing basaltic melts: Results of high temperature decompression experiments

摘要

High pressure and temperature decompression experiments were conducted to provide experimental information on the conditions of homogeneous bubble nucleation in basaltic melts. Experiments were performed on H2O- and H2O-CO2-bearing natural melts from Stromboli. Three starting volatile compositions were investigated: series #1 (4.91 wt% H2O, no CO2), series #2 (2.37-2.45 wt% H2O, 901-1011 ppm CO2) and series #3 (0.80-1.09 wt% H2O, 840-923 ppm CO2). The volatile-bearing glasses were first synthesized at 1200 degrees C and 200 MPa, and second continuously decompressed in the pressure range 150-25 MPa and rapidly quenched. A fast decompression rate of 78 kPa/s (or 3 m/s) was applied to limit the water loss from the glass cylinder and the formation of a H2O-depleted rim. Post-decompression glasses were characterized texturally by X-ray microtomography. The results demonstrate that homogenous bubble nucleation requires supersaturation pressures (difference between saturation pressure and pressure at which homogeneous bubble nucleation is observed, Delta P-HoN) <= 50- 100 MPa. Delta P-HoN varies with the dissolved CO2 concentration, from 50 MPa (no CO2, series #1) to <= 50 MPa (872 +/- 45 ppm CO2, series #3) to <100 MPa (973 +/- 63 ppm CO2, series #2). In series #1 melts, homogeneous bubble nucleation occurs as two distinct events, the first at high pressure (200 < P< 150 MPa) and the second at low pressure (50