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

Abstract

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 (4.91wt% H2O, no CO2), series #2 (2.37–2.45wt% H2O, 901–1011ppm CO2) and series #3 (0.80–1.09wt% H2O, 840–923ppm CO2). The volatile-bearing glasses were first synthesized at 1200°C and 200MPa, and second continuously decompressed in the pressure range 150–25MPa and rapidly quenched. A fast decompression rate of 78kPa/s (or 3m/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, ∆PHoN) ≤50–100MPa. ∆PHoN varies with the dissolved CO2 concentration, from ≪50MPa (no CO2, series #1) to ≤50MPa (872±45ppm CO2, series #3) to <100MPa (973±63ppm CO2, series #2). In series #1 melts, homogeneous bubble nucleation occurs as two distinct events, the first at high pressure (200<P<150MPa) and the second at low pressure (50<P<25MPa), just below the fragmentation level. In contrast, homogenous nucleation in series #2 and #3 melts is a continuous process. As well, chemical near-equilibrium degassing occurs in the series #1 melts, unlike in the series #2 and #3 melts which retain high CO2 concentrations even for higher vesicularities (up to 23% at 25MPa). Thus, our experimental observations underline a significant effect of CO2 on the physical mechanisms of bubble vesiculation in basaltic melts. Our experimental decompression textures either reproduce or approach the characteristics of explosive basaltic eruptions, in terms of vesicularity, bubble shapes, sizes and number densities. Unimodal, exponential to power law bubble size distributions were encountered and correlated with the different melt series, textural characteristics and types of degassing.