Kinetics of heterogeneous bubble nucleation in rhyolitic melts: implications for the number density of bubbles in volcanic conduits and for pumice textures

We performed decompression experiments to simulate the ascent of a phenocryst-bearing rhyolitic magma in a volcanic conduit. The starting materials were bubble-free rhyolites water-saturated at 200 MPa-800 degrees C under oxidizing conditions: they contained 6.0 wt% dissolved H2O and a dense population of hematite crystals (8.7 +/- 2 x 10(5) mm(-3)). Pressure was decreased from the saturation value to a final value ranging from 99 to 20 MPa, at constant temperature (800 degrees C); the rate of decompression was either 1,000 or 27.8 kPa/s. In all experiments, we observed a single event of heterogeneous bubble nucleation beginning at a pressure PN equal to 63 +/- 3 MPa in the 1,000 kPa/s series, and to 69 +/- 1 MPa in the 27.8 kPa/s series. Below P-N, the degree of water supersaturation in the liquid rapidly decreased to a few 0.1 wt%, the nucleation rate dropped, and the bubble number density (BND) stabilized to a value strongly sensitive to decompression rate: 80 mm(-3) at 27.8 kPa/s vs. 5,900 mm(-3) at 1,000 kPa/s. This behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite-H2O system and in numerical simulations of vesiculation in ascending magmas. Similar degrees of water supersaturation were measured at 27.8 and 1,000 kPa/s, implying that a faster decompression rate does not result in a larger departure from equilibrium. Our experimental results imply that BNDs in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate vertical bar dP/dt vertical bar and the number density of phenocrysts, especially the number density of magnetite microphenocrysts (1-100 mm(-3)), which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation. Very low BNDs (approximate to 1 mm(-3)) are predicted in the case of effusive eruptions vertical bar dP/dt vertical bar approximate to 0.1 kPa/s). High BNDs (up to 10(7) mm(-3)) and bimodal bubble size distributions are expected in the case of explosive eruptions: (1) a relatively small number density of bubbles (1-100 mm(-3)) will first nucleate in the lower part of the conduit vertical bar dP/dt vertical bar approximate to 10 kPa/s), either at high pressure on magnetite or at lower pressure on quartz and feldspar (or by homogeneous nucleation in the liquid) and (2) then, extreme decompression rates near the fragmentation level vertical bar dP/dt vertical bar approximate to 10 3 kPa/s) will trigger a major nucleation event leading to the multitude of small bubbles, typically a few micrometers to a few tens of micrometers in diameter, which characterizes most silicic pumices.