Kinetics of bubble nucleation in a rhyolitic melt: an experimental study of the effect of ascent rate

摘要

In order to characterize the effect of ascent rate on the kinetics of bubble nucleation in a rhyolitic magma, we performed three series of experiments decompressed at rates of either 1000, 167, or 27.8 kPa/s. The experiments were carried out in an externally heated pressure vessel at 800 ℃ and in the pressure range 260–59 MPa; the starting material was a crystal-free and bubble-free rhyolitic glass containing 7.0 wt% dissolved H_2O. In all the decompression experiments, homogeneous bubble nucleation began at 90±2 MPa, that is, ≈150 MPa below the water saturation pressure of the silicate liquid, 240 MPa. The degree of supersaturation △P_(HoN) required to trigger homogeneous bubble nucleation was almost independent of decompression rate (△P_(HoN) is the difference between the saturation pressure and the nucleation pressure): nucleation pressure decreased by ≤ 3 MPa for a 36-fold increase in decompression rate. These results are in good agreement with the classical theory of nucleation assuming a rhyolite–H_2O surface tension of 0.106 N m~(-1). Our major experimental finding is that, after a short nucleation event, the nucleation rate dropped and the bubble number density N reached a stationary value that was strongly sensitive to decompression rate: 6.8 mm~(-3) at 27.8 kPa/s, 470 mm~(-3) at 167 kPa/s, and 5800 mm~(-3) at 1000 kPa/s. The smaller value of N at low decompression rate was compensated by a larger mean bubble size, so that, at a given pressure, vesicularity was almost independent of decompression rate. The experimental values of N can be reproduced within a factor 0.3–1.4 using a relationship derived from published numerical simulations of vesiculation in ascending magmas. The nucleation behavior in our experiments is dictated by a competition between bubble nucleation and diffusive bubble growth, which depletes in water the surrounding liquid and therefore reduces the degree of volatile supersaturation. Once a critical value of N is attained, diffusive bubble growth can keep pace with decompression and prevent the degree of volatile supersaturation in the liquid to increase with decreasing pressure. The strong correlation between bubble number density and decompression rate has fundamental volcanological implications. If we extrapolate the experimental data to the typical ascent rates of silicic magmas, we obtain bubble number densities (10~(-3)–10~1 mm~(-3) for homogeneous nucleation; ≈10~(-1)–103~ mm~(-3) for heterogeneous nucleation) that are orders of magnitude smaller than those measured in most natural pumices. We therefore propose that the large values of N in silicic pumices may be due to two successive nucleation events: (1) a first event, which occurs relatively deep in the volcanic conduit and which yields a moderate number of bubbles; and (2) a second nucleation event, yielding a very large number of small bubbles, and presumably related to the dramatic increase of decompression rate that precedes fragmentation. The small bubble number densities associated with homogeneous nucleation suggest that a strong departure from equilibrium degassing should be the rule even at slow ascent rates.