The Effect of Ascent Rate on the Kinetics of Bubble Nucleation in a Rhyolitic Melt.

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 1000, 167, or 27.8 kPa/s. The experiments were carried out in an externally heated pressure vessel at 800°C; the starting material was a crystal-free and bubble-free rhyolitic glass containing 7.0 wt.% H2O. In the three decompression series, homogeneous bubble nucleation began at 90 MPa, that is, ≡ 150 MPa below the water saturation pressure of the silicate liquid, 240 MPa. 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. This behaviour was dictated by a competition between nucleation and diffusive bubble growth, which depleted in water the surrounding liquid and so reduced the degree of volatile supersaturation. With increasing N, the length scale of diffusion of water molecules to growing bubbles decreased: nucleation stopped when N attained a critical value, at which the degree of volatile supersaturation in the liquid was everywhere below the value required for nucleation and no longer increased 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 that are orders of magnitude smaller than those measured in most natural pumices. We 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