The influence of Fe-Ti oxide microlites on bubble nucleation in rhyolitic melts

We conducted a set of high-temperature decompression experiments to constrain the mechanisms of heterogeneous bubble nucleation in high-silica rhyolitic melt that contained 4.6-4.8 wt% H2O. The melt was seeded with two different size fractions of magnetite crystals: 1-2 mu m crystals and large crystals of 32-135 mu m (long axis). The number density of bubbles (BND) that nucleated on the small crystals was found to increase from 10(6.5) to 10(8.7) cm(-3) as H2O increasingly supersaturated (Delta P) in the melt from 3 to 23 MPa. At Delta P >23 MPs, however, the number of bubbles nucleated equals the number of small magnetite and no more nucleated with increased Delta P. At the same conditions, the number of bubbles that nucleated on the large crystals increases, from <1 bubble per crystal at Delta P = 3 MPa to 14 +/- 4 bubbles per crystal at 58 MPa. We thus find that Delta P has a significant influence on the mechanisms of heterogenous nucleation, but the observed increases in BND are much greater than would be predicted solely from the increase in Delta P. The discrepancy can be reconciled if there are different sites on the crystals that become activated at greater Delta P, leading to greater numbers of bubbles nucleating. The cumulative BND nucleated on small crystals, however, is capped by the number of crystals present. The BND values generated at Delta P >23 MPa in our experiments overlap with those found in similar to 80 % of naturally occurring pumice. Assuming our experiments are representative of natural pumice, this suggests that explosively erupted magmas either become significantly volatile supersaturated before heterogeneously nucleating bubbles, or that the number of nucleation sites in natural magmas greatly exceed 10(9) cm(-3).