Bubbleless Glass Pockets and Natural Bubble Nucleation in Rhyolitic Magma

Bubbleless Glass Pockets and Natural Bubble Nucleation in Rhyolitic Magma. It is common for quartz phenocrysts to have bubble-free reentrant pockets of glass that connect to and are continuous with highly vesicular pumiceous glass surrounding the phenocryst. Shapes of such pockets range from broad openings which widen outwards toward the crystal rim to hourglass-shaped lenses of glass that connect to the crystal rim via a narrow neck. The shape is important because the narrow neck of an hourglass retards decompression and maintains a greater pressure within than without. On the other hand a melt pocket that opens outward is free to decompress and expand as in the surrounding pumiceous melt. The puzzle is that such pockets commonly are bubble-poor or bubble-free contrary to the intuitive expectation that the more volatile-rich and thus most volatile-supersaturated, decompressed melt should have the greatest rate of bubble nucleation. An assumption is that initially, at the beginning of decompression and bubble nucleation and growth, the volatile concentration in the melt is at least as great in melt pockets as in surrounding melt. With decompression volatiles diffuse toward growing bubbles. Bits of bubble-free melt in reentrant pockets thus retain a greater concentration of dissolved volatiles, but the mechanical pressure on the melt in open pockets will be the same as that in the adjacent bubbly magma. Thus the volatile supersaturation will be greatest in open pockets where no bubble has yet formed. The bubble-free pockets, being more supersaturated, should nucleate bubbles more readily than the less supersaturated bubbly surrounding melt. A qualitative explanation of this apparently anomalous absence of bubbles in melt pockets is bubble nucleation caused by local tensile stresses. For example the viscous melt between bubbles might be stretched to the point of brittle failure thus creating a local region with lower pressure and consequent greater volatile supersaturation and bubble nucleation. Adjacent bubbles of different size will tend to expand at different rates due to diffusive growth and this may cause shearing stresses on melt between bubbles. Melt in pockets is protected from such stresses. Thus the nucleation of bubbles in decompressing silicic magma may be enhanced by eruptive deformation, and this may allow bubble nucleation at a level of bulk volatile supersaturation that is less than expected based on experiments.