Crystal Sinking and Bubble Rising in the Bishop Tuff Rhyolitic Magma

The Bishop Tuff is a large volume pyroclastic deposit of rhyolitic composition. We studied five pumice clasts from three early to middle stratigraphic units: fall units F7 and F8, and ash-flow unit Ig2E. Our determinations included pumice bulk densities, crystal contents and size distributions. The latter two were obtained by a crushing, sieving and winnowing procedure, supplemented by X-ray tomography and microscopy. A combination of pumice density and crystal contents was used to determine the volumetric abundances of glass, crystals and vesicles in each pumice clast. Our data suggest a positive correlation between pumice densities and crystal contents. Three stages of evolution are evident based on density and crystal content. The three stages are also distinct in the distribution of crystal mass fraction with size. All samples have similar amounts of small crystals (< 200 microns), but the abundance of large crystals (800-1800 microns) varies greatly and is well illustrated by a marked positive correlation between the mass of large crystals and the total crystal contents (and also pumice density). As a result, there is a strong negative correlation between the volumetric abundance of vesicles (porosity) and the amount of large crystals. Accumulation of large crystals (in accordance with evidence from melt inclusion and sanidine zoning) causes a large increase in the mass fraction in this size category with a corresponding slight decrease in porosity, resulting in a slightly negative slope, much smaller than that observed. The effect of growth is not easily portrayed, but for a magma cooling under gas-saturated conditions, both porosity and crystal mass should increase simultaneously, generating a positive correlation; clearly, this expected behavior cannot explain the observed correlation. Hence, in order to explain the negative correlation between porosity and abundance of large crystals we invoke a combination of crystal accumulation and bubble rising in the preeruptive Bishop magma.