HR: 1340h
AN: V43D-1448    [Abstracts]
TI: Pre-eruptive Bubbles in the Bishop magma? An X-ray Tomography Study of Vesicle Size Distributions
AU: * Gualda, G A
EM: ggualda@uchicago.edu
AF: Dept. of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 United States
AU: Anderson, A T
EM: canderso@uchicago.edu
AF: Dept. of the Geophysical Sciences, The University of Chicago, 5734 S. Ellis Ave., Chicago, IL 60637 United States
AB: The presence of water and other volatiles in magmas strongly affects their physical properties. However, it is extremely difficult to properly assess the volatile contents of magmas. The study of melt inclusions is one of the few reliable ways of determining the amount of volatiles in natural magmas. One drawback of this approach is that it does not readily give information on the shapes and sizes of bubbles in the magma. By studying the size distribution of vesicles in pumice, we found evidence for the presence of bubbles in the pre-eruptive Bishop magma. We have completed work on three samples from fall units F7 and F8 of the early-erupted Bishop Tuff. Work currently underway includes samples from ash-flow unit Ig2Ea. We have previously studied crystal size distributions for all of these samples. We have also obtained the bulk porosity of each one of the studied samples. All tomography datasets were collected using the GeoSoilEnvironCARS beamline at the Advanced Photon Source, Argonne National Laboratory, using a 22 keV monochromatic x-ray beam. Image analysis was performed using the software Blob3D. Samples were cut into cylinders of approximately 1 cm in diameter and height. The resulting datasets have voxels (volume elements) of 17.1 $\mu$m in each linear dimension. In such datasets, we can properly quantify the volumes of vesicles larger than approximately 50 $\mu$m (referred to as "visible" vesicles). The remaining ("invisible") vesicles are indistinguishable from noise in the glassy matrix. One striking observation is that the bulk porosity is strongly correlated with the volume fraction of visible vesicles. This indicates that the bulk porosity is at least partly controlled by the presence of the bigger vesicles. This is reinforced by the fact that bigger vesicles are more abundant in the more porous sample, while the opposite is true for smaller vesicles. In fact, the difference in porosity corresponds to the volume fraction represented by the vesicles larger than ca. 500 $\mu$m in diameter. It seems very likely that these bigger vesicles existed prior to eruption, such that low-density, crystal-poor magma was characterized by larger and more abundant vesicles, while higher-density, crystal-rich magma, was characterized by smaller vesicles. This is consistent with our working model of crystal sinking and bubble rise in the Bishop magma.
DE: 8404 Ash deposits
DE: 8439 Physics and chemistry of magma bodies
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting