USING BUBBLES AND MICROLITES IN OBSIDIAN TO ASSESS FACTORS GOVERNING THE ERUPTION AND EMPLACEMENT OF RHYOLITE

Quantitative textural analysis of vesicles and microlites in glassy volcanic samples provides a powerful means to investigate the kinematics and dynamics of the ascent and eruption of rhyolitic magma. Here I highlight results from a research program inspired and made possible by the pioneering work of Professor Katharine Cashman. Our work exploits exposure afforded by heavily dissected rhyolite lava, pyroclastic deposits, and associate feeder conduits from the Binna Burra and Nimbin Rhyolite formations of the Tweed shield volcano in eastern Australia. Although minor components of pure shear, as well as evidence of bubble-bubble interactions and bubble relaxation, were noted in some populations, bubble geometries in both pyroclastic and effusive obsidian dominantly record simple shear. In general, we observe a systematic increase in shear stress recorded in lava, vent samples associated with fountain-fed agglomerate, and pyroclastic flow and fall units consistent with a general model in which eruption style is governed by magma ascent rate.

Microlites in the basal shear zone of near-vent and flow-front locations of flow-banded rhyolite provide further insight into the ascent and emplacement of degassed magma. The rhyolite contains dark bands of microlite-poor glass and light bands of microlite-rich glass. While microlite number densities correlate with the slopes of crystal size distributions, these properties do not correlate with flow-band thickness, degree of microlite preferred orientation, or position within the basal shear zone. Moreover, number densities and size distributions of microlites vary widely at the thin-section scale. Microlite-defined flow bands therefore appear to record spatially complex variations in ascent rate, extent and/or depth of degassing, and residence time during transport in shallow conduits. Finally, in contrast to microlite preferred orientations reported for samples from the upper surfaces of rhyolite lavas elsewhere, which have been interpreted to reflect strain associated with collapsing permeable foam in the conduit, we find that microlites in basal obsidian samples register measurable re-orientation during subaerial flow. Most of the strain associated with emplacement of viscous rhyolite lava appears to be accommodated within a basal zone of shear, while the main mass of lava is rafted above.