Flow Dynamics of the Pitchstone Plateau Obsidian Flow in Yellowstone National Park, WY

Extremely large volume rhyolite flows are known to exist, but have never been witnessed, and so little is known about how such massive, high viscosity lava flows erupt. To better elucidate their behavior, we focus on the 70 km3 Pitchstone Plateau obsidian flow, erupted 72 ka from the Yellowstone caldera (Christiansen, 2007). We have examined samples collected relatively evenly from near vent to the flow front, characterizing the number densities and sizes of groundmass microlites. Phenocryst content and a phenocryst assemblage of >95% quartz and sanidine do not change within the flow, indicating that the magma is homogenous. We find that microlite number densities remain relatively constant in the flow, with the exception of the flow front, which has a density several orders of magnitude higher. Total densities range from 9.65x108 cm-3 to 1.43x1011 cm-3. Average size of all crystal types range from 1.6 um3 to 47 um3. To constrain viscosity and kinetics of groundmass crystallization of the magma, we investigated the pre-eruptive storage conditions of the magma. The temperature was about 700-740 degrees Celsius based on [Ti] in quartz phenocryst rims and fayalite-augite-magnetite equilibrium. Volatiles dissolved in glass inclusions range up to 3.5 wt.% water and, when detected, up to 200 ppm CO2, which implies fluid saturated pressures of up to 100 MPa. Hydrothermal phase equilibrium experiments confirm that quartz and sanidine co-precipitate at those temperatures only when water pressures are below 125 MPa. Groundmass water contents of 0.13 wt.% show that the melt degassed completely, which led to an increase in viscosity from <107 Pa s in the chamber to 1011.6-1012.2 Pa s during extrusion. We will combine our estimates of viscosity with size and number data to investigate the kinetics of groundmass crystallization and estimate flow dynamics of the lava. 1Christiansen, 2007 USGS Open-file Report