WERE THE VOLATILE CONTENTS OF YELLOWSTONE CALDERAS LAVA CREEK TUFF THE SAME?

The Lava Creek Tuff represents ~1000 km³ of rhyolitic ash-fall and pyroclastic flow deposits from the most recent super eruption of Yellowstone caldera. The eruptive sequence is separated into Lava Creek Tuff A and B, based on stratigraphic constraints and compositional differences. The primary compositional difference between the two units is the dominance of iron-rich amphibole as the mafic phase in Lava Creek Tuff A. Amphibole indicates a difference in the effect of dissolved water within the two magmas, which may in turn have controlled other volcanic and petrologic processes. We used FTIR spectroscopy to measure the H₂O and CO₂ content of quartz-hosted melt inclusions from pumice clasts in the unwelded ignimbrite of Lava Creek Tuff A and the basal fall unit of Lava Creek Tuff B. For each sample, we handpicked quartz grains for analysis from crushed pumice clasts. Melt inclusions prior to homogenization are opaque and highly decrepitated. To return them to their initial, pristine condition, we homogenized the melt inclusions in a suite of experiments at 750-800 °C and 2000-2500 bars for 24-48 hours. After homogenization, inclusions are transparent glass with up to 5% microlites and sparse bubbles. Melt inclusions from Lava Creek Tuff A contain 1.4-3.8 wt.% H₂O and 9-65 ppm CO₂, and contained a fluid that was 90 mol% H₂0. Melt inclusions from Lava Creek Tuff B have 0.57-1.06 wt.% H₂O and 2.7-4.3 ppm CO₂, and has a fluid greater than 90 mol% H₂0. Assuming volatile saturation, those volatile contents correlate with a storage pressure of approximately 100 MPa. Such values indicate similar storage pressures to Huckleberry Ridge Tuff and Central Plateau Rhyolites.