MAGMATIC VOLATILE CONTENTS OF YELLOWSTONE CALDERA

We characterize the volatile contents in homogenized, quartz-hosted glass inclusions from rhyolitic lava flows and tuffs from Yellowstone Caldera. Pre-eruptive magmatic volatile abundances are largely functions of chamber pressure and the availability of H₂O and CO₂ in the regional magma-tectonic system. Magmatic degassing exerts first-order control on the eruptive behavior of volcanic systems. Yellowstone caldera is a type-example both for non-eruptive and eruptive degassing, evidenced by the modern hydrothermal/fumarolic system and eruptions of “supervolcano” explosions and volumetrically diverse lava flows and domes over the past 3 Ma. Despite the significance of Yellowstone Caldera’s diverse degassing activity, the pre-eruptive volatile contents of the system remain largely unknown. Thus, we have measured pre-eruptive H₂O and CO₂ concentrations in tuffs and lavas from the youngest episode of volcanism at Yellowstone (175 to 80 kya), which were chosen to encompass the entire range of eruptive style (explosive to effusive) and eruption volume (0.01 km³ to 70 km³). Volatile data from each unit are dispersed in clusters that appear to display minor evidence for closed-system degassing. Water contents from all units are similar (1.3 to 2.5 wt.% H₂O). Conversely, CO₂ contents vary, and can be used to distinctly separate the units into CO₂-rich and CO₂-poor populations. CO₂-rich units contain 300 to 600 ppm CO₂, and suggest the magmas were in equilibrium with fluids composed of 60 to 75 mol.% CO₂ (Tuff of Bluff Point, Pitchstone Plateau, Trischmann Knob, Buffalo Lake, Summit Lake). CO₂-poor units contain 50 to 200 ppm CO₂, and were likely in equilibrium with fluids composed of 40 to 60 mol.% CO₂ (Solfatara Plateau, Cold Mountain Creek Tuff, Douglas Knob, West Yellowstone). Assuming the magmas were fluid saturated and maximum values represent storage conditions, the CO₂-rich magmas were stored at 90 to 130 MPa, whereas the CO₂-poor magmas were stored at 50 to 60 MPa. Interestingly, pre-eruptive volatile contents do not control the eruptive behavior, nor do they correlate with eruptive age, volume, or position in the caldera. Finally, the Central Plateau Member magma are compositionally distinct from the Lava Creek Tuff magma, which contained 3.2 to 4.0 wt.% H₂O and 100 to 600 ppm CO₂ (Gansecki, 1998).