H2O, CO2, CI, and gas in Plinian and ash-flow Bishop rhyolite

Infrared spectroscopic (H₂O, CO₂ and electron microprobe (CI) analyses of glass inclusions in Plinian and ash-flow quartz phenocrysts from the Bishop Tuff reveal the preeruption concentrations of volatiles in separate parts of the body of magma. There is an inverse relation between H₂O, CO₂ that can be explained (1) by closed-system, gas-saturated crystallization of parent magma to yield Plinian magma + (lost) crystals or (2) by the rise of CO₂-rich bubbles through water-rich magma. Our estimated pressures of gas-saturation range from about 1.6 kbar (Plinian) to 2.3 kbar (ash flow) and accord with geologic evidence for 3 km of magma withdrawal and caldera subsidence.