Water concentration profiles around bubbles offer a new kind of geobarometer. We measure H2O and CO2 concentrations in glass adjacent to bubbles in pyroclastic obsidian from Mono Craters, California (United States). H2O and CO2 concentration gradients are preserved during the eruption and record nonequilibrium degassing. A key result is that H2O is enriched in the glass surrounding the bubbles, indicating that bubbles were resorbing into the melt just prior to the eruption. The required pressure increase for the observed water enrichment is inferred to be the last in a series of pressure cycles with amplitude 5–30 MPa that are caused by repeated fragmentation and annealing. CO2 concentrations vary substantially in individual obsidian clasts, suggesting that slow diffusion of CO2 and nonequilibrium degassing contributes to high CO2/H2O ratios in pyroclastic obsidian from Mono Craters. These data are direct evidence for vapor-melt disequilibrium and demonstrate that degassing paths from a single parental melt need not be unidirectional. Hence volatile concentration gradients offer a tool for evaluating degassing models and inferring time scales of magmatic processes.

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