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Published March 1989 | public
Journal Article

H_2O, CO_2, Cl, and gas in Plinian and ash-flow Bishop rhyolite


Infrared spectroscopic (H_2O, CO_2 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 H_2O, CO_2 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 CO_2-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.

Additional Information

© 1989 Geological Society of America. Manuscript received July 1, 1988; Revised manuscript received October 21, 1988; Manuscript accepted November 9, 1988. Supported by Department of Energy Contract ER10763 to Anderson and National Science Foundation Grants EAR-8618229 to Stolper and EAR-8520934 to Anderson. We thank Wes Hildreth for samples and inspiration, and Roy Bailey, Tom Chacko, Bob Newton, Mike Sheridan, Robert L. Smith, and Gerhard Wörner for helpful advice and discussions. Fred Nagle, Clint Mullins, Rob Lander, Fred Marton, Fangquiong Lu, and Barbara Couglin helped in the field and/ or lab at Chicago, and Lynn Silver and Paula Rosener helped in the lab at Caltech.

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