Abstract

Obsidian pyroclasts, a common component of rhyolitic tephra, preserve a range of volatile contents, which has been used to infer syneruptive conditions of magmatic degassing. Here we show that the textures of obsidian pyroclasts provide information on physical mechanisms of magma flow and degassing along conduit margins. Obsidian clasts often contain xenoliths, sheared bands of lithic powder, and textures consistent with magma autobrecciation. These features suggest that pyroclastic obsidian primarily forms near conduit walls where magma fragments and reanneals during ascent. We use these observations to develop a degassing model for pyroclastic obsidian from the A.D. 1340 Mono Craters, California, eruptions. We suggest that degassing was buffered by continual flux of vapor through highly permeable, brecciated magma along conduit walls. Continuous reequilibration of magma with vapor of relatively constant composition not only explains the CO2-H2O and δD-H2O data from Mono Craters pyroclastic obsidian, but also requires much lower magmatic CO2 values than the commonly accepted model of closed-system degassing. Taken together, the chemical and physical evidence suggests that magma brecciation along conduit walls aids the degassing of ascending rhyolite.

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