摘要：

The reasons for obsidian's lack of phenocrysts (<1 vol%) are poorly understood. Modal analyses of obsidian from the Long Valley and Coso volcanic areas in California were completed to better understand the crystallization history of obsidian. Crystal-poor samples (< 2 vol%) contain plagioclase but lack sanidine and often quartz, whereas crystal-rich samples (>2 vol%) contain plagioclase and sanidine but sometimes lack quartz. Crystal-poor samples contain dominantly rounded phenocrysts indicative of resorption, whereas phenocrysts in crystal-rich samples contain sharp, defined corners. Thermal models of ascending, high-silica magma were run using COMSOL Multiphysics to assess temperature increases caused by shear heating, which could potentially resorb crystals. Models show large (>300 K) temperature increases at the conduit edges capable of resorbing crystals. Though model dike geometry is unrealistic, obsidian mineralogy and thermal modeling are consistent with shear heating-induced resorption during ascent of high-silica magmas.

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