Crystallization history of Obsidian Dome,Inyo Domes,California

Samples obtained by U.S. Department of Energy research drilling at the 600-year-old Obsidian Dome volcano provide the rare opportunity to examine the transition from volcanic (dome) to plutonic (intrusion) textures in a silicic magma system. Textures in the lavas from Obsidian Dome record multiple periods of crystallization initiated in response to changes in undercooling (T) related to variable degassing in the mag-ma. Phenocr)ysts formed first at low T. A drastic increase in T, related to loss of a vapor phase during initial stages of eruption, caused nucleation of microlites. All of the lavas thus contain phenocrysts and microlites. Extrusion and subsequent devitrification of the dry (0.1 wt% H₂O) magma crystallized spherulites and fine-grained rhyolite at high T. A granophyric texture, representing crystallization at a moderate T, formed in the intrusions beneath Obsidian Dome. Textures in the intrusion apparently represent crystallization of hydrous (1–2 wt% H₂O) rhyolitic magma at shallow depths.