The role of shear heating in obsidian formation within volcanic conduits

While most volcanic rocks contain a significant amount of crystals (15-35 vol%), obsidian is unusual because it contains < 2 vol% phenocrysts. The few phenocrysts in obsidian are evidence for some crystallization, but the relative paucity reflects conditions in which crystallization is inhibited. The causes of these conditions in obsidian magmas are poorly understood. One way to inhibit crystallization and resorb crystals is to increase temperature. Shear heating is a potentially important source of heat in high-silica rhyolites due to their high viscosity, yet it is seldom accounted for in the thermal budgets of ascending magmas. This study combines mineralogical analysis of obsidian with numerical models of ascending, high-silica magma in order to examine an alternate hypothesis for obsidian formation in which shear heating inhibits crystallization and resorbs crystals. Using the finite-element solver COMSOL Multiphysics, this study models a planar dike 5 m wide and 1 km long. Temperatures increase up to 300 K above initial magma temperatures at conduit edges, which enable velocities and fluxes above Poiseuille solutions. These temperature increases are 150-200 K higher than those found by existing numerical models that account for shear heating in volcanic conduits. Based on velocities in the outer edge of the conduit, residence times of crystals in hotter magma range from 6 minutes to 58 days in a 1 km conduit; longer conduits increase residence time. Furthermore, complex conduit geometry can cause separation of laminar flow lines which would distribute hotter magma to other parts of the conduit. Longer residence times and higher temperatures favor crystal resorption. Modal analyses of obsidian in this study reflect a regional lack of quartz and sanidine phenocrysts in eastern California obsidian. This regional lack is unpredicted by the dominant hypotheses of obsidian formation and unexpected based on the mineralogy of other high-silica rhyolites. Phenocryst morphologies are dominantly rounded in crystal-poor obsidian, but not in crystal-rich obsidian (>2 vol% phenocrysts). Decompression is thought to cause quartz resorption during magma ascent, causing disequilibrium rounding and embayments in quartz. However, decompression does not account for the lack of sanidine in crystal-poor obsidian. Sanidine is stable in pre-eruptive conditions in some obsidian magmas, but experiments show that it is unstable at high-temperature, 1 atm conditions. Near-surface thermal disequilibrium caused by shear heating could dissolve unstable sanidine phenocrysts and cause rounding in other phenocrysts. By increasing temperatures to near liquidus conditions, shear heating potentially plays a large role in the formation of obsidian by inhibiting crystallization and resorbing existing crystals.