The role of melt-fracture degassing in defusing explosive rhyolite eruptions at volcán Chaitén

Explosive volcanic eruptions of silicic magma often evolve towards non-explosive emissions of lava. The mechanisms underlying this transition remain unclear, however, a widely cited idea holds that shear-induced magma fragmentation plays a critical role by fostering volatile loss from fragmentary magma and through ash-filled cracks termed tuffisite. We test this hypothesis by measuring H2O concentrations within glassy tuffisite from the 2008-2011 rhyolitic eruption at volcán Chaitén, Chile. We show that while H2O concentrations decrease next to tuffisite veins and at the margins of obsidian fragments, the depletions cannot account for the disparity in H2O between explosively and effusively erupted rhyolite. Tuffisite vein lifetimes derived from diffusion modeling (min to h) imply degassing rates that are too slow to effectively degas magma, unless the magma was entirely fragmented to mm or smaller particles. This level of brecciation may locally develop near conduit margins, but is unrealistic for entire conduits. The primary role of melt fracturing may therefore be to provide gas-escape pathways for more efficient degassing of permeable vesicular magma in the conduit interior.

► Magmatic degassing of H2O occurs by diffusive release into fractures. ► Numerical models show diffusion is spatially and temporally limited. ► Melt-fracture degassing is important where magma is highly pulverized. ► Brittle failure may aid release of distally derived gas. ► Melt-fracture degassing does not cause the explosive-efffusive eruption transition.