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

Castro, Jonathan M. and Cordonnier, Benoit and Tuffen, Hugh and Tobin, Mark and Puskar, Liljana and Martin, Michael C. and Bechtel, Hans (2012) The role of melt-fracture degassing in defusing explosive rhyolite eruptions at volcán Chaitén. Earth and Planetary Science Letters, 333-33 (n/a). pp. 63-69. ISSN 0012-821X

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Explosive volcanic eruptions of silcic 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- 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 hrs) 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.

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Journal Article
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Earth and Planetary Science Letters
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09 Mar 2012 02:59
Last Modified:
17 Sep 2023 00:59