Browning, J; Tuffen, H; James, MR; Owen, J; Castro, JM; Halliwell, S; Wehbe, K; (2020) Post-fragmentation vesiculation timescales in hydrous rhyolitic bombs from Chaitén volcano. Journal of South American Earth Sciences , 104 , Article 102807. 10.1016/j.jsames.2020.102807.

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Abstract

Bubble nucleation and growth dynamics exert a primary control on the explosivity of volcanic eruptions. Numerous theoretical and experimental studies aim to capture the complex process of melt vesiculation, whereas textural studies use vesicle populations to reconstruct magma behaviour. However, post-fragmentation vesiculation in rhyolitic bombs can create final quenched bubble (vesicle) textures that are not representative of the nature of fragmenting magma within the conduit. To examine bubble growth in hydrous rhyolitic bombs, we have used heated stage microscopy to directly observe vesiculation of a Chaitén rhyolite melt (with an initial dissolved water content of ~0.95 wt %) at atmospheric pressure and magmatic temperatures upon reheating. Thin wafers of obsidian were held from 5 min up to two days in the heated stage at temperatures between 575 °C and 875 °C. We found that bubble growth rates, measured through changes in bubble diameter, increased with both temperature and bubble size. The average growth rate at the highest temperature of 875 °C is ~1.27 μm s^{-1}, which is substantially faster than the lowest detected growth rate of ~0.02 μm s^{-1} at 725 °C; below this temperature no growth was observed. Average growth rate V_{r} follows an exponential relationship with temperature, T and inferred melt viscosity η, where V_{r} = 5.57×10^{-7}e^{0.016T} and V_{r} = 3270e^{−1.117η}. Several stages of evolving bubble morphology were directly observed, including initial relaxation of deformed bubbles into spheres, extensive growth of spheres, and, at higher temperatures, close packing and foam formation. Bubble deformation due to bubble-bubble interaction and coalescence was observed in most experiments. We use our simple, experimentally-determined relationship between melt viscosity and bubble growth rates to model post-fragmentation vesicle growth in a cooling 1 m-diameter rhyolitic bomb. The results, which indicate negligible vesicle growth within 2–3 cm of the bomb surface, correspond well with the observed dense margin thickness of a Chaitén bomb of comparable dimensions. The experiments described can be used to effectively reconstruct the post-fragmentation vesiculation history of bombs through simple analytical expressions which provide a useful tool for aiding in the interpretation of pumiceous endmember textures in hydrous rhyolitic bombs.

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