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 Chaiten 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 Vr follows an exponential relationship with temperature, T and inferred melt viscosity η, where Vr = 5.57×10−7e0.016T and Vr = 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 Chaiten 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.

中文翻译：

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Chaitén 火山含水流纹质炸弹的破碎后气泡时间尺度

气泡成核和生长动力学对火山喷发的爆炸性起到主要控制作用。许多理论和实验研究旨在捕捉熔体成泡的复杂过程，而结构研究则使用泡囊群来重建岩浆行为。然而，流纹岩炸弹中的破碎后气泡会产生最终淬火的气泡（囊泡）纹理，这不代表管道内破碎岩浆的性质。为了检查含水流纹岩炸弹中气泡的生长，我们使用加热的载物台显微镜直接观察了 Chaiten 流纹岩熔体（初始溶解水含量约为 0.95 重量％）在大气压和岩浆温度下再加热时的气泡形成。在 575 °C 和 875 °C 之间的温度下，将黑曜石薄晶片在加热阶段保持 5 分钟至两天。我们发现通过气泡直径的变化测量的气泡增长率随着温度和气泡尺寸的增加而增加。875 °C 最高温度下的平均生长速率为~1.27 μm s-1，大大快于 725 °C 时检测到的最低~0.02 μm s-1 生长速率；低于此温度未观察到生长。平均增长率 Vr 与温度、T 和推断的熔体粘度 η 呈指数关系，其中 Vr = 5.57×10−7e0.016T 和 Vr = 3270e−1.117η。直接观察到气泡形态演变的几个阶段，包括变形气泡初始松弛成球体、球体的广泛生长以及在较高温度下紧密堆积和泡沫形成。在大多数实验中观察到由于气泡-气泡相互作用和聚结引起的气泡变形。我们使用我们简单的、实验确定的熔体粘度和气泡生长速率之间的关系来模拟直径为 1 m 的流纹岩冷却弹中的破碎后囊泡生长。结果表明，炸弹表面 2-3 厘米内的囊泡生长可以忽略不计，这与观察到的具有可比尺寸的柴腾炸弹的致密边缘厚度非常吻合。所描述的实验可用于通过简单的分析表达式有效地重建炸弹的破碎后气泡历史，这为帮助解释含水流纹质炸弹中的浮石端元纹理提供了有用的工具。我们使用我们简单的、实验确定的熔体粘度和气泡生长速率之间的关系来模拟直径为 1 m 的流纹岩冷却弹中的破碎后囊泡生长。结果表明，炸弹表面 2-3 厘米内的囊泡生长可以忽略不计，这与观察到的具有可比尺寸的柴腾炸弹的致密边缘厚度非常吻合。所描述的实验可用于通过简单的分析表达式有效地重建炸弹的破碎后气泡历史，这为帮助解释含水流纹质炸弹中的浮石端元纹理提供了有用的工具。我们使用我们简单的、实验确定的熔体粘度和气泡生长速率之间的关系来模拟直径为 1 m 的流纹岩冷却弹中的破碎后囊泡生长。结果表明，炸弹表面 2-3 厘米内的囊泡生长可以忽略不计，这与观察到的具有可比尺寸的柴腾炸弹的致密边缘厚度非常吻合。所描述的实验可用于通过简单的分析表达式有效地重建炸弹的破碎后气泡历史，这为帮助解释含水流纹质炸弹中的浮石端元纹理提供了有用的工具。这表明炸弹表面 2-3 厘米内的囊泡生长可以忽略不计，这与观察到的具有可比尺寸的柴腾炸弹的致密边缘厚度非常吻合。所描述的实验可用于通过简单的分析表达式有效地重建炸弹的破碎后气泡历史，这为帮助解释含水流纹质炸弹中的浮石端元纹理提供了有用的工具。这表明炸弹表面 2-3 厘米内的囊泡生长可以忽略不计，这与观察到的具有可比尺寸的柴腾炸弹的致密边缘厚度非常吻合。所描述的实验可用于通过简单的分析表达式有效地重建炸弹的破碎后气泡历史，这为帮助解释含水流纹质炸弹中的浮石端元纹理提供了有用的工具。