Magma ascent during silicic dome-forming eruptions is characterized by significant changes in magma viscosity, permeability, and gas overpressure in the conduit. These changes depend on a set of parameters such as ascent rate, outgassing and crystallization efficiency, and magma viscosity, which in turn may influence the prevailing conditions for effusive versus explosive activity. Here, we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive-explosive transitions during Vulcanian phases of the 9.4 ka eruption of Kilian Volcano, Chaîne des Puys, France. Our results suggest that effusive activity at the onset of Vulcanian episodes at Kilian Volcano was promoted by (i) rapid ascent of initially crystal-poor and volatile-rich trachytic magma, (ii) a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization, and (iii) efficient degassing/outgassing in a crystal-rich magma at shallow depths. Trachytic magma repeatedly replenished the upper conduit, and variations in the amount of decompression and cooling caused vertical textural stratification, leading to variable degrees of crystallization and outgassing. Outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles, fractures, and tuffisite veins, fostering the formation of cristobalite in the carapace and talus regions. Build-up of overpressure was likely caused by closing of pore space (bubbles and fractures) in the dome through a combination of pore collapse, cristobalite formation, sintering in tuffisite veins, and limited pre-fragmentation coalescence in the dome or underlying hot vesicular magma. Sealing of the carapace may have caused a transition from open- to closed- system degassing and to renewed explosive activity. We generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity, rapid microlite growth, and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents. We show that trachytes may erupt with a similar style (e.g., Vulcanian) but at significantly higher ascent rates than their andesitic, dacitic, and rhyolitic counterparts. This suggests that the periodicity of effusive-explosive transitions at trachytic volcanoes may differ from that observed at the well-monitored andesitic, dacitic, and rhyolitic volcanoes, which has implications for hazard assessment associated with trachytic eruptions.

中文翻译：

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法国 Chaîne des Puys 的 Kilian 火山粗暴 Vulcanian 喷发期间的流变变化和脱气

硅质圆顶形成喷发期间岩浆上升的特征是岩浆粘度、渗透率和管道中的气体超压发生显着变化。这些变化取决于一组参数，例如上升速率、除气和结晶效率以及岩浆粘度，这些参数反过来可能会影响喷出活动与爆炸活动的主要条件。在这里，我们将 tephra 的化学和结构分析与粘度模型相结合，以更好地了解法国 Chaîne des Puys 的 Kilian 火山 9.4 ka 喷发的 Vulcanian 阶段期间的喷发-爆炸转变。我们的结果表明，在 Kilian 火山的 Vulcanian 事件开始时的喷发活动是由 (i) 最初缺乏结晶和富含挥发性的粗面岩浆的快速上升所促进的，(ii) 由大量挥发物损失和结晶驱动的体积和熔体粘度显着增加，以及 (iii) 在浅层富含晶体的岩浆中有效脱气/除气。粗质岩浆反复补充上部管道，减压和冷却量的变化导致垂直结构分层，导致不同程度的结晶和释气。除气促进了喷出的穹窿生长，气体通过大的相互连接的囊泡、裂缝和凝灰岩静脉渗漏而发生，促进了在甲壳和距骨区域方石英的形成。超压的增加可能是由于圆顶中的孔隙空间（气泡和裂缝）通过孔隙坍塌、方石英形成、凝灰岩脉中的烧结、并且在圆顶或下面的热囊泡岩浆中有限的预碎裂聚结。甲壳的密封可能导致从开放系统脱气到封闭系统脱气以及重新爆炸活动的转变。我们概括了我们的研究结果，提出粗面岩浆的广泛喷发样式可能继承自粗面熔体的特征组合，包括高水溶性和扩散性、快速微晶生长和与其更进化的亚碱性英安岩相比的低熔体粘度和流纹岩等价物。我们表明粗面岩可能以类似的风格喷发（例如瓦肯岩），但​​上升速度明显高于安山岩、英安岩和流纹岩对应物。