Prolonged ascent and episodic venting of discrete magma batches at the onset of the Huckleberry Ridge supereruption, Yellowstone
Prolonged ascent and episodic venting of discrete magma batches at the onset of the Huckleberry Ridge supereruption, Yellowstone
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黄石公园哈克贝利岭超级喷发时，离散岩浆批次的长时间上升和间歇性喷发
DOI：
10.1016/j.epsl.2016.07.023
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发表时间：
2016
影响因子：
5.3
通讯作者：
Elliot J. Swallow
中科院分区：
文献类型：
--
作者：
M. Myers;P. Wallace;C. Wilson;B. K. Morter;Elliot J. Swallow
超大规模的超级火山爆发是如何开始的，这为研究在灾难性事件中富含硅的岩浆的储存、上升和释放提供了至关重要的信息。2.08 Ma， 2500 km 3 Huckleberry Ridge喷发的初始瀑布沉积物是多层层状的，在一些地方包含重新加工的间隔，表明喷发开始阶段的时间中断。这些瀑布沉积物的2.5米剖面在埃弗茨山（怀俄明州猛犸）最早的褐煤（成员A）以下9个层次取样。我们分析了石英含矿熔融包裹体（MIs）、再入物（REs；未密封熔融包裹体）和伴生的黑曜岩火山碎屑（厚壁碎片）中的主微量元素和挥发物，以建立石英结晶、储存深度和熔体成分分组。Rb和其他不相容元素（U, Cl， B）之间的系统关系表明，最少和最进化的玻璃成分之间存在~ 55%的分数结晶。相比之下，MIs中的h2o浓度与微量元素呈分散关系，反映了岩浆上升过程中通过石英宿主扩散导致的h2o损失。在单个下降层的MIs中，h2o的宽变化（1.0-4.7 wt.%）意味着多达14天的弥漫性损失，反映了高度变化和令人惊讶的缓慢的减压条件。然而，再入大气层的水和CO 2梯度与最终上升时间< 1至4小时（上升速率为~ 0.3-1.5 m/s）一致，与我们推断经历很少或没有弥漫性h 2 O损失的MIs所代表的相似。共同喷发的结晶上升速率的大范围反映了1980年夏圣海伦火山间歇爆炸活动的情况，这表明哈克贝利岭岩浆体在喷发开始时并没有受到强烈的超压。恢复的圈闭压力和MIs的地球化学数据为六种不同的石英种群提供了证据，这些石英最初在地球化学上不同的岩浆域中结晶。矿石和黑曜岩火山碎屑的组成对比表明，在火山喷发开始时，石英被带到了三个分离的岩浆体中，我们认为这是主岩浆体顶部的冲天炉。这些冲天炉先后从不同的火山口喷发而出，形成了陨落沉积物，然后逐渐升级的活动导致了大量火山碎屑流的产生，这种活动模式表明，构造触发可能使多个岩浆体不稳定。如果母岩浆体没有强烈的超压，那么像哈克贝利岭事件这样的超级火山爆发可能会犹豫地开始，并且受到外部控制的小规模幕式活动可能不会留下其他地质证据。
How exceedingly large, volcanic supereruptions begin provides crucial information on the storage, ascent and release of silica-rich magma in catastrophic events. Initial fall deposits of the 2.08 Ma, 2500 km 3 Huckleberry Ridge eruption are multiply bedded and in several places contain reworked intervals, indicating time breaks in the opening phases of the eruption. A 2.5 m section of these fall deposits was sampled at nine levels below the earliest ignimbrite (member A) at Mount Everts (Mammoth, Wyoming). We analyzed major and trace elements and volatiles in quartz-hosted melt inclusions (MIs), reentrants (REs; unsealed melt inclusions) and associated obsidian pyroclasts (thick-walled shards) to establish quartz crystallization and storage depths and melt compositional groupings. Systematic relationships between Rb and other incompatible elements (U, Cl, B) indicate∼ 55% fractional crystallization between the least and most evolved glass compositions. In contrast, H 2 O concentrations in MIs show scattered relationships with trace elements and are interpreted to reflect variable loss of H 2 O by diffusion through the quartz host during magma ascent. The wide H 2 O variations (1.0–4.7 wt.%) in MIs from individual fall horizons imply as much as∼ 14 days of diffusive loss, reflecting highly variable and surprisingly slow decompression conditions. Water and CO 2 gradients in reentrants, however, are consistent with final ascent times of< 1 to 4 h (ascent rates of∼ 0.3–1.5 m/s), similar to those represented by MIs that we infer to have experienced little to no diffusive H 2 O loss. The wide range of ascent rates for co-erupted crystals mirrors that of intermittent explosive activity at Mount St. Helens in summer 1980, and implies that the Huckleberry Ridge magma body was not strongly overpressured at eruption onset. Restored entrapment pressures and geochemical data for MIs provide evidence for six distinct populations of quartz that originally crystallized in geochemically distinct magma domains. The compositions of REs and obsidian pyroclasts, by comparison, show that by the onset of eruption, the quartz had been brought together into three discrete magma bodies, which we interpret to have been cupolas on the roof of the main magma body. These cupolas were erupted sequentially and episodically from separate vents to generate the fall deposits before escalating activity led to generation of voluminous pyroclastic flows, and this pattern of activity suggests that tectonic triggering may have destabilized multiple magma bodies. Supereruptions as large as the Huckleberry Ridge event may start hesitatingly if the parental magma bodies are not strongly overpressured, with small-scale episodic activity that is modulated by external controls that may leave no other geological evidence for their presence.
影响因子：
5.3
作者：
Blundy, Jonathan;Cashman, Katharine V.;Witham, Fred
通讯作者：
Witham, Fred
影响因子：
18.3
作者：
Caricchi, Luca;Annen, Catherine;Pinel, Virginie
通讯作者：
Pinel, Virginie