Multiple origins of obsidian pyroclasts and implications for changes in the dynamics of the 1300 B.P. eruption of Newberry Volcano, USA
Multiple origins of obsidian pyroclasts and implications for changes in the dynamics of the 1300 B.P. eruption of Newberry Volcano, USA
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黑曜石火山碎屑的多重起源及其对距今 1300 年美国纽伯里火山喷发动态变化的影响
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发表时间：
2007
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影响因子：
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通讯作者：
K. Cashman
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作者：
A. Rust;K. Cashman
美国纽伯里火山1300年B.P.喷发的火山碎屑沉积含有少量的黑铁矿(1-6wt.%)。这些碎屑的挥发性(H2O和CO2)含量和结构差异很大。红外光谱分析表明，黑麻岩中的H2O含量在0.1%到1.5%之间，表明平衡压力≤为20 Mpa。二氧化碳含量很低(<；10ppm)，除了碎屑岩中也含有捕虏体粉末，提供了当地的二氧化碳来源。黑铁矿碎屑显示了一系列的颜色和结构类型，随着喷发的进行，这些类型的颜色和结构类型发生了相对比例的变化。总而言之，这些数据表明黑铁矿有多个来源，并且主要来源在喷发期间发生了变化。在喷发早期，黑岩几乎完全是黑色或灰色(含微粒岩)，可能来自充满先锋岩浆或凝灰岩的岩脉或围岩裂隙，随着喷口的扩大，这些岩浆或凝灰岩与围岩一起被侵蚀并并入喷发柱中。在喷发后期，在短暂的活动停止后，黑岩和围岩岩屑的比例增加，新类型的黑岩占主导地位，代表浅部管道堵塞的残余类型，管道内的焊接后备材料，以及来自管道墙壁附近的剪切和脱气岩浆。对保存在黑铁矿中的气泡形状的分析表明，剪应力和剪切率的变化超过两个数量级，根据假设的岩浆温度850°C，最大剪应力和剪切率分别为88−−1和10 kPa2.3。此外，最高的剪切率和应力以及最短的流动时间(10-20 min)保存在也含有围岩的碎屑中。最长的变形时间(5h和8h)对应于两个富含微晶石的碎屑，这表明较高的微晶石含量是由于较慢的上升速率和/或较长的浅层岩浆停留时间所致。纽伯里火山喷发的黑铁矿火山碎屑与莫诺陨石坑的火山碎屑之间的差异可能与为这两个事件提供物质来源的管道的性质有关。通过这一对比，我们认为黑岩可以提供关于碎裂前岩浆上升的时间尺度和机制的信息。
The pyroclastic deposits of the 1300 B.P. eruption of Newberry Volcano, OR, USA, contain minor amounts of obsidian (1–6 wt.%). The volatile (H2O and CO2) contents and textures of these clasts vary considerably. FTIR measurements of H2O in obsidian pyroclasts range from 0.1 to 1.5 wt.% indicating equilibration pressures ≤20 MPa. CO2 contents are low (<10 ppm) except in clasts that also contain xenolith powder that provided a local CO2 source. Obsidian clasts exhibit a range of color and textural types that changed in relative proportion as the eruption progressed. Together these data indicate that there were multiple origins of obsidian and that the dominant source changed during the eruption. Early in the eruption, obsidian was almost entirely black or grey (microlite-bearing) and probably derived from dikes or wall rock fractures filled with vanguard magma or tuffisite that, together with wall rocks, were eroded and incorporated into the eruption column as the vent widened. Later in the eruption, following a brief cessation of activity, the proportion of obsidian to wallrock lithic clasts increased and new types of obsidian dominated, types that represent remnants of a shallow conduit plug, welded fallback material from within the conduit, and sheared and degassed magma from near the conduit walls. Analysis of bubble shapes preserved within obsidian indicates that shear stresses and shear rates varied by over two orders of magnitude, with maxima of 88 kPa and 10−2.3 s−1, respectively, based on an assumed magma temperature of 850°C. Furthermore, the highest shear rates and stresses, and the shortest flow times (10–20 min), are preserved in clasts that also contain wall rock. The longest deformation times (5 and 8 h) correspond to two microlite-rich clasts, suggesting that the higher microlite content results from slower ascent rates and/or longer magma residence times at shallow levels. Differences between obsidian pyroclasts from the Newberry eruption and those of the Mono Craters may relate to the nature of the conduit feeding the two events. From this comparison, we conclude that obsidian can provide information on time scales and mechanisms of pre-fragmentation magma ascent.