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首页> 外文期刊>Journal of Volcanology and Geothermal Research >High and highly variable cooling rates during pyroclastic eruptions on Axial Seamount, Juan de Fuca Ridge
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High and highly variable cooling rates during pyroclastic eruptions on Axial Seamount, Juan de Fuca Ridge

机译：胡安德富卡山脊的轴向海山火山碎屑爆发期间的冷却速率高且变化很大

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摘要

We present a calorimetric analysis of pyroclastic glasses and glassy sheet lava flow crusts collected on Axial Seamount, Juan de Fuca Ridge, NE Pacific Ocean, at a water depth of about 1400 m. The pyroclastic glasses, subdivided into thin limu o Pele fragments and angular, blocky clasts, were retrieved from various strati-graphic horizons of volcanidastic deposits on the upper flanks of the volcanic edifice. Each analysed pyroclastic sample consists of a single type of fragment from one individual horizon. The heat capacity (c_p) was measured via differential scanning calorimetry (DSC) and analysed using relaxation geospeedometry to obtain the natural cooling rate across the glass transition. The limu o Pele samples (1 mm grain size fraction) and angular fragments (0.5 mm grain size fraction) exhibit cooling rates of 10~(43) to 10~(6.0) K s~(-1) and 10~(3.9) to 10~(5.1) K s(-1) respectively. A coarser grain size fraction, 2 mm for limu o Pele and 1 mm for the angular clasts yields cooling rates at the order of 10~(3.7) K s~(-1) The range of cooling rates determined for the different pyroclastic deposits presumably relates to the size or intensity of the individual eruptions. The outer glassy crusts of the sheet lava flows were naturally quenched at rates between 63 K s~(-1) and 10~3 K s~(-1). By comparing our results with published data on the very slow quenching of lava flow crusts, we suggest that (1) fragmentation and cooling appear to be coupled dynamically and (2) ductile deformation upon the onset of cooling is restricted due to the rapid increase in viscosity. Lastly, we suggest that thermally buoyant plumes that may arise from rapid heat transfer efficiently separate clasts based on their capability to rise within the plume and as they subsequently settle from it.
机译：我们提出了量热分析，分析了火山碎屑玻璃和玻璃状薄片熔岩流壳，收集于西北太平洋胡安德富卡岭的轴向海山上，水深约1400 m。火山碎屑玻璃细分为薄的Limu o Pele碎片和成角的块状碎屑，是从火山大厦上翼的火山岩沉积物的各种地层学视野中获得的。每个分析的火山碎屑样本均由来自一个单独层的单一类型的碎片组成。通过差示扫描量热法（DSC）测量热容量（c_p），并使用弛豫地球速度分析法进行分析以获得整个玻璃化转变的自然冷却速率。 Limu o Pele样品（1毫米粒度级）和角碎屑（0.5毫米粒度级）的冷却速率分别为10〜（43）至10〜（6.0）K s〜（-1）和10〜（3.9）分别为10〜（5.1）K s（-1）较粗的晶粒度分数，对于limu o Pele为2 mm，对于角质碎屑为1 mm，冷却速度约为10〜（3.7）K s〜（-1）。与单个喷发的大小或强度有关。片状熔岩流的外部玻璃结壳以63 K s〜（-1）和10〜3 K s〜（-1）之间的速率自然淬火。通过将我们的结果与关于熔岩流壳非常缓慢的淬火的公开数据进行比较，我们建议（1）破碎和冷却似乎是动态耦合的；（2）由于冷却液的快速增加，冷却开始时的延性变形受到限制粘度。最后，我们建议快速传热产生的热浮羽可以有效地将碎屑分离，这取决于碎屑在碎屑中上升以及随后从碎屑中沉降的能力。

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