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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Diffusive Dehydration And Bubble Resorption During Open-system Degassing Of Rhyolitic Melts
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Diffusive Dehydration And Bubble Resorption During Open-system Degassing Of Rhyolitic Melts

机译：流纹熔体的开放系统脱气过程中的扩散脱水和气泡吸收

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Degassing through open paths such as bubble and/or fracture networks is considered to be the principal mode of degassing in silicic magmas. However, its detailed mechanisms remain unclear. To investigate the behavior of bubbles in a hypothetical open-system condition, we performed a series of vesiculation experiments on natural rhyolitic obsidian using a newly designed semipermeable cell, which artificially maintains a pressure difference between its inside and outside. The thick-wall cell maintains a constant volume within the sample chamber, while allowing water vapor to escape the cell during the experimental runs. The cells containing obsidian cores with ca. 0.66 wt.% initial water content were externally heated to 1000 ℃ for a period of 1-288 h. The run charges generally showed a zonal structure composed of two contrasting regions: a central region within which the bubbles were concentrated (bubble-rich core, BC) and a bubble-free melt region surrounding the BC (bubble-free margin, BFM). With increased heating duration, the thickness of the BFM increased via dissolution of the outermost bubbles in the BC. The water content was nearly uniform throughout the BC, whereas an outward-decreasing gradient was observed in the BFM. We found that diffusive dehydration occurred from the sample surface, and the bubbles were resorbed into the melt. Thus, the BFM-BC boundary moved inwards. These processes were modelled numerically, and the calculation results were in good agreement with the experimental data. If a "lifetime" of open paths is approximated as the relaxation time of a melt in a shallow volcanic environment, then the paths have to be pinched off quickly (1.2 h at maximum pressure difference between the open path and the melt) and thus the thickness of the bubble-free layer reaches at most -0.1 mm. For the formation of bubble-free obsidian layers with a width of a few millimeters, which are often observed in natural obsidian flows, open paths should be maintained for at least a few hundred hours.
机译：通过诸如气泡和/或裂缝网络之类的开放路径进行脱气被认为是硅质岩浆中脱气的主要方式。但是，其详细机制仍不清楚。为了研究在假设的开放系统条件下气泡的行为，我们使用新设计的半渗透性细胞对天然流纹黑曜石进行了一系列囊泡化实验，该人工渗透性维持了其内部和外部之间的压力差。厚壁样品池在样品室内保持恒定的体积，同时在实验过程中允许水蒸气逸出样品池。含有黑曜石核心的细胞约。将0.66 wt。％的初始水分在外部加热至1000℃，持续1-288小时。运行进料通常显示出由两个对比区域组成的带状结构：气泡集中的中央区域（富气泡核心，BC）和围绕BC的无气泡熔体区域（无气泡边界，BFM）。随着加热持续时间的增加，BFM的厚度通过BC中最外层气泡的溶解而增加。在整个卑诗省，水含量几乎均匀，而在BFM中观察到了向外减少的梯度。我们发现从样品表面发生扩散性脱水，气泡重新吸收到熔体中。因此，BFM-BC边界向内移动。对这些过程进行了数值建模，计算结果与实验数据吻合良好。如果将开放路径的“寿命”估算为浅火山环境中熔体的弛豫时间，则必须迅速切断路径（在开放路径与熔体之间的最大压差下为1.2小时），因此无气泡层的厚度最大为-0.1mm。为了形成通常在天然黑曜石流中观察到的，宽度为几毫米的无气泡黑曜石层，应保持开放路径至少几百小时。

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