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首页> 外文期刊>Journal of Volcanology and Geothermal Research >The role of water in cooling ignimbrites
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The role of water in cooling ignimbrites

机译：水在冷却火成岩中的作用

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A summary of observational literature on ignimbrites provides the basis for the development of a two-dimensional numerical model of ignimbrite cooling processes. Factors include emplacement conditions, post-emplacement processes, and the nature and timing of interactions with water during cooling. The model uses the multiphase finite element heat and mass transfer (FEHM) code, which has been enhanced to handle conditions up to 1500℃. The instantaneous emplacement of a 750℃ ignimbrite with internal gas pressures of up to 0.5 MPa (lithostatic) has a great effect on the variably saturated substrate. A water table present within a few tens of meters of the base of the ignimbrite produces a region of high pressure and temperature that exists for about 20 years, driving vapor upward through the ignimbrite as diffuse flow and in gas escape structures and enhancing cooling at the base of the ignimbrite. Variations in initial gas pressure between atmospheric and lithostatic conditions have little effect on the thermal evolution. The results of the numerical modeling of 20- and 40-m-thick ignimbrites indicate that, even for moderate pore water saturations in the substrate, vaporization and resultant pressurization may exceed lithostatic confining pressures in the upper substrate and basal ignimbrite, and explosive pressure release may occur, resulting in the development of discrete fumarole conduits or phreatic explosions. The likelihood for explosive pressure release appears to be greater when the nominal ignimbrite thickness is on the order of the depth of a buried valley. The pressure buildup is enhanced by the geometry of the ignimbrite-substrate interface, especially at convex corners such as on the edges of a buried valley. The boiling zones at the top and bottom of a cooling ignimbrite involve the development of a heat-pipe, which provides an efficient means of transporting heat from the superheated tephra out tens of meters into the ambient environment. The predicted temporal evolution of temperature, pressure, and vapor flow in a 40-m ignimbrite support the conceptual model of degassing, welding and compaction, devitrification, and alteration occurring concomitantly in the first several years after emplacement and driven in part by production and migration of meteoric steam. This vapor flowing through the ignimbrite matrix at 5x10~(-5) kg s~(-1) in the first 10 years enhances devitrification in any part of the ignimbrile above the base in nonwelded deposits. In the case where welding occurs, lower permeability limits the diffuse flow of gas upward through the ignimbrite from the region of boiling and pore pressurization at the base, and enhanced devitrification in the basal parts of the ignimbrite may occur where pore vapors circulate in abundance. Immediately above the welded zone, a devitrified horizon may develop where the upper boiling/condensation zone and perched meteoric infiltration results in enhanced saturations.
机译：关于火成岩的观测文献的总结为发展火成岩冷却过程的二维数值模型提供了基础。影响因素包括安置条件，安置后过程以及冷却过程中与水相互作用的性质和时间。该模型使用多相有限元传热传质（FEHM）代码，该代码已得到增强，可以处理高达1500℃的条件。内部气体压力高达0.5 MPa（静压）的750℃燃烧物的瞬时放置对可变饱和的基材有很大的影响。距火成岩底部几十米内的地下水位产生的高压和高温区域存在约20年，以扩散流的形式驱使蒸气向上通过火成岩并进入气体逸出结构，并增强了冷却效果。着火的基础。大气和岩石静力学条件之间的初始气压变化对热演化影响很小。 20和40 m厚火成岩的数值模拟结果表明，即使基质中孔隙水饱和度适中，汽化和最终加压也可能超过上基质和基底着火岩中的岩石静压限制压力，并释放爆炸性压力可能会发生，导致形成不连续的喷气孔导管或潜水爆炸。当标称火成岩厚度约为埋谷深度时，爆炸压力释放的可能性似乎更大。易燃物-基底界面的几何形状增强了压力积累，尤其是在凸角处，例如在埋谷的边缘上。冷却火成岩顶部和底部的沸腾区涉及热管的发展，该热管提供了一种将热量从过热的特弗拉传递数十米到周围环境的有效手段。在40 m的火成岩中温度，压力和蒸汽流的预测时间演变支持脱气，焊接和压实，失透和蚀变的概念模型，该模型在就位后的头几年同时发生，并且部分受到生产和迁移的驱动流蒸汽。在最初的10年中，这种蒸汽以5x10〜（-5）kg s〜（-1）的速度通过火成岩基质，从而增强了非焊接沉积物中基体上方任何一部分火成岩的失透性。在发生焊接的情况下，较低的磁导率会限制气体从底部的沸腾和孔隙加压区域向上穿过火成岩的扩散流，并且在火成岩的基础部分中可能发生增强的失透，这是因为孔隙蒸气大量循环。在焊接区的正上方，可能会出现失透的层位，在该处较高的沸腾/冷凝区和栖息的陨石渗透导致饱和度增加。

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