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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Fiamme degassing structures and their implications for the post- emplacement temperatures and H_2O contents of high-grade ignimbrites
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Fiamme degassing structures and their implications for the post- emplacement temperatures and H_2O contents of high-grade ignimbrites

机译：Fiamme脱气结构及其对高级火成岩的镶嵌后温度和H_2O含量的影响

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摘要

A new process in post-emplacement ignimbrite degassing is proposed from observations of unusual structures in a high-grade ignimbrite in Sardinia. The structures consist of fiamme decorated above and below by vesicles, termed here Fiamme Degassing Structures (FDS). The relative volumes of vesicles and fiamme have a positive, linear correlation, and the ratio between the volume of vesicles above and below fiamme (asymmetry ratio) is <1 for a significant number of structures. The positive relationship between the volume of fiamme and vesicles implies that the vesicles were produced by the degassing of fiamme post-emplacement. An equal volume of vesicles would be expected above and below fiamme under simple degassing under compression, contrary to observation in many FDS. The low asymmetry ratio is, therefore, related to a secondary process; vesicle loss and capture is proposed as an explanation. A numerical model of vesicle migration within rhyolitic ignimbrites is used to show that vesicles can rise further than the average separation distance between the fiamme, allowing them to be trapped by overlying fiamme. Sufficient migration of vesicles to result in trapping, however, requires a restricted range of H2O contents and temperatures on emplacement (e.g > 1 wt% at <860 degrees C), explaining the rare occurrence of FDS. The H2O contents required to form these structures post-emplacement are relatively high for pyroclasts, suggesting they are indicators of unusually large supersaturation in water resulting from rapid excavation rate. These structures also illustrate that interaction of volatiles with fiamme occurs during post-emplacement degassing; FDS may act as baffles that channel volatiles into fumarole columns, slowing volatile loss. (C) 2019 Elsevier B.V. All rights reserved.
机译：根据对撒丁岛高级火成岩中异常结构的观察，提出了一种新的置放后火成岩脱气方法。该结构由在上方和下方由小泡装饰的菲安姆组成，此处称为菲安姆脱气结构（FDS）。囊泡和fi虫的相对体积具有正线性相关性，并且对于大量结构而言，am囊上方和下方的囊泡体积之比（不对称比）<1。膜丝和囊泡的体积之间呈正相关关系，这意味着囊泡是通过在植入后对膜丝进行脱气而产生的。与在许多FDS中观察到的情况相反，在压缩条件下进行简单脱气时，可以预期在等高的膜上和下有相等体积的囊泡。因此，低不对称率与二次加工有关。提出了囊泡丢失和捕获的解释。流型火成岩中囊泡迁移的数值模型用于表明，囊泡的上升幅度可能会大于膜丝之间的平均分离距离，从而使它们被上层膜丝捕获。囊泡的充分迁移以导致捕集，然而，需要有限的H2O含量和进样温度范围（例如，在<860摄氏度时> 1 wt％），这解释了FDS的罕见发生。火山碎裂后形成这些结构所需的H2O含量相对较高，表明它们是快速开挖速度导致水中异常过饱和的指标。这些结构还说明，在放置后脱气期间会发生挥发物与火焰状分子的相互作用。 FDS可能充当挡板，将挥发物引导至喷气孔中，从而减缓了挥发物的损失。 （C）2019 Elsevier B.V.保留所有权利。

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