Vesicle shrinkage in hydrous phonolitic melt during cooling
Vesicle shrinkage in hydrous phonolitic melt during cooling
复制标题
水合酚醛树脂熔体冷却过程中的囊泡收缩
DOI：
10.1007/s00410-020-1658-3
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发表时间：
2020
期刊：
影响因子：
--
通讯作者：
Nowak M
中科院分区：
文献类型：
--
作者：
Allabar A;Dobson;Bauer CC;Nowak M
在火山爆发之前，含水岩浆的上升主要是由H2O囊泡的形成及其随后在进一步减压时的生长所驱动的。孔隙度控制浮力以及囊泡聚结和渗流，并且在从喷发产物的结构分析中识别平衡或不平衡脱气之间的差异时是重要的。减压实验通常用于模拟岩浆上升。将暴露于高温（T）和高压（P）的样品减压并快速冷却至环境温度T以进行分析。在冷却期间，流体囊泡可能由于H2O的摩尔体积的减小以及由于随着TatP < 300 MPa降低溶解度增加而驱动的H2O再吸收回到熔体中而收缩。在这里，我们量化的程度，在冷却过程中，囊泡收缩，使用一系列的减压实验与含水phonolitic熔体（5.3-3.3重量% H2O，T之间1323和1373 K，从200到110-20 MPa减压）。大多数样品在减压过程中在接近平衡的条件下脱气。然而，淬火样品的孔隙率显着低于冷却前的预期平衡孔隙率。在44 K·s-1的冷却速率下，囊泡收缩停止的假想温度Tf比玻璃化转变温度（Tg）高出200 K。我们评估这些研究结果对以前的实验脱气研究使用响岩熔体的影响，并强调正确解释实验孔隙度数据的重要性，在任何比较天然火山喷出物可以尝试。
The ascent of hydrous magma prior to volcanic eruptions is largely driven by the formation of H2O vesicles and their subsequent growth upon further decompression. Porosity controls buoyancy as well as vesicle coalescence and percolation, and is important when identifying the differences between equilibrium or disequilibrium degassing from textural analysis of eruptive products. Decompression experiments are routinely used to simulate magma ascent. Samples exposed to high temperature (T) and pressure (P) are decompressed and rapidly cooled to ambientTfor analysis. During cooling, fluid vesicles may shrink due to decrease of the molar volume of H2O and by resorption of H2O back into the melt driven by solubility increase with decreasingTatP< 300 MPa. Here, we quantify the extent to which vesicles shrink during cooling, using a series of decompression experiments with hydrous phonolitic melt (5.3–3.3 wt% H2O,Tbetween 1323 and 1373 K, decompressed from 200 to 110–20 MPa). Most samples degassed at near-equilibrium conditions during decompression. However, the porosities of quenched samples are significantly lower than expected equilibrium porosities prior to cooling. At a cooling rate of 44 K·s−1, the fictive temperatureTf, where vesicle shrinkage stops, is up to 200 K above the glass transition temperature (Tg), Furthermore, decreasing cooling rate enhances vesicles shrinkage. We assess the implications of these findings on previous experimental degassing studies using phonolitic melt, and highlight the importance of correctly interpreting experimental porosity data, before any comparison to natural volcanic ejecta can be attempted.
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