The ascent of hydrous magma prior to volcanic eruptions is largely driven by the formation of H 2 O 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 ambient T for analysis. During cooling, fluid vesicles may shrink due to decrease of the molar volume of H 2 O and by resorption of H 2 O back into the melt driven by solubility increase with decreasing T at P < 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% H 2 O, T between 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 temperature T f , where vesicle shrinkage stops, is up to 200 K above the glass transition temperature ( T g ), 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.

中文翻译：

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冷却过程中含水音脂熔体中的囊泡收缩

火山喷发前含水岩浆的上升主要是由 H 2 O 囊泡的形成及其随后进一步减压后的生长所驱动。孔隙度控制浮力以及囊泡聚结和渗透，并且在通过喷发产物的结构分析确定平衡或不平衡脱气之间的差异时很重要。减压实验通常用于模拟岩浆上升。将暴露于高温 (T) 和压力 (P) 的样品减压并迅速冷却至环境 T 以进行分析。在冷却过程中，由于 H 2 O 的摩尔体积减少，以及在 P < 300 MPa 时溶解度随着 T 的降低而增加，H 2 O 重新吸收回到熔体中，因此流体囊泡可能会收缩。在这里，我们量化了囊泡在冷却过程中收缩的程度，使用一系列水合音质熔体减压实验（5.3-3.3 wt% H 2 O，T 在 1323 和 1373 K 之间，从 200 减压到 110-20 MPa）。大多数样品在减压期间在接近平衡的条件下脱气。然而，淬火样品的孔隙率明显低于冷却前的预期平衡孔隙率。在 44 K·s -1 的冷却速率下，泡囊收缩停止的假想温度 T f 比玻璃化转变温度 (T g ) 高 200 K。此外，降低冷却速率会增强泡囊收缩。我们评估了这些发现对先前使用音质熔体的实验脱气研究的影响，并强调了在尝试与天然火山喷发物进行任何比较之前正确解释实验孔隙度数据的重要性。