Supersaturation of H2O during magma ascent leads to degassing of melt by formation and growth of vesicles that may power explosive volcanic eruptions. Here, we present experiments to study the effect of initially dissolved H2O concentration (cH2Oini) on vesicle formation, growth, and coalescence in phonolitic melt. Vesuvius phonolitic melts with cH2Oini ranging between 3.3 and 6.3 wt% were decompressed at rates of 1.7 and 0.17 MPa·s−1 and at temperatures ≥ 1323 K. Decompression started from 270 and 200 MPa to final pressures of 150–20 MPa, where samples were quenched isobarically. Optical microscopy and Raman spectroscopic measurements confirm that the glasses obtained were free of microcrystals and Fe-oxide nanolites, implying that the experiments were superliquidus and phase separation of the hydrous melt was homogeneous. A minimum number of the initially formed vesicles, defined by the number density normalized to vesicle-free glass volume (VND), is observed at ~ 5 wt% cH2Oini with a logVND of ~ 5 (in mm−3). The logVND increases strongly towards lower and higher cH2Oini by one order of magnitude. Furthermore, an important transition in evolution of vesiculation occurs at ~ 5.6 wt% cH2Oini. At lower cH2Oini, the initial VND is preserved during further decompression up to melt porosities of 30–50%. At higher cH2Oini, the initial vesicle population is erased at low melt porosities of 15–21% during further decompression. This observation is attributed to vesicle coalescence favored by low melt viscosity. In conclusion, cH2Oini determines the VND of initial phase separation and the evolution of vesiculation during decompression that controls the style of volcanic eruptions.

中文翻译：

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初始 H2O 浓度对含水音质熔体减压诱导相分离和脱气的影响

岩浆上升过程中 H2O 的过饱和导致熔体脱气，形成和生长的囊泡可能为火山爆发提供动力。在这里，我们提出了一些实验来研究初始溶解的 H2O 浓度 (cH2Oini) 对音素熔体中囊泡形成、生长和聚结的影响。具有 3.3 至 6.3 wt% 的 cH2Oini 的维苏威火山岩溶体以 1.7 和 0.17 MPa·s-1 的速率和温度≥ 1323 K 减压。减压从 270 和 200 MPa 开始，最终压力为 150-20 MPa，其中样品被等压淬灭。光学显微镜和拉曼光谱测量证实所获得的玻璃不含微晶和氧化铁纳米晶，这意味着实验是超液相线并且含水熔体的相分离是均匀的。初始形成的囊泡的最小数量，由归一化为无囊泡玻璃体积 (VND) 的数密度定义，在 ~ 5 wt% cH2Oini 下观察到，logVND 为 ~ 5（mm-3）。logVND 向较低和较高的 cH2Oini 强烈增加一个数量级。此外，在~ 5.6 wt% cH2Oini 处发生囊泡演变的重要转变。在较低的 cH2Oini 下，初始 VND 在进一步减压至 30-50% 的熔体孔隙率期间得以保留。在较高的 cH2Oini 下，在进一步减压期间，初始囊泡数量会在 15-21% 的低熔体孔隙率下消失。该观察结果归因于低熔体粘度有利于囊泡聚结。综上所述，