Experimental investigation of the effect of bulk composition on the H2O bubble formation in hydrous silicate melts

含水岩浆量超过100立方公里的大规模火山爆发是重大的自然灾害，其发生间隔可能超过100年。火山灰和含硫气体的排放在全球范围内显著降低了地表温度。这种爆发性火山爆发的发生是由于压力释放过程中岩浆中大量H2O气泡的快速形成和生长。这些喷发的特点是岩浆破碎成浮石和大量的细灰颗粒，气泡大小呈双峰分布。从流纹岩、响岩、响岩-软玉、安山岩和英安岩等各种成分的含水岩浆倾向于爆发。在减压实验的基础上，确定了含水硅酸盐熔体中H_2O气泡的两种形成机理。在含水流纹质熔体中形成的具有宽尺寸分布的气泡的数密度随着减压速率的增加而增加，这表明气泡的成核。相反，在响岩熔体中形成的几乎相同大小的H2O气泡的数密度保持独立的减压速率，并与自发相分离的模型是一致的。H2O气泡形成机制对本体熔体组成的依赖性还没有很好地理解。除流纹岩和响岩外，体相成分中气泡形成机制的部分未解决可能是由于以前研究中实验结果的不一致。这可能与罕见的实验数据和不同的实验方案有关。为了阐明气泡的形成机理，将对含水音纹质、安山质和英安质熔体进行减压实验。将确定以下参数：（1）气泡数密度和尺寸分布，其指示成核或自发相分离，（2）气泡形成所需的压力释放，和（3）在成核的情况下，表面张力σ，以及（4）囊泡玻璃的残余H2O含量。收集到的数据将有助于回答基本问题的H2O气泡形成的动力学，预计将取决于硅酸盐熔体组合物。这些数据，加上以前发表的流纹岩和响岩熔体的数据，将改善泡沫形成的建模。预期的结果将有助于量化所需的压力释放和减压速率范围，以形成双峰气泡尺寸群体，其中在少数较大气泡之间具有许多小气泡。这种结构通常保存在爆炸喷出的火山浮石和灰烬中，具有各种散装成分。基于这些数据的气泡形成模型是喷发动力学建模的重要组成部分，这对于爆炸性火山系统的风险评估至关重要。

Large explosive volcanic eruptions of hydrous magma volumes exceeding 100 km3 are significant natural hazards, which may occur with a recurrence interval of over 100 years. The emission of ash and sulfur gases are suggested to significantly decrease the surface temperature on a global scale. The occurrence of such explosive volcanic eruptions is attributed to the rapid formation and growth of numerous H2O bubbles in the magma during pressure release. These eruptions are characterized by the fragmentation of magma into pumice and large amounts of fine ash particles, with a bimodal distribution of bubble sizes. Hydrous magmas with various compositions ranging from rhyolite, phonolite, phono-tephrite, andesite, and dacite tend to erupt explosively. Based on decompression experiments, two H2O-bubble formation mechanisms of hydrous silicate melt have been identified. The number density of bubbles with a broad size distribution formed in hydrous rhyolitic melt increases with decompression rate, suggesting nucleation of bubbles. Conversely, the number density of nearly equally sized H2O bubbles formed in phonolitic melt remains independent of the decompression rate and is consistent with the model of spontaneous phase separation. The dependence of H2O bubble formation mechanisms on bulk melt composition is not well understood. Part of the unresolved mechanisms of bubble formation in bulk compositions other than rhyolite and phonolite may be due to the inconsistency of experimental results from previous studies. This may be related to rare experimental data and different experimental protocols. To characterise the bubble formation mechanisms, hydrous phono-tephritic, andesitic, and dacitic melts will be decompressed experimentally. The following parameters will be determined (1) the bubble number density and size distribution, which are indicative of nucleation or spontaneous phase separation, (2) the pressure release required for bubble formation, and (3) in case of nucleation, the surface tension σ, as well as (4) the residual H2O contents of the vesiculated glasses. The collected data will help to answer fundamental questions on the dynamics of H2O bubble formation, which is expected to depend on silicate melt composition. These data, together with previously published data for rhyolitic and phonolitic melts, will improve the modelling of bubble formation. The expected results will help to quantify the required pressure release and the decompression rate ranges for the formation of bimodal bubble size populations with numerous small bubbles between few larger bubbles. This texture is often preserved in explosively ejected volcanic pumice and ashes with a variety of bulk compositions. The bubble formation model based on these data is an important part of the modelling of the eruption dynamics, which is essential for the risk assessment of explosive volcanic systems.