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

含水岩浆体积超过100 km3的大型爆发性火山喷发是重大的自然灾害，其复发周期可能超过100年。在全球范围内，烟尘和硫磺气体的排放将显著降低地表温度。这种爆发性火山爆发的发生是由于岩浆在压力释放过程中迅速形成和增长了大量的H2O气泡。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。含水岩浆的成分多种多样，包括流纹岩、phonolite、phono-tephrite、安山岩和英安岩。在减压实验的基础上，确定了含水硅酸盐熔体的两种水泡形成机制。含水流纹岩熔体中气泡的数量密度随着减压速率的增加而增加，气泡的大小分布广泛，表明气泡有成核作用。相反，在声olitic熔体中形成的几乎相等大小的H2O气泡的数量密度与减压速率无关，与自发相分离模型一致。H2O气泡形成机制与体熔体成分的关系尚不清楚。在流纹岩和phonolite以外的大块成分中气泡形成的部分未解机制可能是由于先前研究的实验结果不一致。这可能与实验数据少、实验方案不同有关。为了描述气泡的形成机制，我们将对含水音质、安山岩和英安岩熔体进行减压实验。将确定以下参数：(1)气泡数、密度和大小分布，它们表示成核或自发相分离；(2)气泡形成所需的压力释放；(3)成核时的表面张力σ；以及(4)泡化玻璃的残余水含量。收集到的数据将有助于回答有关水泡形成动力学的基本问题，预计这取决于硅酸盐熔体成分。这些数据，连同先前发表的流纹岩和声纹岩熔体的数据，将改进气泡形成的模型。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，以形成双峰气泡大小种群，在几个大气泡之间有许多小气泡。这种结构通常保存在爆炸喷出的火山浮石和各种大块成分的灰烬中。基于这些数据建立的气泡形成模型是火山喷发动力学建模的重要组成部分，对火山喷发系统的风险评估具有重要意义。

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.