Abstract Open vent basaltic volcanoes account for a substantial portion of the global atmospheric outgassing flux, largely through passive degassing and mild explosive activity. We present volcanic gas flux and composition data from Yasur Volcano, Vanuatu collected in July 2018. The average volcanic plume chemistry is characterised by a mean molar CO2/SO2 ratio of 2.14, H2O/SO2 of 148 and SO2/HCl of 1.02. The measured mean SO2 flux in the period of 6th to 9th July is 4.9 kg s−1. Therefore, the mean fluxes of the other species are 7.5 kg∙s−1 CO2, 208 kg∙s−1 H2O and 4.8 kg∙s−1 HCl. The degassing regime at Yasur volcano ranges from ‘passive’ to ‘active’ styles, with the latter including Strombolian activity and spattering. Gases emitted during active degassing are enriched in SO2 over HCl and CO2 over SO2 relative to passive degassing, with CO2/SO2 ratios of 2.85 ± 0.17, SO2/HCl of 1.6 ± 0.22, and H2O/SO2 of 315 ± 78.8. Gases emitted during passive degassing have CO2/SO2 ratios of 1.96 ± 0.12, SO2/HCl of 0.50 ± 0.07 and H2O/SO2 of 174 ± 43.5. We use a model of volatile degassing derived from melt inclusion studies (Metrich et al., 2011), combined with our observations of chemical variations in the outgassing bubbles to propose a mechanism for magma degassing in the conduit at Yasur. We envisage a shallow conduit filled with crystal-rich magma, forming a viscous and mobile plug that develops an effective yield strength from the surface to a depth of at least 2000 m, in which bubbles are trapped, grow, ascend towards the surface and burst in a typical Strombolian eruption. Deeper bubbles released during active degassing are enriched in CO2 and SO2 compared to bubbles released during ‘passive degassing’, which are sourced from close to the surface, and are, consequently, HCl-rich.

中文翻译：

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Yasur 火山（瓦努阿图）的 Strombolian 喷发和岩浆脱气动力学

摘要 开放式玄武岩火山占全球大气释气通量的很大一部分，主要是通过被动脱气和轻度爆炸活动。我们提供了 2018 年 7 月从瓦努阿图亚苏尔火山收集的火山气体通量和成分数据。平均火山羽流化学特征为平均摩尔 CO2/SO2 比为 2.14，H2O/SO2 为 148，SO2/HCl 为 1.02。7 月 6 日至 9 日期间测得的平均 SO2 通量为 4.9 kg s-1。因此，其他物种的平均通量为 7.5 kg∙s−1 CO2、208 kg∙s−1 H2O 和 4.8 kg∙s−1 HCl。Yasur 火山的脱气机制范围从“被动”到“主动”，后者包括斯特龙博利活动和飞溅。与被动脱气相比，主动脱气过程中排放的气体在 HCl 上富含 SO2，在 SO2 上富含 CO2，CO2/SO2 比率为 2.85 ± 0.17，SO2/HCl 为 1.6 ± 0.22，H2O/SO2 比率为 315 ± 78.8。被动脱气过程中排放的气体的 CO2/SO2 比率为 1.96 ± 0.12，SO2/HCl 为 0.50 ± 0.07，H2O/SO2 比率为 174 ± 43.5。我们使用源自熔体包裹体研究的挥发性脱气模型（Metrich 等，2011），结合我们对脱气气泡中化学变化的观察，提出了 Yasur 管道中岩浆脱气的机制。我们设想了一个充满富含晶体的岩浆的浅管道，形成一个粘性且可移动的塞子，从地表到至少 2000 m 的深度产生有效的屈服强度，气泡在其中被捕获、生长、上升到地表并破裂在典型的斯特龙博利火山喷发中。