RAPID: The effusive to explosive transition of La Soufrière, St. Vincent: Insights from petrology and hygrometry

La soufri<e:1>正在进行的喷发为了解和评估火山事件提供了独特的机会，因为它几乎是实时发生和演变的。本次研究的独特之处在于，矿物湿度测定法将被应用于评估喷发前熔体中溶解的水含量，这是由4月9日和10日第一次喷发阶段的矿渣中的斑晶边缘和/或微岩屑的结晶记录的。如果是挥发物的积累导致了火山喷发，人们就会期望看到矿物相记录的水含量反映出这一点。人们还可以通过比较共存矿物的分带剖面，很容易地确定岩浆混合是否是引发喷发的原因，因为岩浆混合和脱气会产生截然不同的模式。利用微晶岩的晶体尺寸分布和结构可以估计减压路径，更好地了解爆炸相的上升速率。喷发前岩浆室的状况也可以估计出来。结合2021年熔岩穹丘或爆炸沉积物中夹带的岩屑的类似数据，可以确定喷发风格差异的原因。这一努力将对监测火山喷发的当局有用。这是一项实质性的国际合作，将资助本科生的研究。2020年12月下旬，加勒比东部圣文森特岛上的层状火山La soufri<e:1>的山顶火山口开始喷出一个熔岩圆顶，自1979年以来一直没有火山活动。2021年4月9日，喷发方式转变为爆发阶段，在喷发的最初几个小时内发生了多次普林尼事件。根据监测正在进行的火山喷发的官方机构UWI地震研究中心（UWI- src）的建议，圣文森特政府在火山喷发前不到24小时发布了疏散红色区域（16000人）的命令。从喷发到爆发的转变是突然的，目前还不清楚是什么机制引发了火山灰喷发阶段，这严重影响了圣文森特的大量火山灰（也影响了邻近的巴巴多斯和圣卢西亚）、火山碎屑密度流和火山泥流。在不断变化的喷发风格中，La soufri<e:1>火山已经通过地震活动、地面变形、气体排放和UWI-SRC的遥感进行了广泛的监测。根据pi在喷发阶段获得的气体排放和水地球化学数据，我们假设岩浆系统没有脱气，导致挥发物积聚，最终引发了火山喷发。利用各种岩石学工具，对这一假设进行检验，并评估未来类似行为的可能性。这一努力将对监测火山喷发的当局有用。这是一项实质性的国际合作，将资助本科生的研究。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The on-going eruption of La Soufrière provides the unique opportunity to understand and evaluate a volcanic event as it is occurring and evolving in almost real-time. Unique to this study, mineral hygrometry will be applied to assess the pre-eruptive water content dissolved in the melt, as recorded by the crystallization of phenocryst rims and/or microlites in scoria from the first eruptive phases on April 9th and 10th. If a buildup of volatiles is driving the eruption, one would expect to see that reflected in the water content recorded by the mineral phases. One can also readily determine if magma mixing was an eruption trigger by comparing the zoning profiles of co-existing minerals, as magma mixing and degassing result in contrasting patterns. Using crystal size distribution and textures of microlites will enable decompression pathways to be estimated and ascent rates of the explosive phase better understood. The magma chamber conditions just prior to the eruption can also be estimated. When combined with similar data from the effusive 2021 lava dome or lithics entrained in the explosive deposits, the causes of the differences in eruptive style can be established. This effort will be useful to the authorities monitoring the eruption. It is a substantive international collaboration and will fund the research of undergraduates.In late December 2020, a lava dome began extruding in the summit crater of La Soufrière, a stratovolcano on the eastern Caribbean island of St. Vincent, after exhibiting no volcanic activity since 1979. On April 9th, 2021, the eruptive style transitioned to an explosive phase, with multiple Plinian events in the first few hours of eruption. Based on the recommendation of the UWI Seismic Research Centre (UWI-SRC), the official agency for monitoring the ongoing eruption, the government of St. Vincent issued an evacuation of the red zone (16,000 people) less than 24 hours prior to the eruption. The transition from effusive to explosive behavior was abrupt and it is unclear what mechanism triggered the ash venting phase, which has severely affected St. Vincent with significant ashfall (also affecting neighboring Barbados and St. Lucia), pyroclastic density currents, and lahars. Throughout the changing eruption styles, La Soufrière has been extensively monitored using seismicity, ground deformation, gas emissions, and remote sensing by the UWI-SRC. Based on gas emission and water geochemistry data obtained by the PIs during the effusive phase, we hypothesize that the magmatic system was not degassing, leading to a buildup of volatiles which eventually triggered the eruption. Using various petrologic tools, this hypothesis shall be tested and the likelihood of similar behavior in the future shall be assessed. This effort will be useful to the authorities monitoring the eruption. It is a substantive international collaboration and will fund the research of undergraduates.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.