Magma ascent and eruption in the Aleutian arc

阿留申弧区岩浆上升和喷发阿留申弧区是世界上火山活动最活跃的地区之一。阿留申火山频繁喷发的高大火山灰云对当地社区和来自北美、欧洲和亚洲的旅行者构成了重大威胁。为了减轻这种危害，阿拉斯加火山观测站(AVO)利用地震数据、大地测量数据和卫星图像对阿拉斯加火山进行了广泛监测。这些技术在很大程度上成功地预测了火山喷发；然而，喷发的前兆信号(根据地震和大地测量数据推断)与喷发风格之间的联系却知之甚少。被认为在控制火山喷发活力方面发挥作用的因素包括岩浆的挥发性含量、火山管道中岩浆的上升速度以及岩浆的温度。这些参数很难直接测量，但可以从对喷发物质的分析中推断出来。这项拟议工作的目标是测量三座阿留申弧形火山喷发的晶体和玻璃中水和其他化学物种的梯度。这些化学梯度是岩浆上升、脱气和冷却的标志。最近开发的解释这种化学梯度的技术将被用来限制岩浆在喷发前几分钟到几个小时内的上升速度和温度。这项研究整合了最近发展起来的岩石学方法，以确定三座阿留申弧形火山(Okmok、Sguam和Pavlof)喷发的岩浆的上升速率和协同热历史，这些火山的爆炸性(火山爆炸性指数为1？3)。三次喷发的岩浆上升速率将使用三种独立的技术计算：(1)不同大小的橄榄石寄主熔体包裹体的水分损失；(2)橄榄石中的水分带；(3)单斜辉石中的水分带。上升和喷发期间的岩浆冷却历史将使用橄榄石承载的熔体包裹体中的氧化镁浓度梯度来计算。在不同喷发风格的火山样本上整合这些技术，将能够识别岩浆挥发分浓度、上升速度、热历史和喷发风格之间的关系。拟议的分析将加强我们对橄榄石熔体包裹体和更广泛的弧形火山中水和其他挥发性元素行为的理解，并将使我们能够更可靠和准确地确定橄榄石熔体包裹体喷发前的挥发性浓度。

Magma ascent and eruption in the Aleutian arcThe Aleutian arc is one of the most volcanically active regions of the world. Frequent eruptions of tall ash clouds by Aleutian volcanoes present a significant hazard to local communities and to travelers flying from North America, Europe, and Asia. In order to mitigate this hazard, Alaskan volcanoes are extensively monitored by the Alaska Volcano Observatory (AVO), using seismic data, geodetic data and satellite imagery. These techniques are largely successful in enabling the prediction of volcanic eruptions; however, the links between precursor signals of an eruption (as inferred from seismic and geodetic data) and eruptive style are poorly understood. Factors that are thought to play a role in controlling the vigor of a volcanic eruption include the volatile content of the magma, the ascent rate of the magma in the volcanic conduit, and the temperature of the magma. These parameters are difficult to measure directly, but they can be inferred a posteriori from the analysis of erupted materials. The goal of the proposed work is to measure gradients of water and other chemical species in erupted crystals and glasses from three Aleutian arc volcanoes. These chemical gradients are signatures of magma ascent, degassing, and cooling. Recently developed techniques for interpreting such chemical gradients will be used to constrain ascent rates and temperatures of magmas in the minutes to hours prior to their eruption. Such measurements will further our understanding of processes operating in the volcanic conduit in the build-up to an eruption, and will provide a physical framework in which to place real-time volcano monitoring data.The proposed study integrates recently developed petrologic methods for determining the ascent rates and syneruptive thermal histories of magmas erupted at three Aleutian arc volcanoes (Okmok, Seguam, and Pavlof) with contrasting explosivities (Volcanic Explosivity Index, VEI, of 1 ? 3). Magma ascent rates for each of the three eruptions will be calculated using three independent techniques: (1) water loss from olivine-hosted melt inclusions of different sizes; (2) water zonation in olivine; and (3) water zonation in clinopyroxene. Magma cooling histories during ascent and eruption will be calculated using MgO concentration gradients inside olivine-hosted melt inclusions. Integration of these techniques on samples from volcanoes with different eruptive styles will allow identification of relationships between magma volatile concentrations, ascent rate, thermal history, and eruptive style. The proposed analyses will enhance our understanding of the behavior of water and other volatile elements in olivine-hosted melt inclusions and in arc volcanoes more broadly, and they will enable more reliable and accurate determination of pre-eruptive volatile concentrations from olivine-hosted melt inclusions.