What Controls Mafic Eruption Style? The Causes and Consequences of Coupled Vesiculation and Crystallization Kinetics in Ascending Basaltic Andesite Magmas

什么控制镁铁质喷发类型？

• 批准号: 1145194
• 负责人: Jessica Larsen
• 金额: $ 39.97万
• 依托单位: University of Alaska Fairbanks Campus
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-01-15 至 2016-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1145194&HistoricalAwards=false
• 关键词: What; Controls; Mafic; Eruption; Style

What controls mafic eruption style? The causes and consequences of coupled vesiculation and crystallization kinetics in ascending basaltic andesite magmas. Explosive eruptions involving mafic magmas pose important implications for hazards near active volcanoes. In particular, Strombolian eruptions produce large volumes of fine ash, high ash plumes, and large dispersal areas that may severely impact surrounding populations. Numerous studies suggest that explosivity is related to expulsion of gases from ascending, decompressing magmas that in turn depends on porosity-permeability properties of the magma and fragmentation style. In an attempt to elucidate the degassing process and its effect on eruptive style, it is proposed to investigate experimentally the development of permeability in experimentally decompressed mafic silicate melts with particular focus on the role of crystallization in mafic magmas as a control on eruptive style. Specifically, how does syn-ascent microlite crystallization influence development of connected vesicle networks and the percolation threshold, and how do these factors influence gas separation in the conduit and transitions in eruptive style? Hypotheses to be tested include: [1] Are crystal number densities related directly to decompression rate (i.e., due to rapid crystallization of hydrous mafic melts during vesiculation)? 2) Is melt permeability inversely related to magma viscosity? 3) Does decompression­driven crystallization change the percolation threshold (i.e., by limiting bubble expansion or by spatial control on bubble placement)? 4) Does decompression rate affect the style of mafic eruptions by controlling degassing efficiency and the depth and efficiency of syn­eruptive crystallization? These questions will be investigated via decompression experiments on synthetic basaltic andesite melts similar in composition to those erupted at Okmok volcano (Alaska), and with both pure water and mixed H2O­CO2 fluid phase to examine the influence of CO2 on crystallization and vesiculation kinetics. The experiments will be compared with natural samples from the mid­Holocene Middle Scoria Okmok volcano, which produced violent strombolian to subplinian and vulcanian eruption deposits. A ?permeameter? will be constructed to measure permeabilities of both experimental and natural samples to estimate percolation thresholds, and the results will be used to model gas segregation and fragmentation during explosive mafic eruptions. Broader Impacts: This project will support a PhD student, and undergraduate student assistant at UAF. A new link will be forged with the UAF Alaska Native Science and Engineering Program (ANSEP) with the aim to hire Alaska Native undergraduate student(s) to perform research for the project. The project will also involve international collaboration with volcanologist Kathy Cashman (University of Bristol). The PI?s association with the Alaska Volcano Observatory also facilitates application of this research to assessment of volcanic hazards related to violent mafic eruptions at active volcanoes in Alaska and Cascadia. Development of new research equipment in the UAF Experimental Petrology lab will also enhance the infrastructure at UAF and provide new equipment that could fill research needs beyond geology and volcanology studies, including applications in other physical sciences and engineering disciplines.

是什么控制了镁铁质喷发方式？上升玄武岩安山岩岩浆中气泡作用和结晶动力学耦合的原因和后果。涉及镁铁质岩浆的爆炸性喷发对活火山附近的危险产生了重要影响。特别是，斯特龙博利亚火山喷发产生大量细灰、高火山灰羽流和大片散布区域，这可能会严重影响周围的人口。许多研究表明，爆炸性与从上升的减压岩浆中排出气体有关，而这反过来又取决于岩浆的孔隙度-渗透性属性和破碎方式。为了阐明脱气过程及其对喷发样式的影响，建议对实验减压镁铁质硅酸盐熔体的渗透性发展进行实验研究，特别是关注镁铁质岩浆中的结晶对喷发样式的控制作用。具体地说，同向上升的微晶结晶如何影响连通的囊泡网络的发展和渗流阈值，以及这些因素如何影响管道中的气体分离和喷发式的转变？有待检验的假设包括：[1]晶体数密度是否与减压速率直接相关(即，由于含水镁铁质熔体在气泡形成过程中快速结晶)？2)熔体渗透率是否与岩浆粘度成反比？3)减压驱动结晶是否改变了渗流阈值(即，通过限制气泡膨胀或通过空间控制气泡的位置)？4)减压速率是否通过控制脱气效率和同步结晶的深度和效率来影响镁铁质喷发的类型？这些问题将通过对成分类似于阿拉斯加奥克莫克火山喷发的合成玄武岩安山岩熔体的减压实验来研究，并用纯水和混合H2O-CO2流体相来研究二氧化碳对结晶和泡化动力学的影响。这些实验将与全新世中期中斯科利亚奥克莫克火山的自然样本进行比较，该火山产生了猛烈的斯特龙博尔式到次普里伯里期和火山岩喷发沉积物。渗透仪？将被建造来测量实验和自然样品的渗透率，以估计渗流阈值，结果将被用于模拟爆炸镁铁质喷发期间的气体分离和破碎。更广泛的影响：该项目将支持UAF的一名博士生和本科生助理。将与UAF阿拉斯加原住民科学与工程项目建立新的联系，目的是聘请阿拉斯加原住民本科生(S)为该项目进行研究。该项目还将涉及与布里斯托尔大学火山学家凯西·卡什曼的国际合作。派？S与阿拉斯加火山观测站的合作也促进了这项研究在评估与阿拉斯加和卡斯卡迪亚活火山猛烈的镁铁质喷发有关的火山灾害方面的应用。UAF实验岩石学实验室新研究设备的开发还将加强UAF的基础设施，并提供新设备，以满足地质学和火山学研究以外的研究需求，包括在其他物理科学和工程学科中的应用。