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.

是什么控制镁铁质喷发风格？在上升的玄武岩和现场岩浆中耦合囊泡和结晶动力学的原因和后果。涉及镁铁质岩浆的爆炸爆发对活动火山附近的危害产生了重要意义。尤其是，斯特罗姆派爆发会产生大量的细灰，高灰羽和大型分散区域，这些区域可能会严重影响周围的人群。大量研究表明，爆炸性与驱逐气体上升有关，这反过来取决于岩浆的孔隙率 - 渗透性特性和碎裂方式。为了阐明脱气过程及其对喷发风格的影响，建议通过实验研究实验中的渗透性在实验压缩的镁硅硅酸盐融化中的发展，尤其关注结晶在镁铁质岩浆中的作用，以控制爆发风格。具体而言，合成的微层结晶如何影响连接的场地网络和渗透阈值的发展，这些因素如何以喷发风格影响导管和过渡的气体分离？要测试的假设包括：[1]晶体数密度是否与减压率直接相关（即，由于囊泡过程中的含水镁铁质熔体的快速结晶）？ 2）熔体渗透性与岩浆粘度成反比吗？ 3）减压驱动的结晶是否会改变阈值的百分比（即，通过限制气泡膨胀或通过空间控制气泡放置）？ 4）解压缩率是否通过控制脱气效率以及突发性结晶的深度和效率来影响镁铁质爆发的样式？这些问题将通过解压缩实验进行研究，以与Okmok火山（阿拉斯加）爆发的合成基底层融化，以及纯水和混合H2OCO2液相，以检查CO2对结晶和囊泡动力学的影响。将将实验与中元中元中期的天生样本进行比较，后者俄克斯岛中部火山将剧烈的Stromborian与Subplinian和vulcanian喷发沉积物相提并论。 a？premeameter？将构建以测量实验和天然样品的渗透率以估计渗透阈值，结果将用于模拟爆炸性镁铁质喷发期间的气体分离和碎片化。更广泛的影响：该项目将支持博士生和UAF的本科生助理。 UAF阿拉斯加本地科学与工程计划（ANSEP）将宽恕新的链接，目的是雇用阿拉斯加本地本科生（S）为该项目进行研究。该项目还将涉及与火山学家凯西·卡什曼（布里斯托尔大学）的国际合作。 PI与阿拉斯加火山天文台的关联还促进了这项研究的应用，以评估与阿拉斯加和卡斯卡迪亚活跃火山的暴力镁铁质爆发有关的火山危害。在UAF实验性岩石学实验室中开发新的研究设备还将增强UAF的基础设施，并提供新的设备，这些设备可以满足地质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。