Experimental Constraints on Gas Permeability Development in Hydrous Intermediate Magmas: Implications for Explosive Versus Effusive Eruption Styles

地球上的许多活火山都会产生猛烈的喷发，将火山灰送入大气，造成危险的现象，威胁到飞机、人和基础设施。美国拥有一些最近活跃的、具有潜在危险的火山，其中大多数位于阿拉斯加。喷发中等二氧化硅成分岩浆的火山在阿拉斯加很常见(例如，奥古斯丁)和世界其他地方。它们通常是富含水和晶体的，经常活跃，并产生在小熔岩穹顶和猛烈的、产生火山灰的火山式爆炸之间循环的喷发。岩浆随着气泡的逸出、长大、结合成更大的气泡而释放，然后连接在一起，形成通过岩浆的可渗透通道，从而允许天然气外泄。爆炸性喷发背后的驱动力是，岩浆在管道中上升时释放出的气体压力是多么容易，这与岩浆上升到地表的速度相平衡。先前的结果表明，随着岩浆晶体含量的增加，固体晶体可以通过允许气泡连接和岩浆在较低的气体含量下变得可渗透来改变脱气过程，尽管发生这种情况的机制尚不清楚。这项研究的主要目标是利用与次火山管道系统中岩浆上升条件近似的实验，检查和量化晶体含量如何影响岩浆脱气。这些实验将被设计成普遍适用于世界上任何地方的中等成分火山。然而，实验结果将通过应用于2006年阿拉斯加奥古斯丁火山喷发与该喷发的自然样本进行比较来检验。这项研究将使地球科学家能够更好地了解俯冲带火山中富含晶体的中等成分岩浆中经常出现的喷发方式的机制，比如阿拉斯加和世界其他地方的火山。这项研究将使用高压、高温、冷密封减压实验来研究晶体数量和/或基质熔体成分如何显著影响岩浆中的渗透率发展。这项研究的具体目标包括：1)定量斑晶与微晶结晶度的相对重要性；2)含水中间岩浆渗透性发展和排气的时间尺度相对于喷发的时间尺度；3)晶体如何影响控制可渗透气体流动的孔隙微结构；4)晶体和/或熔体成分可能对喷发过程中和喷发后岩浆渗透性各向异性发展的影响。实验将在接近管道中岩浆上升的受控条件下进行，并快速冷却以保存减压过程中演化的囊泡结构。在实验中，将使用一种基于实验室的电导率测量的新组合来分析淬火样品，以探测孔结构的形态，并结合三维X射线层析分析来对脱气路径的结构进行成像。这些结果将被用来限制在含水中弧形火山的火山-熔岩穹隆旋回中岩浆脱气的时间尺度和深度。例如，这些实验可以用来限制岩浆脱气和放气的速度，从而形成覆盖在管道上的致密塞子或熔岩穹顶，然后在下面积累气体压力，导致火山灰产生火山喷发。如果纳入更广泛的火山监测，这项研究的结果将有助于我们更好地确定弧形火山发生喷涌到爆炸循环的时间尺度，并可以与喷发前和喷发中的地球物理监测数据进行比较，从而改进未来的喷发预测。

Many of Earth's active volcanoes produce violent eruptions that send ash into the atmosphere, creating hazardous phenomena that threaten aircraft, people, and infrastructure. The United States hosts a number of recently active and potentially hazardous volcanoes with most located in Alaska. Volcanoes that erupt intermediate SiO2 composition magmas are common in Alaska (e.g., Mt. Augustine) and elsewhere in the world. They are typically water and crystal-rich, frequently active and produce eruptions that cycle between small lava domes and violent, ash-producing, Vulcanian-style explosions. Magmas degas as gas bubbles exsolve, grow, coalesce into larger bubbles, and then connect together to form permeable pathways through the magma that allow gas to escape. The driving force behind explosive eruptions is how easily the magma can release the gas pressure that builds as magma rises in the conduit, balanced against how fast the magma ascends to the surface. Prior results indicate that as the magma's crystal content increases, the solid crystals could modify the degassing process by allowing the bubbles to connect and the magma to become permeable at lower gas contents, although the mechanism by which this happens is poorly understood. The primary goal of this study is to examine and quantify how crystal content may influence magma degassing, using experiments that approximate the conditions of magma ascent in the sub-volcanic plumbing system. The experiments will be designed to apply generally to intermediate composition volcanoes anywhere in the world. However, the experimental results will be tested by application to the 2006 eruption of Augustine Volcano, Alaska through a comparison with natural samples from that eruption. This research will allow geoscientists to better understand the mechanisms responsible for the effusive to explosive eruption style that occurs often in crystal-rich, intermediate composition magmas in subduction zone volcanoes, like those in Alaska and elsewhere around the world. This research will employ high-pressure and temperature, cold-seal decompression experiments to examine how crystal populations and/or matrix melt compositions may significantly influence permeability development in magmas. Specific goals of the research include: 1) quantification of the relative importance of phenocryst versus microlite crystallinity; 2) the timescales of permeability development and degassing in hydrous intermediate magmas relative to the timescales of eruption; 3) how crystals influence pore microstructure that controls permeable gas flow; 4) the influence crystals and/or melt composition may have on the development of permeability anisotropy in magmas during and after eruption. The experiments will be conducted under controlled conditions approximating magma ascent in the conduit, and rapidly quenched to preserve vesicle structures that evolve during decompression. The quenched samples will be analyzed using a novel combination of lab-based electrical conductivity measurements to probe the morphology of the pore structure, combined with 3-D X-ray tomography analyses to 'image' the structure of the degassing pathways in the experiments. The results will be used to constrain the timescales and depths of magma degassing in the context of Vulcanian - lava dome cycles at hydrous intermediate arc volcanoes. For example, the experiments can be used to constrain how fast magma degassing and outgassing can lead to the formation of a dense plug or lava dome capping the conduit, and then the subsequent build up of gas pressure beneath that leads to ash producing Vulcanian explosions. When included in the broader context of volcano monitoring, the results from this study will help us better define the timescales over which effusive to explosive cycling occurs in arc volcanoes, and can be compared with pre and syn-eruptive geophysical monitoring data, leading to improved eruption forecasting in the future.