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产生剧烈的喷发，将火山灰送入大气，造成威胁飞机、人类和基础设施的危险现象。美国拥有许多最近活跃和潜在危险的火山，其中大多数位于阿拉斯加。喷发中等SiO2成分岩浆的火山在阿拉斯加（如奥古斯丁山）和世界其他地方很常见。它们通常富含水和晶体，经常活跃，并产生喷发，在小熔岩穹丘和猛烈的、产生灰烬的火山式爆炸之间循环。岩浆的脱气过程是气泡溶解、生长、合并成更大的气泡，然后连接在一起，形成贯穿岩浆的可渗透通道，让气体逸出。爆炸性喷发背后的驱动力是岩浆释放气体压力的容易程度，当岩浆在管道中上升时，气体压力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气体含量下变得可渗透来改变脱气过程，尽管这种情况发生的机制尚不清楚。本研究的主要目标是通过模拟次火山管道系统中岩浆上升的条件的实验，检验和量化晶体含量如何影响岩浆脱气。这些实验将被设计用于世界上任何地方的中等成分火山。然而，实验结果将通过与2006年阿拉斯加奥古斯丁火山喷发的自然样本进行比较来验证。这项研究将使地球科学家能够更好地了解火山喷发的机制，这种喷发方式经常发生在俯冲带火山中富含晶体的中间成分岩浆中，就像阿拉斯加和世界其他地方的火山一样。这项研究将采用高压、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的渗透率发育。具体的研究目标包括：1)量化斑晶与微晶结晶度的相对重要性；2)含水中间岩浆渗透率发育和脱气的时间尺度与喷发时间尺度的关系；3)晶体如何影响孔隙微观结构，从而控制可渗透气体的流动；4)晶体和/或熔体成分可能对喷发期间和喷发后岩浆渗透率各向异性发育的影响。实验将在接近岩浆在管道中上升的受控条件下进行，并迅速淬火以保存减压过程中演变的囊泡结构。将使用基于实验室的电导率测量的新组合来分析淬火样品，以探测孔隙结构的形态，并结合3-D x射线断层扫描分析来“成像”实验中脱气路径的结构。这些结果将用于在含水中间弧火山的火山-熔岩穹窿旋回背景下限制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，这些速度可以导致形成覆盖管道的密集塞或熔岩圆顶，然后随后在下面积聚气体压力，导致产生灰烬的火山爆发。当纳入更广泛的火山监测背景时，本研究的结果将帮助我们更好地定义弧火山喷发到爆炸循环发生的时间尺度，并可以与喷发前和喷发时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.