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剧烈的喷发，将火山灰送入大气层，造成危险现象，威胁飞机，人员和基础设施。美国有许多最近活跃和潜在危险的火山，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很常见（例如，山奥古斯丁）和世界其他地方。它们通常富含水和晶体，经常活跃，并产生喷发，在小熔岩圆顶和暴力，产生灰烬的火山式爆炸之间循环。当气泡溶解、增长、合并成更大的气泡，然后连接在一起形成穿过岩浆的可渗透通道，允许气体逸出时，岩浆脱气。爆炸性喷发背后的驱动力是岩浆在管道中上升时释放气体压力的容易程度，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修改脱气过程，尽管这种情况发生的机制知之甚少。这项研究的主要目标是研究和量化晶体含量如何影响岩浆脱气，使用近似的岩浆上升的条件下，在次火山管道系统的实验。这些实验的设计将普遍适用于世界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，通过与该喷发的天然样品进行比较来进行测试。这项研究将使地球科学家能够更好地了解在俯冲带火山（如阿拉斯加和世界其他地方的火山）中富含晶体的中间成分岩浆中经常发生的爆发式喷发的机制。这项研究将采用高压和高温，冷密封减压实验，以研究如何晶体人口和/或矩阵熔体组合物可能会显着影响渗透性的发展岩浆。 研究的具体目标包括：1）斑晶与微晶结晶度的相对重要性的量化; 2）相对于喷发时间尺度的含水中间岩浆的渗透性发展和脱气的时间尺度; 3）晶体如何影响控制可渗透气体流动的孔隙微结构;（4）岩浆喷发过程中和喷发后，晶体和熔体成分对渗透率各向异性发展的影响。实验将在受控条件下进行，接近管道中的岩浆上升，并迅速淬火以保存减压过程中演变的囊泡结构。淬火样品将使用基于实验室的电导率测量的新组合进行分析，以探测孔结构的形态，结合3-D X射线断层扫描分析，以“成像”实验中脱气途径的结构。结果将被用来限制的时间尺度和深度的岩浆脱气的背景下，在火山熔岩圆顶循环在含水中间弧火山。例如，这些实验可以用来限制岩浆脱气和脱气的速度有多快，从而导致形成致密的塞子或覆盖管道的熔岩圆顶，然后在下面建立气体压力，导致火山灰产生火山爆发。当包括在更广泛的范围内的火山监测，从这项研究的结果将帮助我们更好地定义的时间尺度，在喷发到爆炸循环发生在弧火山，并可以与前和同步喷发地球物理监测数据进行比较，从而改善未来的喷发预测。

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.