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

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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.