abstract: Volcanic eruptions can be serious geologic hazards, and have the potential to effect human life, infrastructure, and climate. Therefore, an understanding of the evolution and conditions of the magmas stored beneath volcanoes prior to their eruption is crucial for the ability to monitor such systems and develop effective hazard mitigation plans. This dissertation combines classic petrologic tools such as mineral chemistry and thermometry with novel techniques such as diffusion chronometry and statistical modeling in order to better understand the processes and timing associated with volcanic eruptions. By examining zoned crystals from the fallout ash of Yellowstone’s most recent supereruption, my work shows that the rejuvenation of magma has the ability to trigger a catastrophic supereruption at Yellowstone caldera in the years (decades at most) prior to eruption. This provides one of the first studies to thoroughly identify a specific eruption trigger of a past eruption using the crystal record. Additionally, through experimental investigation, I created a novel diffusion chronometer with application to determine magmatic timescales in silicic volcanic systems (i.e., rhyolite/dacite). My results show that Mg-in-sanidine diffusion operates simultaneously by both a fast and slow diffusion path suggesting that experimentally-derived diffusion chronometers may be more complex than previously thought. When applying Mg-in-sanidine chronometry to zoned sanidine from the same supereruption at Yellowstone, the timing between rejuvenation and eruption is further resolved to as short as five months, providing a greater understanding of the timing of supereruption triggers. Additionally, I developed a new statistical model to examine the controls on a single volcano’s distribution of eruptions through time, therefore the controls on the timing between successive eruptions, or repose time. When examining six Cascade volcanoes with variable distribution patterns through time, my model shows these distributions are not result of sampling bias, rather may represent geologic processes. There is a robust negative correlation between average repose time and average magma composition (i.e., SiO2), suggesting this may be a controlling factor of long-term repose time at Cascade volcanoes. Together, my work provides a better vision for forecasting models to mitigate potential destruction. / Dissertation/Thesis / Doctoral Dissertation Geological Sciences 2020

abstract: Primitive arc magmas provide a critical glimpse into the geochemical evolution of subduction zone magmas, as they represent the most unadulterated mantle-derived magmas observed in nature in these tectonic environments and are the precursors of the more abundant andesites and dacites typical in arcs. To date, the study of primitive arc magmas has largely focused on their origins at depth, while significantly less is known about pre-eruptive crustal storage and ascent history. This study examines the crustal storage and ascent history of the Mt. Shasta primitive magnesian andesite (PMA), the demonstrated dominant parent magma for the abundant mixed andesites erupted at Mt. Shasta. Petrographic and geochemical observations of the PMA identify a mid-crustal magma mixing event with a less evolved relative of the PMA recorded in multiple populations of reversely zoned clinopyroxene and orthopyroxene phenocrysts. Prior phase equilibrium experiments and thermobarometric calculations as part of this study suggest the PMA experienced storage, mixing with a less evolved version of itself, and subsequent crystallization at 5kbar and 975°C. Modeling of Fe-Mg interdiffusion between the rims and cores of the reversely-zoned clinopyroxene and orthopyroxenes suggest this mixing, crystallization and subsequent ascent occurred within 10 years, or ~2.9 (+6.5 / -2.5) years, prior to eruption. Ascent from 5kbar or ~15 km, with no meaningful shallower storage, suggests minimum crustal transit rates of ~5 km/year. This rate is comparable to only a couple of other similar types of crustal transit rates (and slower than the much faster, syn-eruptive ascent rates measured through methods like olivine-hosted melt embayment volatile gradients and U-series isotope measurements on other arc magmas). The results of this study help to constrain the pre-eruptive history and ascent rates of hydrous primitive arc magmas, illuminating their magmatic processes during ascent. When combined with geophysical signals of magma movement, mixing to eruption timescales such as this have the power to inform volcanic hazard models for monogenetic, cinder cone eruptions in the Southern Cascades. / Dissertation/Thesis / Masters Thesis Geological Sciences 2019

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