The physical properties and dynamic processes that control effusions of rhyolitic lavas are poorly constrained because of a paucity of direct observations. To assess the pre-eruptive storage conditions, eruptive ascent, and subaerial emplacement for a suite of volumetrically diverse rhyolitic lavas, I studied 10 obsidian lavas from Yellowstone Caldera, Wyoming and Mono Craters, California. Storage, ascent, and emplacement of those lavas were quantitatively constrained using phenocryst compositions, high temperature experiments, microlite textures, and compositional gradients surrounding spherulites. Compositions of phenocrysts and quartz-hosted glass inclusions indicate the magmas at Yellowstone were stored at 750±25 °C in the shallow crust (<7 km), in agreement with phase equilibria experiments. Following the initiation of an eruption, magma leaves the chamber and ascends in a conduit. Microlite number density can be used to quantify eruptive ascent rates. To generate the observed microlite number densities (10⁸·¹¹±⁰·⁰³) to 10⁹·⁴⁵±⁰·¹⁵ cm⁻³), the magmas decompressed at ~1 MPa hour⁻¹, equivalent to ascent rates of ~10 mm s⁻¹. Upon subaerial emplacement, microlites act as rigid particles in a deforming fluid (lava), and hence their 3D orientations could indicate flow direction and how strain accumulates in the fluid during flow. Microlites are strongly aligned in samples from all flows, but variations in alignment were found to be independent of flow volume or distance travelled. Together, those observations suggest that strains accumulated during subaerial transport must be small (<2). Instead, microlites most likely aligned in response to strain in the conduit, which can be generated by collapse and flattening. Upon reaching the surface, the cooling history and longevity of rhyolitic lavas are critical for developing models of emplacement and hazard assessment. Compositional gradients surrounding spherulites provide one method to assess such temporal characteristics. Spherulites, crystalline spheres of radiating quartz and feldspar, form by crystallization of obsidian glass in response to cooling. An advection-diffusion model was developed to simulate the growth of spherulites and compositional gradients that develop in the surrounding glass during spherulite growth. Observed gradients are consistent with spherulites growing between ~700 and ~400 °C, and cooling at rates of 10⁻⁵·²±⁰·³) °C s⁻¹. / text

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