Elucidating the Role of Titanomagnetite in the Vesiculation of Silicic Magmas from Observations of Natural and Experimental Samples
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2024
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The onset of an eruption is preceded by the nucleation of bubbles in a magma, which in turn reduces its density and increases its buoyancy and compressibility. These factors relate directly the ascent rate of a magma, which controls the style and intensity of an eruption. Therefore, it is critical to understand the mechanism of bubble nucleation, and what factors contribute to the efficiency of vesiculation in silicic magmas. Of particular interest is bubble nucleation in high silica magmas, as the process of bubble nucleation is delayed until the supersaturation of volatiles (e.g., H2O, CO2, and S) occurs. This process requires magma chambers of significant storage depths for bubble nucleation to occur homogeneously (without the aid of a bubble nucleation substrates), or the presence of abundant heterogeneous bubble nucleation substrates for bubble nucleation to occur in magma staged at shallow storage depths. In this dissertation, we first examine pumice and obsidian from the 1060 CE Glass Mountain subplinian eruption and investigate the presence of sub-micron scale heterogenous bubble nucleation substrates (crystals) using applications of magnetic methodologies and physical characterization techniques, as a means to explain how bubble nucleation occurred in an otherwise aphyric (apparent homogeneous nucleation dominated) shallow storage system. We find titanomagnetite in number densities greater than bubble number densities and confirm early titanomagnetite stability in both the pumice and obsidian. Therefore, sub-micron titanomagnetite is present and available to act as a heterogeneous bubble nucleation site in this system. We extend our application of rock magnetic methodologies and physical characterization techniques to aphyric pumice from other rhyolitic eruptions (1912CE Novarupta, 181CE Taupo, and 0.45 Ma Pudahuel) that represent a range of magma storage depths. We also find titanomagnetite in number densities greater than bubble number densities in these systems and confirm early titanomagnetite stability. Sub-micron titanomagnetite may, therefore, act as a heterogeneous bubble nucleation site in magmatic systems where it is abundant and stable, regardless of magma chamber storage depths. From this effort, and observations from past work, we suggest that titanomagnetite is an efficient bubble nucleation site, but the specific effects of increasing titanomagnetite number density on bubble nucleation efficiency are still poorly constrained. So, we conducted a series of 1-atmosphere gas mixing furnace vesiculation experiments to investigate how nucleation of new titanomagnetite crystals affects bubble nucleation efficiency and bubble number density in a natural rhyolite obsidian. Our experimental results confirm that bubble nucleation efficiency is dependent on the nucleation of new titanomagnetite in our experimental samples. The reason for titanomagnetites dominance as a heterogeneous bubble nucleation site has been theorized to be the result of its composition, morphology, and/or its crystal structure. We performed a series of sessile drop experiments involving the melting of rhyolite glass in contact with several mineral substrates, to constrain which of the above factors control the interfacial energy between the solid and liquid phases by observing wetting angles. We find that titanomagnetite is likely the preferred bubble nucleation site due to its composition, morphology, and crystal structure, in addition to its abundant presence and common stability across rhyolitic systems. The work described in this dissertation suggests that heterogeneous bubble nucleation may dominate in previously defined aphyric systems and encourages the use of magnetic analytical methodologies for future investigations into determining bubble nucleation style. In addition, this work provides new insights into the influence of heterogeneous bubble nucleation substrate abundance and composition on bubble nucleation efficiency in high silica systems. Future work should draw from the magnetic analytical and experimental methods described in this dissertation to further our comprehension of bubble nucleation and solidify our understanding of the early stages of volcanic eruptions.
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Petrology, Geology, bubble nucleation, rhyolite, titanomagnetite, volcanology
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234 pages
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