Browsing by Author "Johnson, Elizabeth Baedke"

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    Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets
    Moore, Lowell (Virginia Tech, 2014-04-29)
    H2O and CO2 concentrations of the glass phase in melt inclusions (MI) are commonly used both as a barometer and to track magma degassing behavior during ascent due to the strong pressure dependence of H2O and CO2 solubilities in silicate melts. A requirement for this method to be valid is that the glass phase in the MI must represent the composition of the melt that was originally trapped. However, melt inclusions commonly contain a vapor bubble that formed after trapping. Such bubbles may contain CO2 that was originally dissolved in the melt. In this study, we determined the contribution of CO2 in the vapor bubble to the overall CO2 content of MI based on quantitative Raman analysis of the vapor bubbles in MI from the 1959 Kilauea Iki, 1960 Kapoho, 1974 Fuego volcano, and 1977 Seguam Island eruptions. The bubbles contain up to 90% or more of the total CO2 in some MI. Reconstructing the original CO2 content by adding the CO2 in the bubble back into the melt results in an increase in CO2 concentration by as much an order of magnitude (1000s of ppm), corresponding to trapping pressures that are significantly greater (by 1 to >3 kbars) than one would predict based on analysis of the volatiles in the glass alone. Many MI also showed the presence of a carbonate mineral phase; failure to include its contained CO2 when reconstructing the CO2 content of the originally trapped melt may introduce significant errors in the calculated volatile budget.
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    Understanding Non-Plume Related Intraplate Volcanism
    Mazza, Sarah Elizabeth (Virginia Tech, 2016-12-21)
    Intraplate volcanism is a worldwide phenomenon producing volcanoes away from active plate boundaries, a process that cannot yet be sufficiently explained by plate tectonic processes, and thus is still a missing piece in the understanding of the dynamics and evolution of our planet. Models for the formation of intraplate volcanism are dominated by mantle plumes, but alternative explanations, such as adiabatic decompression triggered by lithospheric delamination, and edge driven convection (EDC), could be responsible for magmatism. This dissertation explores intraplate volcanic locations that do not fit the mantle plume model, and presents geochemical evidence for lithospheric delamination and edge driven convection for the cause of volcanism. I studied an Eocene volcanic swarm exposed in the Appalachian Valley and Ridge Province of Virginia and West Virginia, which are the youngest known igneous rocks along the Eastern North American Margin (ENAM). These magmas provide the only window into the most recent deep processes contributing to the post-rift evolution of this margin. This study presents the first high precision 40Ar/39Ar ages along with new geochemical data, and radiogenic isotopes that constrain the melting conditions and the timing of emplacement. Modeling of the melting conditions suggests that melting occurred under conditions slightly higher than average mantle beneath mid-ocean ridges. Asthenosphere upwelling related to localized lithospheric delamination is a possible process that can explain the intraplate signature of these magmas that lack evidence of a thermal anomaly. The Virginia-West Virginia region of the ENAM also preserves a second post-rift magmatic event in the Late Jurassic. By studying both the Late Jurassic and Eocene magmatic events we can better understand the post-rift evolution of passive margins. This study presents a comprehensive set of geochemical data that includes new 40Ar/39Ar ages, major and trace-element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long-term evolution of this margin, resulting in a 'passive-aggressive' margin that records multiple magmatic events long after rifting ended. Finally, Bermuda is an intraplate volcano that has been historically classified as mantle plume related but evidence to support the plume model is lacking. Instead, geophysics have argued that EDC is the best model to explain Bermuda volcanism. This study presents the first geochemical analysis of Bermuda volcanism, and found that Bermuda was built by two different magmatic processes: melting of carbonated peridotite to produce silica under-saturated, trace element enriched volcanics and melting of an enriched upper mantle component that produced silica saturated volcanics. We attribute the cyclicity of silica under-saturated and silica saturated volcanics to EDC melting.