Theses and Dissertations at Montana State University (MSU)
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Item The rise of caldera forming eruptions: refining tools for understanding magma ascent(Montana State University - Bozeman, College of Letters & Science, 2022) Harris, Megan Ann; Chairperson, Graduate Committee: Madison Myers; This is a manuscript style paper that includes co-authored chapters.The rate at which magma moves from the magma chamber to the surface influences the amount of degassing and crystallization that occurs, which in turn controls the style and intensity of the ensuing eruption. Thus, our understanding of magma ascent rates is crucial to understanding and mitigating future volcanic hazards. The bulk of this dissertation revolves around using the diffusion of water through melt-filled pockets (embayments) in quartz crystals as an ascent speedometer, coupled with geochemical and textural analysis of co-erupted material. In Chapter Two, I apply and refine the water diffusion speedometer to establish timescales of ascent for the two eruptions that formed the modern-day Valles Caldera, with the aim of understanding whether these rates change during subsequent eruptions from the same caldera. In Chapter Three, I apply the diffusion speedometer to the opening behavior of the 1991 eruption of Mount. Pinatubo and compare the results to ascent rates obtained using independent petrologic methods (bubble number density and microlite number density). This chapter seeks to reconcile the several orders of magnitude offset in ascent rates produced by various geospeedometers. Finally, in Chapter Four, I explore the mechanisms by which embayments are formed in magmatic systems. I do this by conducting a survey of cathodoluminescence images of crystals taken from five volcanic systems to determine how the embayments interact with the internal zoning of the crystal. I then attempt to form embayments experimentally using a cold-seal pressure vessel under variable magmatic conditions. The culmination of this work emphasizes that embayments are robust and faithful recorders of a magma's journey from its source to the surface and may be a critical piece of evidence for unraveling the magmatic history leading to eruption. This dissertation includes both previously published and unpublished co-authored work.Item Petrology and geochemistry of the Lazufre volcanic complex : evidence for diverse petrogenetic processes and sources in the Andean Central Volcanic Zone(Montana State University - Bozeman, College of Letters & Science, 2015) Wilder, Alicia Diane; Chairperson, Graduate Committee: David W. MogkThe Lazufre volcanic complex is an area of active surface uplift (~25°14'S) situated between two potentially active Quaternary volcanic centers, Lastarria and Cordon del Azufre, in the Andean Central Volcanic Zone. Studies incorporated geologic field relationships, mineral compositions, textures, and whole rock geochemical and isotopic data to develop a petrogenetic model to identify the source area and petrogenetic processes for the Lazufre magmatic system. Whole rock K-Ar dates of lavas from Cordon del Azufre place the most recent eruptions at 0.6-0.3 Ma + or = 0.3 Ma. The most recent eruptive activity at Lastarria has been dated at ~0.5-0.1 Ma. Volcanic rocks erupted from Lazufre are andesites to dacites and conform to a medium- to high-K calc-alkaline suite. Typical phenocryst assemblage is plagioclase-orthopyroxene-clinopyroxene-amphibole. Magmatic inclusions and mafic glomerocryst are present in most lava flow samples. Plagioclase and pyroxene phenocrysts in all rocks exhibit textures consistent with thermal disequilibrium. Important geochemical characteristics of these rocks include negative correlations for Mg, Fe, Ca and increased K and Na with increasing SiO 2 suggesting limited crystal fractionation. High Cr and Ni in some of the more mafic samples indicate mingling of a more mafic magma with a large volume of more silicic magma. Large ion lithophile elements are elevated at higher SiO 2 content, suggesting assimilation of more felsic rocks. A low range in 206 Pb/204 Pb, 87 Sr/86 Sr, and 143 Nd/144 Nd suggest partial melting of lower mafic crust as the dominant process in the generation of Lazufre extrusive rocks and indicate that there was relatively little involvement of ancient or felsic continental crust in magmagenesis of the area. The original magma was modified by homogenization and small degrees of mixing and assimilation and fractional crystallization during differentiation through ascent of the mid and upper crust. The results from this study are significant in that a multitude of differentiation processes and magma sources, specifically, a considerable mafic lower crustal component were involved in the generation of Lazufre Volcanic Complex magmas in the Andean Central Volcanic Zone.Item The geochemical evolution of the Cerro Uturuncu magma chamber, sw Bolivia and its relation to the Andean Central Volcanic Zone(Montana State University - Bozeman, College of Letters & Science, 2015) Michelfelder, Gary Scott; Chairperson, Graduate Committee: Todd Feeley; Todd C. Feeley and Alicia D. Wilder were co-authors of the article, 'The volcanic evolution of Cerro Uturuncu: a high-K, composite volcano in the back-arc of the central Andes of sw Bolivia' in the journal 'International journal of geosciences' which is contained within this thesis.; Todd C. Feeley was a co-author of the article, 'Crustal differentiation processes at Cerro Uturuncu, Andean central volcanic zone, sw Bolivia: insights from in situ SR isotopic analyses of plagioclase phenocrysts' submitted to the journal 'Geosphere' which is contained within this thesis.; Todd C. Feeley, Alicia D. Wilder and Erik C. Klemetti were co-authors of the article, 'Modification of the continental crust by subduction zone magmatism and vice-versa: across-strike geochemical variations of silicic lavas from individual eruptive centers in the Andean central volcanic zone' in the journal 'Geosciences' which is contained within this thesis.Cerro Uturuncu is a composite volcano located in the back-arc of the Andean Central Volcanic Zone (22.27°S, 67.18°W). The volcano has exclusively erupted crystal-rich andesite and dacite lava flows over the ~800,000 year life span of eruptive activity. This study provides new bulk-rock major- and trace- element, 18 O/16 O isotope ratios, Sr, Nd, and Pb radiogenic isotopic ratios of lava flows, domes, magmatic inclusions and xenoliths. The study also adds major and trace element and Sr isotopic ratios of plagioclase phenocrysts in order to examine the evolution of the magmatic plumbing system beneath Cerro Uturuncu and place the volcanic center in the regional context of the CVZ. Plagioclase crystals from silicic (andesitic to dacitic) lavas and domes at Volcan Uturuncu exhibit large variations in An contents, textures, and core to rim 87 Sr/86 Sr ratios. Many of the isotopic variations cannot have existed at magmatic temperatures for more than a few thousand years. The crystals likely derived from different locations in the crustal magmatic system and mixed just prior to eruption. Uturuncu magmas initially assimilated crustal rocks with high 87 Sr/86 Sr ratios. The magmas were subsequently modified by frequent recharge of more mafic magmas with lower 87 Sr/86 Sr ratios. A typical Uturuncu silicic magma therefore only attains its final composition just prior to or during eruption. On an arc-wide scale silicic lavas erupted from three well-characterized composite volcanoes between 21°S and 22°S (Aucanquilcha, Ollague, and Uturuncu) display systematically higher K 2O, LILE, REE and HFSE contents and 87 Sr/86 Sr ratios with increasing distance from the arc-front. In contrast, the lavas have systematically lower Al 2O 3, Na 2O, Sr, and Ba contents; LILE/HFSE ratios; 143 Nd/144 Nd ratios; and more negative Eu anomalies. Silicic magmas along the arc-front apparently reflect melting of relatively young, mafic composition amphibolitic source rocks with the continental crust becoming increasingly older with a more felsic bulk composition toward the east. This results from progressively smaller degrees of mantle partial melting, primary melt generation, and crustal hybridization with distance from the arc-front.Item Miocene calc-alkaline volcanism in southern Jackson Hole, Wyoming : evidence of subduction-related volcanism(Montana State University - Bozeman, College of Letters & Science, 1997) Adams, David CongdonItem Volcanism and associated sedimentation in a retroarc foreland basin : the Upper Cretaceous Two Medicine formation of west-central Montana(Montana State University - Bozeman, College of Letters & Science, 1998) Smith, Angela ElaineItem The Structural, volcanic, and hydrothermal geology of the Warm Springs Creek Area, eastern Garnet Range, Powell County Montana(Montana State University - Bozeman, College of Letters & Science, 1984) Callmeyer, Thomas J.Item St. Paul Island, Pribilof Islands, Alaska : geology, volcanic evolution, and volcanic hazards(Montana State University - Bozeman, College of Letters & Science, 2001) Winer, Grace SherwoodItem Sedimentary deposits and processes of the Late Cretaceous Adel Mountain volcaniclastic apron, west-central Montana(Montana State University - Bozeman, College of Letters & Science, 1999) LaBranche, Julie VeronicaItem Stable isotope (18 O/16 O and D/H) studies of cascade volcanic arc magmatism(Montana State University - Bozeman, College of Letters & Science, 2009) Underwood, Sandra Jean; Chairperson, Graduate Committee: Todd FeeleyStable isotope ratios (18 O/16 O and D/H) potentially constrain origins of magma and volatile sources in igneous rocks. Modification of magmas by closed system processes (fractional crystallization and closed system devolatilization) or open-system processes (assimilation and fractional crystallization, magma mixing, degassing) affect 18 O/16 O and D/H ratios in known ways. Magma degassing during volcanic eruptions and subsequent rehydration or alteration of groundmass glass reduces accuracy of whole rock stable isotope measurements, the traditional method of measuring glassy volcanic rocks. However, phenocrysts separated from fresh volcanic rocks may retain magmatic 18 O/16 O and D/H values. Accordingly, oxygen isotopes in olivine, clinopyroxene, and plagioclase crystals were measured to determine whether Quaternary little-evolved mafic lavas of the Southernmost Cascades (SMC) represent only melts of heterogeneously contaminated mantle sources. In variably degassed silicic volcanic rocks from the 1980-1986 eruptions at Mount St. Helens (MSH) and the 1915 eruptions at Lassen Volcanic Center (LVC), hydrogen isotopes in amphibole and biotite phenocrysts were measured to evaluate shallow subvolcano magmatic processes. Magmas in each study are strongly influenced by crustal stress fields associated with each tectonic setting. The mantle source of the SMC little-evolved mafic lavas is heterogeneous, but these continental arc magmas acquire crustal contamination that reflects vent location across a region of extended heterogeneous crust. Hydrous phenocrysts in comparatively shallow, sill-like LVC silicic magma bodies recorded heating and devolatilization associated with the periodic injections of mafic magma. The broadly distributed crustal extension enables crystallizing silicic magmas to devolatilize as they are variably remobilized by heat and volatiles of recharging mafic magmas. At MSH, numerous small dacite magma pulses crowded into the narrow extensional volume below the vent. This focusing of all magmas and exsolved volatiles from a deeper main magma body produced the explosive May 18, 1980, eruption that also extensively damaged at least the upper 5 km of subvolcano plumbing. Through 1986, pulses of volatile-rich dacite magma degassed in a structurally and thermally evolving plumbing system. [Mineral data are located in separate Supplemental Data Files.].