Theses and Dissertations at Montana State University (MSU)
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Item 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.].Item Nutrient allocation to egg formation of lesser scaup(Montana State University - Bozeman, College of Letters & Science, 2010) Cutting, Kyle Anthony; Chairperson, Graduate Committee: Jay J. RotellaLesser Scaup (Aythya affinis) populations have declined for nearly three-decades. Recent evidence suggests that decreases in habitat quality and availability of spring staging areas may have resulted in a decline of recruitment. Recently, stable isotopes analysis has emerged as a powerful ecological tool to measure the degree of cross-seasonal effects of birds. In 2006 through 2008 in southwestern Montana, I used carbon (13C) and nitrogen (15N) stable isotopes to assess how red blood cells (RBC), which is a proxy for stored body reserves (endogenous), change over time as local foods (exogenous) are consumed, and to estimate the relative contributions of endogenous reserves and exogenous foods for egg formation. From arrival through the egg laying period, 15N values of RBC decreased while 13C values became more stable, a pattern consistent with expectations of endogenous tissues equilibrating with local dietary sources. In 2006 and 2008, isotopic values for egg albumen and yolk protein were similar to those expected from local dietary sources, which indicated that most protein used for producing egg albumen and yolk protein was obtained on the breeding grounds (exogenous sources). In 2007, endogenous reserves contributed on average 26% and 10% more for producing albumen and yolk protein, respectively, than in 2006 and 2008 combined. Due to small differences in 13C values between female endogenous lipids upon arrival to the breeding grounds and those of local invertebrate lipids, it was not possible to separately estimate the contributions of endogenous and invertebrate lipids to egg lipid formation. My results suggest that local invertebrates and endogenous lipid reserves contributed on average 51% (SE = 7%) to egg lipid production. The remaining contributions to eggs were derived from local seed sources. Despite recent findings of reduced endogenous reserves during spring migration, results from the females in this study suggest that the amount of time that females spend on the breeding grounds prior to nest initiation may be adequate in some years to allow them to attain adequate exogenous foods for reproduction. Future isotopic research is now needed across latitudinal gradients, allowing length of prebreeding season to vary, while separating out the contribution from endogenous versus exogenous sources.