UMD Theses and Dissertations
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
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Item Lithium and its isotopes as a tracer of fluid flow mechanisms in the Catalina Schist melange zone(2014) Roble, Leigh Anne; Penniston-Dorland, Sarah; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mélange zones are areas of highly mixed and deformed rock believed to form from shearing between subducting slab and peridotite mantle wedge. They have high -pressure/low-temperature mineral assemblages and contain a fine-grained matrix with centimeter to meter scale blocks surrounded by rinds, thought to represent a reaction zone between the block and matrix. These rinds are not well understood, but could be formed due to mechanical mixing, diffusion, or infiltration. Lithium is used to determine the role played by fluid-mediated processes in the Catalina Schist mélange zone because it is fluid mobile and has high diffusivity. Samples from amphibolite, lawsonite-blueschist, and lawsonite-albite facies were retrieved from the Catalina Schist subduction complex on Santa Catalina Island. Lithium isotopic compositions and concentrations were determined using mass spectrometry techniques. One-dimensional diffusion models were applied to the data to determine the extent of the different mechanisms responsible for fluid transport throughout the subduction complex.Item Lithium and delta7Li behavior during metamorphic dehydration processes and crustal evolution(2011) Qiu, Lin; Rudnick, Roberta L; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Li isotopes have been used as a tracer of a wide range of geological processes, because the two stable isotopes of Li have significant relative mass difference and large elemental dispersion in Earth. In order to understand better Li behavior during the processes of sub-greenschist to granulite facies metamorphism and partial melting, fluid transport in subduction zones, weathering of continental crust and the evolution of juvenile continental crust, a large variety of samples have been studied in this thesis. These samples include mudrocks from British Caledonides, schists from Otago, New Zealand, high-grade metapelites and metabasites from Ivrea-Verbano Zone, Italy, Archean shales from the Kaapvaal Craton, South Africa, and Archean granitoids from Scotland, South Africa and Canada. Investigations of metapelites from several localities show that: 1) sedimentary provenance exerts the greatest control on Li in fine grained sediments, and Li concentrations generally increase while delta7Li decreases with CIA (Chemical Index of Alteration) in post Archean shales, 2) sub-greenschist facies metamorphism has negligible effect on Li concentrations and isotopic compositions, 3) metamorphic dehydration from greenschist to granulite facies may cause significant Li depletion, but has had little influence on delta7Li, and 4) a key factor controlling [Li] in metapelites during metamorphism is the stability of Mg-bearing phyllosilicates. Furthermore, based on the understanding gained of the Li behavior above, the Li signature in Archean shales and granitoids may indicate that: 1) more severe weathering conditions prevailed during the Archean, 2) the Archean upper continental crust had a heavier Li isotopic composition than post Archean upper continental crust, and 3) Archean juvenile granitoids have heavier Li isotopic compositions than post-Archean equivalents. Collectively, the results for Archean samples may indicate that Li isotopes can be used as tracers of paleoclimate change, and also can be used to understand the nature of the source of Archean juvenile magmas.Item MOLYBDENUM ISOTOPE SYSTEMATICS IN NATURAL AND EXPERIMENTAL SETTINGS(2010) Scheiderich, Kathleen Dwyer; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Molybdenum isotopes have a broad potential applicability for paleoenvironmental analysis, particularly with respect to questions of eutrophication history, development of anoxia, and sedimentation under conditions of varying oxygenation. Using a double-spike method, the Mo isotope proxy was applied to sediments and water samples from the Chesapeake Bay, where the severity of seasonal anoxic episodes has been increasing over the last century. It was discovered that isotopic fractionation is occurring in the estuary, as indicated by the large differences between the δ98Mo of Mo dissolved in the water and authigenic Mo in the sediments. Increased variability of δ98Mo values and increased authigenic Mo deposition were likely related to the onset of coastal anoxic episodes in the Bay. Sediment samples from the Eastern Mediterranean were also analyzed for δ98Mo, along with redox-sensitive element concentrations (Re, Mo, V, Ba, and Fe). Over the past 5 million years, climatic shifts have driven cyclic oceanographic changes in the Mediterranean, specifically basin-wide anoxic episodes, which are visible in the sedimentary sequence as layers that are highly enriched in redox-sensitive elements and organic matter (sapropels). I investigated whether δ98Mo values, in conjunction with other proxies, could be used to infer the degree to which the deep basin was affected by anoxic conditions, and how this may have changed between individual anoxic episodes. There were clear temporal differences in the apparent severity of anoxia in the Mediterranean, as reflected by the proxies in the sapropels. The amount of Mo in Mediterranean seawater did not change during sapropel deposition, and therefore, the basin likely remained open to circulation. I collaborated in a project to determine whether Mo isotopes could be fractionated at high temperature and pressure in an experimental system, designed to mimic natural hydrothermal-type porphyry systems. It was found that Mo isotopes are fractionated between a melt and vapor phase under the experimental conditions, and in a manner consistent with equilibrium exchange processes. Molybdenum entering the melt phase undergoes a coordination change to higher coordination number, thus preferentially enriching the vapor phase in the heavier Mo isotopes.Item An empirical re-evaluation of the boron isotope/pH proxy in marine carbonates(2009) Klochko, Kateryna; Kaufman, Alan J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The boron isotopic composition measured in marine carbonates is considered to be a tracer of seawater pH. However, an accurate application of this proxy has been hampered by our lack of intimate understanding of chemical kinetics and thermodynamic isotope exchange between the two dominant boron-bearing species in seawater: boric acid B(OH)3o and borate ions B(OH)4-, as well as their subsequent partitioning into a carbonate lattice. In this dissertation I have taken on a task of a systematic empirical re-evaluation of the fundamental parameters and assumptions on which the boron isotope paleo-pH proxy is based. As a result of this research strikingly different values of the boron isotope exchange constant in solution (Klochko et al., 2006) and boron speciation and partitioning in carbonates (Klochko et al., 2009) were determined, suggesting that the most parameters and assumptions that were believed to be previously constrained and have been widely applied to the 11B-pH reconstructions were incorrect. Recognizing that both biological and inorganic processes may potentially affect boron speciation and isotopic composition in carbonates, to isolate purely inorganic effects on the boron isotope co-precipitation with carbonates, we have designed a series of pH-controlled 11B calibration experiments of inorganic calcite and inorganic aragonite. Results to date reveal that precipitates from our experiments at pH = 8.7 fall exactly along the borate ion 11B curve predicted by our empirically determined boron isotope fractionation factor (Byrne et al., 2005; Klochko et al., 2006). Extending these experiments to wider range of pH conditions will provide the necessary inorganic baseline for paleo-studies of inorganic carbonate and future investigations of the purely biological effects on the boron isotope distributions in carbonates.