Theses and Dissertations from UMD
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Item Anatexis and crustal differentiation: Insights from the Fosdick migmatite-granite complex, West Antarctica(2014) Yakymchuk, Christopher; Brown, Michael; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the Fosdick migmatite-granite complex of West Antarctica, U-Pb geochronology of monazite in migmatites and zircon in granites records two episodes of high-temperature metamorphism, one in the Devonian-Carboniferous and another in the Cretaceous. For the older lower-grade event, whole-rock and zircon isotope geochemistry of granites within the Fosdick complex are interpreted to record crustal reworking during metamorphism associated with continental arc magmatism along the East Gondwana convergent plate margin. By contrast, the geochemistry of correlative granites suites from along and across the same margin indicates a greater proportion of crustal growth. This suggests prominent arc-parallel and arc-normal variations in the mechanisms of crustal reworking versus growth in continental arc systems. Based on garnet Lu-Hf ages, the timing of peak metamorphism in the younger higher-grade event has been determined as c. 116-111 Ma. U-Pb ages of monazite from migmatites and zircon from anatectic granites suggest that exhumation of the complex as a gneiss dome occurred during the interval c. 107-100 Ma. Contemporaneous exhumation of high-grade metamorphic rocks in the Western Province of New Zealand suggests that intracontinental extension preceded the final breakup of New Zealand from West Antarctica by c. 25 Myr. Melt migration through and emplacement within the Fosdick complex during Cretaceous metamorphism was accomplished via a self-organized melt network controlled by the regional stress field and anisotropy of the host rocks. Granites within this network and in sills at shallower crustal levels have microstructures and chemistry consistent with a cumulate origin, and are interpreted to record fractional crystallization during magma ascent and doming related to exhumation. Phase equilibria modeling of open system melting during prograde metamorphism is used to quantify the reduced fertility of source rocks during high-temperature exhumation and later overprinting orogenic events. Quantitative modelling of the dissolution of zircon and monazite during prograde melting demonstrates that accessory minerals are expected to be partially to completely consumed up to the metamorphic peak. New growth of these minerals in migmatite melanosomes is predicted to be limited during cooling, whereas leucosomes and anatectic granites are predicted to contain new zircon and monazite growth.Item Sulfur isotopic evolution of Phanerozoic and Ediacaran seawater sulfate(2013) Wu, Nanping; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Dynamics of &sigma34S and &sigma33S of oceanic sulfate and sedimentary pyrite over geologic time has been used to understand and extract information about the marine sulfur cycle. My Ph.D project uses this approach to study the evolution of the marine sulfur cycle and focuses on: 1)Providing temporal &sigma34S and &sigma33S records for Phanerozoic and Ediacaran seawater sulfate based on the analysis of carbonate associated sulfate 2) Reinterpreting previously documented variations of sulfur isotope fractionation using new measurements of &sigma33S of oceanic sulfate. The sulfur isotope fractionation between sulfate and pyrite appears to have varied widely, between 25 / to 40 / over the course of Phanerozoic. For the earlier part of Phanerozoic, the values of sulfur isotope fractionation are approximately 35 /. The fractionation then decreases to 25 / during the Carboniferous Period. Following this, the sulfur isotope fractionations progressively increase, reaching approximately 40 / during the Cenozoic Period. 3) Evaluating the connection between sulfate concentration, sulfide re-oxidation and sulfate exchange between water column and marine sediment systems using steady state and also non steady state models. The model results and the data presented here suggest the sulfur cycle in the Ediacaran Oman basin evolved from one that was similar to the global sulfur cycle to a sulfur cycle that was disconnected or partially was disconnected from the open ocean sulfur cycle. It also implies that the sulfate levels drop at this transition due to weakening of vertical bioturbation or weakening of other physical processes that involve in mixing of sulfate in pore water and overlying sulfate in marine settings. The significance of these three directions is the new information that they provide about the evolution of the sulfur cycle. It is relevant to understanding the environmental changes and their connections to sulfur ecosystem evolution for the geological time interval extending from present until latest Precambrian.Item A Study of the Sulfur Isotopic Composition of Martian Meteorites(2012) Franz, Heather; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)ABSTRACT Title of Document: A STUDY OF THE SULFUR ISOTOPIC COMPOSITION OF MARTIAN METEORITES Heather B. Franz, Ph.D., 2012 Directed By: Professor James Farquhar, Department of Geology and ESSIC Sulfur is an important tracer for geochemical processes because it possesses four stable isotopes and forms natural compounds in a range of oxidation states. This element has been shown to undergo mass-independent isotopic fractionation (S-MIF) during laboratory photochemical experiments, which may provide clues to processes that have occurred both in the solar nebula and in planetary atmospheres. The surface of Mars has been found to contain ubiquitous sulfate minerals, marking this planet as an ideal candidate for sulfur isotope study. The shergottites comprise the youngest group of martian meteorites and the most representative of mantle-derived igneous rocks. Extraction and isotopic measurement of sulfur from 30 shergottites yield the first estimate of the juvenile martian sulfur composition, which matches within uncertainties that of Cañon Diablo Troilite. Analysis of martian meteorites spanning a range of ages from the shergottites, as young as ~150 Ma, to the nakhlites, ~1.3 Ga, reveals the presence of sulfur characterized by S-MIF compositions. These findings are interpreted as evidence for cycling of sulfur between an atmospheric reservoir where photochemical processing of sulfur-bearing gases occurred and a surface reservoir in which photochemical products were ultimately deposited. Anomalous sulfur has been detected in both sulfate and sulfide minerals, implying assimilation of sulfur from the martian surface into magmas. Differences in the S-MIF compositions of the nakhlites and shergottites may preserve a record of complementary sulfur formed by a single process or may indicate the operation of multiple photochemical processes at different times or geographical locations. Identification of the photochemical mechanism responsible for producing the anomalous sulfur observed in martian meteorites is important for constraining the atmospheric composition at the time the S-MIF signals were generated. Results of laboratory experiments with pure SO2 gas suggest that self-shielding is insufficient to explain the anomalous sulfur isotopic composition. This implies that an optically thick SO2 column in the martian atmosphere may not have been required for production of the observed signalsItem Whole stream nitrogen uptake and denitrification in a restored stream of the Chesapeake Bay(2007-08-10) Klocker, Carolyn Ann; Kaushal, Sujay S; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Little is currently known about the effects of stream restoration practices on in-stream processing and nitrogen removal. This study quantified nitrate retention in a survey of two restored and two unrestored streams in Baltimore, MD using unenriched nitrate additions, denitrification enzyme assays, and a 15N isotope tracer addition in one of the urban restored streams, Minebank Run. Denitrification potential in sediments was variable across streams, whereas nitrate uptake length was significantly correlated to surface water velocity, which was lowest in restored streams. In situ denitrification rates in Minbank Run were 153 mg NO3--N m-2 d-1, and approximately 40% of the daily load of nitrate could be removed over a distance of 220.5 m. Stream restoration projects that decrease water velocity and increase residence time may lead to considerable rates of nitrate removal through denitrification.Item DETERMINATION OF SULFUR ISOTOPE COMPOSITION IN SULFATE FROM TWO HIGH ELEVATION SNOWPITS BY MULTI-COLLECTOR THERMAL IONIZATION MASS SPECTROMETRY USING A DOUBLE SPIKE(2005-05-12) Mann, Jacqueline Lorraine; Prestegaard, Karen L.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The variability of stable sulfur isotopes in nature provides a chemical tool for tracing the various sources of sulfur and a useful tool for understanding the sulfur cycle. It is also well established that snow and ice preserve a record of the sources, sinks, and processing of sulfur that reflect changes in this cycle through time. Our ability to sample this record is however limited by the total sample concentration and the analytical requirements for isotopic analysis. A high-resolution double spike technique using multi-collector thermal ionization mass spectrometry was developed for stable sulfur isotope composition measurements of small concentration sulfate samples (ppb level). The capability of this new technique was demonstrated by measuring internationally recognized standards of known isotopic composition and by measuring snowpit samples with low sulfate concentrations collected from the Inilchek Glacier, Kyrgyzstan and Summit, Greenland. The elemental and high resolution sulfur isotope data for the snowpit samples were used to calculate the relative seasonal contributions of anthropogenic and natural sulfur sources to sulfate at these high-elevation Northern Hemisphere sites. The isotope composition results for the standards demonstrate the double spike technique to be competitive in accuracy and precision with the traditional methods but the sample requirement is smaller. The average uncertainties on the individual isotope composition measurements for the Inilchek and Summit samples were approximately ± 0.10 (2s) and ± 1.5 (2s), respectively. The larger uncertainties for the Greenland samples resulted from increased blank and the smaller sample size used for analysis. Decreasing the blank concentrations by an order of magnitude show that a factor of two to three improvement in the uncertainties on small sample sizes is attainable with the double spike technique. The sulfur isotope values in the Inilchek snowpit demonstrate no seasonality; while the values observed in the Greenland snowpit exhibit strong seasonality, where the values are 34S-depleted in the winter months and are 34S-enriched in the summer months. Mass balance calculations indicate that anthropogenic sources are the main contributor (75%) to sulfate during most of the year for both locations.Item THE INVESTIGATION OF MERCURY REDOX CHEMISTRY IN NATURAL WATERS AND THE DEVELOPMENT OF A NEW METHOD FOR INCUBATION EXPERIMENTS(2005-04-26) Whalin, Lindsay; Mason, Robert P; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The redox processes that control Hg speciation in natural waters are poorly understood and study results often disagree, primarily a consequence of varied and often flawed methodologies. An incubation method was developed utilizing PFA Teflon® bag reaction vessels to reduce sources of error, and additions of isotopically labeled Hg to quantify rate constants. With low measures of error and duplicate bag reproducibility, this method was applied via incubations of natural waters in ambient sunlight to test three theories; 1) Hg oxidation and reduction are photochemically mediated, 2) Hg reduction is correlated to [DOC], and 3) Hg oxidation is enhanced by halides. The former was proven through the detection of redox chemistry during daylight and its absence in the dark. Results indicate the importance of both redox processes in natural waters, but failed to prove the latter two theories, potentially due to greater than expected [DOC] in one experiment.