Geology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2774
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Item Pressure-Temperature-time-Deformation (P-T-t-D) History of High-Grade Gneisses of the Port aux Basque Area, Southwest Newfoundland, Canada(1994) Burgess, Jerry Lee; Brown, Michael; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)A polyphase deformation history (D1-D4) and upper amphibolite facies metamorphism characterize the Port aux Basques Gneisses. Late-D1 to early D2 kyanite porphyroblasts each contain inclusion trails that preserve S1. Reaction out of muscovite, staurolite and kyanite in favor of sillimanite + garnet + alkali feldspar-bearing assemblages in the matapelitic gneisses record syn- to late- D2 peak metamorphic conditions. Isograd surfaces related to syn-D2 metamorphism were probably subhorizontal to inclined but not their metamorphism were probably subhorizontal to incline but now their map pattern reflects subsequent deformation by D3. Fluid-present melting initiated in the kyanite zone and continued into the sillimanite zone. Metamorphic conditions increase to the southeast with 'peak' temperatures of c. 700-750° at 8-9 kbar associated with the D2/M2 thermal regime. A Pb207/Pb206 date of c. 417 Ma was obtained from titanite in high-grade rocks of the Harbour le Cou Group. This date provides a minimum contraint for the M2 event. Hornblende from a nearby amphibolite yields an 40Ar/39Ar isotope correlation date of c. 419 Ma. Muscovite at the same locality records a 40Ar/39Ar plateau date of 391 Ma. Hornblende and muscovite separates from rocks of the Port aux Basques Complex yield similar 40Ar/39Ar dates. Calculations indicate that post- D3 cooling rates of approximately 8-1°C/Ma are required for the area. They kyanite to sillimanite transition and D2 structures suggest a clockwise trajectory in P-T space as a result of Silurian orogenesis.Item The Kinetics and Mechanism of Sedimentary Iron Sulfide Formation(1976) Pyzik, Albert John; Sommer, Sheldon E.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)The reaction between goethite, ∝-FeOOH, and aqueous bisulfide ion, HS, was studied under conditions representative of estuarine sediments. The concentration-time curves of the following species were determined by spectrophotometric methods: total sulfide, dissolved sulfide, precipitated sulfide, thiosulfate ion, sulfite ion, elemental sulfur , and dissolved (<0.1μ) iron. Polysulfides were monitored by ultraviolet absorbance measurements, while the hydrogen ion concentration was determined with a pH electrode. Elemental sulfur, both as free and polysulfide sulfur was found to be the major sulfide oxidation product. Thiosulfate ion comprised about 14±8% (electron balance-wise) of the oxidation products. Concentration-time curves of precipitated sulfide sulfur were analyzed by the initial rate method to determine the rate expression. The rate expression for the reaction between ∝-FeOOH and HS- is d [FeS]/dt = k [HS-]i^97 (H+)i^82 A1.1FeOOHi where d [FeS]/dt is the rate of precipitated iron sulfide formation, (H+)i is the initial hydrogen ion activity, AFeOOHi is the initial geothite surface are in m^2/1, and k is the rate constant with the value 31±10 M^-1 1^-1 m^-2 min ^-1. 0.97, 0.82, and 1.1 are the reaction orders for the species bisulfide ion, hydrogen ion, and goethite surface area respectively. A combination of hydrogen balance and electron transfer balance and stoichiometric reactions were studied in view of the rate expression to yield a mechanism. The multistep mechanism consisted of several parallel and consecutive reactions: (1) the protonation reaction of the goethite surface, (2) the parallel reduction reactions of ferric iron to yield elemental sulfur and thiosulfate as oxidation products, (3) the dissolution of the ferrous hydroxide, and (4) the precipitation reaction of dissolved ferrous species and aqueous bisulfide ion. The rate determining step in the reaction sequence was the dissolution step. Results of this study indicate that the oxidation of sulfide species by ferric iron may be a significant source of elemental sulfur in the sediment. Elemental sulfur is necessary for the formation of pyrite (FeS2), the thermodynamically stable iron sulfide. In addition, the previous studies of the interstitial waters of anoxic sediments showed an excess of "dissolved" iron which was greater than calculated from equilibrium solubility products. It is suggested from particle size studies of the precipitated iron sulfide that these high concentration are a result of the submicron particles of ferrous sulfide (<0.1μ). These particles would obviously pass through the .45μ filters which are traditionally used as the dividing line for dissolved and particulate species.Item Impact of Isotopic Heterogeneity in Shallow Systems on Modeling of Stormflow Generation(1993) Kendall, Carol; Candela, Philip A.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)A major uncertainty in hydrologic and geochemical modeling of stormflow generation in watersheds has been quantification of the contributions of water and solutes from different sources and hydrologic pathways to streamwater. Isotopic techniques have recently gained widespread acceptance as useful tools in the investigation of sources of stream flow, but considerable debate still surrounds the question of whether the spatial and temporal variations in the isotopic and chemical compositions of water components are negligible. At Panola Mountain, Georgia, a 2-year study of temporal and spatial variability in rain and throughfall has determined that average throughfall is enriched by 0.5% in 0^18O and 3.0% in 0D relative to rain; site-specific differences in canopy cause up to 1.2 % variation in 0^18O among collectors for the same storm; and throughfall ^18O enrichment takes place throughout the storm, not just at the beginning. Evaporative losses are greater and throughfall is generally slightly enriched in ^18O in conifer forests relative to deciduous forests. However, throughfall shows little evidence of evaporative fractionation; instead, the high deuterium-excess values suggest considerable exchange with re-evaporated waters. A 490-m^2 artificial catchment in China was used to investigate the effects of temporal variations in rain composition, and temporal and spatial variations in dominant water flowpath, on the development of isotopic and chemical heterogeneity in soil waters and groundwater. In response to changes in storm intensity, variability in the amounts of water transported via piston versus macropore flow caused a 4% range in 0^18O of groundwaters. Selective storage of early rain in shallow soils makes characterization of the isotopic composition of infiltrating rain water problematic. Seasonal and hydrologic differences in the sources of alkalinity were investigated at four watersheds at Catoctin Mountain, Maryland, by analyzing the dissolved inorganic carbon in streamwater for 0^13C. Because of short residence times, the isotopic signatures of the two primary carbon sources, calcite and soil-derived carbonic acid, do not appear to be appreciably overprinted by exchange reactions, biological recycling, or degassing; hence, 0^13C seems to be a useful semi-conservative tracer of water flowpaths and carbon sources.