Browsing Geology Theses and Dissertations by Issue Date
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- ItemThe 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.
- ItemImpact 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.
- ItemPressure-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.
- ItemA PETROLOGIC, GEOCHEMICAL AND OSMIUM ISOTOPIC STUDY OF SELECTED PRECAMBRIAN KOMATIITES(2004-11-17) Gangopadhyay, Amitava; Walker, Richard J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The major and trace elements, and Re-Os isotope systematics of the ca. 2.7-Ga komatiites from the Alexo and Dundonald Beach areas in the Abitibi greenstone belt, Canada, and ca. 2.0-Ga komatiites from Jeesiörova, Finnish Lapland were examined in order to constrain the long-term Os isotopic evolution of their mantle sources and also to evaluate different petrogenetic models for their generation. The Re-Os isotope results for whole-rock komatiites and chromite separates from Alexo, Dundonald Beach and the Jeesiörova areas, all yield precisely chondritic intial Os isotopic compositions of their mantle sources (gamma-Os = -0.1± 1.0, 0.0 ± 0.6 and 0.1± 0.5, respectively), consistent with that for the projected primitive upper mantle. The uniform chondritic initial Os isotopic compositions of these komatiites suggest their derivation from mantle sources that had undergone neither long-term Re-depletion nor enrichments in radiogenic Os-187. The uniform chondritic initial Os isotopic compositions of the Ti-rich Finnish komatiites also suggest that the processes responsible for high Ti concentrations in the Finnish rocks must have either been coeval with the generation of the rocks or Re and Os were not significantly fractionated during the processes. The uniform Os isotopic compositions of the Precambrian mantle source regions for these komatiites are in contrast to the variably radiogenic Os isotopic compositions, commonly reported for the present-day ocean island basalts and modern arc-related rocks. This suggests that the Precambrian mantle was significantly more homogeneous than the present-day mantle. Finally, the high-precision Os isotopic results of this study, combined with those previously published for Precambrian ultramafic rocks suggest that although Os isotopic heterogeneities began to appear in the terrestrial mantle as early as ~2.8 Ga, large portions of the Precambrian mantle were evidently not affected by the processes responsible for long-term Os isotopic heterogeneities.
- ItemCharacterization of the spatial differences in hydrological functioning in a tidal marsh, Patuxent River, MD: A framework for understanding nutrient dynamics(2004-12-08) Phemister, Karen; Prestegaard, Karen L.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study investigates spatial variations in sediment hydraulic conductivity (K), network channel shape and horizontal groundwater flux magnitude toward tidal network channels in a freshwater tidal marsh. Results showed the average value of K at zero meters from the creekbank was significantly higher than the K at both 5 and 15 meters from the network channel creekbank. Creekbank gradient did increase with increasing distance from the main channel and some data indicated that channel width-to-depth ratio (F), which is inversely related to creekbank gradient, correlates well with K. In addition, horizontal groundwater flux magnitude at a depth of 11 cm was significantly greater than flux magnitude at 22 cm below the ground surface at the first-order network channel location. Horizontal flux magnitude was also significantly higher from 5 to 0 meters than from 15 to 5 meters from the network channel creekbank at both the first- and second-order channel locations.
- ItemROCK FABRIC ANALYSIS OF THE SIERRA CREST SHEAR ZONE SYSTEM, CALIFORNIA: IMPLICATIONS FOR CRUSTAL-SCALE TRANSPRESSIONAL SHEAR ZONES(2004-12-08) Bentley, Callan; Jiang, Dazhi; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Crustal-scale tabular shear zones are common features of transpressional orogens and arcs. Though foliation development in shear zones is well understood, the development of another rock fabric element, lineation, is not. Computer modeling shows that monoclinic solutions for lineations are unstable; triclinic models better explain the variation of lineation in natural transpressional zones. This thesis investigates an example of natural lineation variation in three segments of the Sierra Crest shear zone system, Sierra Nevada, California. Field data collected during large-scale mapping indicate an early (Paleozoic?) generation of deformation overprinted by a Cretaceous, dextral, reverse (west-over east), steeply-dipping shear zone active for ~20 Ma. A third generation of deformation overprints previous fabrics. Estimates of shear zone volume reduction are correlated with volumetrically-complementary emplacement of the Sierra Nevada batholith across strike. Disagreement between single-domain model predictions and field observations is interpreted as being due to heterogeneous strain localization on many scales.
- ItemKinematics and Deformation History of the Cross Lake Greenstone Belt(2004-12-17) Dai, Tianhuan; Jiang, Dazhi; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Greenstone belts are a common feature of Archean terrains. However, the tectonic environment for Archean earth remains in hot debate: did vertical or horizontal crustal movement dominate the Archean Eon? Small-scale structural analysis is applied to a late Archean greenstone belt in the Superior Province of the Canadian Shield in order to test the two end member hypotheses. Detailed structural analysis reveals that the Cross Lake greenstone belt has undergone three major events of deformation. The early event of ESE-WNW convergence and crustal thickening initiated folding and produced northeast-trending shear zones. The configuration of the northwestern Cross Lake area is largely due to this event. This was followed by the juxtaposition of the Nelson River - Pipestone Lake high-strain zone. The last event was the juxtaposition of the Eves Rapids Complex with the development of a major northeast-trending fault, which overprints all previous deformation. The strain geometry and structural features suggest that the first event of crustal thickening likely involved vertical tectonics induced by gravitational instability rather than transpression.
- ItemExamination of the Thermal Decomposition of Chrysotile(2005-04-22) Crummett, Courtney; Candela, Philip A; Wylie, Ann G; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The decomposition of pure chrysotile from Thetford, Quebec heated at constant temperature in air from 200-1000°C for 4 to 720 hours was studied by using X-ray diffraction and optical microscopy techniques. No morphological changes were observed optically below 450ºC and 24 hours, although X-ray diffraction data suggest that chrysotile degrades then recrystallizes below 450˚C. Throughout the temperature range of 500-1000ºC, changes in the refractive indices observed included several cycles of increasing and decreasing magnitudes and ranges. Chrysotile was no longer present above 575˚C and 24 hours. The lowest temperature of forsterite appearance was at 500˚C and 720 hours and the lowest temperature of enstatite appearance was at 800˚C for 8 hours. Broad reflections were observed within 500-750˚C at 16-8Å, 4Å, and 3Å spacings. These reflections suggested the possible presence of talc and tridymite-like mineral phases. X-ray diffraction and optical microscopy results of this study show that the decomposition of chrysotile is more complex than previously understood.
- ItemDETERMINATION 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.
- ItemA CHEMICAL AND ISOTOPIC COMPARISON OF TWO 1ST-ORDER AGRICULTURAL TRIBUTARIES, KENT COUNTY, MARYLAND(2005-05-27) Teerlink, Jennifer; Candela, Philip; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Nitrate derived predominantly from agricultural fertilizers results in algal blooms and wide-spread anoxia in estuarine environments. Roughly half the nitrate delivered to the Chesapeake Bay is derived from groundwater (residence time up to 40 years). Two 1st-order tributaries in eastern Maryland were sampled on five dates. Over the study interval, average nitrate, alkalinity, and delta 13C were 207 µM, 212 µM, and -12.1, respectively, in the ditched tributary, and 106 µM, 451 µM, and -9.7, respectively, in the unaltered tributary. Ditching of the western tributary results in discharge of less anthropogenically-altered groundwater. Nitrate, calcium, and magnesium concentrations decrease and 13C abundance of DIC becomes enriched along the reach. The unaltered tributary is stagnant in the headwaters, resulting in consumption of dissolved oxygen and denitrification. Alkalinity correspondingly increases suggesting reduced carbon as the electron source for this microbial process. Alteration of 1st-order tributaries influences the processing and delivery of nutrients.
- ItemTerrestrial Alteration of CM2 Chondritic Carbonates in a Suite of Paired Antarctic Meteorites.(2005-06-01) Tyra, Mark Anthony; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The δ18O, δ17O, δ13C, and δ14C compositions of carbonate grains were measured from paired Antarctic CM2 chondrites (EET96006, EET96016, EET96017, and EET96019). Oxygen isotopic compositions reveal both terrestrial and extraterrestrial carbonate sources. Bulk δ13C and δ14C measurements suggest at least two terrestrial carbonate components: 1) carbonate derived from equilibration of atmospheric CO2 with silicates during weathering reactions, and 2) carbonate derived from a second undefined source. Oxygen and carbon isotope data suggest that silicate weathering reactions drove oxygen isotopic composition of Antarctic water away from the terrestrial fractionation line (TFL) (D17O < 3.6). Further oxygen isotopic compositional constraints were placed by Secondary Ionization Mass Spectrometry (SIMS) of in-situ carbonates. We suggest three sources for meteoritic carbonate: 1) primary calcite formed in the presence of evolving planetesimal water and 2) secondary calcite derived from alkalinity laden terrestrial water and 3) alkalinity formed as the result of weathering of meteoritic silicates.
- ItemAn experimental study of silver partitioning in sulfide-oxide-melt systems at 800 degrees C(2005-06-01) Englander, Leah; Candela, Philip A; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Silver solubility, and partitioning of silver between pyrrhotite and magnetite at a(Ag) = 0.13 ± 0.04 (1σ) has been determined at 800°C, and a range of fS2 between -4.06 and -0.18 and fO2 between -14.90 and -11.62 . Solubility is reported at 2800 ± 700 (1σ) ppm for pyrrhotite and 200 ± 100 (1σ) ppm for magnetite, at an a(Ag) = 0.13 ± 0.04 (1σ). Data indicate that silver partitions to a larger extent into pyrrhotite as compared to magnetite, = 13 ± 14 (1σ). Silver solubility in a melt, and vapor and brine present at 800°C, 140 MPa and fO2 ranging between QFM and NNO in Ag-metal saturated hydrothermal experiments were quantified by using LA-ICP-MS. Approximately 1 ppm silver is soluble in peralkaline silicate melts at an a(Ag) = 0.10. is approximated at 7 at 800ºC, 140 MPa, fO2 ranging between QFM and NNO, and a(Ag) = 0.10. Models presented in this study indicate that Au, Ag, and Cu concentrations in silicate melts are impacted by the fractionation of magnetite and pyrrhotite. Sulfide mineral fractionation has a larger effect on ore metal budgets in silicate melts than magnetite. Magnetite does not remove as significant quantities of ore metals as does pyrrhotite; in high fO2 melts the minimal effects of Au, Ag, and Cu sequestering by magnetite should yield a high removal of these metals by the MVP. Small variations in fO2 can affect metal budgets in magmatic-hydrothermal systems because of the partially overlapping stability fields of pyrrhotite and magnetite.
- ItemTRACING SULFUR SOURCES IN AN ARCHEAN HYDROTHERMAL SYSTEM USING SULFUR MULTIPLE ISOTOPES - A CASE STUDY FROM THE KIDD CREEK VOLCANOGENIC MASSIVE SULFIDE DEPOSIT(2005-07-07) Jamieson, John William; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mass-independent fractionation of sulfur isotopes in the Archean atmosphere resulted in surface sulfur reservoirs with distinct isotopic signatures. These signatures are used to trace the movement of sulfur through an Archean seafloor hydrothermal system associated with the Kidd Creek volcanogenic massive sulfide (VMS) deposit. Isotopic measurements of sulfides from the 2.7 Ga Kidd Creek VMS deposit reflect two separate sulfur sources for ore precipitation. Subseafloor ore has a predominantly juvenile sulfur source. with a small (~3%) component of seawater sulfate, which was transported through the hydrothermal system to the site of precipitation. Surface sulfides contain a significant proportion of sulfur that was stripped from coeval seawater sulfate or native sulfur at the site of precipitation. Mass-independent isotopic signatures are also used in a sulfur multiple-isotope framework to evaluate isotopic disequilibrium and to assess the suitability of mineral pairs for paleothermometric calculations in the Kidd Creek VMS deposit.
- ItemA Stable Isotope Investigation of Precipitation Nitrate(2005-08-15) Cooney, Katherine Suzanne; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The isotopic composition of precipitation nitrate reflects the processes involved in its formation. Silver nitrates prepared from Maryland precipitation were thermally decomposed to gases for mass spectrometric analyses. Nitrate δ15N ranged from -5.1±0.5 to +5.9±0.5 (1σ), δ18O ranged from +42.6±0.5 to +81.9±0.5, and Δ17O ranged from +15.33±0.05 to +31.71±0.05. Precipitation samples were re-analyzed for δ15N and δ18O using a method in which bacteria convert nitrate to N2O for analyses, and d15N ranged from -3.6±0.2 to +7.1±0.2 and δ18O ranged from +61.6±0.3 to +86.8±0.3. Differences between the methods were attributed to organic contaminants in the silver nitrates. Nitrate δ15N, δ18O, and Δ17O were highest in the winter and lowest in the summer. Ion concentrations, storm track data, and the δ34S of precipitation sulfate were used for interpretations. Most likely, δ15N varied due to seasonal changes in the NOx photo-stationary state, and δ18O and Δ17O varied due to changes in oxidation chemistry.
- ItemLITHIUM ISOTOPIC SYSTEMATICS OF THE CONTINENTAL CRUST(2005-12-05) Teng, Fang-Zhen; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In order to fully utilize Li isotopes as a geochemical tracer, it is necessary to characterize the Li isotopic compositions of different geological reservoirs, and quantify the magnitude of isotopic fractionations for various conditions and compositions. However, our knowledge of Li isotope geochemistry is mostly limited to the hydrosphere and mantle. Little is known about either the Li isotopic composition of the continental crust or the mechanisms by which Li isotopes are fractionated. The primary objective of this thesis is to characterize the Li isotopic composition of the continental crust. Over 50 upper crustal rocks including loess, shale, granite, and upper crustal composites, have been measured and show a limited range of Li isotopic composition (-5 to +5), with an average (0 ± 2 at 1s) that is lighter than the average upper mantle (+4 ± 2). More than 70 high-grade metamorphic rocks, including granulite xenoliths and composite samples from high-grade metamorphosed terranes have been analyzed to constrain the Li isotopic composition of the deep crust. Thirty composite samples from eight Archean terranes show mantle-like Li isotopic composition (+4 ± 1.4 (at 1σ)) while 44 granulite xenoliths display a much larger Li isotopic range from -17.9 to +15.7 with an average of -1± 7 (1σ), isotopically lighter than the mantle. These data indicate that the continental crust on average has a lighter Li isotopic composition than the upper mantle from which it was derived. Given that Li isotopes do not fractionate during high-T magmatism, juvenile crust and the mantle should have identical Li isotopic compositions. Therefore, the isotopically light continental crust is likely the result of secondary processes, e.g., weathering, metamorphism and low-T intracrustal melting. Previous studies have shown that weathering can strongly fractionate Li isotopes, with heavy Li leaching into the hydrosphere, leaving the rock residue isotopically light. Studies carried out in this thesis indicate that Li isotopes can be fractionated by diffusion, metamorphic dehydration and granite differentiation. Collectively, these processes shift the continental crust to isotopically lighter and the hydrosphere heavier than the mantle with respect to δ7Li.
- ItemRhenium-Osmium Age Determinations of Glaciogenic Shales from the Mesoproterozoic Vazante Formation, Brazil(2006-12-12) Geboy, Nicholas J.; Kaufman, Alan J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Vazante Group is a sedimentary package of carbonate interbedded with diamictite and organic-rich shale, potentially representing a Pleistocene-style glacial cycle of Proterozoic age in Brazil. Lacking volcanic ashes, the age of the Vazante strata has eluded researchers. This study applied the Re-Os geochronometer to successive organic-rich horizons, revealing pre- and synglacial ages of ca. 1.35 and 1.13 Ga. Time-series stable isotopic studies were conducted throughout the succession to investigate active metabolisms and depositional redox conditions. Given the radiometric constraints, the Vazante Group contains the only recognized glacial deposit of Mesoproterozoic age, but may be similar to Neoproterozoic events. Vazante strata were deposited on a low-latitude passive margin formed during the diachronous break-up of the supercontinent Rodinia, with local ice-sheets potentially developing on uplifted rift margins. A low-latitude Mesoproterozoic glaciation suggests that the "climate paradox" of the Neoproterozoic extends back further than previously believed.
- ItemStable Isotopic Evidence for the Rise of Oxygen and Reorganization of the Sulfur Cycle from the ca. 2.3 Ga Duitschland Formation, South Africa(2006-12-13) Baker, Margaret Anne; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Metasedimentary rocks from the Transvaal Supergroup, South Africa, were used in a stable isotopic study to investigate the evolution of the Earth's early atmosphere and early biogeochemical cycles. Sulfur isotopic evidence indicates that the transition from an anoxic to and oxic surface environment is recorded in the Duitschland Formation. The mass-independent fractionation signal (Δ33S and Δ36S) of Precambrian sulfide and sulfate samples diminishes from the Malmani Subgroup at the base of the study area to the top of the Duitschland Formation. Coincidental with the reduced mass-independent fractionation signal is a sizable jump in δ34S and δ13C fractionation related to biological activity. These data suggest that the rise of atmospheric oxygen to levels above which mass-independent fractionation in sulfur species in not effectively recorded occurred at the time of the deposition of the upper Duitschland Formation and was associated with a fundamental reorganization of the carbon and sulfur cycles.
- ItemA Geochemical and Osmium Isotopic Study of Group IVB Irons and Ungrouped Irons Chinga, Tishomingo and Willow Grove(2007-01-23) Honesto, Jenise Marie; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Re-Os isotopic and highly siderophile element (HSE) systematics of IVB irons and ungrouped iron meteorites Chinga, Tishomingo and Willow Grove are presented. IVB iron composition has been constrained by HSE (Re, Os, Ru, Ir, Pt, and Pd) abundances, showing a limited range of Pt concentration (31.15 to 32.75 ppb) despite a wide range in nickel (Ni) content (15.64 to 18.02 wt %). Using the Re-Os system, a crystallization age of 4575 ± 56 Ma was determined for the group. Fractional crystallization modeling suggests these samples crystallized from a non-chondritic parent-body. Chinga is shown to be unrelated to the IVBs through HSE abundances, and fractional crystallization modeling. Willow Grove and Tishomingo have HSE patterns comparable to the IVBs. Tishomingo has a platinum concentration within the range of the IVBs. The high Ni content of these two irons (>27 wt %) poses a problem for their formation by simple fractional crystallization.
- ItemIsotopic systematics of ultramafic and mafic rocks of the Taitao ophiolite, southern Chile(2007-03-14) Schulte, Ruth Frost; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A variety of ultramafic and mafic rocks from a well-mapped sampling of the ~6 Ma Taitao Ophiolite, Chile, have been examined. Calculated initial 187Os/188Os ratios of the peridotitic rocks range from 0.1168 to 0.1282. Similar ranges of Os isotopic compositions have been reported for abyssal peridotites, peridotites, and chromites from other ophiolites. A correlation between Mg # of primary olivine and Os isotopic compositions of whole rock peridotites suggests that the isotopic variability is due to variable extents of partial melting at ~1.5 Ga. This conclusion is also supported by a linear correlation between 187Re/188Os and 187Os/188Os. The ancient melting event requires that this km-scale block of the mantle remained isolated and unmixed within the convecting upper mantle for a period of ~1.5 Ga. The results are comparable to global observations of Os isotopic compositions in mid-ocean ridge basalts versus abyssal peridotites, but the geologic relations here are well-defined.
- ItemSulfur isotope fractionations in biological systems: insight into the Proterozoic biosphere(2007-03-26) Johnston, David Thomas; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study focuses on developing a framework for interpreting isotopic fractionations in the four stable sulfur isotopes, with a special focus on microbial effects. Calculations of low temperature equilibrium fractionations between various sulfur species were performed and suggest a tightly constrained fractionation relationship between the isotopes. This work was followed by conducting a series of batch culture experiments with two prominent sulfur metabolisms: sulfate reduction and sulfur disproportionation. The results of these experiments illustrate that measurable deviations from the equilibrium predictions exist, and further serve to highlight the ability of these measurements to differentiate between fractionations produced by these two metabolisms. This is a distinction not possible from traditional isotope measurements alone (34S/32S). In addition, models constructed to describe these observations provide insight into the intra-workings of these metabolic pathways, with the most recent experiments suggesting that the traditionally accepted model for isotope fractionation produced by sulfate reducers be re-evaluated. The microbial fractionations were also used to calibrate global sulfur cycle box models. These models have been coupled with the measurement of natural samples to illustrate the onset of microbial disproportionation ~800 million years earlier in Earth history than previously thought and to describe the sulfur cycle of three Proterozoic marine basins.