Geology
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Item CONSTRAINTS ON THE DEPOSITIONAL AGES OF LESSER HIMALAYAN ROCKS IN CENTRAL NEPAL AND THEIR STRUCTURAL IMPLICATIONS(2009) Burgy, Katherine Diane; Martin, Aaron; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The lack of good exposures and paucity of datable horizons in central Nepal has hindered the ability of geologists to piece together a relatively cohesive and straightforward stratigraphic succession within the Lesser Himalaya. U-Pb isotopic analyses of detrital zircons from the Modi Khola valley indicates maximum depositional ages of ~1875 Ma for the Kuncha Formation, ~1800 Ma for the Fagfog Formation, and ~ 1780 Ma for the Kushma Formation. The intrusive 1831 ± 17 Ma Ulleri augen gneiss provides a minimum depositional age bound for the Kuncha. Combined, these data suggest the Kuncha Formation is the oldest member of the Lesser Himalayan series in central Nepal. Additionally, 13C data suggest the Malekhu Formation of the Lakharpata Group was deposited before ca. 1250 Ma. A field mapping comparison based on the redefined stratigraphy indicates the Ramgarh thrust is located >10 km farther south than previously mapped, potentially reducing regional shortening estimates.Item Advection-Diffusion Controlled Lithium Isotopic Distribution in Contact Aureoles: A Case Study from the Florence County Pegmatites, Wisconsin(2009) Liu, Xiaoming; Hier-Majumder, Saswata; Rudnick, Roberta L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Stable isotopes are useful tracers of fluid-rock interactions in contact aureole settings. To date, only a few case studies have used Li isotopes to study fluid-rock interactions in contact aureole settings. These studies highlight the very large Li isotopic fractionation that can be generated in these settings via diffusion of Li from the pluton into the country rocks, but none of these studies have generated a complete and detailed section of the contact aureole needed to understand the Li distributions. Here, I report the results from a combination of Li isotope analyses and 2-D advection-diffusion modeling for two detailed profiles through country rocks adjacent to Li-rich pegmatite dikes in the Florence County pegmatite field. The results show that the Li concentration and isotopic distribution in the two contact profiles is consistent with fluid infiltration and diffusion of Li through a grain boundary fluid from the pegmatites into their country rocks.Item Highly Siderophile Element and Tungsten Systematics of Hawaiian Picrites(2009) Ireland, Thomas James; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A suite of Hawaiian picrites (MgO > 13 wt.%), and associated basalts, that represent some of the most primitive melts from the Hawaiian mantle source regions were analyzed for their W, highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd and Re) and 186Os-187Os isotope systematics. These picritic samples are among the most primitive samples produced from the Hawaiian main-shield stage volcanoes. As such, they may preserve considerable information about the mantle source regions from which they were derived. Hawaii is of particular interest because there is geochemical and geophysical evidence that suggest that the Hawaiian plume may originate at the core-mantle boundary. If any outer core material is incorporated into plume lavas, it could carry important geochemical information. The primary goal of this study is to improve our understanding of the processes and materials that may affect the mantle source regions of the Hawaiian volcanoes. Abundances of HSE and W, as well as Os isotopes, are useful tools for evaluating the mantle source regions of ocean island basalts because their absolute and relative abundances may be affected by various mantle processes, including the recycling of oceanic crust and sediment, mantle metasomatism, and other forms of crystal-liquid fractionation. In addition, these elements may be suitable for addressing the question of core-mantle interaction, because the core is highly concentrated in both the moderately siderophile and highly siderophile elements, and may have a distinct Os isotopic composition relative to the mantle. The collected data imply that W abundances in the Hawaiian mantle sources are similar for all volcanic centers, and enriched relative to depleted MORB mantle. This suggests that W may be controlled by a primary source component that is less depleted in incompatible elements than the depleted mantle. HSE abundances in the picrites are controlled predominantly by crystal-liquid fractionation processes, and may reflect the presence of residual sulfides in the mantle sources. Lastly, the 187Os/188Os variations are consistent with some proportion of a recycled oceanic crust component; however, variations in 186Os/188Os require another process, such as the incorporation of variable Pt-enriched base-metal sulfides, or mixing with an 186Os-187Os enriched reservoir.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.Item Diffusion of Oxygen and Lithium Isotopes at a Contact between the Bushveld Complex and Metasedimentary Rock: Implications for the Timescale of Phepane Dome Diapirism(2009) Potter, Rachel; Penniston-Dorland, Sarah; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Within the Eastern Lobe of the Bushveld Complex, the Phepane Dome is a circular structure of metasedimentary rock hypothesized to have formed as a wallrock diapir. To constrain the duration of Phepane Dome formation using one-dimensional diffusion models of oxygen and lithium exchange between the Bushveld Complex and the Phepane Dome, samples taken across the contact between these two lithologies were measured for their O and Li isotopic compositions and Li concentrations. Models of O and Li diffusion through melt and through aqueous fluid were fit to the data, resulting in a diffusive distance of 1.0 m for oxygen and 14.1 m for lithium. Using experimentally constrained parameters for O and Li diffusion, a range of 2 kyrs to 2 Myrs was calculated from the diffusive distances. This is consistent with previous studies of the time for crystallization of the Bushveld Complex and a model of Phepane diapir development.Item Age, composition, and origin of the lower continental crust, northern Tanzania(2008) Mansur, Adam T.; Rudnick, Roberta L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Granulite xenoliths from the East African rift in Tanzania sample the lower continental crust beneath both the stable Tanzanian craton and the adjacent, polymetamorphic Mozambique belt. Protoliths of surface rocks in both terranes formed 2.8 to 2.6 Ga ago, but only the Mozambique belt underwent significant post-Archean metamorphism. Most granulite xenoliths represent mafic magmas formed in a convergent margin at ca. 2660 Ma that were subsequently metamorphosed at lower crustal conditions. However, a small number of xenoliths are more similar to intraplate magmas, suggesting that at least two episodes of lower crustal growth occurred during the Archean. No post-Archean additions to the crust were detected beneath either terrane, but xenoliths from the Mozambique belt record high P-T conditions, suggesting equilibration at the base of orogenically thickened crust. By contrast, xenoliths from beneath the craton equilibrated under conditions typical of unthickened continental crust.Item Experimental study of the partitioning of Cu, Ag, Au, Mo and W among pyrrhotite and immiscible Fe-S-O and silicate melts(2007-08-14) Mengason, Michael James; Candela, Philip A; Piccoli, Philip M; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Partition coefficients have been determined for several transition metals amongst a rhyolitic silicate melt, pyrrhotite, and an immiscible Fe-S-O melt. Two sets of experiments were performed in sealed silica tubes at 1035°C-1045°C, FMQ-NNO O2, and logS2 ~1 bar, and analyzed by EMPA and LA-ICP-MS. Experiments yielded DFe-S-O/melt ±1σ(mean) for Au =300±100, Ag =120±20, Cu >200, Mo =90±10 and W =9±3, and Dpo/melt for Au =120±50, Ag =58±8, Cu >200, Mo =35±3 and W =1.2±0.6x10-3. Some preliminary data are also presented on the partitioning of Si, Ti, V, Co, Zn, Re, Th and U. Modeling predicts the loss to a daughter melt of up to 50% of the initial Au, >37% Cu, 24% Ag, 19% Mo, and 2% W when Fe-S-O melts are removed with other cumulate phases during fractionation. This would reduce the metal available to potential intrusion-related hydrothermal ore deposits.Item Isotope and Organic Geochemistry of a Unique Proterozoic, Postglacial Succession: The Lapa Formation, Vazante Group, Brazil(2007-05-09) Brody, Kristina; Kaufman, Alan J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study is the second to investigate biological characteristics associated with Proterozoic glaciation via molecular fossils from organic matter preserved in shale. In the Vazante Group, Minas Gerais, Brazil, the Serra da Lapa Formation unconformably overlies a formation recently dated to ca. 1.13 Ga. Lithologic and isotopic data suggest the Lapa represents deposition immediately after either an early Neoproterozoic "snowball Earth" ice age or a possible regional, but still low-latitude, late Mesoproterozoic ice age. The relative abundances of biomarkers and other organic molecules show variations that match lithologic and isotopic changes observed in 40 meters of exploration drill core studied. Inconsistencies among biomarker abundances as well as differences between the organic matter of the Lapa Formation and that of the underlying formation hint at heterogeneity among and within formations. A more complete picture of the Vazante Group is warranted before characteristics of preserved organic matter can be interpreted in the context of depositional environments or postdepositional processes.Item Sulfur 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.Item Isotopic 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.