Browsing by Author "Bodnar, Robert J."

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    Advances in Subduction Zone Processes
    Gorce, Jennifer Shannon (Virginia Tech, 2018-06-29)
    Subduction zones are an important recycling center at which material from the exterior of the Earth is transported to Earth's interior. The processes that occur along subduction zones have important implications for elemental cycles, geodynamics, and material mass transport. The cold, dense subducting lithosphere experiences prograde metamorphism as it transitions from blueschist to eclogite facies resulting in the breakdown of volatile-bearing minerals and producing anhydrous minerals and a free fluid phase. Previous works attempting to understand the evolution of subducted lithologies have provided a firm foundation in which to apply field work, computational thermodynamic modeling, and geochronological techniques in order to better constraint the Pressure-Temperature-time (P-T-t) paths and dehydration of subducted lithologies. This dissertation; 1.) Explores novel approaches to modeling and predicting fluid/rock interactions during deep (>60km) subduction, and 2.) Questions what the calculated P-T-t path from eclogite lithologies reveals about early exhumation of subducted terrains. The second chapter focuses on how externally-derived hydrous fluids can decarbonate subducted basalt, liberate carbon and transfer it to the overlying mantle wedge, where it can be incorporated into melt that forms volcanic arcs. Here, the thermodynamic response to the infiltration of external fluids assuming open system, pervasive fluid flow, is quantified. It was determined that while hotter subduction zones have more favorable P-T conditions in which to facilitate decarbonation than colder subduction, the extent of decarbonation is largely dependent on the availability of fluid from the dehydration of underlying serpentine. The third chapter constrains the P-T-t paths of subducted lithologies from Syros, Greece using a combination of thermodynamic modeling, 147Sm/144Nd garnet geochronology, and quartz-in-garnet geobarometry. This provides insight into early exhumation of subducted lithologies, and allows for the exploration of assumptions made in thermodynamic modeling and in quartz-in-garnet geobarometry. Results suggest that garnet grew over a 4.31my period from 45.71±0.98Ma to 41.4±1.7Ma, during initial exhumation from maximum subducted depths. Calculated exhumation rates are a relatively rapid, 0.4-1.7 cm/yr. Because field relationships on Syros suggest the width of the subduction channel along the slab/mantle interface is not adequate to facilitate buoyancy-driven ascension of metabasic blocks, initiation of southward retreat of the Hellenic Subduction Zone and subsequent slab rollback is proposed to have played an important role in the exhumation of subducted lithologies. The final chapter investigates the compositional controls on the P-T conditions at which dehydration due to the breakdown of hydrous minerals occur during subduction (blueschist/eclogite boundary), and the implications they have on the rheology, seismicity, and densification of the down going slab. Total Alkali Silica (TAS) diagrams reveal that eclogites are more alkali rich, implying that initial alteration of the seafloor controls the mineral evolution of subducted basalt in many cases.
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    Application of Electromagnetic Methods to Identify and Characterize Sub-surface Structures Associated with the Coles Hill Uranium Deposit
    Whitney, Joshua Andrew (Virginia Tech, 2009-04-22)
    The Coles Hill uranium deposit in Pittsylvania County, Virginia represents the largest unmined uranium resource in the United States, with an estimated resource of 110 million pounds of U3O8 in place with a cutoff grade of 0.025 wt% U3O8. The deposit is localized along a geologic unit that parallels the Chatham Fault, which separates the Triassic Danville Basin to the east from the older crystalline rocks to the west. The location of the Chatham Fault is important to understanding distribution of ore and for developing an effective mine plan. In this study the Chatham Fault location has been inferred from ground conductivity and ground penetrating radar (GPR) surveys. Anomalies in the data are consistent with previously mapped fault locations based on drillhole and geophysical data, such as gravity and magnetic surveys, collected in the 1980s. These results confirm that the strike of the Chatham Fault is approximately N40ºE and dips to the southeast with dip values ranging from 70º, in the northeast, to 50º, in the southwest.
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    Application of Fluid Inclusions and Mineral Textures in Exploration for Epithermal Precious Metals Deposits
    Moncada de la Rosa, Jorge Daniel (Virginia Tech, 2008-12-09)
    Fluid inclusion and mineralogical features indicative of boiling have been characterized in 855 samples from epithermal precious metals deposits along the Veta Madre at Guanajuato, Mexico. Features associated with boiling that have been identified at Guanajuato include colloform texture silica, plumose texture silica, moss texture silica, ghost-sphere texture silica, lattice-bladed calcite, lattice-bladed calcite replaced by quartz and pseudo-acicular quartz after calcite and coexisting liquid-rich and vapor-rich fluid inclusions. Most samples were assayed for Au, Ag, Cu, Pb, Zn, As and Sb, and were divided into high-grade and low-grade samples based on the gold and silver concentrations. For silver, the cutoff for high grade was 100 ppm Ag, and for gold the cutoff was 1 ppm Au. The feature that is most closely associated with high grades of both gold and silver is colloform texture silica, and this feature also shows the largest difference in grade between the presence or absence of that feature (178.8 ppm Ag versus 17.2 ppm Ag, and 1.1 ppm Au versus 0.2 ppm Au). For both Ag and Au, there is no significant difference in average grade as a function of whether or not coexisting liquid-rich and vapor-rich fluid inclusions are present. The textural and fluid inclusion data obtained in this study were analyzed using the binary classifier within SPSS Clementine. The models that correctly predicted high versus low grade samples most consistently (~70-75% of the tests) for both Ag and Au were the neural network, the C5 decision tree and Quest decision tree models. For both Au and Ag, the presence of colloform silica texture was the variable with the greatest importance, i.e., the variable that has the greatest predictive power. Boiling features are absent or rare in samples collected along a traverse perpendicular to the Veta Madre. This suggests that if an explorationist observes these features in samples collected during exploration that an environment favorable to precious metal mineralization is nearby. Similarly, good evidence for boiling is observed in the deepest levels of the Veta Madre that have been sampled in the mines and drill cores, suggesting that additional precious metal reserves are likely beneath the deepest levels sampled.
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    Application of fluid inclusions in geological thermometry
    Fall, Andras (Virginia Tech, 2008-12-10)
    Many geologic processes occur in association with hydrothermal fluids and some of these fluids are eventually trapped as fluid inclusions in minerals formed during the process. Fluid inclusions provide valuable information on the pressure, temperature and fluid composition (PTX) of the environment of formation, hence understanding PTX properties of the fluid inclusions is required. The most important step of a fluid inclusion study is the identification of Fluid Inclusion Assemblages (FIA) that represent the finest (shortest time duration) geologic event that can be constrained using fluid inclusions. Homogenization temperature data obtained from fluid inclusions is often used to reconstruct temperature history of a geologic event. The precision with which fluid inclusions constrain the temperatures of geologic events depends on the precision with which the temperature of a fluid inclusion assemblage can be determined. Synthetic fluid inclusions trapped in the one-fluid-phase field are formed at a known and relatively constant temperature. However, microthermometry of synthetic fluid inclusions often reveals Th variations of about ± 1- 4 degrees Centigrade, or one order of magnitude larger than the precision of the measurement for an individual inclusion. The same range in Th was observed in well-constrained natural FIAs where the inclusions are assumed to have been trapped at the same time. The observed small variations are the result of the effect of the fluid inclusion size on the bubble collapsing temperature. As inclusions are heated the vapor bubble is getting smaller until the pressure difference between the pressure of the vapor and the confining pressure reaches a critical value and the bubble collapses. It was observed that smaller inclusions reach critical bubble radius and critical pressure differences at lower temperatures than larger inclusions within the same FIA. Homogenization temperature (Th) variations depend on many factors that vary within different geological environments. In order to determine minimum and acceptable Th ranges fro FIAs formed in different environments we investigated several geologic environments including sedimentary, metamorphic, and magmatic hydrothermal environments. The observed minimum Th ranges range from 1-4 degrees Centigrade and acceptable Th range from 5-25 degrees Centigrade. The variations are mostly caused by the fluid inclusion size, natural temperature and pressure fluctuations during the formation of an FIA and reequilibration after trapping. Fluid inclusions containing H₂O-CO₂-NaCl are common in many geologic environments and knowing the salinity of these inclusions is important to interpret PVTX properties of the fluids. A technique that combines Raman spectroscopy and microthermometry of individual inclusions was developed to determine the salinity of these inclusions. In order to determine the salinity, the pressure and temperature within the inclusion must be known. The pressure within the inclusions is determined using the splitting in the Fermi diad of the Raman spectra of the CO₂ at the clathrate melting temperature. Applying the technique with to synthetic fluid inclusions with known salinity suggests that the technique is valid and useable to determine salinity of H₂O-CO₂-NaCl fluid inclusions with unknown salinity.
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    Application of the Linkam TS1400XY heating stage to melt inclusion studies
    Esposito, Rosario; Klébesz, Rita; Bartoli, Omar; Klyukin, Yury I.; Moncada, Daniel; Doherty, Angela L.; Bodnar, Robert J. (De Gruyter, 2012-05-13)
    Melt inclusions (MI) trapped in igneous phenocrysts provide one of the best tools available for characterizing magmatic processes. Some MI experience post-entrapment modifications, including crystallization of material on the walls, formation of a vapor bubble containing volatiles originally dissolved in the melt, or partial to complete crystallization of the melt. In these cases, laboratory heating may be necessary to return the MI to its original homogeneous melt state, followed by rapid quenching of the melt to produce a homogeneous glass phase, before microanalyses can be undertaken. Here we describe a series of heating experiments that have been performed on crystallized MI hosted in olivine, clinopyroxene and quartz phenocrysts, using the Linkam TS1400XY microscope heating stage. During the experiments, we have recorded the melting behaviors of the MI up to a maximum temperature of 1360°C. In most of the experiments, the MI were homogenized completely (without crystals or bubbles) and remained homogeneous during quenching to room temperature. The resulting single phase MI contained a homogeneous glass phase. These tests demonstrate the applicability of the Linkam TS1400XY microscope heating stage to homogenize and quench MI to produce homogeneous glasses that can be analyzed with various techniques such as Electron Microprobe (EMP), Secondary Ion Mass Spectrometry (SIMS), Laser ablation Inductively Coupled Plasma Mass Spectrometry (LA ICP-MS), Raman spectroscopy, FTIR spectroscopy, etc. During heating experiments, the optical quality varied greatly between samples and was a function of not only the temperature of observation, but also on the amount of matrix glass attached to the phenocryst, the presence of other MI in the sample which are connected to the outside of the crystal, and the existence of mineral inclusions in the host.
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    Applications of Melt Inclusions to Problems in Igneous Petrogenesis
    Severs, Matthew Jeremiah (Virginia Tech, 2007-06-22)
    Understanding the different igneous processes that magmas undergo is important for a variety of reasons including potential hazards associated with volcanoes in populated regions, magmatic hydrothermal ore deposition, and tectonic processes. One method of obtaining geochemical data that can help constrain petrogenetic processes is through the study of melt and fluid inclusions. The research presented here examines melt inclusions through experimental, analytical and field studies to better understand igneous petrogenesis. One potential problem associated with melt inclusions is water-loss during laboratory heating. A Raman spectroscopic technique was developed to determine water contents of silicate glasses, and this technique was applied to monitor water loss from natural melt inclusions that were heated for varying lengths of time. The results suggest that water loss is insignificant when heated for less than 12 hours but significant water loss can occur with longer duration heating. The distribution of trace elements between silicate melts and phenocrysts growing from that melt can constrain igneous processes such as fractional crystallization, assimilation, and partial melting. Partition coefficients were determined for syngenetic clinopyroxene, orthopyroxene, and plagioclase in equilibrium with a dacitic melt using the Melt Inclusion-Mineral (MIM) technique. Melt inclusion chemistry is the same regardless of mineral host phase, suggesting that the melt inclusions have not been subjected to re-equilibration processes or boundary layer development. Partition coefficients from this study are similar but typically lower than published values. Three closely-spaced monogenetic eruptive units from the active Campi Flegrei volcanic system (Italy) with similar eruptive styles were examined to better understand the evolution of the magmatic system. Results suggest fractional crystallization as the dominant process taking place over time but that magma mixing was significant for one of the eruptions. Trace element geochemical data suggest a mixed magma source of within-plate and volcanic arc components, and still retain a T-MORB signature from the subducting slab.
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    Assessing the Reactive Surface Area of Phlogopite during Acid Dissolution: An Atomic Force Microscopy, X-ray Photoelectron Spectroscopy, and Low Energy Electron Diffraction Study
    Rufe, Eric (Virginia Tech, 2000-01-14)
    The behavior during dissolution of edge and basal surfaces of the mica phlogopite were examined using in situ atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED) in an attempt to characterize the reactive surface area during dissolution. Mica minerals are the ideal material for this study because they offer a high degree of structural anisotropy. Therefore surfaces with different structures are easily identified. Dissolution is shown to proceed preferentially by removal of material from {hk0} edges. Dissolution rates were calculated by measuring the volume of material removed from etch pits, and normalizing to either the "reactive" surface area of {hk0} edges exposed at pit walls, or to a total "BET-equivalent" surface area. Rates normalized to total surface area are in the range of dissolution rates reported in the literature. Edge surface normalized rates are about 100 times faster. Long-term in situ AFM observations of phlogopite dissolution reveal that exposed (001) surfaces also display a distinct reactivity, though it operates on a different time scale. The top layer is shown to expand between 39 and 63 hours in contact with pH 2 HCl solution. Subsequent LEED analysis shows that the (001) surface becomes amorphous upon reacting with pH 2 HCl. Compositional characterization of the phlogopite after reaction shows that for pitted phlogopite surfaces, dissolution is characterized by leaching of octahedral cations and polymerization of the silica-enriched residual layer. No chemical changes or polymerization are observed for freshly cleaved unpitted phlogopite after reaction with pH 2 HCl for 24 hours. This suggests a gallery access mechanism is facilitated by edge attack, and is only significant on exposed (001) surfaces after a certain amount of dissolution by edge attack.
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    Biogenic formation of amorphous carbon by anaerobic methanotrophs and select methanogens
    Allen, Kylie D.; Wegener, Gunter; Sublett, D. Matthew, Jr.; Bodnar, Robert J.; Feng, Xu; Wendt, Jenny; White, Robert H. (AAAS, 2021-10-27)
    Elemental carbon exists in different structural forms including graphite, diamond, fullerenes, and amorphous carbon. In nature, these materials are produced through abiotic chemical processes under high temperature and pressure but are considered generally inaccessible to biochemical synthesis or breakdown. Here, we identified and characterized elemental carbon isolated from consortia of anaerobic methanotrophic archaea (ANME) and sulfate-reducing bacteria (SRB), which together carry out the anaerobic oxidation of methane (AOM). Two different AOM consortia, ANME-1a/HotSeep-1 and ANME-2a/c/Seep-SRB, produce a black material with similar characteristics to disordered graphite and amorphous carbon. Stable isotope probing studies revealed that the carbon is microbially generated during AOM. In addition, we found that select methanogens also produce amorphous carbon with similar characteristics to the carbon from AOM consortia. Biogenic amorphous carbon may serve as a conductive element to facilitate electron transfer, or redox active functional groups associated with the carbon could act as electron donors and acceptors.
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    Bubbles matter: An assessment of the contribution of vapor bubbles to melt inclusion volatile budgets
    Moore, Lowell (Virginia Tech, 2014-04-29)
    H2O and CO2 concentrations of the glass phase in melt inclusions (MI) are commonly used both as a barometer and to track magma degassing behavior during ascent due to the strong pressure dependence of H2O and CO2 solubilities in silicate melts. A requirement for this method to be valid is that the glass phase in the MI must represent the composition of the melt that was originally trapped. However, melt inclusions commonly contain a vapor bubble that formed after trapping. Such bubbles may contain CO2 that was originally dissolved in the melt. In this study, we determined the contribution of CO2 in the vapor bubble to the overall CO2 content of MI based on quantitative Raman analysis of the vapor bubbles in MI from the 1959 Kilauea Iki, 1960 Kapoho, 1974 Fuego volcano, and 1977 Seguam Island eruptions. The bubbles contain up to 90% or more of the total CO2 in some MI. Reconstructing the original CO2 content by adding the CO2 in the bubble back into the melt results in an increase in CO2 concentration by as much an order of magnitude (1000s of ppm), corresponding to trapping pressures that are significantly greater (by 1 to >3 kbars) than one would predict based on analysis of the volatiles in the glass alone. Many MI also showed the presence of a carbonate mineral phase; failure to include its contained CO2 when reconstructing the CO2 content of the originally trapped melt may introduce significant errors in the calculated volatile budget.
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    Carbon dioxide (CO2) sorption to Na-rich montmorillonite at Carbon Capture, Utilization and Storage (CCUS) P-T conditions in saline formations
    Krukowski, Elizabeth Gayle (Virginia Tech, 2013-01-24)
    Carbon capture, utilization and storage (CCUS) in confined saline aquifers in sedimentary formations has the potential to reduce the impact of fossil fuel combustion on climate change by storing CO2 in geologic formations in perpetuity. At PT conditions relevant to CCUS, CO2 is less dense than the pre-existing brine in the formation, and the more buoyant CO2 will migrate to the top of the formation where it will be in contact with cap rock. A typical cap rock is clay-rich shale, and interactions between shales and CO2 are poorly understood at PT conditions appropriate for CCUS in saline formations. In this study, the interaction of CO2 with clay minerals in the cap rock overlying a saline formation has been examined, using Na-rich montmorillonite as an analog for clay-rich shale. Attenuated Total Reflectance -- Fourier Transform Infrared Spectroscopy (ATR -FTIR) was used to identify potential crystallographic sites (AlAlOH, AlMgOH and interlayer space) where CO2 could interact with montmorillonite at 35"C and 50"C and from 0-1200 psi.  Analysis of the data indicates that CO2 that is preferentially incorporated into the interlayer space, with dehydrated montmorillonite capable of incorporating more CO2 than hydrated montmorillonite. No evidence of chemical interactions between CO2 and montmorillonite were identified, and no spectroscopic evidence for carbonate mineral formation was observed.  Further work is needed to determine if reservoir seal quality is more likely to be degraded or enhanced by CO2 - montmorillonite interactions.
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    Carbonatite and highly peralkaline nephelinite melts from Oldoinyo Lengai Volcano, Tanzania: The role of natrite-normative fluid degassing
    Berkesi, Marta; Bali, Eniko; Bodnar, Robert J.; Szabo, Abel; Guzmics, Tibor (2020-09)
    Oldoinyo Lengai, located in the Gregory Rift in Tanzania, is a world-famous volcano owing to its uniqueness in producing natrocarbonatite melts and because of its extremely high CO2 flux. The volcano is constructed of highly peralkaline [PI = molar (Na2O + K2O)/Al2O3 > 2-3] nephelinite and phonolites, both of which likely coexisted with carbonate melt and a CO2-rich fluid before eruption. Results of a detailed melt inclusion study of the Oldoinyo Lengai nephelinite provide insights into the important role of degassing of CO2-rich vapor in the formation of natrocarbonatite and highly peralkaline nephelinites. Nepheline phenocrysts trapped primary melt inclusions at 750-800 degrees C, representing an evolved state of the magmas beneath Oldoinyo Lengai. Raman spectroscopy, heating-quenching experiments, low current EDS and EPMA analyses of quenched melt inclusions suggest that at this temperature, a dominantly natritess-normative, F-rich (7-14 wt%) carbonate melt and an extremely peralkaline (PI = 3.2-7.9), iron-rich nephelinite melt coexisted following degassing of a CO2+H2O-vapor. We furthermore hypothesize that the degassing led to re-equilibration between the melt and liquid phases that remained and involved 1/mixing between the residual (after degassing) alkali carbonate liquid and an F-rich carbonate melt and 2/enrichment of the coexisting nephelinite melt in alkalis. We suggest that in the geological past similar processes were responsible for generating highly peralkaline silicate melts in continental rift tectonic settings worldwide. (C) 2020 The Author(s). Published by Elsevier B.V. on behalf of International Association for Gondwana Research.
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    The Coles Hill Uranium Deposit, Virginia, USA: Geology, Geochemistry, Geochronology, and Genetic Model
    Hall, S. M.; Beard, J. S.; Potter, C. J.; Bodnar, Robert J.; Neymark, L. A.; Paces, J. B.; Johnson, C. A.; Breit, G. N.; Zielinksi, R. A.; Aylor, G. J. (Society of Economic Geologists, 2022-03)
    The Coles Hill uranium deposit, with an indicated resource of about 130 Mlb of U3O8, is the largest unmined uranium deposit in the United States. The deposit is hosted in the Taconian (approx. 480-450 Ma) Martinsville igneous complex, which consists of the Ordovician Leatherwood Granite (granodiorite) and the Silurian Rich Acres Formation (diorite). The host rock was metamorphosed to orthogneiss during the Alleghanian orogeny (approx. 325-260 Ma), when it also underwent dextral strike-slip movement along the Brookneal shear zone. During the Triassic, extensional tectonics led to the development of the Dan River Basin that lies east of Coles Hill. The mineralized zone is hosted in brittle structures in the footwall of the Triassic Chatham fault that forms the western edge of the basin. Within brittle fracture zones, uranium silicate and uranium-bearing fluorapatite with traces of brannerite form veins and breccia-fill with chlorite, quartz, titanium oxide, pyrite, and calcite. Uranium silicates also coat and replace primary titanite, zircon, ilmenite, and sulfides. Sodium metasomatism preceded and accompanied uranium mineralization, pervasively altering host rock and forming albite from primary feldspar, depositing limpid albite rims on igneous feldspar, altering titanite to titanium oxide and calcite, and forming riebeckite. Various geothermometers indicate temperatures of less than similar to 200 degrees C during mineralization. In situ U-Pb analyses of titanite, Ti-oxide, and apatite, along with Rb/Sr and U/Pb isotope systematics of whole-rock samples, resolve the timing of geologic processes affecting Coles Hill. The host Leatherwood Granite containing primary euhedral titanite is dated at 450 to 445 Ma, in agreement with previously obtained ages from zircon in the Martinsville igneous complex. A regional metamorphic event at 330 to 310 Ma formed anhedral titanite and some apatite, reequilibrated whole-rock Rb/Sr and U-Pb isotopes, and is interpreted to have coincided with movement along the Brookneal shear zone. During shearing and metamorphism, primary refractory uranium-bearing minerals including titanite, zircon, and uranothorite were recrystallized, and uranium was liberated and mixed locally with hematite, clay, and other fine-grained minerals. Uranium mineralization was accompanied by a metasomatic episode between 250 and 200 Ma that reset the Rb-Sr and U-Pb isotope systems and formed titanium oxide and apatite that are associated and, in places, intimately intergrown with uranium silicate dating mineralization. This event coincides with rifting that formed the Dan River Basin and was a precursor to the breakup of Pangea. The orientation of late-stage tectonic stylolites is compatible with their formation during Late Triassic to Early Jurassic basin inversion, postdating the main stage of uranium mineralization and effectively dating mineralization as Mesozoic. Based on the close spatial and temporal association of uranium with apatite, we propose that uranium was carried as a uranyl-phosphate complex. Uranium was locally reduced by coupled redox reactions with ferrous iron and sulfide minerals in the host rock, forming uranium silicates. The release of calcium during sodium metasomatic alteration of primary calcic feldspar and titanite in the host rock initiated successive reactions in which uranium and phosphate in mineralizing fluids combined with calcium to form U-enriched fluorapatite. Based on the deposit mineralogy, oxygen isotope geochemistry, and trace element characteristics of uranium silicate and gangue minerals, the primary mineralizing fluids likely included connate and/or meteoric water sourced from the adjacent Dan River Basin. High heat flow related to Mesozoic rifting may have driven these (P-Na-F- rich) fluids through local aquifers and into basin margin faults, transporting uranium from the basin or mobilizing uranium from previously formed U minerals in the Brookneal shear zone, or from U-enriched older basement rock.
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    Composition and origin of nodules from the ≈20 ka Pomici di Base (PB)-Sarno eruption of Mt. Somma – Vesuvius, Italy
    Klébesz, Rita; Bodnar, Robert J.; De Vivo, Benedetto; Török, Kálmán; Lima, Annamaria; Petrosino, Paola (De Gruyter, 2012-05-13)
    Nodules (coarse-grain “plutonic” rocks) were collected from the ca. 20 ka Pomici di Base (PB)-Sarno eruption of Mt. Somma-Vesuvius, Italy. The nodules are classified as monzonite-monzogabbro based on their modal composition. The nodules have porphyrogranular texture, and consist of An-rich plagioclase, K-feldspar, clinopyroxene (ferroan-diopside), mica (phlogopite-biotite) ± olivine and amphibole. Aggregates of irregular intergrowths of mostly alkali feldspar and plagioclase, along with mica, Fe-Ti-oxides and clinopyroxene, in the nodules are interpreted as crystallized melt pockets. Crystallized silicate melt inclusions (MI) are common in the nodules, especially in clinopyroxenes. Two types of MI have been identified. Type I consists of mica, Fe-Ti-oxides and/or dark green spinel, clinopyroxene, feldspar and a vapor bubble. Volatiles (CO2, H2O) could not be detected in the vapor bubbles by Raman spectroscopy. Type II inclusions are generally lighter in color and contain subhedral feldspar and/or glass and several opaque phases, most of which are confirmed to be oxide minerals by SEM analysis. Some of the opaque-appearing phases that are below the surface may be tiny vapor bubbles. The two types of MI have different chemical compositions. Type I MI are classified as phono-tephrite — tephri-phonolite — basaltic trachy-andesite, while Type II MI have basaltic composition. The petrography and MI geochemistry led us to conclude that the nodules represent samples of the crystal mush zone in the active plumbing system of Mt. Somma-Vesuvius that were entrained into the upwelling magma during the PB-Sarno eruption.
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    Constraining Metamorphic and Tectonic Evolution in Convergent Terranes: How Trace Elements and Mineral Inclusions Shape Mechanical and Reconstructive Models
    Ashley, Kyle T. (Virginia Tech, 2015-06-01)
    Conventional thermobarometry in metamorphic systems has been primarily limited to constraining peak temperature (or pressure) along a generalized P-T loop. This is largely attributed to the assumption that mineral assemblages and chemistries achieve a state closest to equilibrium with the maximum thermal (and therefore energetic) input at these peak conditions. However, this traditional approach is limited in providing much information about the evolution of a metamorphic terrane, which is modified by tectonic (kinematic) forces, fluid and component mobility, and heating duration. The ubiquity of quartz in the continental crust has driven much interest in using the phase for thermobarometric purposes. In this dissertation, I discss the application of elastic theory in reconstructing conditions of inclusion encapsulation through inclusion pressure estimation with Raman spectroscopy. In some instances, overpressuring of quartz inclusions in garnet give evidence for high-pressure formation conditions. When analyses are collected from garnet core to rim, pressure paths along garnet growth can be inferred (if temperature can be reasonably estimated). In high-T, low-P terranes, quartz may become dilated if the inclusion adheres to the host. If a quartz inclusion is sufficiently stretched, transformation to a low-density polymorph may occur. Trace element uptake, particularly Ti, have been characterized in quartz and understood to be the result of a temperature- (and to a lesser extent, pressure-) sensitive substitution for Si4+. However, the application of the Ti-in-quartz thermobarometer in quartz mylonites has led to mixed results due to the low-Ti resetting that occurs with dynamic recrystallization. We applied defect energy simulations and took a global assessment of deformed quartz trace element chemistries to infer that sweeping grain boundaries provide short pathways that allows localized re-equilibration with a Ti-undersaturated medium, resulting in Ti removal from the quartz lattice. In addition, thermodynamic pseudosection modeling has provided a method to assess Ti activity as a dynamic parameter – one that evolves as the phase stability changes through prograde and retrograde metamorphic reactions. With this understanding, better growth-composition models can be derived to infer complex pressure-temperature-time-deformation (P-T-t-D) histories of metamorphic rocks. These techniques and results are coupled with conventional thermobarometry techniques to provide a more comprehensive picture of the conditions experienced by a rock through the evolution, from burial to exhumation to the Earth's surface. The thermal evolution is used to provide conceptual thermal-kinematic models to explain tectonic evolution and heat advection in the continental lithosphere in ancient mountain belts.
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    Contributions and New Methods in Paleontology: Geochemical, Ultrastructural, and Microstructural Characterization of Archean, Proterozoic, and Phanerozoic Fossils
    Schiffbauer, James Daniel (Virginia Tech, 2009-05-01)
    Over the past decade, the study of organismal or systematic paleobiology has been progressing into a new age of digital paleobiology, in which advanced instrumentation is utilized for primary data collection and analyses. Having been progressing throughout this field of study, advanced instruments–commonly electron- and ion- microbeam equipment–have been employed for numerous fossils over the entire range of geologic time, from microfossils to macrofossils and from the Archean (beginning at 3800 Ma) to the Cenozoic (ending at the recent). These techniques, predominantly used for geochemical, morphological, and ultra-/micro-structural analyses, have unlocked an incredible amount of detail contributing to our understanding of fossil organisms, their modes of life, and their biological affinities. But further, as these techniques continue to grow and become popularized in various fields of paleobiological study, they are certain to significantly progress our comprehension and knowledge of the evolution of life through time. While the chapters presented in this dissertation may not have a unifying theme in terms of a distinct fossil organism or specific time in Earth's history, furthering the use of electron- and ion- microbeam instrumentation and expanding the paleo-genres to which digital paleobiological approaches may be applied encompasses the fundamental intention of my research. Two of the chapters reported here focus on the geochemical, ultrastructural, and microstructural investigation of organic-walled microfossils, or acritarchs, from the Paleoproterozoic (2500–1600 Ma) and Mesoproterozoic (1600–1000 Ma), using a range of advanced instrumentation including field emission scanning electron microscopy, transmission electron microscopy, laser Raman spectroscopy, electron microprobe, secondary ion mass spectroscopy, and focused ion beam electron microscopy. Moving into the Neoproterozoic (1000–542 Ma), the third primary research chapter utilizes field emission scanning electron microscopy for high-resolution, high magnification imaging and quantitative evaluation of an entire fossil assemblage–from acritarchs and algal fossils to the earliest metazoan embryos. This study was conducted in an effort to examine and describe the phosphatization taphonomic window of the Doushantuo Formation of South China, which is a prime example of exceptional preservation. Finally, the fourth primary research chapter reported here uses field emission scanning electron microscopy and environmental scanning electron microscopy in a field of paleobiology in which advanced instrumentation has been highly underutilized – predatory-prey interactions. This research examines microstructural characteristics of predatory drill holes in both modern and fossil organisms in an attempt to mitigate the identification of predation traces in the fossil record.
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    Contributions to the Neoproterozoic Geobiology
    Shen, Bing (Virginia Tech, 2007-11-29)
    This thesis makes several contributions to improve our understanding of the Neoproterozoic Paleobiology. In chapter 1, a comprehensive quantitative analysis of the Ediacara fossils indicates that the oldest Ediacara assemblage "the Avalon assemblage" already encompassed the full range of Ediacara morphospace. A comparable morphospace range was occupied by the subsequent White Sea and Nama assemblages, although it was populated differently. In contrast, taxonomic richness increased in the White Sea assemblage and declined in the Nama assemblage. The Avalon morphospace expansion mirrors the Cambrian explosion, and both may reflect similar underlying mechanisms. Chapter 2 describes problematic macrofossils collected from the Neoproterozoic slate of the upper Zhengmuguan Formation in North China and sandstone of the Zhoujieshan Formation in Chaidam. Some of these fossils were previously interpreted as animal traces. Our study of these fossils recognizes four genera and five species. None of these taxa can be interpreted as animal traces. Instead, they are problematic body fossils of unresolved phylogenetic affinities. Chapter 3 reports stable isotopes of the Zhamoketi cap dolostone atop the Tereeken diamictite in the Quruqtagh area, eastern Chinese Tianshan. Our new data indicate that carbonate associated sulfate (CAS) abundance decreases rapidly in the basal cap dolostone and δ34SCAS composition varies between +9â ° and +15â ° in the lower 2.5 m. In the overlying interval, CAS abundance remains low while δ34SCAS rises ~5â ° and varies more widely between +10â ° and +21â °. δ34Spy is typically greater than δ34SCAS measured from the same samples. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin. Chapter 4 studies δ13C, δ18O, δ34SCAS, and δ34Spy of the Zhoujieshan cap carbonate that overlies the Ediacaran Hongtiegou glaciation. The Zhoujieshan cap dolostone shows positive δ13C values (0 â 1.7â °). δ34SCAS shows rapid stratigraphic variations from +13.9 to +24.1â °, probably due to relatively low oceanic sulfate concentrations. δ34Spy shows a steady stratigraphic trend. Thus, the δ34SCAS and δ34Spy trends are decoupled from each other. The decoupling of δ34SCAS and δ34Spy trends suggests that CAS and pyrite were derived from different sulfur pools, which were probably due to the postglacial basin stratification.
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    Critical properties of NaCI-H₂O Solutions
    Knight, Cheryl L. Erickson (Virginia Tech, 1988-01-15)
    Critical properties of the NaCI-H₂0 fluid system are of fundamental interest to a variety of geochemical applications including fluid inclusion studies, numerical modeling of hydrothermal systems, and development of theoretical models for two·component fluid systems. Although many workers have expressed interest in NaCl·H₂0 fluid critical properties, most studies have been limited to small compositional ranges with little agreement among data sets at higher salinities. Critical densities are recorded in only one of these reports, and no studies have determined the locations of NaCl-H₂0 critical isochores (PT projections of critical densities). Furthermore, no studies to date have determined critical properties of NaCl·H₂0 solutions in excess of room temperature saturation (26.4 wt.% NaCl).
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    The crystal chemistry of MTO₄ compounds with the zicron, scheelite, and monazite structure types
    Macey, Brett Jarrod (Virginia Tech, 1995-09-15)
    The crystal structures of zircon, scheelite, and monazite are very closely related. All three have chains of alternating polyhedra and planes of closest packed or pseudo-closest packed cations. Using these similarities the unit cells of these structures can be placed in analogous orientations. This in turn leads to a better understanding of the geometrical aspects of the reconstructive phase transformations that occur among the structure types as functions of temperature and pressure. In essence the phase transformations require the cation planes of one structure to the cation planes of another. Phase transformations also occur via compositional pathways. Crystal structure parameters were modeled for compounds with the zircon, scheelite and monazite structure types using multiple regression techniques. Data consisted of structure refinements of 26 zircon-, 13 scheelite-, and 13 monazite-type compounds. These compounds include but are not limited to the lanthanide vanadates and phosphates, the alkali earth molybdates and tungstates, and KTcO₄. The structural parameters studied included bond lengths, bond angles, polyhedral volumes, unit cell edge lengths, tetrahedral quadratic elongations and atomic coordinates of individual atoms; they were modeled as a function of the Shannon radii of the cations and the product of the M and T cation charges. Correlation coefficients for these regressions exceeded 0.9 for nearly all parameters studied except for the y coordinate of M, the z coordinate of O1, and T-O1-M2 angle of the monazite compounds.
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    Detection limits of CO₂in fluid inclusions using microthermometry and Raman spectroscopy and the spectroscopic characterization of CO₂
    Rosso, Kevin M. (Virginia Tech, 1994)
    In many geologic environments, dominantly aqueous solutions contain low concentrations of CO₂. At ambient temperature, in fluid inclusions which trap these solutions, the typical phase assemblage consists of a CO₂-rich vapor (where PCO₂ ≈ PinternaI) and an aqueous phase containing dissolved salts and CO₂. In this study, the CO₂ minimum detection limits (MDLs) using microthermometry and laser Raman spectroscopy are established in terms of PCO₂ using synthetic H₂O-CO₂ inclusions. The purpose of the microthermometric experiments was to examine the diagnostic CO₂ phase changes and determine the quantity of CO₂ necessary to result in observable solid CO₂ melting. The results of these experiments show that an observable solid CO₂ melting event requires PCO₂ ≥ 45 bar at 25°C. The Raman spectroscopic detection limits were investigated using a multichannel Raman spectrometer. Because the Raman spectroscopic MDLs are a function of counts, the CO₂ MDLs were determined by collecting signal-to-noise ratios for both the upper and lower v₁-2v₂ bands as a function of CO₂ pressure (5-60 bars) and over a range of integration times and incident laser power to predict the optimal instrument settings. The resulting CO₂ MDLs are on the order of 1 bar for our instrument. The band splitting of the v₁-2v₂ diad as a function of CO₂ pressure was measured up to 500 bar at ambient temperature. The CO₂ pressures were converted to ρCO₂ and the results are given in terms of the frequency separation between the upper and lower bands. These results are compared to those of previous studies. An analysis of the estimated errors indicates that the technique can be used to determine CO₂ densities in fluid inclusions containing a homogenous, free CO₂ phase to a precision of approximately ± 0.02 g/cm³. The temperature dependence of the intensity ratio of the hot bands to the v₁-2v₂ diad was measured from 270 to 315 K. The close agreement between the calculated and observed results indicate that laser induced sample heating is not significant. The intensity ratio can be used to estimate the CO₂ temperature and, combined with the Raman density determination, allows calculation of the CO₂ pressure.
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    Development of novel computational techniques for phase identification and thermodynamic modeling, and a case study of contact metamorphism in the Triassic Culpeper Basin of Virginia
    Prouty, Jonathan Michael (Virginia Tech, 2024-08-12)
    This dissertation develops computational techniques to aid in efficiently studying petrologic systems that would otherwise be challenging. It then focuses on a case study in which the transition from diagenesis to syn-magmatic heating led to a recrystallization and sulfur mobilization. A Markov-chain Montecarlo-based methodology is developed to allow for the assessment of uncertainty in calculated phase assemblage diagrams. Such phase equilibria are ubiquitous in modern petrology, but uncertainties are rarely considered. Methods are discussed for visualizing and quantifying emergent patterns as phase diagrams are re-calculated with input data modified within permitted uncertainty bounds, and these are implemented in a new code. Results show that uncertainty varies significantly across pressure-temperature space and that in some conditions, estimates of stable mineral assemblage are known with very little confidence. A Machine-Learning (ML) based methodology is developed for automatically identifying unknown phases using Electron-dispersion spectra (EDS) in concert with a Random Forest Classification algorithm. This methodology allows for phase identification that it is insensitive to overfitting and noisy spectra. However, this tool is limited by the amount of reference spectra available in the dataset on which the ML algorithm is trained. The approximately 250 EDS spectra in the current training database must be supplemented to make the tool more widely useful, though it currently has an excellent success rate for correctly identifying various sulfide and oxide minerals. An analysis of paragenesis associated with Central Atlantic Magmatic Province (CAMP) intrusions helps to better constrain the dynamics of magma emplacement, while also providing a method for estimating the amount of sedimentary sulfide-sequestered sulfur mobilized as a result of magnetite formation associated with igneous activity. This method demonstrates that dike emplacement can trigger liberation of sedimentary sulfur with no direct cooling impact on climate.