Geology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2774
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Item The Redox History of the Earth's Mantle: Evidence from Ultramafic Lavas(2019) Nicklas, Robert William; Walker, Richard J; Puchtel, Igor S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In order to determine the evolution of the redox state of the mantle, the oxygen fugacities of sixteen mantle-derived komatiitic and picritic systems, ranging in age from 3.55 Ga to present day, were determined using the redox-sensitive partitioning of V between olivine and komatiitic/picritic magma, a method refined by this study. The oxygen fugacity data for the studied systems was determined to reflect that of their respective mantle source regions. The dataset defines a well-constrained trend indicating an increase in oxygen fugacity of the bulk convecting mantle of 1.33±0.43 FMQ log units from 3.48 to 1.87 Ga, and nearly constant oxygen fugacity from 1.87 Ga to the present. The oxygen fugacity data for the 3.55 Ga Schapenburg komatiites, the mantle source region of which was shown to have been isolated from mantle convection within the first 30 Ma of the Solar System history, plot well above the trend defined by the data for the contemporaneous lavas. This anamolous data point likely reflects preservation of early-formed magma ocean redox heterogeneities until at least the Paleoarchean. The observed increase in the oxygen fugacity of the mantle requires admixture of a likely geochemically detectable amount of oxidized material. Three mechanisms were considered to account for the observed change in mantle redox state. The first two mechanisms: recycling of altered oceanic crust and venting of oxygen from the core due to inner core crystallization, were found to be unfeasible due to the large mass of recycled crust required, and the likely young age of the inner core, respectively. The third accessible mantle oxidation mechanism: convection-driven homogenization of an initially redox-heterogeneous mantle, is the most likely given available geochemical constraints. The new data presented here provide evidence for the mantle having triggered the Great Oxidation Event at ~2.4 Ga. We have additionally determined the Os isotopic and HSE systematics of 89 Ma komatiites from Gorgona Island, Colombia. The subset of these Gorgona samples that were also analyzed for oxygen fugacity shows BSE-like Os isotopes and HSE abundances in their mantle source, showing that their oxygen fugacity is likely representative of the mantle at 89 Ma.Item Origin of the anomalous sulfur isotope composition of the Rustenburg Layered Suite (Bushveld Complex), South Africa(2019) de Assis Magalhaes, Nivea Maria; Penniston-Dorland, Sarah C; Farquhar, James; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The 2.06 Ga Bushveld Magmatic Province (BMP) hosts the largest platinum group element (PGE) reserve of the world that occurs mainly as sulfide-rich layers within the Rustenburg Layered Suite (RLS), and also in mineralized layers of the Waterberg Project (WP). Despite extensive studies, many questions remain on the origin and evolution of this large igneous province, and on the source of sulfur that allowed for the extensive PGE mineralization. This study looks systematically into the multiple sulfur isotope composition of the RLS, finding that all layers show the presence of a mass-independently fractionated sulfur component (Δ33S≠0), which are all distinguishable from the expected Δ33S value of the mantle. The exogenic sulfur reflects contamination by Archean surface-derived material (e.g. sediments, altered oceanic crust). Such contamination can occur in many different stages of the evolution of these intrusions: either by assimilation of wall rock during ascent and emplacement, or in a staging chamber in the lower crust, or by recycling of crustal material in an ancient subduction zone. The WP, an intrusion related to the BMP that was emplaced off-craton, has a similar sulfur composition to the Main Bushveld Series of the RLS. It is, however, a separate intrusion that crystallized in a separate magma chamber and was emplaced in a different unit than the RLS, which suggests that the contamination of the parental magma occurred at a deeper level, prior to emplacement of magma in the upper crust. Rocks from the Vredefort Dome, used as a proxy for the sulfur composition of the lower crust underneath that region, yield a sulfur composition that cannot account for the composition of the RLS or the WP. Finally, the sub-continental lithospheric mantle has been studied through xenoliths carried by the Premier Kimberlite. These xenoliths, such as what was observed in sulfide inclusions in diamond, also have Δ33S≠0, evidencing that the sub-continental lithospheric mantle may contain recycled sulfur that contributed this sulfur to primitive magmas during the Bushveld magmatic event.Item Anatexis and crustal differentiation: Insights from the Fosdick migmatite-granite complex, West Antarctica(2014) Yakymchuk, Christopher; Brown, Michael; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In the Fosdick migmatite-granite complex of West Antarctica, U-Pb geochronology of monazite in migmatites and zircon in granites records two episodes of high-temperature metamorphism, one in the Devonian-Carboniferous and another in the Cretaceous. For the older lower-grade event, whole-rock and zircon isotope geochemistry of granites within the Fosdick complex are interpreted to record crustal reworking during metamorphism associated with continental arc magmatism along the East Gondwana convergent plate margin. By contrast, the geochemistry of correlative granites suites from along and across the same margin indicates a greater proportion of crustal growth. This suggests prominent arc-parallel and arc-normal variations in the mechanisms of crustal reworking versus growth in continental arc systems. Based on garnet Lu-Hf ages, the timing of peak metamorphism in the younger higher-grade event has been determined as c. 116-111 Ma. U-Pb ages of monazite from migmatites and zircon from anatectic granites suggest that exhumation of the complex as a gneiss dome occurred during the interval c. 107-100 Ma. Contemporaneous exhumation of high-grade metamorphic rocks in the Western Province of New Zealand suggests that intracontinental extension preceded the final breakup of New Zealand from West Antarctica by c. 25 Myr. Melt migration through and emplacement within the Fosdick complex during Cretaceous metamorphism was accomplished via a self-organized melt network controlled by the regional stress field and anisotropy of the host rocks. Granites within this network and in sills at shallower crustal levels have microstructures and chemistry consistent with a cumulate origin, and are interpreted to record fractional crystallization during magma ascent and doming related to exhumation. Phase equilibria modeling of open system melting during prograde metamorphism is used to quantify the reduced fertility of source rocks during high-temperature exhumation and later overprinting orogenic events. Quantitative modelling of the dissolution of zircon and monazite during prograde melting demonstrates that accessory minerals are expected to be partially to completely consumed up to the metamorphic peak. New growth of these minerals in migmatite melanosomes is predicted to be limited during cooling, whereas leucosomes and anatectic granites are predicted to contain new zircon and monazite growth.Item Petrogenesis of Peraluminous Granites from the Fosdick Mountains, Marie Byrd Land, West Antarctica(2013) Brown, Caitlin R.; Brown, Michael; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Granites from the Fosdick Mountains, West Antarctica were analyzed for major and trace elements as well as Sr-Nd isotopes ratios in order to investigate the sources and processes associated with granite formation at a former convergent margin. U-Pb ages from zircon separates are consistent with previous results and yield ages of ~360 Ma and ~100Ma. Major and trace elements indicate that paragneiss and orthogneiss samples are the high-grade equivalents of the Swanson Formation and the Ford Granodiorite site. Granites produced from the Devonian-Carboniferous melting event are derived primarily from the Ford Granodiorite suite while granites produced from the Cretaceous melting event are derived from melting of the Ford Granodiorite suite or mixing between the two putative sources. Cretaceous granites show evidence of early crystallized minerals. There is no chemical evidence for a source other than the Ford Granodiorite suite or the Swanson Formation.Item The role of mechanical mixing in the formation of reaction zones in subduction-related melange(2013) Gorman, Julia; Penniston-Dorland, Sarah C; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Traverses across metamorphic reaction zones at contacts between mafic rocks and ultramafic mélange matrix were analyzed in two mélange zones (Catalina Schist, Santa Catalina Island, CA, and Attic-Cycladic Complex, Syros, Greece) in order to investigate a mechanically mixed contribution to subduction-related reaction zones. Elements enriched in peridotite relative to mafic crust such as Os, Ir, Ru, Ni and Cr were used to assess the addition of a peridotitic component to reaction zones. Results showed co-varying concentrations of Ni, Cr, Os, Ir, and Ru in reaction zones, suggesting these components were added via mixing with mantle peridotite. 187Os/188Os ratios were lower in reaction zones relative to block cores, consistent with the addition of peridotite to mafic rock. Reaction zone samples plot along mixing trends between peridotite and basalt, indicating that mechanical mixing, along with fluid-mediated mass transfer of fluid-mobile elements, contributes to reaction zone formation in the Catalina Schist mélange zone.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 The Effect of CO2 on Copper Partitioning in Sulfur- Free and Sulfur-Bearing Felsic Melt-Vapor-Brine Assemblages(2012) Tattitch, Brian Christopher; Candela, Philip; Piccoli, Philip; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Analysis of fluid inclusions from porphyry copper deposits (PCD) reveals that magmatic vapor and/or brine are vital for the removal of copper from arc magmas and its transport to the site of ore formation. Experiments in melt-vapor-brine systems allow for investigating the partitioning of copper between silicate melts and volatile phases under magmatic conditions. The presence of CO2 affects both the pressure of vapor saturation and the composition of exsolving volatile phases. However, PCD are primarily sulfide ore deposits, and the role of sulfur must also be examined as part of magmatic-hydrothermal experiments. Therefore, the partitioning of copper in CO2 ± S-bearing experiments was examined in an attempt to provide insights into copper partitioning and the generation of PCD. I present the results from experiments performed at 800 °C and 100 MPa in CO2-bearing melt-vapor-brine systems with XCO2 = 0.10 and 0.38. The compositions of vapor and brine inclusions and run-product glasses were used to determine the compositions of the magmatic phases. The partitioning of copper between brine and vapor (DCu b/v ±2σ) increases from 25(±6) to 100 (±30) for sulfur-free experiments and increases from 11(±3) to 95(±23) for sulfur-bearing experiments as XCO2 is increased from 0.10 to 0.38. The partitioning of copper between vapor and melt (DCu v/m ±2σ) decreases from 9.6(±3.3) (sulfur-free, HCl-bearing), 18(±8) (sulfur-bearing, HCl-free), and 30(±11) (sulfur-bearing, HCl-bearing) at XCO2 = 0.10, to 2(±0.8)(HCl-free) at XCO2 = 0.38, sulfur-free or sulfur-bearing. These data demonstrate that copper partitioning in sulfur-free, CO2-bearing systems is controlled by the changes in the salinity of the vapor and brine corresponding to changes in XCO2. Sulfur-bearing experiments demonstrate that magmatic vapors are enriched in copper in the presence of sulfur at low XCO2. However, the enrichment of copper in the magmatic vapor is suppressed for sulfur-bearing systems at high XCO2. The MVPart model presented by Candela and Piccoli (1998) was modified to incorporate CO2 to predict trends in efficiency of removal of copper into exsolving CO2-bearing magmatic volatile phases. The CO2-MVPart model predicts two to three times lower efficiency for CO2-rich (XCO2 = 0.38) magmatic volatile phases compared to low-CO2 (XCO2 ≤ 0.10) systems.Item Metals in Arc Magmas: The Role of Cu-Rich Sulfide Phases(2011) Mengason, Michael James; Candela, Philip A; Piccoli, Philip M; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Based on experiments performed on hydrous andesitic melts at 1000°C, 150 MPa, fO2 from the Co-CoO to Ni-NiO buffer, and log fS2 equal to -0.5 to -1.5 (bar), greater than 32 ± 4 ppm copper (all uncertainties = 1 sigma, standard deviation of the mean) in the silicate melt favors the formation of a Cu-Fe sulfide liquid (CFSL) relative to pyrrhotite at sulfide saturation. This concentration is well within the range encountered in intrusive and extrusive rocks suggesting that saturation by sulfide liquids is a common occurrence in magmatic arc systems consistent with observations in naturally occurring andesites. Nernst-type partition coefficients determined from these experiments highlight the importance of accurately modeling the composition of the sulfide phase present during partial melting or fractional crystallization: Dpyrrhotite/melt = 1320 ± 220 for Cu, 1.73 ± 0.37 for Mo, 90 ± 19 for Ag, and 500 ± 87 for Au, whereas DCFSL/melt = 7,800 ± 1,400 for Cu, 0.45 ± 0.14 for Mo, 6,800 ± 1,300 for Ag, and 84,000 ± 19,000 for Au. Data from these experiments support a direct correlation between the solubility of gold and the concentration of sulfur in the silicate melt at low fO2, as well as a dependence of the solubility of gold on fS20.25 in pyrrhotite and CFSL. As a part of this research, pyrrhotite of variable copper concentration was equilibrated at 1000°C in sealed evacuated silica tubes to determine a method that allows the equation of Toulmin and Barton (1964) to be used to calculate fS2 for Cu-bearing pyrrhotite. This method is consistent for pyrrhotite with up to 6 wt % Cu by using N=2*[(XCu+XFe)/(1.5XCu+XFe+XS)]. These data suggest that separation of CFSL from the magma along with crystalline phases during fractional crystallization can reduce the likelihood of magmatic hydrothermal ore formation. For example, modeling 30 % Rayleigh fractional crystallization (F=1.0 to F=0.7), with 0.1% sulfide among the separating phases, and an initial 65 ppm Cu in the silicate melt, would result in the sequestration of up to 50% of the initial Ag, 60 % Cu, and > 99 % Au.Item Reaction rates and textural development of hydrolysis reactions in the system MgO-SiO2-H2O(2011) Kerrigan, Ryan Jason; Candela, Philip A; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experiments in the simplified systems MgO-SiO2-H2O (MSH) and MgO-SiO2-H2O-CO2 (MSHC) have been conducted by using hydrothermal diamond anvil cells to investigate reaction rates and the resulting textures at temperatures and pressures consistent with the temperatures and pressures of the Earth's crust. The conditions and simplified systems of the experiments serve as approximations for geologic environments wherein magnesium-rich rocks (i.e., mafic, ultramafic, and magnesium-rich carbonate rocks) are hydrothermally altered by silica-rich fluids. The hydrolysis reaction rates and textures that result from the irreversible interactions of olivine and magnesite with aqueous fluids in the presence of quartz have been characterized. Reaction rates have been determined by a new approach developed during this study, which uses in situ observation of reactant volume loss to determine the growth rate of the products of hydrolysis reactions. In addition, some experiments were analyzed by real-time synchrotron radiation analysis to identify the phases in the reactions and to provide semi-quantitative constraints on the kinetic data. Experiments performed in this study resulted in the development of several textural varieties. Talc grown during this study exhibited both fibrous and platy habits, textural variations that appeared to be controlled by: variations in the density of the aqueous phase, surface area of starting materials, rate of temperature increase, and the presence of strong chemical gradients. The primary growth of fibrous talc in these experiments demonstrates that the production of fibrous talc does not require the pseudomorphism of a fibrous precursor as previously suggested.Item Quantitative modeling of mantle heterogeneity and structure(2010) Arevalo, Jr., Ricardo David; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mantle-derived rocks, particularly mid-ocean ridge basalts (MORB) and intraplate ocean island basalts (OIB), provide insights into the compositional heterogeneity and first-order structural make-up of the modern mantle; laser ablation (LA-) ICP-MS analysis provides the ideal analytical tool for the in situ chemical characterization of these materials. The silicate Earth, as defined by the MORB and OIB source regions plus the continental crust, is determined to have a representative W/U and K/U ratio of 0.65 ± 0.45 (2σ) and 13,800 ± 2600 (2σ), respectively, equating to 13 ± 10 ng/g W and 280 ± 120 μg/g K in the silicate Earth. Although both the isotopic composition of W and the constancy of the terrestrial W/U ratio may serve as tracers of putative core-mantle interactions, both of these proxies are sensitive to the chemical composition of the mantle source and have yet to resolve a core signal in Hawaiian picrites. The abundance of K in the silicate Earth indicates a current convective Urey ratio of ~0.34 and mantle cooling rate of 70-130 K*Gyr−1, after taking into account potential heat flux across the core-mantle boundary. The Earth's balance of radiogenic heat and budget of 40Ar necessitate a lower mantle reservoir enriched in radioactive elements. The bulk Earth Pb/U ratio, determined here to be ~85, suggests ~1200 ng/g Pb in the bulk Earth and ≥3300 ng/g Pb in the core. A compositional model of MORB, which is derived from a suite of sample measurements augmented by a critically compiled data set, shows that Atlantic, Pacific and Indian MORB can be distinguished based on both trace element abundances and ratios. The geochemical signatures associated with global MORB are not entirely complementary to the continental crust, and require an under-sampled reservoir enriched in Ti, Nb and Ta. A compositional model of OIB, which is based on the inferred chemical composition of OIB parental melts from Hawaiian shield volcanoes as well as the Austral-Cook islands, indicates that the OIB source region may only be ≥1.0x as enriched in incompatible elements as the unfractionated silicate Earth, and constitute up to ≤50% of the modern mantle mass.