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 Characterization of the regional, crustal, and global distribution and abundance of the heat producing elements and their geoneutrino flux(2019) Wipperfurth, Scott Alan; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The amount and distribution of radiogenic power generation from the heat producing elements (HPE) U, Th, and K in the Earth is not well constrained. Compositional estimates of these elements vary by a factor of three in the bulk-Earth and 30 in the mantle after removal of the continental crust contribution. Understanding the total power derived from these elements is critical to understanding the power driving the Earth as they supply fuel to the geodynamo and mantle convection. The decay of HPE's produce particles called geoneutrinos and the measurement of the geoneutrino flux reveals the frequency of decay and the abundances of these elements in the Earth. The total geoneutrino flux can be categorized into three major contributors: the dominant component from the nearest 500 km of continental crust surrounding the detector and slightly smaller sub-equal contributions from the remaining global continental crust and the mantle. The negligible amount of HPE's within the core was tested by a mass-balance of the Th/U derived from Pb isotopes (κ_Pb). Each Earth layer was attributed a κ_Pb from representative samples with associated weighting factor from the estimated mass of U in each reservoir. The radiogenic power in the core from U and Th was constrained to ~0.03 terra-watts (median), emphasizing the core's negligible geoneutrino luminosity. To unravel the contribution from the inaccessible mantle to the signal at a detector one must build a physical and chemical description of the local and global crust. The 50x50 km regional geoneutrino flux surrounding the SNO+ detector (Sudbury, Canada) was modeled. 112 geologic samples were analyzed for their U, Th, K abundances and combined with a 3D physical model of the region. To supplement this, the methodology of Huang et al. (2013) was applied to an updated geophysical model for the bulk-crust to predict the global crustal signal at SNO+ and other detectors. Variable correlation is addressed and uncertainties from density, seismic velocity, crustal thickness, and abundances propagated. This dissertation explores the amount and distribution of HPE's within the Earth and their geoneutrino flux through geochemical and geophysical modeling on regional, crustal, and global scales. Together, the results update our understanding of the Earth's geoneutrino flux and the uncertainties still in the system.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 The Earth’s Thorium and Uranium Abundance and Distribution(2018) Guo, Meng; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The abundance and distribution of thorium (Th) and uranium (U) in the Earth can provide important data for constraining its composition, heat budget, and processes of differentiation. This project seeks to constrain the 232Th/238U (К) ratio in different domains of the Earth. We reports more than one hundred thousand 232Th/238U ratios and more than ten thousand time-integrated Pb isotopic ratios (КPb) for rocks from the continental crust (CC) and modern mantle (MM). The results reveal that these two complementary reservoirs MMКPb = 3.87 +0.15-0.07 and CCКPb = 3.94 +0.20-0.11 tightly bracket the solar system (SS) initial SSКPb = 3.890 ± 0.015 (Blichert -Toft et al., 2010), defining a bulk silicate Earth (BSE) composition of BSEКPb = 3.90 +0.13-0.07. The CCКPb, MMКPb and BSEКPb are indistinguishable statistically, which indicates that negligible Th/U fractionation accompanied crust-mantle segregation, accretion and core-mantle segregation. Open system crustal growth modeling suggests that the changing incompatibility of Pb during the formation of the continents could on its own account for the kappa conundrum (i.e., К < КPb). The timing of Great Oxidation Event (GOE) coincidently overlapped with the peak of continental crust recycling, but may have no causal relationship or trivially contribution to the kappa conundrum.Item Hydrothermally Altered Basalt as a Source for Indium in Ore Deposits(2018) Hollingsworth, John Walter; Piccoli, Philip M; Candela, Philip A; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Indium is an element integral to various emerging technologies. Despite the demand for indium, little is known about its geochemical behavior in hydrothermal systems that generate economic deposits. The aim of this study is to estimate the efficiency with which indium can be removed from basalt during hydrothermal alteration in order to develop better exploration vectors. This can be accomplished by studying how indium partitions between an aqueous solution and the products of hydrothermal alteration. To evaluate the magnitude of this efficiency, two sets of experiments were performed. The first set used a low initial pH, indium+HCl-bearing aqueous solution, whereas the second used a higher initial pH, indium+sulfur-bearing seawater solution. The water/rock mass ratio ranged from 1-10. Experiments were performed at 500°C and 50 MPa for 2-8 weeks. Water/rock was found to have the greatest affect on the partition coefficient of indium between the altered basalt and the aqueous solution.Item SULFUR ISOTOPE RECORDS IN NEOARCHEAN CARBONATES: IMPLICATIONS FOR THE EARLY PRECAMBRIAN SULFUR CYCLE(2017) Zhelezinskaia, Iadviga; Farquhar, James; Kaufman, Alan J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mass-independent fractionation of sulfur isotopes found in Early Precambrian records is the main evidence supporting an oxygen-poor atmosphere before ~2.4 Ga when ancient sulfur cycling was different than today. In previous studies, shale facies formed in deep-water environments have been the main target that were used to constraint the ancient sulfur cycle using sulfur isotopes, even though, among sedimentary Neoarchean strata, carbonate rocks are found to be more abundant. In order to follow previous observations and reveal processes operating in shallow water environments, I conducted a series of systematic studies of Neoarchean carbonate archives. Elemental and isotope measurements of sulfur and carbon in carbonate (and some shale) facies were obtained from multiple cores drilled through ~2.7 to 2.5 Ga successions of South Africa (GKF01, GKP01, and BH1-Sacha), Western Australia (AIDP-2, AIDP-3, BB, PR, RP, and RG) and Brazil (GDR-117). This study demonstrates that carbonate facies preserve distinctive MIF-S compositions relative to shale facies. Drilled pyrites in carbonate formations mostly preserved negative Δ33S values suggesting that the major sulfur source to shallow environments was atmospheric sulfate that also was isotopically redistributed through microbial sulfate reduction producing δ34S > 35‰ isotope fractionation. Atmospheric sulfate was the main source for seawater sulfate making its concentration in the Neoarchean ocean of less than 10µM/l. At this low concentration, reservoir effects would be pronounced leading to the formation of carbonate associated pyrites with highly positive δ34S compositions ranging to > +30‰. The bulk pyrites in most carbonate formations from South African and Western Australian cores possess small positive Δ33S signals (<+3.0‰) suggesting the incorporation of 20-35% of photolytic elemental sulfur. Photolytic sulfate with Δ36S/Δ33S deviations found in macroscopic pyrites with negative Δ33S from the Carawine Formation provide evidence for changes in atmospheric reactions during periods of an organic hazy atmosphere. My study of Δ36S/Δ33S in contemporaneous Jeerinah shale indicates the possible temporal decoupling of the MIF-S signal on a basinal scale implying heterogeneous haze structure. Integration of sulfur and carbon isotopes measured in carbonate facies suggests that sulfur-metabolizing microbes such as sulfur phototrophs and sulfate reducers were actively recycling these elements in shallow marine environments.Item INDIUM PARTITIONING BETWEEN FERROMAGNESIAN PHASES AND FELSIC MELTS: SIGNIFICANCE FOR ORE FORMATION AND EXPLORATION(2017) Gion, Austin Michael; Candela, Philip A; Piccoli, Philip M; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Indium demand has increased due to the production of cell phone screens, solar cells, alloys, and LED displays. This suggests a need for increased exploration, which can aide in constraining where in space and time indium-bearing deposits are likely to form. Exploration vectors are suggested based on results of experiments conducted on the partitioning behavior of indium between ferromagnesian (biotite and amphibole), a felsic melt, and vapor phases. D_In^(Bt/Melt) ranges from 0.6 ± 0.1 (1 σm) to 16 ± 3 (1 σm) and is a function of the biotite composition, with D_In^(Bt/Melt) decreasing with increasing X_Annite^Bt. D_In^(Am/Melt) is 36 ± 4 (1σm) and D_In^(Vapor/Melt) is ~17 ± 5 (1σm). Exploration vectors suggest that granites that lack amphibole and contain iron-rich biotite have a higher potential to be associated with indium-bearing deposits.Item Meter Scale Heterogeneities in the Oceanic Mantle Revealed in Ophiolite Peridotites(2017) Haller, Mitchell Bruce; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The upper oceanic mantle is the largest accessible terrestrial geochemical reservoir. Numerous aspects of the upper oceanic mantle’s current state, as well as its chemical evolution through time, remain obscure. Although studies of Mid Ocean Ridge Basalts (MORB), and other oceanic mantle derived melts have provided important insights into the nature of their sources, previous studies have shown that they fail to capture the full range of end-member compositions present in oceanic peridotites. Ophiolites are especially useful in interrogating this issue as field-based observations can be paired with geochemical investigations over a wide range of geologic time. Grid sampling methods (3m x 3m) at the 497 Ma Leka Ophiolite Complex (LOC), Norway, and the 1.95 Ga Jormua Ophiolite Complex (JOC), Finland, offer an opportunity to study mantle domains at the meter and kilometer scale, and over a one billion year timespan. The lithology of each locality predominately comprises harzburgite, hosting layers and lenses of dunite and pyroxenite. Here, we combine highly siderophile elements (HSE) and Re-Os isotopic analysis of these rocks with major and trace element measurements. Two grids sites are studied within the LOC harzburgite mantle section. Harzburgites at individual LOC grid sites show variations in initial γOs(497 Ma) (-2.1 to +2.2) at the meter scale. Analyses of dunites within the same LOC grid, reveal that dunites may either have similar γOs to their host harzburgite, or different, implying interactions between spatially associated rock types may differ at the meter scale. A harzburgite sample is characterized by low initial 187Os/188Os (<0.121), reflecting Proterozoic melt depletion. Preservation of Os isotopic compositions consistent with ancient melt depletion is a common characteristic in oceanic peridotites. Grid sampling of adjacent harzburgites and dunites reveal that the geometry of these refractory domains can be constrained to be < 1 m3. TMA model ages of an LOC websterite reveals at least one other stage of partial melting in the LOC, which broadly corresponds to the opening of the Iapetus Ocean (~620 – 550 Myr). Averaged γOs values between the mantle sections of two LOC grid sites (+1.3 and -0.4) separated by ~5 km, indicate km-scale heterogeneity in the convecting upper mantle. Major and trace element compositions suggest that the km-scale heterogeneity in the LOC, is a result of variable melt-extraction at different depths, and local scale processes. Analyses of two, 1 cm thick orthopyroxenite veins, hosted by harzburgite near Kvaløya-moen, are more radiogenic than host harzburgites, and suggest vein formation had minimal impact on the host harzburgite. Whole rock major and trace element data, and thin sections of relict olivine grains, are also examined to shed light on the causes of the isotopic heterogeneities in the LOC. Two grids sites are studied within the JOC serpentinite mantle section. Serpentinites at JOC grid JU15-16, display modest heterogeneities at the meter scale in γOs(1.95 Ga) (-0.5 to -3.0). TRD model ages show evidence of melt depletion at least 400 Ma prior to the accepted age of the ophiolite. Re-Os systematics of a separate JOC grid site, JU15-18 (~3 km away), show evidence of Re addition/loss at the age of the ophiolite (~1.95 Ga). LREE-enriched REE patterns suggest that this grid location was subsequently affected by metasomatic processes possibly associated with gabbroic dykes, affecting the geochemical and Os isotopic compositions of these JOC serpentinites. Enrichments of fluid mobile elements including Re, Ba, and Sr, may implicate recent Re mobilization caused by weathering and ground-water interactions. Trends in major elements show signs of variable MgO and SiO2 loss by serpentinization.Item Highly Siderophile Elements, 187Re-187Os and 182Hf-182W Isotopic Systematics of Early Solar System Materials: Constraining the Early Evolution of Chondritic and Achondritic Parent Bodies(2016) Archer, Gregory Jude; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Highly siderophile element (HSE) abundances and 187Re-187Os isotopic systematics for H chondrites and ungrouped achondrites, as well as 182Hf-182W isotopic systematics of H and CR chondrites are reported. Achondrite fractions with higher HSE abundances show little disturbance of 187Re-187Os isotopic systematics. By contrast, isotopic systematics for lower abundance fractions are consistent with minor Re mobilization. For magnetically separated H chondrite fractions, the magnitudes of disturbance for the 187Re-187Os isotopic system follow the trend coarse-metalItem Siderophile elements and molybdenum, tungsten, and osmium isotopes as tracers of planetary genetics and differentiation: Implications for the IAB iron meteorite complex(2016) Worsham, Emily Anne; Walker, Richard J; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Isotopic and trace element abundance data for iron meteorites and chondrites are presented in order to investigate the nature of genetic relations between and among meteorite groups. Nebular and planetary diversity and processes, such as differentiation, can be better understood through study of IAB complex iron meteorites. This is a large group of chemically and texturally similar meteorites that likely represent metals with a thermal history unlike most other iron meteorite groups, which sample the cores of differentiated planetesimals. The IAB complex contains a number of chemical subgroups. Trace element determination and modeling, Re/Os isotopic systematics, nucleosynthetic Mo isotopic data, and 182Hf-182W geochronology are used to determine the crystallization history, genetics, and relative metal-silicate segregation ages of the IAB iron meteorite complex. Highly siderophile element abundances in IAB complex meteorites demonstrate that diverse crystallization mechanisms are represented in the IAB complex. Relative abundances of volatile siderophile elements also suggest late condensation of some IAB precursor materials. Improvements in the procedures for the separation, purification, and high-precision analysis of Mo have led to a ~2 fold increase in the precision of 97Mo/96Mo isotope ratio measurements, compared to previously published methods. Cosmic ray exposure-corrected Mo isotopic compositions of IAB complex irons indicate that at least three parent bodies are represented in the complex. The Hf-W metal-silicate segregation model ages of IAB complex subgroups suggests that at least four metal segregation events occurred among the various IAB parent bodies. The IAB complex samples reservoirs that were isotopically identical, but chemically distinct, and reservoirs that were chemically similar in some respects, yet isotopically different. Some IAB subgroups are genetically distinct from all other iron meteorite groups, and are the closest genetic relations to the Earth. The chemical differences between magmatic iron meteorite groups and some IAB subgroups likely originated as a result of their formation on undifferentiated parent bodies where impacting and mixing processes were important. This, in addition to the genetic difference between some IAB irons and magmatic groups, implies that these IAB parent bodies and magmatic parent bodies formed in a different location and/or time in the protoplanetary disk.