Geology Theses and Dissertations
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Item THE METAMORPHIC HISTORY OF A SUBDUCTED SLAB: NEW INSIGHTS FROM THE CATALINA SCHIST(2023) Taylor, Alexander Theodore; Penniston-Dorland, Sarah C.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rocks exhumed from ancient subduction zone interfaces often contain both coherent terranes and block-in-matrix mélanges, but the relationship between these two endmembers and its implications for underplating have not been closely examined. The Catalina Schist is a Cretaceous paleosubduction complex on Santa Catalina Island (California) that contains an amphibolite facies mélange with an underlying unit of coherent amphibolite. In this thesis, I present a metamorphic history of the coherent amphibolite that is based on field and petrographic evidence and pressure-temperature estimates from Raman elastic barometry and trace element thermometry. Rocks from the coherent amphibolite record peak metamorphic conditions of 1.20 to 1.29 GPa and 665 to 691 °C. This is consistent with several amphibolite mélange blocks, but the range of block conditions suggests that some upwards movement of the coherent amphibolite may have occurred at the subduction interface prior to the current juxtaposition of the two units.Item Rapid destabilization of deep, superhydrous magma prior to the largest known Plinian eruption of Cerro Machin volcano, Colombia(2022) Castilla Montagut , Silvia Camila; Newcombe, Megan; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A detailed stratigraphy of the pyroclastic fall deposits associated with Cerro Machin volcano (CMV) is presented following a previously defined categorization of the different eruptive units: El Espartillal, P0, P1, El Guaico, P2 and El Anillo. For the largest Plinian eruption in the sequence (P1), two lithofacies were distinguished on the basis of sedimentary features, grain size and componentry analysis. Early stages of the eruption could have been associated with vulcanian-type phases characterized by conduit/plug clearing explosions, producing a monolithologic lithic-rich laminated basal layer. The climactic event is represented by a white to grey, clast-supported, reverse to normally graded pumice-rich lapilli layer formed by a sustained eruptive column that gradually waned towards the end of the eruption. Associated deposits were identified up to 40 km from the vent. Pumice clasts from the most explosive phase were sampled along thedeposit layer in order to characterize storage conditions and ascent rates for the magma erupted. Pumice samples were classified as medium-K, calc-alkaline dacites (63-67 wt.% SiO2). The mineral assemblage includes plagioclase+amphibole+biotite+quartz and olivine and orthopyroxene (Fo 89-92) as accessory phases with amphibole overgrowths. Geothermobarometry of unzoned amphiboles, cores of reversely zoned crystals and rims of normally zoned crystals indicate a temperature range from 825±17°C and 913±45°C, a pressure range from 270±75 MPa to 1000±320 MPa indicating crystallization depths of 8-29 km. Thermobarometry of minor populations of unzoned amphibole crystals, cores of normally zoned crystals and rims of reversely zoned amphiboles show the same crystallization pressures as the dominant amphibole populations, but higher temperatures between 850±17°C and 978±29°C. The presence of these small populations of high-temperature amphiboles suggests a minor recharge event that did not drastically change the average crystallization conditions of the main dacitic reservoir but that could have been the trigger mechanism for the explosive eruption. CMV dacites display several geochemical signatures of adakites (low Y < 14 ppm, high Sr >700ppm, high Al2O3 > 16 wt.%, low MgO < 3 wt.%) which suggest that CMV magmas are produced by fractional crystallization of primitive hydrous magmas at the Moho boundary. The primitive magmas could have been the result of the interaction between Ba-enriched fluids dehydrated from the subducted slab with mantle peridotite in the mantle wedge. Three lines of evidence support the presence of superhydrous magma (containing >~8 wt% H2O) beneath Cerro Machin: 1) water concentrations of 2–11 wt% measured in plagioclase- and quartz-hosted melt inclusions; 2) the presence of Fo89-92 olivine rimmed by high Mg# amphibole; and 3) measurements of ~100–167 ppm H2O in plagioclase phenocrysts. Assuming a partition coefficient of 0.002, measured plagioclase crystals crystallized in a melt with 5-8 wt% H2O. Water-diffusion modelling in plagioclase crystals indicates a minimum time of 1 day for the magma to ascend from shallow depth (<250 MPa, ~5 km) before the eruption. Rapid ascent times are also suggested by the absence of breakdown rims in amphibole crystals which indicate a magma ascent timescale of <4 days from 8 km to the surface (Rutherford & Hill, 1993). Results from this study indicate that the 3600 yr BP eruption was preceded by rapid mobilization and ascent of superhydrous dacitic magma from mid- to deep-crustal storage regions beneath Cerro Machin. This thesis comprises eight appendices (A-H), Appendix A contains fieldwork information including: map of locations, a description of sample labels and schematic stratigraphic columns. In Appendix B, descriptions of componentry analysis are presented as well as pictures of the different grains identified under binocular observations. Appendix C is conformed by ten petrographic forms in which petrographic observations are detailed. This is followed by Appendix D in which whole rock chemistry of five samples is listed. In Appendix E, water measurements in melt inclusions are presented. In Appendix F, measurements of water in feldspar are detailed while in Appendix G, glass, melt inclusions, mineral chemistry and pictures of each crystal are presented, it also contains plagioclase-melt and amphibole-melt equilibrium tests and geothermobarometry calculations. Finally, Appendix H contains the link with the repository of the Matlab code used for volatile diffusion modelling in feldspar.Item Petrologic, Geochemical, and Spectral Characteristics of Oxidized Planetary Differentiation(2021) Crossley, Samuel Dean; Sunshine, Jessica M; Ash, Richard D; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Meteorites provide evidence that planetary formation occurred across a wide range of oxidation environments in the early Solar System. While the process of differentiation for many reduced, oxygen-poor assemblages has been thoroughly explored, significantly less is known about how differentiation occurred in more oxidized regions of the Solar System. Results from petrologic and geochemical investigations of oxidized chondrites (Rumurutiites) and primitive achondrites (brachinites) reveal that significant mineralogic differences occur with increasing degrees of oxidation. As a consequence, the differentiation pathways of oxidized and reduced assemblages diverge during the earliest stages of partial melting. While reduced materials differentiate to form a basaltic crust, magnesian peridotite mantle, and metallic core, oxidized materials may instead form felsic crusts, ferroan peridotite mantles, and sulfide-dominated cores. These pathways are evident in distinct siderophile trace element systematics for oxidized and reduced endmembers of the brachinite meteorite family. The compositions of olivine between oxidation endmembers are resolvable using remote sensing techniques that are applicable to asteroids. Most olivine-dominated asteroids examined in this work are consistent with having formed in oxidized environments, similar to R chondrites and brachinites, or in even more oxidizing environments not recorded among the meteoritic record. This provides strong evidence that environments capable of supporting oxidized, sulfide-dominated core formation are widespread among asteroidal materials. Several of these asteroids are likely mantle restites, based on their olivine composition and the estimated abundances of pyroxene. The predominance of oxidized over reduced environments among olivine-dominated asteroids is likely related to their respective petrogenetic histories: reduced assemblages must reach and sustain much higher temperatures to fully melt and segregate their pyroxene contents from olivine, which requires larger and earlier-accreted parent bodies. Consequently, sampling reduced mantle restites without significant pyroxene contamination would require catastrophic parent body destruction without mixing crustal and mantle materials. Oxidized materials, in contrast, have much higher initial olivine/pyroxene ratios, and thus are much more prone to producing asteroids dominated by olivine.Item CHARACTERIZING THE DURATION, PERIODICITY AND CHEMICAL IMPACT OF FLUID TRANSPORT IN THE SUBDUCTING SLAB: INSIGHTS FROM ISOTOPE GEOCHEMISTRY OF HIGH-PRESSURE METAMORPHOSED OCEANIC CRUST(2021) Hoover, William Floyd; Penniston-Dorland, Sarah C; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Subduction zones are key loci of geochemical cycling and natural hazards on Earth including large earthquakes and explosive volcanic eruptions. Fluids produced during subduction are thought to play a role in all these processes, however, many aspects of fluid transport in subduction zones remain enigmatic. In this dissertation, three types of fluid-related features are examined: 1) an eclogite-facies vein and 2) an eclogite- facies shear zone block and metasomatic rind, both from the Monviso Ophiolite (Western Alps), and 3) two amphibolite-facies mélange blocks and rinds from the Catalina Schist (CA). The mechanisms, episodicity and duration of fluid transport associated with these fluid pathways are investigated with bulk and in situ Li isotope geochemistry, in situ Sr isotope and trace element geochemistry, and quantitative transport modeling. In the eclogite-facies vein, evidence for five distinct locally-derived fluid compositions suggests a complex process of fluid-rock interaction. The unusual geometry of alteration features in the host rock suggests that initial host rock heterogeneity led to the development of reactive porosity channels. A method for in situ measurement of Li isotopes in garnet by secondary ion mass spectrometry is developed to explore the relative chronology of fluid rock interaction preserved in mineral zoning. The equivalence of natural garnet and garnet-like glass reference materials is demonstrated and a correction procedure for instrumental mass fractionation due to MnO and FeO is proposed. The resulting method is highly adaptable and attains 2-4‰ precision at the 20-μm-scale. Application of this method to garnet from the eclogite-facies shear zone block and rind reveals negative ?7Li excursions to values as low as -9‰ that record fluid-driven Li diffusion and rapid garnet growth. Multiple negative excursions within a single garnet require at least four episodes of fluid infiltration in the shear zone. Lastly, the first fluid transport durations for the subduction interface are obtained by inverting Li isotopes profiles from the amphibolite-facies mélange blocks and rinds using an advection-diffusion model. Uniform durations of ~60 years for metasedimentary rock-derived fluid flow near peak metamorphic conditions suggest fluid infiltration was pervasive and episodic, with earlier episodes erased by the expansion of rinds into blocks.Item Petrologic and geochronologic constraints on the thermal and structural evolution of paleo-subduction interfaces(2020) Harvey, Kayleigh; Penniston-Dorland, Sarah C; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In subduction zones, the interface between the downgoing slab and overriding plate controls a number of important dynamic processes. Both the rheologic behavior and thermal structure of the interface influence fluid release and transport, melt generation, seismic phenomena and viscous coupling between the downgoing and overriding plates. In this dissertation I present field, petrologic and geochronologic observations of paleo-subduction interface rocks from the Catalina Schist (Santa Catalina Island, CA) and Rio San Juan Complex (Dominican Republic). Methods including trace element thermometry, elastic barometry and Sm-Nd garnet geochronology are used to place constraints on spatial and temporal scales of deformation and to understand long-term changes in thermal structure. The applicability of new thermobarometric methods to reconstruct the pressure-temperature evolution of a sample are also assessed. Mélange, a block-in-matrix structure, is an important constituent of the subduction interface. Blocks within mélange can be mechanically and metasomatically mixed over several kilometers along the subduction interface, likely as a result of rheologic contrasts in the matrix. In the Catalina Schist amphibolite-facies mélange zone, blocks record up to 7 million years of variation in peak metamorphic age, placing constraints on the timescale over which the zone developed prior to underplating and rapid cooling of the system. This mixing process fundamentally changes the composition of the interface and may be an important driver of seismic phenomena including episodic tremor and slip. Changes in the thermal structure of the subduction interface can be constrained by thermobarometry of exhumed metamorphic rocks and the timescales of those changes can be constrained by geochronology. Here, new age constraints on the timing of amphibolite-facies metamorphism of the Catalina Schist are presented as well as the first evidence of an earlier eclogite-facies metamorphic event that is approximately coeval with the earliest records of subduction in related exhumed terranes. Finally, trace element and elastic thermobarometers are compared to major element thermobarometry and phase equilibria modeling in order to assess the utility of trace element and elastic thermobarometry to reconstructing the metamorphic history of a sample using an eclogite from the Rio San Juan Complex as a type-example.Item DYNAMIC MELT PROCESSES IN THE LITHOSPHERES OF MARS AND IO(2020) Schools, Joseph William; Montesi, Laurent G. J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The interior structures of planetary bodies beyond the Earth are broadly unknown. Our observational capacity is largely limited to surface imagery from spacecraft. The work presented in this dissertation uses novel modeling methods of melt migration and melt focusing processes to illuminate the thermal and structural characteristics of the lithospheres of Mars and Jupiter’s moon Io. Models are constrained by, and inform observations of, surface volcanism. Coupled petrological-geodynamical models of one-dimensional melt migration are performed to determine the depth of impermeable boundaries, known as permeability barriers, in the lithosphere of Mars. Relatively deep permeability barriers are found to be prevalent throughout Martian history unless in regions of high strain rate (10^-13 s^-1), or a wet mantle (25-1000 ppm H2O). Permeability barrier depth is suggested to be linked to the style of volcanic edifice seen at the surface, with deep barriers creating larger edifices like shield volcanoes, and shallower barriers creating widespread flows. Similar petrological-geodynamical models performed for the lithosphere of Io reveal that permeability barriers always form at the base of the lithosphere due to the cold temperatures caused by geologically rapid resurfacing (~1 cm/yr) and subsidence. Melt may ascend closer to the surface in areas with a low subsidence rate (0.02 cm/yr) Two-dimensional numerical models of melt migration in the Martian lithosphere suggest that convection in a highly porous layer beneath a permeability layer (a decompaction channel) focuses melt over the convective wavelength. Melt ascends in the lithosphere at this wavelength which is reflective of volcano spacing at the surface for Hesperian aged terrains. Numerical and analytical models of melt flow through the asthenosphere and lithosphere of Io constrain the lifespan of its volcanic plumbing systems. A 1 km conduit will fully close within ~10,000 years while a 25 km conduit of melt will close within 6-7 million years. Solid convection in the asthenosphere is found to be necessary for melt focusing to heat pipe centers at the base of the lithosphere, however it is counterintuitively found that an arrangement with downwelling undeath the eruptive center is the most efficient for melt extraction.Item THE GEOCHEMICAL BEHAVIOR OF SCANDIUM DURING FRACTIONAL CRYSTALLIZATION AND IMPLICATIONS FOR ORE FORMATION(2020) Gion, Austin Michael; Piccoli, Philip M; Candela, Philip A; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Demand for scandium is increasing due to the use of scandium-aluminum alloys in the aerospace and sporting goods industries, and in solid-oxide fuel cells. Scandium deposits are associated with mafic intrusions and laterites, carbonatites, and granitic pegmatites and the element is recovered as a byproduct of uranium, titanium, apatite, and rare earth element mining. Understanding the processes that form scandium-rich deposits is vital in order to inform exploration for such deposits. The deposits and mafic intrusions at Kiviniemi, Finland and Nyngan, Australia, as well as the granitic pegmatites of Evje-Iveland, Norway are of particular interest. Experiments and thermodynamic modeling of magmatic-hydrothermal systems have been performed in order to constrain the petrogenesis of these deposits. Cold-seal pressure vessel experiments have been performed on systems with basaltic to rhyolitic compositions in order to evaluate the behavior of scandium in upper crustal magmas. Partition coefficients for scandium between olivine, pyroxene, plagioclase, biotite, spinel, cordierite, aluminosilicates, ilmenite, rutile, apatite and silicate melts, were determined and found to vary as a function of mineral and melt compositions. These partition coefficients were combined with MELTS modeling (MELTS is a software package that is used for performing thermodynamically constrained phase equilibria calculations) to evaluate the behavior of scandium during fractional crystallization of a mafic melt and formation of a cumulate, the subsequent partial melting of that cumulate, then the isothermal decompression and final cooling of that melt. Fractional crystallization can produce scandium-rich cumulates, such as those found at Kiviniemi and Nyngan. However, felsic melts produced by partial melting of a scandium-rich cumulate have, at most, scandium concentrations consistent with the upper continental crust. Amphibolite partial melting experiments were performed in a piston-cylinder to constrain the petrogenesis of the Evje-Iveland pegmatites. These experiments are inconsistent with the long-held hypothesis that the pegmatites formed by partial melting of their host amphibolite. Instead, magmatic differentiation is the preferred petrogenic model. This model requires that few ferromagnesian phases occur during crystallization of a felsic melt or the presence of scandium complexes that reduce scandium partition coefficients.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.