UMD Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/3
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a given thesis/dissertation in DRUM.
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
4 results
Search Results
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 Lithium and its isotopes as a tracer of fluid flow mechanisms in the Catalina Schist melange zone(2014) Roble, Leigh Anne; Penniston-Dorland, Sarah; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mélange zones are areas of highly mixed and deformed rock believed to form from shearing between subducting slab and peridotite mantle wedge. They have high -pressure/low-temperature mineral assemblages and contain a fine-grained matrix with centimeter to meter scale blocks surrounded by rinds, thought to represent a reaction zone between the block and matrix. These rinds are not well understood, but could be formed due to mechanical mixing, diffusion, or infiltration. Lithium is used to determine the role played by fluid-mediated processes in the Catalina Schist mélange zone because it is fluid mobile and has high diffusivity. Samples from amphibolite, lawsonite-blueschist, and lawsonite-albite facies were retrieved from the Catalina Schist subduction complex on Santa Catalina Island. Lithium isotopic compositions and concentrations were determined using mass spectrometry techniques. One-dimensional diffusion models were applied to the data to determine the extent of the different mechanisms responsible for fluid transport throughout the subduction complex.Item TRACING RETROGRADE METAMORPHIC FLUIDS IN A SUBDUCTION ZONE USING LI: FRANCISCAN COMPLEX, CALIFORNIA(2012) Henriquez, John-Luke; Penniston-Dorland, Sarah C; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Centimeter-scale layers of eclogite and blueschist from Tiburon Peninsula, Franciscan Complex, CA were contrasted with a similarly layered sample from Dos Rios, CA. Eclogites from both localities have similar mineral assemblages (e.g., omphacite, glaucophane, phengite, garnet, epidote, and titanite). However, the Tiburon blueschist shows petrographic evidence for fluid-rock interaction, while the Dos Rios sample does not. Mineral phases common to both samples were contrasted via textural evidence, major and minor element concentrations, and lithium concentrations. Lithium concentrations of omphacite and chlorite decrease from the eclogite to the blueschist domains in the Tiburon sample. These lithium concentration differences are interpreted to be the result of fluid-rock interactions. These differences are not seen in the Dos Rios sample. I propose that a difference in the Dos Rios sample bulk composition produced the alternating eclogite and blueschist lithologies as a result of a process such as seafloor alteration prior to prograde metamorphism.Item Lithium and delta7Li behavior during metamorphic dehydration processes and crustal evolution(2011) Qiu, Lin; Rudnick, Roberta L; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Li isotopes have been used as a tracer of a wide range of geological processes, because the two stable isotopes of Li have significant relative mass difference and large elemental dispersion in Earth. In order to understand better Li behavior during the processes of sub-greenschist to granulite facies metamorphism and partial melting, fluid transport in subduction zones, weathering of continental crust and the evolution of juvenile continental crust, a large variety of samples have been studied in this thesis. These samples include mudrocks from British Caledonides, schists from Otago, New Zealand, high-grade metapelites and metabasites from Ivrea-Verbano Zone, Italy, Archean shales from the Kaapvaal Craton, South Africa, and Archean granitoids from Scotland, South Africa and Canada. Investigations of metapelites from several localities show that: 1) sedimentary provenance exerts the greatest control on Li in fine grained sediments, and Li concentrations generally increase while delta7Li decreases with CIA (Chemical Index of Alteration) in post Archean shales, 2) sub-greenschist facies metamorphism has negligible effect on Li concentrations and isotopic compositions, 3) metamorphic dehydration from greenschist to granulite facies may cause significant Li depletion, but has had little influence on delta7Li, and 4) a key factor controlling [Li] in metapelites during metamorphism is the stability of Mg-bearing phyllosilicates. Furthermore, based on the understanding gained of the Li behavior above, the Li signature in Archean shales and granitoids may indicate that: 1) more severe weathering conditions prevailed during the Archean, 2) the Archean upper continental crust had a heavier Li isotopic composition than post Archean upper continental crust, and 3) Archean juvenile granitoids have heavier Li isotopic compositions than post-Archean equivalents. Collectively, the results for Archean samples may indicate that Li isotopes can be used as tracers of paleoclimate change, and also can be used to understand the nature of the source of Archean juvenile magmas.