Geology Theses and Dissertations

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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.
MOBILIZATION OF CHEMICAL COCKTAILS BY FRESHWATER SALINIZATION SYNDROME IN THE CHESAPEAKE BAY WATERSHED
(2023) Galella, Joseph George; Kaushal, Sujay S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Increasing trends in base cations, pH, and salinity of urbanizing freshwaters have been documented in U.S. streams for over 50 years. These patterns, collectively known as Freshwater Salinization Syndrome (FSS), are driven by multiple processes, including applications of road salt and human-accelerated weathering of impervious surfaces, reductions in acid rain, and other anthropogenic legacies of change. FSS mobilizes chemical cocktails of distinct elemental mixtures via ion exchange, and other biogeochemical processes. Urban streams in temperate areas experience chronic salinization throughout the year punctuated by acute salinization during winter storms with associated road salting. My research analyzed impacts of FSS on stream water chemistry in the field with routine bi-weekly and targeted high frequency sampling during road salting events. Field sites were proximal to USGS stream sensors using multiparameter datasondes, allowing for additional parameters to be monitored at 5-15 minute resolution. In the laboratory incubation analyses were also conducted using sediment and water samples to assess the function of stormwater best management practices (BMPs) during road salting events. Acute FSS associated with road salting was found to mobilize chemical cocktails of metals (Mn, Cu, Sr²⁺), base cations (Na+, Ca²⁺, Mg²⁺, K⁺), nutrients (TDN), and organic matter (NPOC). Regression relationships were developed among specific conductance and major ion and trace metal concentrations. These linear relationships were statistically significant in most of the urban streams studied (e.g., R2 = 0.62 and 0.43 for Mn and Cu, respectively), and showed that specific conductance could be used as a proxy to predict concentrations of major ions and trace metals. Principal component analysis (PCA) showed co-mobilization (i.e., correlations among combinations of specific conductance, Mn, Cu, Sr²⁺, and all base cations during certain times of year and hydrologic conditions). Co-mobilization of metals and base cations was strongest during peak snow events but could continue over 24 hours after specific conductance peaked, suggesting ongoing cation exchange in soils and stream sediments. Increased salt concentrations of all three major road salts (NaCl, CaCl₂, and MgCl₂) had profound effects on major and trace element mobilization, with all three salts showing significant positive relationships across nearly all elements analyzed. Salt type showed preferential mobilization of certain elements. NaCl mobilized Cu, a potent toxicant to aquatic biota, at rates over an order of magnitude greater than both CaCl₂ and MgCl₂. Hourly mass fluxes of TDN in streams were also found to be elevated during winter months with peaks coinciding with road salting events. Targeted winter snow event sampling and high-frequency sensor data suggested plateaus in NO₃⁻ / NO₂⁻ and TDN concentrations at the highest peak levels of SC during road salt events between 1,000 and 2,000 μS/cm, which possibly indicated source limitation of TDN after extraction and mobilization of watershed nitrogen reservoirs by road salt ions. My results may help guide future regulations on road salt usage as there are currently no federally enforceable limits. NaCl is the most commonly used deicer in the United States, largely because it is often the least expensive option. Other technologies such as brines and other more efficient deicers (CaCl₂ and MgCl₂) should be considered in order to lessen the deleterious effects of FSS.
Thermal Control on the Location of the Volcanic Arc at Subduction Zones
(2023) Ha, Goeun; Montesi, Laurent G.J.; Zhu, Wenlu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
At subduction zones, where oceanic plates are recycled into the Earth’s interior, water released by the subducting plate initiates partial melts that form volcanic arcs. Partial melts can be present in a broad melting zone below a narrow volcanic arc. The second melting zone can be formed by mantle upwelling induced by active extension behind the arc and subsequent decompression melting. In this dissertation, I explain the locations of the arc in global using a temperature-dependent melt focusing mechanism. I present a simple geometrical model to explain the observed correlation between the location of the arc and the back-arc spreading center (BASC) at five subduction zones. Lastly, I discuss the thermal influence of the BASC on the arc location. The melts rise vertically through the pore spaces in the mantle rock until they encounter a low permeability barrier formed at a temperature where the crystallization rate is maximum. As the melt trajectory is deflected laterally, the melts are focused at the apex of the permeability barrier and the volcano is more likely to form immediately above the magma pool. In the subduction zones without back-arc spreading, the projection of the apex of the barrier-forming isotherm shows good agreement with the observed arc locations. The arc and the BASC location are negatively correlated with the slab dip at five subduction zones. The decoupling depth between the slab and the overlying mantle defines the closest approach of the nose of the isotherm. The horizontal distance from the trench to the decoupling depth is controlled by the slab dip, which produces the negative correlation. The back-arc extension is related to the trench retreat and the slab anchoring at 660 km discontinuity, which results in a decrease in the slab dip. The relation between the slab anchoring depth and the slab dip generates the observed negative correlation. When the BASC develops near the trench, the thermal structure is disrupted by the mantle upwelling and thereby the predicted arc location moves toward the spreading center.
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.
Freshwater salinization syndrome limits management efforts to improve water quality
(2022) Maas, Carly Marcella; Kaushal, Sujay S; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Freshwater Salinization Syndrome (FSS) refers to the interactive effects of salt ions on the degradation of the natural, built, and social systems. FSS can mobilize chemical mixtures, termed ‘chemical cocktails’, in watersheds. The formation of chemical cocktails across space and time depends on the amounts and types of salt pollution, the surrounding land use including conservation and restoration areas, and the location along the flowpath in the watershed. We investigated (1) the formation of chemical cocktails temporally and spatially and (2) the natural capacity of watersheds and streams to attenuate salt ions along flowpaths with conservation and restoration efforts. We monitored high-frequency temporal and longitudinal spatial chemical changes in stream water in response to different pollution events (i.e., road salt, stormwater runoff, wastewater effluent, and baseflow conditions) and several types of watershed management efforts (i.e., national parks, regional parks, and floodplain reconnection) in six urban watersheds in the Chesapeake Bay region. There were significant relationships between watershed impervious surface cover and mean concentrations of salt ions (Ca2+, K+, Mg2+), metals (Fe, Mn, Sr2+), and nutrients (total dissolved nitrogen) (p < 0.05). Principal component analysis (PCA) indicates that chemical cocktails which formed along flowpaths in response to winter road salt applications were enriched in salts and metals (e.g., Na+, Mn, and Cu). During most baseflow and stormflow conditions, chemical cocktails that were less enriched in salt ions and trace metals were attenuated downstream. There was also downstream attenuation of FSS ions during baseflow conditions through management efforts including a regional park, national park, and floodplain restoration. Conversely, chemical cocktails that formed in response to multiple road salt applications or prolonged road salt exposure did not show patterns of attenuation downstream. The spatial patterns were quite variable, with increasing, plateauing, or decreasing patterns based on the magnitude, timing, duration of road salt loading, and extent of management efforts. Our results suggest that FSS can mobilize multiple contaminants along watershed flowpaths, however, the capacity of current watershed management strategies such as restoration and conservation areas to attenuate FSS is limited.

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