Geology
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Item USING BAYESIAN ELECTRICAL RESISTIVITY INVERSION TO REVEAL HILLSLOPE DRY-UP PROCESS IN A MEDITERRANEAN CLIMATE(2024) Shahid, Saffat; Huang, Mong-Han; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrologic dynamics in hillslopes is essential for comprehending the processes that shape landscape evolution and sustain the Earth’s critical zone. Electrical resistivity (ER) is considered as one of the best geophysical methods to observe these dynamics due to its sensitivity to subsurface water content. To understand hillslope water dynamics and mitigating the risks of slope instability caused by extreme weather events, we studied how subsurface hydrological processes are being influenced by variations in vegetation type across different aspects of hillslopes. Thus, how accurately ER can quantify the dry-up process during the growing season on hillslopes becomes critical, particularly in regions with distinct dry summers and wet winters (i.e. Mediterranean climates). The Blue Oaks Ranch Reserve (BORR) in Central California provides an ideal location for this study due to its consistent ridge-valley systems, which well represents the regional climatic and topographic conditions. Previous work at BORR used active source seismic refraction (SR) to constrain subsurface structure. To additionally investigate moisture content in regolith, we conduct ER surveys with Schlumberger and Dipole-Dipole configurations to invert for resistivity using Transdimensional Hierarchical Bayesian (THB) inversion framework with reversible-jump Markov Chain Monte Carlo (THB rj-MCMC). We also performed 2D synthetic tests to evaluate how well THB can recover a synthetic model with imposed data uncertainty. The results indicate that Schlumberger outperforms Dipole-Dipole in the THB rj-MCMC inversion. However, these results also reveal limited depth resolution to ~10 m depth using current ERT configurations. Finally, we adopt the THB approach for a series of ER surveys at BORR between June and September 2023. The findings suggest a distinct increase in resistivity on the North-facing slope during growing seasons, indicating reduced moisture content particularly in areas with presences of oak trees as they draw water from deep regolith. On the South-facing slope, resistivity remained stable due to the dominance of grass that lacks deep roots for consuming deep moisture. Our resistivity results show that vegetation type particularly trees play a critical role in regolith moisture distribution. To compare and correlate changes in resistivity over dry periods, we analyzed soil probe data previously collected at the site. The correlation suggested that increases in resistivity are related to decreases in volumetric moisture content. Additionally, we compared ERT data with seismic survey data to better understand changes in subsurface properties like porosity and saturation along depth, as ERT and seismic velocity is sensitive to moisture content and material porosity.Item QUANTIFICATION OF MELT DISTRIBUTION, MELT CONNECTIVITY, AND ANISOTROPIC PERMEABILITY OF DEFORMED PARTIALLY MOLTEN ROCKS USING X-RAY MICROTOMOGRAPHY(2024) Bader, James Alexander; Zhu, Wenlu; Montesi, Laurent G.J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Volcanic activity plays a dominant role in shaping the surface of Earth and other planets. For example, Earth’s ocean floor is created by volcanic activity at mid-ocean ridges. There, magma is sourced from a ~60 km deep, ~100 km wide region of the mantle, from which partial rock ascends and erupts along narrow ridges that run along the middle of Earth’s oceans. Volcanic activity at mid-ocean ridges, the strength of Earth’s mantle, and the geochemical composition of volcanic rocks and ocean water are all influenced by how the melt is distributed in partially molten rocks, and how easily it can flow through the partially molten mantle beneath these ridges. In particular, it is often assumed that most of the melt ascends through isolated channels that direct it towards the mid-ocean ridges, making melt transport localized and anisotropic. A possible origin of these channels is the differential stress induced by upwelling mantle material beneath the ridge, which has been shown in laboratory experiments to localize melt into planar regions named “melt-rich bands.” To date, the development and characteristics of melt-rich bands have been studied principally using theoretical models and two-dimensional (2D) images of sheared partially molten rocks. There has been little experimental research using 3D techniques until now. This thesis uses 3D images of sheared partially molten rocks created in the laboratory, obtained using high-resolution x-ray microtomography (X-ray µCT), to investigate how the distribution of melt, its orientation, its connectivity, and its ability to flow through the rocks changes when stress is applied. This study shows how melt connectivity and, therefore, rock permeability changes as melt changes from being dispersed through a partially molten rock to being localized on well-developed melt-rich bands. This work shows that melt forms melt volumes that are preferentially elongated within the plane of melt-rich bands even before these bands form. This discovery emphasizes the importance of permeability anisotropy at all stages of melt-rich band development. We also measured permeability and melt connectivity at all scales, both inside and outside melt-rich bands. Our results show that melt can hardly flow perpendicular to melt-rich bands over distances larger than a few grains. Additionally, the permeability along the melt-rich bands is also reduced by half compared to that in a partially molten rock that is not subjected to differential stress. This research quantifies the uneven distribution of permeability in a sheared, partially molten rock. It also proposes a scheme to average local permeability estimates, helping us understand and quantify how melt travels along melt-rich bands at various scales. These findings provide valuable insights into how magma flows in the Earth’s mantle, especially at active plate boundaries like mid-ocean ridges. Overall, this research provides the first experimental constraints, based on 3D microtomography images, on the melt network and how melt flows in the presence of stress in partially molten rocks.Item GEOCHEMICAL INDICATORS OF REDOX AND WEATHERING ACROSS THE EDIACARAN-CAMBRIAN TRANSITION IN SIBERIA(2024) Doerrler, Andrew; Kaufman, Alan Jay; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The transition between the Ediacaran and Cambrian periods witnessed the fall of the enigmatic Ediacara biota. The cause of their extinction is poorly understood, but the timing broadly coincides with a significant negative ẟ13C anomaly, the BAsal Cambrian Carbon isotope Excursion (BACE). These macroscopic organisms were likely tolerant of anoxic conditions, so oceanic oxidation may have been a kill mechanism. This study utilizes uranium isotopes, sulfur isotopes, and cerium anomalies to understand oceanic redox conditions from two BACE sections in Siberia, as well as lithium isotopes to constrain weathering intensity. Reconstruction of seawater ẟ238U values from equivalent evaporite and carbonate-dominated successions indicate a notable increase in oxygen during the event supporting the oxidation hypothesis. Global sea level fall and evaporite formation suggest that seawater salinity increased dramatically along continental margins, which may provide an alternative osmotic kill mechanism for the softbodied Ediacaran biota. Support for the salinity hypothesis comes from profound ẟ7Li and ẟ34S compositions of carbonate and pyrite, respectively, which arguably resulted from the distillation of lithium and sulfate from seawater into evaporite-rich lithologies.Item PREDICTING THE SEISMIC SIGNATURE OF LAVA TUBES FOR THE EARTH, THE MOON, AND MARS(2024) Wike, Linden; Schmerr, Nicholas; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lava tubes are a type of volcanically-generated subsurface void structure found on Earth, the Moon, and Mars and hold the potential to serve as shelters for crew members, preservation sites of pristine geological samples, and locations of in situ resources. A key question for lava tube science is how to locate them through geophysical methods. Here, we create a workflow that locates and characterizes the geometry of subsurface voids. We build a suite of subsurface synthetic seismic wavefield models that contain lava tube structures and investigate which seismic method best images them. Our models show that the more readily detectable lava tube simulations have a geophone spacing of 0.5 m, variable diameter, and shallow ceiling depth; and reverse-time migration and phase-shift-plus-interpolation migration techniques produce more accurate lava tube reconstructions than the Kirchhoff method. The synthetic modeling serves as a benchmark for understanding seismic wave propagation around lava tubes and helps answer how voids on the Moon and Mars would be imaged through seismology.Item THE CONCENTRATION OF HYDROGEN IN INCOMPLETELY AND WHOLLY MELTED TERRESTRIAL BUILDING BLOCKS(2024) Peterson, Liam Donald; Newcombe, Megan E; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrogen (H) is the most abundant element in our solar system and exerts a primary control on the habitability, and geochemical and geodynamic evolution of rocky bodies. Therefore, constraining the source(s), timing of accretion, and abundance of H in the Earth and other bodies is of fundamental importance for understanding how planets evolve. Direct constraints on the source(s) of H and other highly volatile elements (HVEs; e.g., H, C, F, Cl, and S) to the bulk Earth can be provided by analyzing meteorites, which are the remnants of early-formed rocky bodies that were present during the accretion of the terrestrial planets. Such samples either directly sample or provide analogs for terrestrial precursor materials.Rocky solar system materials can be subdivided based upon their nucleosynthetic isotopic compositions (“genetic” tracers; e.g., 50Ti, 54Cr) into two groups, which are thought to correspond to the inner- and outer- solar system. Materials may be further subdivided by their extent of thermal processing (i.e., unmelted, incompletely melted, and wholly melted). Earths H budget is commonly accounted for by addition of unmelted (i.e., chondritic) materials, namely carbonaceous chondrite-like (CC-like) materials, thought to be derived from the outer solar system, which have high H concentrations (up to ~14 wt. % H2O; total H as H2O equivalents) and similar H isotopic compositions to the bulk Earth. Furthermore, chondrites derived from the inner solar system (e.g., ordinary and enstatite chondrites) are H-poor relative to carbonaceous chondrites. Similarly, all melted planetesimals are commonly assumed to be anhydrous. However, recent analyses of enstatite chondrites (ECs), which are formed in the inner solar system and are the closest match to the nucleosynthetic isotopic composition of the bulk Earth, suggest that ECs have a similar H isotopic composition to the bulk Earth and can account for its entire H budget. Furthermore, recent analyses suggest that melted (i.e., achondritic) bodies may retain considerable amounts of H, potentially enough to account for Earth’s H budget in the case of the enstatite achondrites (i.e., aubrites). However, achondritic materials are predominantly highly H-poor relative to chondritic materials, and it is unclear if the aubrites are an anomaly, and at which stage of planetesimal evolution H and other HVEs are lost. In chapter 2, I re-examine prior bulk analyses of H in aubrites, and by extension ECs, using in situ methods and suggest that nearly all H measured in aubrites by bulk methods reflects pervasive terrestrial contamination and alteration, a result which may extend to concurrent bulk H analyses of ECs. In chapters 3 and 4, I examine the H content of incompletely melted (i.e., primitive achondritic) planetesimals to constrain at what stage of planetesimal evolution H is lost. Chapter 3 characterizes the H contents of the ureilites, a group of C-rich primitive achondrites, and chapter 4 characterizes the H contents of the acapulcoite-lodranite clan which represents the “prototypical” primitive achondritic parent body. I find that primitive achondritic parent bodies are highly H-depleted relative to chondrites, requiring that H is efficiently lost prior to or at the onset of planetesimal melting, and that Earth’s H budget is accounted for by accretion of thermally primitive materials (e.g., chondrites). Within my primitive achondrite data sets, I observe apparent disequilibrium with respect to H between olivine, pyroxene, and feldspar. In chapter 5, I explore whether this apparent disequilibrium is the result of extrapolating high pressure experimental data to low pressures. I conduct olivine–melt H partitioning experiments at low pressures (10 – 200 MPa) and find that the olivine-melt H partition coefficient increases at low pressures, contrary to extrapolation from high pressure data. This observation is best explained by a control of H speciation in the melt on the partitioning of H between olivine and melt.Item EXPERIMENTAL CONSTRAINTS ON PHYSICAL PROPERTIES OF VOLCANIC ROCKS WITH IMPLICATIONS FOR LUNAR EXPLORATION(2023) Braccia, Casey Marie; Zhu, Wenlu; Schmerr, Nicholas; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The lunar subsurface is a primary science target for future missions to the Moon and serves as a potential host location for resources such as water ice, void spaces for astronaut shelter, and important ore bodies for in-situ manufacturing and building materials (Horz, 1985; Coombs and Hawke, 1992; Wendell, 2017). Here we conducted laboratory experiments to investigate how the seismic signature obtained at or near the lunar surface is related to subsurface material properties and structures. A range of analog basaltic samples collected from the San Francisco Volcanic Field (SFVF) in Arizona, Kilbourne Hole (KH) in Southern New Mexico, and Lava Beds National Monument (LBNM) in Northern California are collected and their geophysical properties are measured. The relationships between the seismic wave velocity and porosity of basaltic rocks from different locales are obtained and the measured velocities are compared to local seismic surveys of the sample locale. Using laboratory techniques, the SFVF basalts P-wave and S-wave velocities range from ~5 km/s to ~6 km/s +/- 0.1 km/s and ~2.1 km/s to ~3 km/s +/- 0.1 km/s, respectively. For the KH basalts P-wave and S-wave velocities range from ~3.5 km/s to ~4.4 km/s +/- 0.1 km/s and ~1.8 km/s to ~2.3 km/s +/- 0.1 km/s, respectively. For LBNM basalts P-wave and S-wave velocities range from ~3.1 km/s to ~6 km/s +/- 0.1 km/s and ~1.2 km/s to ~2.4 km/s +/- 0.1 km/s, respectively. A relationship between the porosity and velocity of basalts from these three locations was determined. Using the field seismic data for each study site and the relationship derived from the laboratory seismic data and porosity, the amount of possible large-scale fracturing is derived. Based on these experimental measurements, we estimate the large-scale fractures and voids are ~84 vol% at the SFVF, ~42 vol% at the KH, and ~39 vol% at the LBNM. These results provide quantitative constraints for subsurface exploration in future lunar surface missions.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 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.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 GENETICS, AGES, AND CHEMICAL COMPOSITIONS OF THE GROUP IIIE IRON METEORITES AND THE IRON METEORITE LIEKSA(2022) Chiappe, Emily; Walker, Richard; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Siderophile element concentrations, genetic isotopic data, and chronological data for ten group IIIE iron meteorites and the recently found iron meteorite Lieksa were determined. The modeling of siderophile element abundances shows that the IIIE irons can be related to one another through a common fractional crystallization process. Highly siderophile element data, however, indicate that the anomalous IIIE iron meteorite Aletai does not sample the same crystallization sequence as the bona fide IIIE irons. The bulk chemical characteristics of Lieksa are distinct from that of the established iron meteorite groups, indicating that, if it is an iron meteorite, it should be classified as an ungrouped iron meteorite. Isotopic data shows that, while the IIIE iron meteorites, Lieksa, and Aletai exhibit different HSE abundances, they exhibit similar genetic characteristics, indicating that they likely originated from the same nebular reservoir. Additionally, isotopic data indicate that all irons analyzed here sample parent bodies that differentiated within the first ~2 Myr of solar system history, which is consistent with other NC-type bodies.