Matching Items (18)

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Shock Effects and Mineral Assemblages in the Genomict Eucrite Northwest Africa 8677

Description

Shock effects in meteorites provide important insights into impacts on their parent bodies. Eucrites are among the Howardite-Eucrite-Diogenite (HED) class of achondrites that likely originate from the intact, differentiated asteroid

Shock effects in meteorites provide important insights into impacts on their parent bodies. Eucrites are among the Howardite-Eucrite-Diogenite (HED) class of achondrites that likely originate from the intact, differentiated asteroid Vesta. Brecciated eucrites provide a record of the impact processes that occurred after the crustal formation of the parent body. Radiometric dating of HEDs has shown that they were affected by resetting events at 3.4 – 4.1 and 4.48 Ga. Therefore, shock effects in HEDs are windows into ancient impacts on asteroids early in solar system history. Northwest Africa (NWA) 8677 is a genomict eucrite with lithologies that are texturally different, but compositionally similar. The clasts in the breccia include strongly shocked (S5) gabbroic fragments and weakly shocked (S3) basaltic clasts. Coesite, a high-pressure polymorph of quartz, is preserved in the core of a large (~250 μm) silica grain, indicating the gabbro was strongly shocked. A large thermal overprint from the surrounding melt resulted in the transformation of coesite to low-pressure silica phases of quartz and cristobalite on the rims of this grain. The shock melt, interstitial to the breccia fragments, exhibits well-developed quench textures and contains a low-pressure mineral assemblage of plagioclase and pyroxene, implying that crystallization occurred after pressure release. The heterogeneity in shock features between the gabbroic and basaltic lithologies suggests that NWA 8677 experienced a variable impact history, which included at least two impact events. An initial impact strongly shocked and brecciated the gabbro and ejected both onto the regolith of the parent body where a more weakly shocked basalt was incorporated. A second impact produced the interstitial melt between the breccia matrix. The temperature of this shock melt remained high after pressure release, resulting in crystallization of a low-pressure assemblage of pyroxene and feldspar, as well as the transformation of quartz + cristobalite rims on coesite

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Date Created
  • 2019-05

Systematics of giant impacts in late-stage planet formation and active neutron experiments on the surface of Mars

Description

Part I – I analyze a database of Smoothed Particle Hydrodynamics (SPH) simulations of collisions between planetary bodies and use the data to define semi-empirical models that reproduce remant masses.

Part I – I analyze a database of Smoothed Particle Hydrodynamics (SPH) simulations of collisions between planetary bodies and use the data to define semi-empirical models that reproduce remant masses. These models may be leveraged when detailed, time-dependent aspects of the collision are not paramount, but analytical intuition or a rapid solution is required, e.g. in ‘N-body simulations’. I find that the stratification of the planet is a non-negligible control on accretion efficiency. I also show that the absolute scale (total mass) of the collision may affect the accretion efficiency, with larger bodies more efficiently disrupting, as a function of gravitational binding energy. This is potentially due to impact velocities above the sound speed. The interplay of these dependencies implies that planet formation, depending on the dynamical environment, may be separated into stages marked by differentiation and the growth of planets more massive than the Moon.

Part II – I examine time-resolved neutron data from the Dynamic Albedo of Neutrons (DAN) instrument on the Mars Science Laboratory (MSL) Curiosity rover. I personally and independently developed a data analysis routine (described in the supplementary material in Chapter 2) that utilizes spectra from Monte Carlo N-Particle Transport models of the experiment and the Markov-chain Monte Carlo method to estimate bulk soil/rock properties. The method also identifies cross-correlation and degeneracies. I use data from two measurement campaigns that I targeted during remote operations at ASU. I find that alteration zones of a sandstone unit in Gale crater are markedly elevated in H content from the parent rock, consistent with the presence of amorphous silica. I posit that these deposits were formed by the most recent aqueous alteration events in the crater, since subsequent events would have produced matured forms of silica that were not observed. I also find that active dunes in Gale crater contain minimal water and I developed a Monte Carlo phase analysis routine to understand the amorphous materials in the dunes.

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Created

Date Created
  • 2019

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Using array seismology to study planetary interiors

Description

Stratification is a dominant feature of all planetary interiors. Fine-scale structure associated with layering, as well as heterogeneities hold important clues on a planet's compositional, thermal, and dynamical state, as

Stratification is a dominant feature of all planetary interiors. Fine-scale structure associated with layering, as well as heterogeneities hold important clues on a planet's compositional, thermal, and dynamical state, as well as its evolution. This research centers on using data from seismic arrays, networks of seismic sensors, and array processing methodologies to map the fine scale structure in the Earth's upper mantle and deep layering in the Moon - Earth and Moon are the only two planetary bodies with seismic available data for such analyses. Small-scale structure in the Earth's upper mantle can give rise to seismic wave scattering. I studied high frequency data from the Warramunga Array in Australia using array seismology. I developed and employed back-projection schemes to map the possible upper mantle scattering or reflection locations. Mapped scatterers show good correlation to strong lateral P-wave velocity gradients in tomography models and may be associated with the complex tectonic history beneath north of Australia. The minimum scale of scatterers relates to the seismic wavelength, which is roughly between 5 and 10 km in the upper mantle for the frequencies we study. The Apollo Passive Seismic Experiment (APSE) consisted of four 3-component seismometers deployed between 1969 and 1972 that continuously recorded lunar ground motion until late 1977. I studied the deep lunar interior with array methods applied to the legacy APSE dataset. The stack results suggest the presence of a solid inner and fluid outer core, overlain by a partially molten boundary layer, but their reflector impedance contrasts and reflector depths are not well constrained. With a rapidly increasing number of available modern broadband data, I developed a package, Discovery Using Ducttape Excessively (DUDE), to quickly generate plots for a comprehensive view of earthquake data. These plots facilitate discovery of unexpected phenomena. This dissertation identifies evidence for small-scale heterogeneities in Earth's upper mantle, and deeper lunar layering structure. Planetary interiors are complex with the heterogeneities on many scales, and discontinuities of variable character. This research demonstrates that seismic array methods are well-suited for interrogating heterogeneous phenomena, especially considering the recent rapid expansion of easily available dense network data.

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Date Created
  • 2011

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Energy transfer between the geosphere and biosphere

Description

One goal of geobiochemistry is to follow geochemical energy supplies from the external environment to the inside of microbial cells. This can be accomplished by combining thermodynamic calculations of energy

One goal of geobiochemistry is to follow geochemical energy supplies from the external environment to the inside of microbial cells. This can be accomplished by combining thermodynamic calculations of energy supplies from geochemical processes and energy demands for biochemical processes. Progress towards this goal is summarized here. A critique of all thermodynamic data for biochemical compounds involved in the citric acid cycle (CAC) and the formulation of metabolite properties allows predictions of the energy involved in each step of the cycle as well as the full forward and reverse cycles over wide ranges of temperature and pressure. These results allow evaluation of energy demands at the center of many microbial metabolic systems. Field work, sampling, and lab analyses from two low-temperature systems, a serpentinizing system, and a subglacial setting, provide the data used in these thermodynamic analyses of energy supplies. An extensive literature summary of microbial and molecular data from serpentinizing systems found is used to guide the evaluation and ranking of energy supplies used by chemolithoautotrophic microbes. These results constrain models of the distribution of microbial metabolisms throughout the low-temperature serpentinization systems in the Samail ophiolite in Oman (including locales of primary and subsequent alteration processes). Data collected from Robertson Glacier in Alberta, Canada, together with literature data from Lake Vida in Antarctica and bottom seawater, allowed thermodynamic analyses of low-temperature energy supplies in a glacial system. Results for 1460 inorganic redox reactions are used to fully inventory the geochemical energy sources that support the globally extensive cold biosphere.

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Date Created
  • 2016

Using micro-scale observations to understand large-scale geophysical phenomena: examples from seismology and mineral physics

Description

Earthquake faulting and the dynamics of subducting lithosphere are among the frontiers of geophysics. Exploring the nature, cause, and implications of geophysical phenomena requires multidisciplinary investigations focused at a range

Earthquake faulting and the dynamics of subducting lithosphere are among the frontiers of geophysics. Exploring the nature, cause, and implications of geophysical phenomena requires multidisciplinary investigations focused at a range of spatial scales. Within this dissertation, I present studies of micro-scale processes using observational seismology and experimental mineral physics to provide important constraints on models for a range of large-scale geophysical phenomena within the crust and mantle.

The Great Basin (GB) in the western U.S. is part of the diffuse North American-Pacific plate boundary. The interior of the GB occasionally produces large earthquakes, yet the current distribution of regional seismic networks poorly samples it. The EarthScope USArray Transportable Array provides unprecedented station density and data quality for the central GB. I use this dataset to develop an earthquake catalog for the region that is complete to M 1.5. The catalog contains small-magnitude seismicity throughout the interior of the GB. The spatial distribution of earthquakes is consistent with recent regional geodetic studies, confirming that the interior of the GB is actively deforming everywhere and all the time. Additionally, improved event detection thresholds reveal that swarms of temporally-clustered repeating earthquakes occur throughout the GB. The swarms are not associated with active volcanism or other swarm triggering mechanisms, and therefore, may represent a common fault behavior.

Enstatite (Mg,Fe)SiO3 is the second most abundant mineral within subducting lithosphere. Previous studies suggest that metastable enstatite within subducting slabs may persist to the base of the mantle transition zone (MTZ) before transforming to high-pressure polymorphs. The metastable persistence of enstatite has been proposed as a potential cause for both deep-focus earthquakes and the stagnation of slabs at the base of the MTZ. I show that natural Al- and Fe-bearing enstatite reacts more readily than previous studies and by multiple transformation mechanisms at conditions as low as 1200°C and 18 GPa. Metastable enstatite is thus unlikely to survive to the base of the MTZ. Additionally, coherent growth of akimotoite and other high-pressure phases along polysynthetic twin boundaries provides a mechanism for the inheritance of crystallographic preferred orientation from previously deformed enstatite-bearing rocks within subducting slabs.

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Created

Date Created
  • 2015

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Comparative evolution of the Shyok and Yarlung suture zones: implications for the collision Between India and Eurasia

Description

The collision between the Indian and Eurasian tectonic plates marked the onset of the rise of the Himalayan-Tibetan orogen, but also brought about profound changes to the Earth's oceans and

The collision between the Indian and Eurasian tectonic plates marked the onset of the rise of the Himalayan-Tibetan orogen, but also brought about profound changes to the Earth's oceans and climate. The exact sequence of events that occurred during this collision is poorly understood, leading to a wide range of estimates of its age. The Indus and Yarlung sutures are generally considered to represent the final collision between India and Eurasia, and together form a mostly continuous belt that can be traced over 2000 km along strike. In the western portions of the orogen the Karakoram Fault introduces a key complexity to the study of timing of collision by offsetting the Indus and Yarlung sutures. Recent work has used the complexities introduced by the Karakoram Fault to suggest that the more northerly Shyok suture, not the Indus suture, represents the India-Eurasia collision zone. Estimates for timing of the India-Eurasia collision fall into one of three groups: 40-34 Ma, 55-50 Ma, and 66-60 Ma. Attempts to reconcile these models have thus far been unsuccessful. In order to provide additional data that might further clarify the timing and location of collision, studies have been performed along the Shyok suture in India and along the Yarlung suture in Tibet at Sangsang. A study along the Shyok suture argues that the suture formed between 92-85 Ma. This timing precludes an interpretation that the Shyok suture marks the location of the India-Eurasia collision. A second study demonstrates the utility of two new geochronometers, (U-Th)/Pb joaquinite and 40Ar/39Ar neptunite, that play an important role in unraveling the tectonic history of the Yarlung suture. A third study is an investigation of the structure and geochronology of the Sangsang ophiolite complex. Here, multiple (U-Th)/Pb and 40Ar/39Ar systems record magmatism and metamorphism spanning ca. 125-52 Ma. By tying these chronometers to tectonic process, a history is reconstructed of the southern margin of Tibet that includes Early Cretaceous to Late Cretaceous forearc rifting associated with mid ocean ridge subduction, Paleocene accretionary wedge uplift and erosion, and finally Eocene metasomatism and collision.

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Date Created
  • 2016

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The ancient rocky surfaces of Mars: analysis of spacecraft data and the development of laboratory instrumentation

Description

Early spacecraft missions to Mars, including the Marnier and Viking orbiters and landers revealed a morphologically and compositionally diverse landscape that reshaped widely held views of Mars. More recent spacecraft

Early spacecraft missions to Mars, including the Marnier and Viking orbiters and landers revealed a morphologically and compositionally diverse landscape that reshaped widely held views of Mars. More recent spacecraft including Mars Global Surveyor, Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter, and the Mars Exploration Rovers have further refined, enhanced, and diversified our understanding of Mars. In this dissertation, I take a multiple-path approach to planetary and Mars science including data analysis and instrument development. First, I present several tools necessary to effectively use new, complex datasets by highlighting unique and innovative data processing techniques that allow for the regional to global scale comparison of multiple datasets. Second, I present three studies that characterize several processes on early Mars, where I identify a regional, compositionally distinct, in situ, stratigraphically significant layer in Ganges and Eos Chasmata that formed early in martian history. This layer represents a unique period in martian history where primitive mantle materials were emplaced over large sections of the martian surface. While I originally characterized this layer as an effusive lava flow, based on the newly identified regional or global extent of this layer, I find the only likely scenario for its emplacement is the ejecta deposit of the Borealis Basin forming impact event. I also re-examine high thermal inertia, flat-floored craters identified in Viking data and conclude they are typically more mafic than the surrounding plains and were likely infilled by primitive volcanic materials during, or shortly after the Late Heavy Bombardment. Furthermore, the only plausible source for these magmas is directly related to the impact process, where mantle decompression melting occurs as result of the removal of overlying material by the impactor. Finally, I developed a new laboratory microscopic emission and reflectance spectrometer designed to help improve the interpretation of current remote sensing or in situ data from planetary bodies. I present the design, implementation, calibration, system performance, and preliminary results of this instrument. This instrument is a strong candidate for the next generation in situ rover instruments designed to definitively assess sample mineralogy and petrology while preserving geologic context.

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Date Created
  • 2012

Investigating Lava Flow Emplacement: Implications for Volcanic Hazards and Planetary Evolution

Description

Lava flow emplacement in the laboratory and on the surface of Mars was investigated. In the laboratory, the effects of unsteady effusion rates at the vent on four modes

Lava flow emplacement in the laboratory and on the surface of Mars was investigated. In the laboratory, the effects of unsteady effusion rates at the vent on four modes of emplacement common to lava flow propagation: resurfacing, marginal breakouts, inflation, and lava tubes was addressed. A total of 222 experiments were conducted using a programmable pump to inject dyed PEG wax into a chilled bath (~ 0° C) in tanks with a roughened base at slopes of 0, 7, 16, and 29°. The experiments were divided into four conditions, which featured increasing or decreasing eruption rates for either 10 or 50 s. The primary controls on modes of emplacement were crust formation, variability in the eruption rate, and duration of the pulsatory flow rate. Resurfacing – although a relatively minor process – is inhibited by an extensive, coherent crust. Inflation requires a competent, flexible crust. Tube formation requires a crust and intermediate to low effusion rates. On Mars, laboratory analogue experiments combined with models that use flow dimensions to estimate emplacement conditions and using high resolution image data and digital terrain models (e.g. THEMIS IR, CTX, HRSC), the eruption rates, viscosities, and yield strengths of 40 lava flows in the Tharsis Volcanic Province have been quantified. These lava flows have lengths, mean widths, and mean thicknesses of 15 – 314 km, 0.5 – 29 km, and 11 – 91 m, respectively. Flow volumes range from ~1 – 430 km3. Based on laboratory experiments, the 40 observed lava flows were erupted at 0.2 – 6.5x103 m3/s, while the Graetz number and Jeffrey’s equation when applied to 34 of 40 lava flows indicates eruption rates and viscosities of 300 – ~3.5 x 104 m3/s and ~105 – 108 Pa s, respectively. Another model which accounts for mass loss to levee formation was applied to a subset of flows, n = 13, and suggests eruption rates and viscosities of ~30 – ~1.2 x 103 m3/s and 4.5 x 106 – ~3 x 107 Pa s, respectively. Emplacement times range from days to centuries indicating the necessity for long-term subsurface conduits capable of delivering enormous volumes of lava to the surface.

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Date Created
  • 2020

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Investigations into Crustal Composition and Oxidative Weathering in the Archean

Description

Archean oxidative weathering reactions were likely important O2 sinks that delayed the oxygenation of Earth’s atmosphere, as well as sources of bio-essential trace metals such as Mo to the biosphere.

Archean oxidative weathering reactions were likely important O2 sinks that delayed the oxygenation of Earth’s atmosphere, as well as sources of bio-essential trace metals such as Mo to the biosphere. However, the rates of these reactions are difficult to quantify experimentally at relevantly low concentrations of O2. With newly developed O2 sensors, weathering experiments were conducted to measure the rate of sulfide oxidation at Archean levels of O2, a level three orders of magnitude lower than previous experiments. The rate laws produced, combined with weathering models, indicate that crustal sulfide oxidation by O2 was possible even in a low O2 Archean atmosphere.

Given the experimental results, it is expected that crustal delivery of bio-essential trace metals (such as Mo) from sulfide weathering was active even prior to the oxygenation of Earth’s atmosphere. Mo is a key metal for biological N2 fixation and its ancient use is evidenced by N isotopes in ancient sedimentary rocks. However, it is typically thought that Mo was too low to be effectively bioavailable early in Earth’s history, given the low abundances of Mo found in ancient sediments. To reconcile these observations, a computational model was built that leverages isotopic constraints to calculate the range of seawater concentrations possible in ancient oceans. Under several scenarios, bioavailable concentrations of seawater Mo were attainable and compatible with the geologic record. These results imply that Mo may not have been limiting for early metabolisms.

Titanium (Ti) isotopes were recently proposed to trace the evolution of the ancient continental crust, and have the potential to trace the distribution of other trace metals during magmatic differentiation. However, significant work remains to understand fully Ti isotope fractionation during crust formation. To calibrate this proxy, I carried out the first direct measurement of mineral-melt fractionation factors for Ti isotopes in Kilauea Iki lava lake and built a multi-variate fractionation law for Ti isotopes during magmatic differentiation. This study allows more accurate forward-modeling of isotope fractionation during crust differentiation, which can now be paired with weathering models and ocean mass balance to further reconstruct the composition of Earth’s early continental crust, atmosphere, and oceans.

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  • 2020

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Amorphous weathering products: evidence for basalt-water interactions and the relevance to paleo-environments on Mars

Description

Amorphous phases are detected over large regions of the Martian surface from orbit and in more localized deposits by rovers on the surface. Amorphous silicates can be primary or secondary

Amorphous phases are detected over large regions of the Martian surface from orbit and in more localized deposits by rovers on the surface. Amorphous silicates can be primary or secondary in origin, both having formed through very different processes, so the unambiguous identification of these phases is important for understanding the geologic history of Mars. Secondary amorphous silicates are poorly understood and underrepresented in spectral libraries because they lack the long-range structural order that makes their crystalline counterparts identifiable in most analytical techniques. Fortunately, even amorphous materials have some degree of short-range order so that distinctions can be made with careful characterization.

Two sets of laboratory experiments were used to produce and characterize amorphous weathering products under probable conditions for the Martian surface, and one global spectral analysis using thermal-infrared (TIR) data from the Thermal Emission Spectrometer (TES) instrument was used to constrain variations in amorphous silicates across the Martian surface. The first set of experiments altered crystalline and glassy basalt samples in an open system under strong (pH 1) and moderate (pH 3) acidic conditions. The second set of experiments simulated a current-day Martian weathering scenario involving transient liquid water where basalt glass weathering solutions, formed in circumneutral (pH ~5.5 and 7) conditions, were rapidly evaporated, precipitating amorphous silicates. The samples were characterized using visible and near-infrared (VNIR) spectroscopy, TIR spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).

All experiments formed amorphous silicate phases that are new to spectral libraries. Moderately acidic alteration experiments produced no visible or spectral evidence of alteration products, whereas exposure of basalt glass to strongly acidic fluids produced silica-rich alteration layers that are spectrally consistent with VNIR and TIR spectra from the circum-polar region of Mars, indicating this region has undergone acidic weathering. Circum-netural pH basalt weathering solution precipitates are consistent with amorphous materials measured by rovers in soil and rock surface samples in Gale and Gusev Craters, suggesting transient water interactions over the last 3 billion years. Global spectral analyses determine that alteration conditions have varied across the Martian surface, and that alteration has been long lasting.

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Agent

Created

Date Created
  • 2016