Matching Items (17)

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Modeling Volatiles at the Lunar Poles

Description

The lunar poles have hydrated materials in their permanently shadowed regions (PSRs), also known as lunar cold traps. These cold traps exist because of the Moon’s slight tilt of 1.5,

The lunar poles have hydrated materials in their permanently shadowed regions (PSRs), also known as lunar cold traps. These cold traps exist because of the Moon’s slight tilt of 1.5, which consequently creates these PSRs. In these shadows, the temperature remains cold enough to prevent the sublimation of volatile materials for timescales spanning that of geologic times [Hayne et. al 2015]. PSRs are significant because they create an environment where water ice can exist within the first meter of regolith at the lunar poles, where many cold traps are present. These volatile materials can be observed through a process called neutron spectroscopy. Neutron spectroscopy is a method of observing the neutron interactions caused by galactic and extragalactic cosmic ray proton collisions. Neutron interactions are more sensitive to hydrogen than other elements found in the regolith, and thus are a good indicator of hydrated materials. Using neutron spectroscopy, it is possible to detect the hydrogen in these cold traps up to a meter deep in the regolith, thus detecting the presence of hydrated materials, water, or ice.
For this study, we used the Monte Carlo Neutral Particle Transport Code (MCNP6) to create a homogenous sphere that represented the PSRs on Moon, and then modeled five differing water contents for the lunar regolith ranging from 0-20 percent weight. These percent weights were modeled after the estimates for Shackleton crater, data from Lunar Reconnaissance Orbiter (LRO) mission, and data from Lunar Orbiter Laser Altimeter (LOLA).
This study was created with the LunaH-Map mission as motivation, seeking to exhibit what neutron data might be observed. The LunaH-Map mission is an array of mini-Neutron Spectrometers that will orbit the Moon 8-20 km away from the lunar surface and map the spatial
distribution of hydrogen at the lunar poles. The plots generated show the relationship between neutron flux and energy from the surface of the Moon as well as from 10km away. This data provides insight into the benefits of collecting orbital data versus surface data, as well as illustrating what LunaH-Map might observe within a PSR.

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Date Created
  • 2020-05

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Hydrogen isotopic systematics of nominally anhydrous phases in martian meteorites

Description

Hydrogen isotope compositions of the martian atmosphere and crustal materials can provide unique insights into the hydrological and geological evolution of Mars. While the present-day deuterium-to-hydrogen ratio (D/H) of the

Hydrogen isotope compositions of the martian atmosphere and crustal materials can provide unique insights into the hydrological and geological evolution of Mars. While the present-day deuterium-to-hydrogen ratio (D/H) of the Mars atmosphere is well constrained (~6 times that of terrestrial ocean water), that of its deep silicate interior (specifically, the mantle) is less so. In fact, the hydrogen isotope composition of the primordial martian mantle is of great interest since it has implications for the origin and abundance of water on that planet. Martian meteorites could provide key constraints in this regard, since they crystallized from melts originating from the martian mantle and contain phases that potentially record the evolution of the H2O content and isotopic composition of the interior of the planet over time. Examined here are the hydrogen isotopic compositions of Nominally Anhydrous Phases (NAPs) in eight martian meteorites (five shergottites and three nakhlites) using Secondary Ion Mass Spectrometry (SIMS).

This study presents a total of 113 individual analyses of H2O contents and hydrogen isotopic compositions of NAPs in the shergottites Zagami, Los Angeles, QUE 94201, SaU 005, and Tissint, and the nakhlites Nakhla, Lafayette, and Yamato 000593. The hydrogen isotopic variation between and within meteorites may be due to one or more processes including: interaction with the martian atmosphere, magmatic degassing, subsolidus alteration (including shock), and/or terrestrial contamination. Taking into consideration the effects of these processes, the hydrogen isotope composition of the martian mantle may be similar to that of the Earth. Additionally, this study calculated upper limits on the H2O contents of the shergottite and nakhlite parent melts based on the measured minimum H2O abundances in their maskelynites and pyroxenes, respectively. These calculations, along with some petrogenetic assumptions based on previous studies, were subsequently used to infer the H2O contents of the mantle source reservoirs of the depleted shergottites (200-700 ppm) and the nakhlites (10-100 ppm). This suggests that mantle source of the nakhlites is systematically drier than that of the depleted shergottites, and the upper mantle of Mars may have preserved significant heterogeneity in its H2O content. Additionally, this range of H2O contents is not dissimilar to the range observed for the Earth’s upper mantle.

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Date Created
  • 2015

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Chronology of Planetesimal Differentiation Based on the Timing of Achondrite Formation in the Early Solar System

Description

During the early Solar System many physiochemical processes were taking place that would shape the formation and evolution of rocky bodies. Growth of these rocky objects was rapid, with some

During the early Solar System many physiochemical processes were taking place that would shape the formation and evolution of rocky bodies. Growth of these rocky objects was rapid, with some growing to sizes of 10s – 1000s km (“planetesimals”) in the first few million years. Because these objects formed early, they contained sufficient 26Al (an isotope of Al with a short half-life of ~705,000 yrs) to heat the interiors to melting temperatures, resulting in the formation of the first igneous rocks in nascent Solar System. Depending on the size and time of accretion, some bodies experienced high degrees of melting (with some having global magma oceans) while others experienced lower degrees of partial melting, and yet others did not experience any melting at all. These varying degrees of heating and melting processes on early-formed planetesimals produced a variety of achondritic meteorite types. These achondrites have bulk compositions ranging from ultramafic to basaltic, with some rare types having more highly “evolved” (i.e., high-SiO2) compositions. Determining the detailed chronology of their formation with fine time resolution is key for understanding the earliest stages of planet formation, and there are high resolution chronometers that are ideally suited for this application. Three such chronometers (i.e., the 26Al-26Mg, 53Mn-53Cr, and 207Pb-206Pb chronometers) are the focus of this work. Based on investigations of these chronometers in several achondritic meteorites, the implications for the formation and evolution of planetesimals in the early Solar System will be discussed.

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Date Created
  • 2020

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Uranium isotope variations in nature: mechanisms, applications, and implications

Description

Historically, uranium has received intense study of its chemical and isotopic properties for use in the nuclear industry, but has been largely ignored by geoscientists despite properties that make it

Historically, uranium has received intense study of its chemical and isotopic properties for use in the nuclear industry, but has been largely ignored by geoscientists despite properties that make it an intriguing target for geochemists and cosmochemists alike. Uranium was long thought to have an invariant 238U/235U ratio in natural samples, making it uninteresting for isotopic work. However, recent advances in mass spectrometry have made it possible to detect slight differences in the 238U/235U ratio, creating many exciting new opportunities for U isotopic research. Using uranium ore samples from diverse depositional settings from around the world, it is shown that the low-temperature redox transition of uranium (U6+ to U4+) causes measurable fractionation of the 238U/235U ratio. Moreover, it is shown experimentally that a coordination change of U can also cause measurable fractionation in the 238U/235U ratio. This improved understanding of the fractionation mechanisms of U allows for the use of the 238U/235U ratio as a paleoredox proxy. The 238U/235U ratios of carbonates deposited spanning the end-Permian extinction horizon provide evidence of pronounced and persistent widespread ocean anoxia at, or immediately preceding, the extinction boundary. Variable 238U/235U ratios correlated with proxies for initial Cm/U in the Solar System's earliest objects demonstrates the existence of 247Cm in the early Solar System. Proof of variable 238U/235U ratios in meteoritic material forces a substantive change in the previously established procedures of Pb-Pb dating, which assumed an invariant 238U/235U ratio. This advancement improves the accuracy of not only the Pb-Pb chronometer that directly utilizes the 238U/235U ratio, but also for short-lived radiometric dating techniques that indirectly use the 238U/235U ratio to calculate ages of Solar System material.

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Created

Date Created
  • 2011

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Extinct radionuclides in the early Solar System: the initial Solar System abundance of ⁶⁰Fe from angrites and unequilibrated ordinary chondrites and ²⁶Al-²⁶Mg chronology of ungrouped achondrites

Description

The presence of a number of extinct radionuclides in the early Solar System (SS) is known from geochemical and isotopic studies of meteorites and their components. The half-lives of these

The presence of a number of extinct radionuclides in the early Solar System (SS) is known from geochemical and isotopic studies of meteorites and their components. The half-lives of these isotopes are short relative to the age of the SS, such that they have now decayed to undetectable levels. They can be inferred to exist in the early SS from the presence of their daughter nuclides in meteoritic materials that formed while they were still extant. The extinct radionuclides are particularly useful as fine-scale chronometers for events in the early SS. They can also be used to help constrain the astrophysical setting of the formation of the SS because their short half-lives and unique formation environments yield information about the sources and timing of delivery of material to the protoplanetary disk. Some extinct radionuclides are considered evidence that the Sun interacted with a massive star (supernova) early in its history. The abundance of 60Fe in the early SS is particularly useful for constraining the astrophysical environment of the Sun's formation because, if present in sufficient abundance, its only likely source is injection from a nearby supernova. The initial SS abundance of 60Fe is poorly constrained at the present time, with estimates varying by 1-2 orders of magnitude. I have determined the 60Fe-60Ni isotope systematics of ancient, well-preserved meteorites using high-precision mass spectrometry to better constrain the initial SS abundance of 60Fe. I find identical estimates of the initial 60Fe abundance from both differentiated basaltic meteorites and from components of primitive chondrites formed in the Solar nebula, which suggest a lower 60Fe abundance than other recent estimates. With recent improved meteorite collection efforts there are more rare ungrouped meteorites being found that hold interesting clues to the origin and evolution of early SS objects. I use the 26Al-26Mg extinct radionuclide chronometer to constrain the ages of several recently recovered meteorites that sample previously unknown asteroid lithologies, including the only know felsic meteorite from an asteroid and two other ungrouped basaltic achondrites. These results help broaden our understanding of the timescales involved in igneous differentiation processes in the early SS.

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Date Created
  • 2012

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Solar wind sodium and potassium abundance analysis in Genesis diamond-on-silicon and silicon bulk solar wind collectors, and how hydration affects the microtexture of olivine phase transformation at 18 GPa

Description

The present work covers two distinct microanalytical studies that address issues in planetary materials: (1) Genesis Na and K solar wind (SW) measurements, and (2) the effect of water on

The present work covers two distinct microanalytical studies that address issues in planetary materials: (1) Genesis Na and K solar wind (SW) measurements, and (2) the effect of water on high-pressure olivine phase transformations.

NASA’s Genesis mission collected SW samples for terrestrial analysis to create a baseline of solar chemical abundances based on direct measurement of solar material. Traditionally, solar abundances are estimated using spectroscopic or meteoritic data. This study measured bulk SW Na and K in two different Genesis SW collector materials (diamond-like carbon (DlC) and silicon) for comparison with these other solar references. Novel techniques were developed for Genesis DlC analysis. Solar wind Na fluence measurements derived from backside depth profiling are generally lower in DlC than Si, despite the use of internal standards. Nevertheless, relative to Mg, the average SW Na and K abundances measured in Genesis wafers are in agreement with solar photospheric and CI chondrite abundances, and with other SW elements with low first ionization potential (within error). The average Genesis SW Na and K fluences are 1.01e11 (+9e09, -2e10) atoms/cm2 and 5.1e09 (+8e08, -8e08) atoms/cm2, respectively. The errors reflect average systematic errors. Results have implications for (1) SW formation models, (2) cosmochemistry based on solar material rather than photospheric measurements or meteorites, and (3) the accurate measurement of solar wind ion abundances in Genesis collectors, particularly DlC and Si.

Deep focus earthquakes have been attributed to rapid transformation of metastable olivine within the mantle transition zone (MTZ). However, the presence of H2O acts to overcome metastability, promoting phase transformation in olivine, so olivine must be relatively anhydrous (<75 ppmw) to remain metastable to depth. A microtextural analysis of olivine phase transformation products was conducted to test the feasibility for subducting olivine to persist metastably to the MTZ. Transformation (as intracrystalline or rim nucleation) shifts from ringwoodite to ringwoodite-wadsleyite nucleation with decreasing H2O content within olivine grains. To provide accurate predictions for olivine metastability at depth, olivine transformation models must reflect how changing H2O distributions lead to complex changes in strain and reaction rates within different parts of a transforming olivine grain.

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Date Created
  • 2015

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The Effect of Bulk Composition on the Sulfur Content of Cores

Description

This study explores how bulk composition and oxygen fugacity (fO2) affect the partitioning of sulfur between the molten mantle and core of an early planetesimal. The model can be used

This study explores how bulk composition and oxygen fugacity (fO2) affect the partitioning of sulfur between the molten mantle and core of an early planetesimal. The model can be used to determine the range of potential sulfur concentrations in the asteroid (16) Psyche, which is the target of the National Aeronautics and Space Administration/Arizona State University Psyche Mission. This mission will be our visit to an M-type asteroid, thought to be dominantly metallic.

The model looks at how oxygen fugacity (fO2), bulk composition, temperature, and pressure affect sulfur partitioning in planetesimals using experimentally derived equations from previous studies. In this model, the bulk chemistry and oxygen fugacity of the parent body is controlled by changing the starting material, using ordinary chondrites (H, L, LL) and carbonaceous chondrites (CM, CI, CO, CK, CV). The temperature of the planetesimal is changed from 1523 K to 1873 K, the silicate mobilization and total melting temperatures, respectively; and pressure from 0.1 to 20 GPa, the core mantle boundary pressures of Vesta and Mars, respectively.

The final sulfur content of a differentiated planetesimal core is strongly dependent on the bulk composition of the original parent body. In all modeled cores, the sulfur content is above 5 weight percent sulfur; this is the point at which the least amount of other light elements is needed to form an immiscible sulfide liquid in a molten core. Early planetesimal cores likely formed an immiscible sulfide liquid, a eutectic sulfide liquid, or potentially were composed of mostly troilite, FeS.

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Date Created
  • 2020

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Early Solar System Processes and Parent Body Relationships Recorded by Chromium and Titanium Isotopes in Meteorites

Description

Meteorites and their components can be used to unravel the history of the early Solar System. Carbonaceous chondrites are meteorites that originated from undifferentiated parent bodies that formed within a

Meteorites and their components can be used to unravel the history of the early Solar System. Carbonaceous chondrites are meteorites that originated from undifferentiated parent bodies that formed within a few million years of the beginning of the Solar System. These meteorites contain calcium-aluminum-rich inclusions (CAIs), which are the oldest dated solids in the Solar System at ~4.567 billion years old and thus preserve a record of the earliest stage of Solar System formation. The isotopic compositions of CAIs and bulk carbonaceous chondrites can be used to identify the sources of material inherited by the protoplanetary disk, assess the degree of mixing in the disk, and evaluate sample origins and potential genetic relationships between parent bodies. In particular, mass-independent Cr and Ti isotopic compositions have proven to be especially useful for these purposes.

In this work, I first developed new methods for the chemical separation of Cr and Ti, improving the reliability of existing methods to ensure consistent yields and accurate isotopic measurements. I then measured the Cr and Ti isotopic compositions of CAIs from CV and CK chondrites to determine the extent of isotopic heterogeneity in the CAI-forming region and assess the role of CAIs in the preservation of planetary-scale isotopic anomalies. My results show that all measured CAIs originated from a common isotopic reservoir that incorporated material from at least three distinct nucleosynthetic sources and preserved limited isotopic heterogeneity. These results also suggest that planetary-scale isotopic anomalies cannot be attributed solely to the transport of CAIs from one part of the solar nebula to another. I finally measured the Cr and Ti isotopic compositions of bulk CM, CO, and ungrouped chondrites to evaluate the relationship between CM and CO chondrites, which have been suggested to originate from either distinct but related parent bodies or a common compositionally heterogeneous parent body. My results suggest that CM, CO, and related ungrouped chondrites originated from distinct parent bodies that formed from similar precursor materials in nearby formation regions. These results may have implications for asteroid samples returned by the OSIRIS-REx and Hayabusa2 missions.

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Date Created
  • 2020

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Refining Earth’s Ocean Oxygenation History using Molybdenum and Thallium Isotopes

Description

Isotope ratios of some trace metals have proven useful for tracking Earth’s ocean oxygenation history. As the limitations of some of these isotope systems are realized, it becomes increasingly important

Isotope ratios of some trace metals have proven useful for tracking Earth’s ocean oxygenation history. As the limitations of some of these isotope systems are realized, it becomes increasingly important to develop new and complementary systems. This dissertation examines the utility of molybdenum (98Mo) and thallium (205Tl) isotope compositions preserved in ancient marine shales to track past ocean oxygenation. My approach is as follows: (1) as an initial exercise, apply the well-established Mo isotope system to a set of ancient shales; (2) validate the use of the newly developed Tl isotope system; and finally (3) examine the potential of applying Mo and Tl isotopes in tandem.

Increasingly heavier 98Mo are found in shales deposited during the Neoarchean (2,800 to 2,500 million years ago, or Ma), which would be a predicted consequence of progressive ocean oxygenation across this timeframe. Increasingly heavier 205Tl across a well-documented Mesozoic Oceanic Anoxic Event (~94 Ma), on the other hand, would be a predicted consequence of progressive ocean de-oxygenation. An anti-correlation in the first combined application of Mo and Tl isotopes in ancient shales provides a strong fingerprint for previously unrecognized levels of ocean oxygenation at ~2,500 Ma. Lastly, neither 98Mo or 205Tl behave as predicted in shales deposited during three Ediacaran Ocean Oxygenation Events (~635 Ma, ~580 Ma, and ~560 Ma). These unexpected trends are due, at least in part, to local-scale overprints that must be taken into consideration when pairing together Mo and Tl isotopes in shales.

The ability of the Mo and Tl isotope systems to track changes in past ocean oxygenation is confirmed in this dissertation. Both isotope systems have the potential to track these changes independently, but their combined utility is particularly powerful. Under ideal conditions, their combined application can provide an even more robust fingerprint for changes in past ocean oxygenation. Even under non-ideal conditions, their combined application makes it possible to decipher local-scale overprints from signals of past ocean oxygenation. It is therefore ideal, whenever possible, to measure both 98Mo and 205Tl in the same shale samples to assess past changes in ocean oxygenation.

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Date Created
  • 2020

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To Be Or Not To B: Meteoritic Implications for the Galactic Environment of Solar System Formation

Description

Short-lived radionuclides (SLRs) once present in the solar nebula can be used to probe the Solar System’s galactic formation environment. Isotopic analyses reveal that the first solids formed in the

Short-lived radionuclides (SLRs) once present in the solar nebula can be used to probe the Solar System’s galactic formation environment. Isotopic analyses reveal that the first solids formed in the Solar System, calcium- and aluminum-rich inclusions (CAIs) in chondritic meteorites, formed with the live SLRs 10Be (t1/2 = 1.4 Ma) and 26Al (t1/2 = 0.7 Ma). Beryllium-10 is produced when high-energy ions, solar energetic particles or galactic cosmic rays (GCRs), spall nuclei in gas or dust. The most likely source of Solar System 10Be is inheritance of GCR-irradiated protosolar molecular cloud material, but only if all CAIs recorded the same initial 10Be abundance. The goal of this dissertation is to assess the homogeneity of 10Be by measuring CAIs for 10Be–10B isotope systematics, correlated to 26Al–26Mg and oxygen isotopes.

I synthesized appropriate standards for secondary ion mass spectrometry (SIMS) measurements of 10Be–10B, necessary for accurate determination of the 10Be/9Be ratio. I then analyzed 32 CAIs for 10Be–10B as well as 6 CAIs for 26Al–26Mg and 5 CAIs for oxygen isotopes within this sample set using SIMS. Previous studies analyzed CAIs primarily from CV3 chondrites, which are known to have experienced thermal metamorphism and aqueous alteration. My work included a variety of CAIs (Type A, B, fine-grained, igneous) from CV3oxidized, CV3reduced, CO3, CR2, and CH/CB chondrites. Finally, after evaluating my data and literature data consistently, I statistically tested whether all CAIs belong to a single 10Be population. I find that the majority (~85%) of the normal (i.e., without large isotopic fractionations or anomalies), 26Al-bearing CAIs recorded a single value, 10Be/9Be = (7.0 ± 0.2) × 10-4. Although 6 CAIs recorded higher or lower values, these are plausibly explained by secondary alteration processes. The galaxy-wide average value of 10Be/9Be from GCR interactions 4.56 billion years ago is predicted to be <2 × 10-4; the value I measured is more than 3 times higher. Because GCRs trace supernovae and star formation, my results suggest a similarly enhanced star formation rate in the molecular cloud within ~1 kpc of the Sun, in the ~15 Ma prior to the Sun’s birth.

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Date Created
  • 2020