Matching Items (11)

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Microanalysis for Oxygen Fugacity by Secondary Ion Mass Spectrometry

Description

Oxygen fugacity (ƒO2) is a thermodynamic variable used to represent the redox state of a material or a system. It is equivalent to the partial pressure of oxygen in a

Oxygen fugacity (ƒO2) is a thermodynamic variable used to represent the redox state of a material or a system. It is equivalent to the partial pressure of oxygen in a particular environment corrected for the non-ideal behavior of the gas. ƒO2 is often used to indicate the potential for iron to occur in a more oxidized or reduced state at a particular temperature and pressure in a natural system. Secondary ion mass spectrometry (SIMS) is a powerful analytical instrument that can be used to analyze elemental and isotopic compositional information about microscopic features within solid materials. SIMS analyses of the secondary ion energy distribution of semi-pure metals demonstrate that the energy spectrum of individual mass lines can provide information about alterations in its surface environment.

The application of high-resolution (see Appendix C) energy spectrum calibrations to natural ilmenite led to the investigation of zirconium (90Zr+) and niobium (93Nb+) as potential indicators of sample ƒO2. Energy spectrum measurements were performed on an array of ilmenite crystals from the earth’s upper mantle retrieved from kimberlites and from a reduced meteorite. In all studied materials, variability in the peak shape and width of the energy spectra has been correlated with inferred sample ƒO2. The best descriptor of this relationship is the full-width at half-maximum (FWHM; see Appendix C) of the energy spectra for each sample. It has been estimated that a 1eV change in the FWHM of 93Nb+ energy spectra is roughly equivalent to 1 log unit ƒO2. Simple estimates of precision suggest the FWHM values can be trusted to  1eV and sample ƒO2 can be predicted to ±1 log unit, assuming the temperature of formation is known.

The work of this thesis also explores the applicability of this technique beyond analysis of semi-pure metals and ilmenite crystals from kimberlites. This technique was applied to titanium oxides experimentally formed at known ƒO2 as well as an ilmenite crystal that showed compositional variations across the grain (i.e., core to rim chemical variations). Analyses of titanium oxides formed at known ƒO2 agree with the estimation that 1 eV change in the FWHM of 93Nb+ is equivalent to ~1 log unit ƒO2 (in all cases but one); this is also true for analyses of a natural ilmenite crystal with compositional variations across the grain.

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Agent

Created

Date Created
  • 2019

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Meteorites on Mars as planetary research tools with special considerations for Martian weathering processes

Description

The occurrence of exogenic, meteoritic materials on the surface of any world presents opportunities to explore a variety of significant problems in the planetary sciences. In the case of Mars,

The occurrence of exogenic, meteoritic materials on the surface of any world presents opportunities to explore a variety of significant problems in the planetary sciences. In the case of Mars, meteorites found on its surface may help to 1) constrain atmospheric conditions during their time of arrival; 2) provide insights into possible variabilities in meteoroid type sampling between Mars and Earth space environments; 3) aid in our understanding of soil, dust, and sedimentary rock chemistry; 4) assist with the calibration of crater-age dating techniques; and 5) provide witness samples for chemical and mechanical weathering processes. The presence of reduced metallic iron in approximately 88 percent of meteorite falls renders the majority of meteorites particularly sensitive to oxidation by H2O interaction. This makes them excellent markers for H2O occurrence. Several large meteorites have been discovered at Gusev Crater and Meridiani Planum by the Mars Exploration Rovers (MERs). Significant morphologic characteristics interpretable as weathering features in the Meridiani suite of iron meteorites include a 1) large pit lined with delicate iron protrusions suggestive of inclusion removal by corrosive interaction; 2) differentially eroded kamacite and taenite lamellae on three of the meteorites, providing relative timing through cross-cutting relationships with deposition of 3) an iron oxide-rich dark coating; and 4) regmaglypted surfaces testifying to regions of minimal surface modification; with other regions in the same meteorites exhibiting 5) large-scale, cavernous weathering. Iron meteorites found by Mini-TES at both Meridiani Planum and Gusev Crater have prompted laboratory experiments designed to explore elements of reflectivity, dust cover, and potential oxide coatings on their surfaces in the thermal infrared using analog samples. Results show that dust thickness on an iron substrate need be only one tenth as great as that on a silicate rock to obscure its infrared signal. In addition, a database of thermal emission spectra for 46 meteorites was prepared to aid in the on-going detection and interpretation of these valuable rocks on Mars using Mini-TES instruments on both MER spacecraft. Applications to the asteroidal sciences are also relevant and intended for this database.

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Agent

Created

Date Created
  • 2011

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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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Created

Date Created
  • 2015

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Analyzing nitrogen in silicate glasses by secondary ion mass spectrometry

Description

Volcanic devolatilization is one of the major processes in the global nitrogen cycle. Past studies have often estimated the magnitude of this flux using volcanic emission measurements, which are limited

Volcanic devolatilization is one of the major processes in the global nitrogen cycle. Past studies have often estimated the magnitude of this flux using volcanic emission measurements, which are limited to currently active systems and sensitive to atmospheric contamination. A different methodological approach requires appropriate analytical parameters for nitrogen analysis in silicate glasses by secondary ion mass spectrometry (SIMS), which have not yet been established. To this end, we analyze various ion implanted basaltic and rhyolitic glasses by SIMS. We demonstrate that water content significantly affects the ion yields of 14N+ and 14N16O−, as well as the background intensity of 14N+ and 12C+. Application of implant-derived calibrations to natural samples provide the first reported concentrations of nitrogen in melt inclusions. These measurements are from samples from the Bishop Tuff in California, the Huckleberry Ridge Tuff of the Yellowstone Volcanic Center, and material from the Okaia and Oruanui eruptions in the Taupo Volcanic Center. In all studied material, we find maximum nitrogen contents of less than 45 ppm and that nitrogen concentration varies positively with CO2 concentration, which is interpreted to reflect partial degassing trend. Using the maximum measured nitrogen contents for each eruption, we find that the Bishop released >3.6 x 1013 g of nitrogen, the Huckleberry Ridge released >1.3 x 1014 g, the Okaia released >1.1 x 1011 g of nitrogen, the Oruanui released >4.7 x 1013 g of nitrogen. Simple calculations suggest that with concentrations such as these, rhyolitic eruptions may ephemerally increase the nitrogen flux to the atmosphere, but are insignificant compared to the 4 x 1021 g of nitrogen stored in the atmosphere.

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Agent

Created

Date Created
  • 2016

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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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Agent

Created

Date Created
  • 2020

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Interactions Between Fluids, Melts, and Rocks in Subduction Zones

Description

My dissertation research broadly focuses on the geochemical and physical exchange of materials between the Earth’s crust and mantle at convergent margins, and how this drives the compositional diversity observed

My dissertation research broadly focuses on the geochemical and physical exchange of materials between the Earth’s crust and mantle at convergent margins, and how this drives the compositional diversity observed on the Earth’s surface. I combine traditional petrologic and geochemical studies of natural and experimental high-pressure mafic rocks, with thermodynamic modeling of high-pressure aqueous fluids and mafic-ultramafic lithologies allowing for more complete understanding of fluid-melt-rock interactions. The results of the research that follows has important implications for: the role of lower crustal foundering in the geochemical origin and evolution of the modern continental crust (Chapter 2; Guild et al., under review), metasomatic processes involving aqueous metal-carbon complexes in high pressure-temperature subduction zone fluids (Chapter 3; Guild & Shock, 2020), natural hydrous mineral stability at the slab-mantle interface (Chapter 4; Guild, et al., in preparation) and water-undersaturated melting in the sub-arc (Chapter 5; Guild & Till, in preparation).

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Agent

Created

Date Created
  • 2020

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Timescales and Characteristics of Magma Generation in Earth and Exoplanets

Description

Volcanic eruptions are serious geological hazards; the aftermath of the explosive eruptions produced at high-silica volcanic systems often results in long-term threats to climate, travel, farming, and human life. To

Volcanic eruptions are serious geological hazards; the aftermath of the explosive eruptions produced at high-silica volcanic systems often results in long-term threats to climate, travel, farming, and human life. To construct models for eruption forecasting, the timescales of events leading up to eruption must be accurately quantified. In the field of igneous petrology, the timing of these events (e.g. periods of magma formation, duration of recharge events) and their influence on eruptive timescales are still poorly constrained.

In this dissertation, I discuss how the new tools and methods I have developed are helping to improve our understanding of these magmatic events. I have developed a method to calculate more accurate timescales for these events from the diffusive relaxation of chemical zoning in individual mineral crystals (i.e., diffusion chronometry), and I use this technique to compare the times recorded by different minerals from the same Yellowstone lava flow, the Scaup Lake rhyolite.

I have also derived a new geothermometer to calculate magma temperature from the compositions of the mineral clinopyroxene and the surrounding liquid. This empirically-derived geothermometer is calibrated for the high FeOtot (Mg# = 56) and low Al2O3 (0.53–0.73 wt%) clinopyroxene found in the Scaup Lake rhyolite and other high-silica igneous systems. A determination of accurate mineral temperatures is crucial to calculate magmatic heat budgets and to use methods such as diffusion chronometry. Together, these tools allow me to paint a more accurate picture of the conditions and tempo of events inside a magma body in the millennia to months leading up to eruption.

Additionally, I conducted petrological experiments to determine the composition of hypothetical exoplanet partial mantle melts, which could become these planets’ new crust, and therefore new surface. Understanding the composition of an exoplanet’s crust is the first step to understanding chemical weathering, surface-atmosphere chemical interactions, the volcanic contribution to any atmosphere present, and biological processes, as life depends on these surfaces for nutrients. The data I have produced can be used to predict differences in crust compositions of exoplanets with similar bulk compositions to those explored herein, as well as to calibrate future exoplanet petrologic models.

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Agent

Created

Date Created
  • 2020

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Boron isotopic composition of the subcontinental lithospheric mantle

Description

Boron concentrations and isotopic composition of phlogopite mica, amphibole, and selected coexisting anhydrous phases in mantle-derived xenoliths from the Kaapvaal Craton were measured by secondary ion mass spectrometry in an

Boron concentrations and isotopic composition of phlogopite mica, amphibole, and selected coexisting anhydrous phases in mantle-derived xenoliths from the Kaapvaal Craton were measured by secondary ion mass spectrometry in an effort to better understand the B isotope geochemistry of the subcontinental lithospheric mantle (SCLM) and its implications for the global geochemical cycle of B in the mantle. These samples display a wide, and previously unrecognized, range in their boron contents and isotopic compositions reflecting a complex history involving melt depletion and metasomatism by subduction- and plume-derived components, as well as late stage isotopic exchange related to kimberlite emplacements. Micas from ancient lithospheric harzburgite metasomatized by slab-derived fluids suggest extensive B-depletion during subduction, resulting in low-B, isotopically light compositions whereas kimberlite-related metasomatic products and a sample from the 2 Ga Palabora carbonatite have boron isotopic compositions similar to proposed primitive mantle. The results suggest that subduction of oceanic lithosphere plays a limited role in the B geochemistry of the convecting mantle.

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Created

Date Created
  • 2014

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Fluorine partitioning between nominally anhydrous minerals (olivine, clinopyroxene, and plagioclase) and silicate melt using secondary ion mass spectrometry and newly synthesized basaltic fluorine microanalytical glass standards

Description

Fluorine (F) is a volatile constituent of magmas and hydrous mantle minerals. Compared to other volatile species, F is highly soluble in silicate melts, allowing F to remain in the

Fluorine (F) is a volatile constituent of magmas and hydrous mantle minerals. Compared to other volatile species, F is highly soluble in silicate melts, allowing F to remain in the melt during magma differentiation and rendering F less subject to disturbance during degassing upon magma ascent. Hence, the association between fluorine in basalts and fluorine in the mantle source region is more robust than for other volatile species. The ionic radius of F- is similar to that of OH- and O2-, and F may substitute for hydroxyl and oxygen in silicate minerals and melt. Fluorine is also incorporated at trace levels within nominally anhydrous minerals (NAMs) such as olivine, clinopyroxene, and plagioclase. Investigating the geochemical behavior of F in NAMs provides a means to estimate the pre-eruptive F contents of degassed magmas and to better understand the degassing behavior of H. The partition coefficients of F were determined for clinopyroxene, olivine, plagioclase, and hornblende within melts of olivine-minette, augite-minette, basaltic andesite, and latite compositions. The samples analyzed were run products from previously-published phase-equilibria experiments. Fluorine was measured by secondary ion mass spectrometry (SIMS) using an 16O- primary beam and detection of negative secondary ions (19F-, 18O-, 28Si-). SIMS ion intensities are converted to concentrations by analyzing matrix-matched microanalytical reference materials and constructing calibration curves. For robust F calibration standards, five basaltic glasses (termed Fba glasses) were synthesized in-house using a natural tholeiite mixed with variable amounts of CaF2. The Fba glasses were characterized for F content and homogeneity, using both SIMS and electron-probe microanalysis (EPMA), and used as F standards. The partition coefficients for clinopyroxene (0.04-028) and olivine (0.01-0.16) varied with melt composition such that DF (olivine-minette) < DF (augite-minette) < DF (basaltic andesite) < DF (latite). Crystal chemical controls were found to influence the incorporation of F into clinopyroxene, but none were found that affected olivine. Fluorine partitioning was compared with that of OH within clinopyroxenes, and the alumina content of clinopyroxene was shown to be a strong influence on the incorporation of both anions. Fluorine substitution into both olivine and clinopyroxene was found to be strongly controlled by melt viscosity and degree of melt polymerization.

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Created

Date Created
  • 2012

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Highly explosive mafic volcanism: the role of volatiles

Description

Explosive mafic (basaltic) volcanism is not easily explained by current eruption models, which predict low energy eruptions from low viscosity magma due to decoupling of volatiles (gases). Sunset Crater volcano

Explosive mafic (basaltic) volcanism is not easily explained by current eruption models, which predict low energy eruptions from low viscosity magma due to decoupling of volatiles (gases). Sunset Crater volcano provides an example of an alkali basalt magma that produced a highly explosive sub-Plinian eruption. I investigate the possible role of magmatic volatiles in the Sunset Crater eruption through study of natural samples of trapped volatiles (melt inclusions) and experiments on mixed-volatile (H2O-CO2) solubility in alkali-rich mafic magmas.

I conducted volatile-saturated experiments in six mafic magma compositions at pressures between 400 MPa and 600 MPa to investigate the influence of alkali elements (sodium and potassium) on volatile solubility. The experiments show that existing volatile solubility models do not accurately describe CO2 solubility at mid-crustal depths. I calculate thermodynamic fits for solubility in each composition and calibrate a general thermodynamic model for application to other mafic magmas. The model shows that the relative percent abundances of sodium, calcium, and potassium have the greatest influence on CO2 solubility in mafic magmas.

I analyzed olivine-hosted melt inclusions (MIs) from Sunset Crater to investigate pre-eruptive volatiles. I compared the early fissure activity to the sub-Plinian eruptive phases. The MIs are similar in major element and volatile composition suggesting a relatively homogeneous magma. The H2O content is relatively low (~1.2 wt%), whereas the dissolved CO2 content is high (~2300 ppm). I explored rehomogenization and Raman spectroscopy to quantify CO2 abundance in MI vapor bubbles. Calculations of post-entrapment bubble growth suggest that some MI bubbles contain excess CO2. This implies that the magma was volatile-saturated and MIs trapped exsolved vapor during their formation. The total volatile contents of MIs, including bubble contents but excluding excess vapor, indicate pre-eruptive magma storage from 10 km to 18 km depth.

The high CO2 abundance found in Sunset Crater MIs allowed the magma to reach volatile-saturation at mid-crustal depths and generate overpressure, driving rapid ascent to produce the explosive eruption. The similarities in MIs and volatiles between the fissure eruption and the sub-Plinian phases indicate that shallow-level processes also likely influenced the final eruptive behavior.

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Agent

Created

Date Created
  • 2018