Identifier

etd-08042016-151320

Degree

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

Document Type

Dissertation

Abstract

The field of isotope geochemistry began with the study of oxygen isotope geothermometry, most famously for carbonates. Traditionally oxygen isotope studies are only concerned with the relationship between one rare isotope, oxygen-18, and the common isotope, oxygen-16. In these cases, the abundance of the third stable isotope, oxygen-17 is ignored because for almost all terrestrial processes the 17O-16O relationship roughly scales with the 18O-16O relationship through a fractionation processes and is thought to not provide any new information. However, the discovery of large “mass independent” isotope effects for ozone chemistry has driven a multitude of uses for triple isotope relationships. Triple stable isotope relationships have found uses including, but not limited to, in geochemistry to determine ancient atmospheric pCO2, atmospheric chemistry to understand the details of ozone cycling, and biochemistry to study enzymatic pathways. These uses rely on a component of “mass-independent” isotope fractionation, previously described by deviation from a “canonical” range of mass-fractionation exponent values. However, recent advances in analytical techniques and precision have allowed for the measurement of small mass dependent variations in three isotope composition that hold information not present in the two isotope composition. The purpose of this work is to re-investigate the generic theoretical boundaries of mass-dependent variations in three isotope composition through fractionation processes and to demonstrate the utility of mass-dependent isotope fractionation. In the first study presented here, the boundaries and behaviors of mass-dependent isotope fractionation are investigated from a theoretical perspective. Previous approximations to the statistical-mechanical models for predicting isotope effects have led to the notion that mass fractionation laws are constant, and later, constrained to a “canonical” range of possible values. Despite previous work indicating that these mass fractionation exponents can be highly variable, the concept of a constant relationship remains common. In the first study presented here, it is demonstrated generically that the mass fractionation exponent, θ, can take any value for small fractionations, that these deviations are measurable and that the half-reaction mass fractionation exponent, κ, is bounded by upper and lower limits to a close approximation. In addition, we characterize and advocate the use of ∆∆‡M or “change/difference in cap-delta” as a necessary and more reliable descriptor of multiple isotope fractionation relationships. Deviations from the “canonical” range are demonstrated by experimental data in the geochemically relevant hematite-water system. The results of this work are valid for any element where nuclear volume effects are not significant. In the second study presented here, theoretical calculations for both two and three isotope fractionations between several common minerals and liquid water are presented for the purpose of calibration the three oxygen isotope geothermometer. The three isotope geothermometer concept used here is applied similarly to the more common two oxygen isotope thermometer, but utilizes a somewhat independent assumption. For mineral-water equilibrium systems, because the two-isotope and three-isotope geothermometers can be treated somewhat independently, there is potential to constrain the isotope composition of water with a single analysis. Alternately, without the assumption of equilibrium, the three isotope composition can be used as a test for equilibrium for the more accurate two isotope thermometer. In this study, new theoretical calibrations are presented for both the traditional two-isotope and the recently introduced three-isotope thermometer for pairs of quartz, calcite, dolomite, fluorapatite, hematite, magnetite and liquid water. The results presented here compare well with previous studies on 18O/16O fractionation where data is available. Of the models given here, pairs of quartz, calcite, dolomite and fluorapatite with water, hematite and magnetite show promise as three isotope thermometers with acceptable uncertainties for surface and low-T hydrothermal environments. As an example, the new quartz-water fractionation curves are applied to triple oxygen isotope date from previously published 2.5 Ga marine chert samples. These results indicate that the water which those chert samples formed from had a temperature of < 1C and a δ’18O of < -23.8‰. In the third study presented here, isotope fractionations associated with two key processes for ceria, an important industrial catalyst, are measured using constraints on triple oxygen isotope fractionation. Ceria (CeO2) is a heavily studied material in catalytic chemistry for use as an oxygen storage medium, oxygen partial pressure regulator, fuel additive, and for the production of syngas, among other applications. Ceria powders are readily reduced and lose structural oxygen when subjected to low pO2 and/or high temperature conditions. Such dis-stoichiometric ceria can then re-oxidize under higher pO2 and/or lower temperature by incorporating new oxygen into the previously formed oxygen site vacancies. Despite extensive studies on ceria, the mechanisms for oxygen adsorption-desorption, dissociation-association, and diffusion of oxygen species on ceria surface and within the crystal structure are not well known. We predict that a large kinetic oxygen isotope effect should accompany the release and incorporation of ceria oxygen. As the first attempt to determine the existence and the magnitude of the isotope effect, this study focuses on a set of simple room-temperature re-oxidation experiments that are also relevant to a laboratory procedure using ceria to measure the triple oxygen isotope composition of CO2. Triple-oxygen-isotope labeled ceria powders are heated at 700°C and cooled under vacuum prior to exposure to air. By combining results from independent experimental sets with different initial oxygen isotope labels and using a combined mass-balance and triangulation approach, we have determined the isotope fractionation factors for both high temperature reduction in vacuum (~10−4 mbar) and room temperature re-oxidation in air. Results indicate that there is a 1.5‰±0.8‰ increase in the δ18O value of ceria after being heated in vacuum at 700°C for one hour. When the vacuum is broken at room temperature, the previously heated ceria incorporates 3% to 19% of its final structural oxygen from air, with a δ18O value of 2.1‰ (−4.1‰; +7.7‰) for the incorporated oxygen. The substantial incorporation of oxygen from air supports that oxygen mobility is high in vacancy-rich ceria during re-oxidation at room temperature. The quantified oxygen isotope fractionation factors are consistent with the dissociation of O2 and association of atomic oxygen species at the surface of ceria being the main rate-limiting steps during ceria oxidation and reduction, respectively. While additional parameters may reduce some of the uncertainties in our approach, this study demonstrates that isotope effects can be an encouraging tool for studying oxygen transport kinetics in ceria and other oxides.

Date

2016

Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Huiming Bao

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