Stable isotopes serve as tracers of processes. Through analysis of the stable isotope compositions of meteorites, we can look back in time and learn about the conditions present in our nascent Solar System as well as the processes that were occurring. Here we characterize aspects of terrestrial planet formation and evolution using stable isotopes as our tool.

To understand how the first rocky material in our Solar System formed, we measure the isotopic compositions of several major rock-forming elements in the oldest surviving solids that formed in the Solar System (calcium-aluminum-rich inclusions). Measurements of 25Mg, 29Si, 44Ca, 49Ti, and anomalous 50Ti in these refractory inclusions indicate that CAIs are not primary condensates, but instead, aggregates of pre-existing materials. Furthermore, they display signs of heterogeneity, which are echoes of larger scale heterogeneity that existed in the precursor material from which they formed. The compositions of CAIs are consistent with averaging of the precursor material; this averaging has the effect of dampening the heterogeneity exhibited by the precursor material.

We also use stable isotopes to understand the nature of core formation in planetesimals. Measurements of the iron isotopic composition, 57Fe, of differentiated aubrite, iron, and chondrite meteorites allow us to form a more complete picture of core formation. The 57Fe of the metal and silicate phases in aubrite meteorites indicate that the metallic phase is enriched in 57Fe/54Fe. The magnitude of this fractionation cannot explain the observed difference in the compositions of iron meteorites and chondrites. Instead, the elevated 57Fe/54Fe of iron meteorites relative to chondrites, is likely due to evaporation and not core-mantle differentiation.

In this work, exoplanet-research is combined with the study of the solar system in order to assess differences and similarities between rocky bodies in the Milky Way. To evaluate rocky bodies outside of the solar system, I utilize optical spectroscopy to study polluted white dwarf stars, dense stars that show accretion of planetary material. By observing polluted white dwarfs, we can measure elemental abundances from the rocky and icy bodies that previously orbited the star. Specifically, I conduct observations using the KAST Spectrograph on the Shane 3-meter telescope at Lick Observatory and the High-Resolution Echelle Spectrometer (HIRES) on the Keck I Telescope, as well as evaluate compiled literature data. Generally, the elemental compositions of extrasolar planetesimals closely resemble those of rocky bodies in the solar system. In this work, a more detailed comparison with solar system meteorites and planets shows that oxidation of planetesimals prior to planet formation is common among extrasolar rocks. Overall, the processes that lead to the geochemistry and much of the geophysics of Earth is normal compared to the current sample of extrasolar planetesimals. Additionally, the origin of excesses in spallogenic nuclides in polluted white dwarfs is investigated. The MeV proton fluence required to form the high Be/O ratio in the accreted parent bodies of two polluted white dwarfs (GALEX J2339-0424 and GD 378) is consistent with irradiation of ice in the rings of a giant planet within its radiation belt, followed by accretion of the ices to form a moon that is later accreted by the WD.

Polluted white dwarfs offer the rare chance to directly measure the bulk compositions of exoplanetary material. These stellar remnants are actively accreting exoplanetary debris, whose chemical abundances can be extrapolated from excess metal lines in white dwarf spectra. In this dissertation I leverage the growing sample of observed polluted white dwarfs to conduct statistical comparisons between exoplanetary rock compositions and objects in our solar system. My work combines data compiled from the literature along with analytical models for accretion and settling in white dwarf atmospheres to test the elemental abundances of white dwarf pollution, while placing constraints on how to best leverage white dwarf data given uncertainties in different accretion processes. I first validate exomoons as a potential source of white dwarf pollution, and show that the bulk objects causing pollution need to be massive, on the order of Vesta or Ceres, the largest objects in our asteroid belt. I then argue that the white dwarf sample is evidence that most nearby exoplanets form from compositional building blocks similar to CI chondrites, the assumed primitive material in our own solar system. I support this conclusion by showing that the relative abundances of rock-forming elements in nearby stars are similarly consistent with chondrites, and demonstrate that on very large scales, the chemical evolution of our galaxy may be encoded in planet compositions. Finally, I show that the oxygen abundances in polluted white dwarfs are consistent with water contents ranging from dry, Earth-like bodies to water-rich objects akin to icy moons in the solar system. These results place important constraints on the compositions of exoplanets, suggest that solar system compositions are not unique compared to our nearest neighbors, and provide the foundation for future studies on the formation and diversity of exoplanets in the Milky Way.

Reactions occurring at enzymes drive all of Earth’s biogeochemical cycles from the oxygen in the atmosphere to methane below the seafloor. Although these gases are critical for life on our planet, they have a multitude of sources and sinks that can be difficult to distinguish from one another, complicating our ability to understand their budgets both in the present and the past. Here, I explore new tracers of the oxygen and methane cycles with a focus on the biologic production and consumption of these gases: photosynthesis/respiration and methanogenesis/methanotrophy, respectively. Isotopes have been used as tracers of these processes since the inception of the field of stable isotope geochemistry, but only the measurement of singly-substituted molecules (i.e., 18O16O and 13CH4) has been possible. Within, I report measurements of the relative abundances of 18O18O and 18O17O for oxygen that has been biologically cycled in a terrarium experiment and respired in lake water as well as 13CH3D and 12CH2D2 of biologically produced and consumed subseafloor methane. These multiply-substituted isotopologues provide a new dimension of information by illuminating the enzyme level chemistry in making and breaking bonds. I find that photosynthesis and methanogenesis produce oxygen and methane respectively that is out of equilibrium with environmental temperatures and the resulting gases have fewer multiply-substituted isotopologues than predicted by chance alone. Respiration of oxygen leaves behind a residue enriched in these rare isotopologues; this unexpected result merits further exploration. However, anaerobic methanotrophy seems to be capable of reordering isotopes by enzymatic back reaction, driving a pool of methane to intra-species equilibrium at low temperature. These findings have consequences both for ongoing work in measuring marine primary productivity as well as exploring the extent of life in the deep biosphere and throughout the solar system.

Primitive chondritic meteorites formed at the same time the planets in our solar system were forming and can offer information about the conditions present in the early solar system, provided that they have not been significantly altered by heat and water. The CR carbonaceous chondrite group contains some of the least-altered meteorites known. The members of the group have undergone minimal thermal alteration, but while on the parent asteroid CR chondrites have interacted with water to varying degrees. A numerical aqueous alteration scale was developed to rank the CR chondrites based on their interactions with water. The majority of the CR carbonaceous chondrites have had minimal interaction with water and this scale is used to select appropriate samples for the study of the early solar system.

Silicon and magnesium isotope ratios were measured in selected CR and CV chondrules using multi-collector inductively-coupled plasma mass spectroscopy to test the hypothesis that a silicon-bearing gas was present in the solar nebula and condensed into fully or partially molten chondrules. The SiO gas could react with olivine in the chondrules to form pyroxene. These data were interpreted using a condensation model for silicon. Some of the chondrules studied did show a non-equilibrium fractionation in δ29Si between olivine and pyroxene that was calculated to have been caused by a supersaturated SiO gas that had condensed into the chondrule. If this SiO gas did condense into the chondrules the process happened at or near equilibrium with a undercooling of 3 K or less below the equilibrium temperature. These results provide additional evidence that a silicon-bearing gas was present in the early solar system during the time of planet formation, but not all chondrules studied show clear evidence of having incorporated this gas.

Making precise measurements of clumped-isotopes in CO₂ (reported as Δ₄₇) is resource-intensive and time-consuming. Instrument and time-related variability of measured Δ₄₇ values have hindered inter-laboratory calibration efforts. Additionally, clumped-isotopes measured in Paleozoic fossils suggest ancient marine temperatures and/or ocean isotopic compositions that are difficult to accept at face value.

This dissertation describes a strategy for measuring Δ₄₇ on a conventional mass spectrometer, with the usual CO₂ set of three Faraday collectors by "multicollector peak hopping" (MPH). The MPH method involves directing m/z = 46 and m/z = 47 ion beams into the Faraday cups used for m/z = 44 and m/z = 45, and then calculating Δ₄₇ from sample δ¹³C, δ¹⁸O, and measured ^47/46CO₂⁺. This method includes a protocol for correcting ion-beam intensities for secondary electrons in order to remove Δ₄₇ dependence on δ⁴⁷CO₂.

Measurements of Δ₄₇ in CO₂ liberated from modern bivalved mollusc shells demonstrate that electron scatter has a significant influence on Δ₄₇ values. As this effect is similar in magnitude to the pairwise differences between carbonate temperature calibrations that were made using different instruments, it is possible that electron scatter is responsible for these discrepancies. The observed relationship between Δ₄₇ and growth temperature preserved in modern mollusc shell is very close to theoretical predictions for Δ₄₇ in calcite.

Measurements of Δ₄₇ in CO2 liberated from shells of the Permian bivalved mollusc Eurydesma cordatum yield paleotemperatures that are too high to conform with geologic indicators for near-freezing water temperatures, yet too low to represent maximum burial temperatures. Cycles in δ¹⁸O that are in-phase with growth bands suggest that this fossil did not exchange oxygen isotopes with water after the precipitation of the carbonate, meaning that ¹³C-¹⁸O bonds were re-ordered at low temperature in the absence of significant amounts of fluid. It is suggested that original seawater was present in trace amount, enough to catalyze ¹³C-¹⁸O bond re-ordering, but without affecting δ¹⁸O of the shell. Such subtle alteration eludes detection with current screening methods. Unless the role of geologic processes in altering Δ₄₇ is better understood, successful application of clumped-isotope paleothermometry to Paleozoic fossils may be limited.

A report on the relative abundances of the three stable isotopes of silicon, 28Si, 29Si and 30Si, across the Galaxy using the v = 0, J = 1 - 0 transition of silicon monoxide. The chosen sources represent a range in Galactocentric radii (RGC) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the Robert C. Byrd Green Bank Telescope (GBT) permit isotope ratios to be corrected for optical depth, using a novel method developed as part of this study. The optical-depth-corrected data indicate that the secondary-to-primary silicon isotope ratios [29Si]/[28Si] and [30Si]/[28Si] vary much less than predicted on the basis of other stable isotope ratios in the Galaxy. Indeed, there is no detectable variation in Si isotope ratios with RGC. This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with RGC in published secondary-to-primary oxygen isotope ratios. These results, when considered in the context of the expectations for galactic chemical evolution, suggest that the reported isotope ratio trends in oxygen, and likely carbon as well, may be in error and require further investigation. The methods developed in this study for SiO isotopologue ratio measurements are equally applicable to Galactic oxygen, carbon and nitrogen isotope ratio measurements, and should prove useful for future observations of these isotope systems.