We proposed to use volcanogenic sandstones for CO₂ sequestration. Such sandstones with a relatively high percentage of volcanic rock fragments (VRF) could be a promising target for CO₂ sequestration in that they have a sufficient percentage of reactive minerals to allow substantial mineralization of injected scCO₂, which provides the most secure form of CO₂ storage, but can also be porous and permeable enough to allow injection at acceptable rates. The limitation in using volcanogenic sandstones as CO₂ reservoir rocks is that porosity and permeability tend to decrease with increase of volcanic rock fragments (VRF) and with the length and complexity of the diagenetic history. Decreased porosity limits the rate at which CO₂ can be injected. We evaluated these tradeoffs to assess the feasibility of using volcanogenic sandstone to achieve highly secure CO₂ storage. Using relationships between VRF percent, porosity and permeability from available geological data, the reactive transport code TOUGHREACT was used to estimate the rate and extent of CO₂ mineralization over 1000 years, and the trade-off between higher reactivity and lower porosity and permeability. For the parameter set that we believe is defensible, the optimal VRF percent for the largest amount of mineralized CO₂ is around 10-20%. The results show that as much as 80% CO₂ mineralization could occur in 1000 years and still allow sufficient injectivity so that ca. 1 megaton of CO₂ can be injected per year per well. The key to estimating how much CO₂ can be injected and mineralized is the relationship between permeability (or injectivity), reactive mineral content, mineral dissolution rates and reactive surface areas. We have sampled examples of volcanogenic sandstones from the Miocene Etchegoin Formation, central California to examine these parameters. Characterizations of these samples by SEM, XRF and XRD show that they are very rich in reactive minerals with around 32% plagioclase, 10% orthopyroxene, 2% clinopyroxene, and 1% ilmenite. Porosities range from 10% to 20%, and permeabilities range from 10 mD to 1000 mD. Batch experiments are also conducted to measure dissolution rates of one of the common minerals in volcanic sandstones, i.e. chlorite. Results show that the released Mg from chlorite in such sandstones could provide enough cations to potentially mineralize about half the stored CO₂ in the pore space within 1000 years. The last chapter presents an extension of the TOUGHREACT code to include carbon 13 isotope into reactive transport modeling. ¹³C isotope proves to be a great tracer for monitering subsurface flow of CO₂.

In the last two decades it has been increasingly recognized that Ca isotope fractionation at high-temperatures can greatly exceed fractionation at surface conditions. These effects have been used to trace recycling of crustal materials into the mantle, to estimate equilibration temperatures in mantle rocks, and to understand kinetic isotope fractionation during diffusion in experimental melts. However, little is known about the competition between kinetic and equilibrium Ca isotope effects in natural samples, which suggests that the use of stable Ca isotope variations as a proxy for carbonate recycling (or for mantle temperature) is significantly underdetermined.

The Ca isotope evolution of continental crust, which is currently understudied, is intimately linked to the geochemical evolution of Earth’s mantle, oceans, and atmosphere. As such, this dissertation focuses dominantly on understanding radiogenic, kinetic, and equilibrium Ca isotope variations in igneous and metamorphic rocks from the continental crust, but also includes radiogenic Sr isotope models of hydrothermal circulation at mid-ocean ridges, which are critical to understanding continental weathering rates and the geochemical evolution of seawater over time.

Chapter 2 explores the potential feed-back between the chemical and isotopic composition of paleoseawater and high-temperature hydrothermal fluids, which has been implied by Sr-isotope measurements in hydrothermal minerals over time (Antonelli et al., 2017). Chapter 3 presents results from radiogenic Ca isotope measurements in lower-crustal rocks and minerals, confirming that K is effectively lost from the lower crust during high-T metamorphism (Antonelli et al., 2018). Chapter 4 focuses on stable Ca isotope measurements in lower-crustal rocks and minerals, demonstrating that kinetic Ca effects are abundant in nature, both at the whole-rock and inter-mineral scales, and that they can be used to understand paragenesis and Ca diffusion in lower-crustal rocks (Antonelli et al., 2019b). Chapter 5 presents stable Ca isotope results from volcanic and sub-volcanic rocks and minerals, which imply that Ca isotopes can be used to estimate crystal growth-rates in igneous crystals (e.g. phenocrysts, comb-layers, and orbicules) and that associated volcanic eruption/recharge events are generally short-lived (Antonelli et al., 2019a).

Chemical speciation in molten silicates is of broad interest for understanding thermodynamic and transport properties. However, it is difficult to elicit the nature of dissolved species in silicate liquids, let alone the role that speciation plays in the diffusion process. This dissertation is about chemical diffusion in silicate melts; it combines experiments with isotopic measurements to infer the physical mechanisms of cation diffusion and the role that diffusion plays in producing isotope effects in nature.

Diffusion of calcium (Ca) between natural volcanic liquids in diffusion-couple experiments leads to ⁴⁴Ca/⁴⁰Ca variations of ca. 5‰ due to a mass dependence of Ca diffusion coefficients. The efficiency of Ca isotope separation by diffusion varies with bulk liquid composition and depends also on the magnitude and direction of aluminum (Al) gradients. Some Ca isotopic fractionations seem to arise solely from gradients in Al. These observations indicate that isotopic discrimination by diffusion reflects the mass difference between isotopically-substituted molecular species, and there is evidence for at least one Ca-bearing and one Ca-Al-bearing diffusing species in the volcanic liquids that were studied. The inferred existence of a Ca-Al complex is consistent with the additional observation that Ca diffuses slowly since it is inferred that Ca atoms interact strongly with their nearby Si and Al "solvent" molecules in the liquid.

The third part of this thesis describes Ca diffusion between silicate liquids of simplified chemical composition. Results from these experiments indicate that the efficiency of diffusive separation of Ca isotopes is systematically related to the normalized diffusivity - the ratio of the mobility of the cation D_Ca to the mobility of the liquid matrix D_Si. A similar dependence is observed for Fe, Li, and Mg isotopes, and in aqueous solutions. This empirical result provides a predictive tool that can be used to understand diffusive isotopic effects in a wide variety of geologic environments and a basis for a more comprehensive theory of isotope separation in liquid solutions. A conceptual model is presented for the relationship between diffusivity and liquid structure that is consistent with available data.

Precipitation of carbonate minerals is ubiquitous in the near-surface environment, and the isotopic and trace element composition of carbonates may be used to reconstruct the conditions of growth. Little is known about the mechanisms controlling isotope and trace element distribution into carbonates. Proposed growth mechanisms are typically inferred from the supersaturation dependence of CaCO₃ precipitation rates. As different experimental techniques often generate different apparent reaction orders, numerous hypothesized mechanisms can be found in the literature. These descriptions of crystallization pathway cannot be used to identify processes controlling trace element and isotope partitioning.

Recent advances in experimental methodology allow us to observe microscopic to nano-scopic structures at the mineral surface during growth. The mechanisms controlling calcite growth have been directly determined by using fluid flow cells placed in an atomic force microscope (AFM; chapter 3). Calcite growth below the threshold oversaturation for amorphous calcium carbonate (ACC) formation &ndash typical of seawater and most terrestrial fluids &ndash occurs primarily through the attachment of ions to kink sites on the surface. The net flux of ions to kink sites governs both overall growth rate and mineral composition.

In this thesis, I present a self-consistent model based on observed calcite growth mechanisms that may be used to predict growth rate (chapter 1), and isotopic (chapter 2; Nielsen et al., 2012) and trace element (chapter 5; Nielsen et al., in prep.) partitioning as a function of solution composition. I apply this model to calcium isotope fractionation during the precipitation of synthetic calcite, which I grew and analyzed using novel secondary ion mass spectrometry (SIMS) methods (chapter 3). In chapter chapter 4 (Nielsen and DePaolo, in review), I model calcium isotope fractionation in carbonate minerals that I collected from the highly alkaline Mono Lake, CA. The mechanistic framework developed here may be extended to multicomponent systems, and may be adapted for use in reactive transport models. When interpreted through the lens of this model, trace element and isotope signatures preserved in carbonates may eventually be used to reconstruct the chemistry of natural aqueous fluids.

Characterizing the reactivity and fate of solutes in groundwater presents a formidable challenge in that these environments are inherently isolated from the observer. Samples are commonly limited to sediment cores and fluid collected from discrete well bores, while the measurements from these locations must be used to infer geochemical processes occurring across continuous, dynamic and heterogeneous systems. Stable isotope fractionations are an important tool in the quantitative analysis of reactive transport processes in porous media because they provide information coupled to, but distinct from solute concentrations. Examples of the combined analysis of solute concentrations and stable isotope fractionations abound in the literature, but almost entirely involve analytical models that decouple isotopic fractionation from the full extent of reactive pathways contributing to water-rock interaction in porous media flow.

It is the purpose of this dissertation to apply analysis of stable isotopes to elucidate the processes of biogeochemical reactivity contributing to in situ bioreduction of uranium during electron donor augmentation in the Old Rifle aquifer in western Colorado. Bio-induced stabilization of heavy metals in subsurface systems involves a complex and concurrent series of dissimilatory reduction reactions, each with a unique metabolism and contaminant affinity. In such a complex system, application of simplified fractionation-reactivity relationships is shown to be inadequate to describe the behavior of multiple stable isotope systems. As a result, the methodology is developed to explicitly incorporate the isotopologues of these species into a multi-component reactive transport model of the system. The result is a means of mechanistically analyzing isotopic fractionation within an integrated framework encompassing the complete reactive network associated with electron-donor augmented bioreduction.

The chapters herein are roughly divided into two sections. In the first half, sulfur and calcium isotopes are measured during field scale acetate amendments at Old Rifle. The results of these analyses demonstrate the utility of isotopic fractionation to track the development of terminal electron accepting processes and the formation of secondary mineral precipitates. However, these studies also demonstrate the limitations incurred by restriction to discrete well bores. In the second half, a flow-though column study is designed to overcome these limitations and test the extent to which individual isotopic fractionations are conserved during reactive transport under Old Rifle conditions. The results provide the first carbon budget during electron donor augmentation, and demonstrate that while the &delta³⁴S fractionation induced by biogenic sulfate reduction is conservative, ion exchange effects dampen the &delta⁴⁴Ca fractionation associated with carbonate precipitation.

Magmatism has played an integral part in the evolution of the Earth through geologic time. Mantle-derived mafic magmatism is a primary mechanism by which the bulk silicate Earth segregated into mantle and crust, and the continental crust has further differentiated through metamorphism, anatexis, and assimilation. The study of magmatic processes has given rise to our understanding of how the Earth has evolved.

Central to this understanding are chemical, isotopic, and geochronological studies. Chemical and isotopic studies provide unparalleled richness of information; this is especially true now, as technological advances extend our analytical and computational capabilities. The difficulties presented in geochemical field studies of magmatic processes are primarily twofold: we are mostly limited to surface samples at the end of their evolutionary story (while the processes of interest are those occurring from source to surface), and it can be difficult to disentangle the myriad causes that produce the chemical and isotopic effects observed. These difficulties can be addressed by combined geochemical-geophysical-geospatial studies, by computational modeling based on an extensive (and growing) body of thermodynamic data, and by application of novel geochemical techniques.

As with chemical data, geochronological data are fundamental descriptors in their own right, but they are also crucial because they anchor rates. Almost all geologic processes or features are rate-dependent, from growth of crystals on an atomic scale, to morphologies of volcanoes, to the evolution of the crust. An understanding of chronologies and rates is therefore fundamental to understanding the processes of magmatic evolution.

The diverse chapters in this thesis reflect my diverse interests in the Earth sciences, but all are motivated by desire to understand both fundamental aspects of the chemical evolution of magmas, and how that evolution informs our broader understanding of volcanoes, volcanic fields, and continental crust evolution.

The first chapter is a geochemical study of the San Francisco Volcanic Field in central Arizona. In this study, I combine geochemical, geochronological, geospatial, and isotopic data with the computational thermodynamic modeling package alphaMELTS to understand the chemical evolution of the volcanic magmas from source to surface. I show that the isotopic and chemical characteristics of the SFVF can be largely explained by assimilation of the deep crust by parent magma derived from fertilized asthenospheric mantle. The computational treatment with alphaMELTS allows for quantitative limits to be placed on the mineralogy, chemistry, and dynamics of the source of the volcanic field.

The second chapter is a geochronological study of magmas from the Hawaii Scientific Drilling Project phase 2 core. This study works to extend the U--Th/He dating technique to olivine phenocrysts in relatively undegassed submarine basalts, rocks which are normally difficult to date by other chronometers.

The third chapter investigates the evolution of felsic magma chemistries and the changing nature of continental crust source rocks during the Himalayan collision. In this study, I apply relatively new (Mg, Ca) and established (Sr, Nd) isotopic techniques to a sample suite of diverse crustal lithologies (primarily granitoids and felsic volcanics) from the Lhasa, Tibet region. I show that Ca and Mg isotopes are sensitive to specific mineralogies and processes, including carbonate-bearing shale assimilation, and that the Ca and Mg isotopic systems are promising tools in studies of granite petrogenesis, metamorphism, assimilation, and subduction recycling.

This dissertation is about trace element and isotope fractionation in natural and laboratory-synthesized calcite and aragonite. Quantifying various isotope (δ13C and δ44Ca) and trace element (Sr/Ca and Mg/Ca) ratios in carbonates allows us to probe the kinetics of various reactions at the mineral-fluid interface. Understanding the way in which these signals depend on various parameters (e.g., precipitation rate, temperature, fluid composition) elucidates molecular-scale fundamentals of crystal growth. Similarly, this framework allows us to use these isotopic and trace element probes as proxies for interpreting natural carbonates, and in many cases, understand ancient environments. Furthermore, by identifying the nature of carbonate growth in solutions of varying chemistry and temperature, we can best harness the ability of these minerals to sequester CO2 and fluid contaminants in response to anthropogenic pollution and climate change.

In Chapter 2, I use the Ca and Sr concentrations of marine sedimentary pore fluids to estimate rates of authigenic carbonate precipitation in modern sediments. Using the extensive Ocean Drilling Program database, these rates of authigenic carbonate production can be compared to coeval rates of organic carbon and biomineralized carbonate deposition, thereby quantifying relative fractions of each contributing carbon flux. While there are many sedimentary environments in which the relative authigenic carbonate fraction is significant in comparison to the organic and biomineralized fractions, the δ13C of these carbonates is almost identical to the biomineralized fraction in nearly all cases. Within the context of a carbon isotope mass balance framework, I demonstrate that the biomineralized and authigenic carbonate fractions can be suitably accounted for via a single inorganic carbonate pool. This finding has implications for interpreting the geologic record, indicating that some of the fluctuations in δ13C over the course of Earth history cannot be explained by the ongoing and extensive production of authigenic carbonate. I explore the few marine sedimentary environments that do produce authigenic carbonate with δ13C distinct from that of the biomineralized fraction, and thereby could be possible exceptions to the main finding of this work. The processes responsible for creating this isotopically anomalous authigenic carbonate are not thought to be steady-state phenomena throughout most of Earth history. The production of authigenic carbonate in these environments therefore suggests that δ13C variations in the sedimentary record could not have been the result of authigenic carbonate formation. Rather, the formation of authigenic carbonate with a δ13C composition distinct from that of biomineralized carbonate would be a consequence of a non-steady state perturbation to the carbon cycle. Consequently, these exceptional environments do not change the finding that a single inorganic carbon reservoir can be used to encapsulate the ongoing biomineralized and authigenic fluxes in marine sediments from an isotopic mass balance standpoint.

In Chapter 3, I study the formation of calcite and aragonite from the travertine-precipitating hot springs of Bridgeport, California. These travertines form as an elongate fissure ridge of intergrown calcite and aragonite, precipitating along the edges of a thin fluid conduit with systematically changing fluid pH, temperature, and CO2 degassing. The ability to readily measure modern precipitates from predictably evolving ambient fluids makes this system a unique natural laboratory in which to probe kinetics of carbonate growth in a high-temperature (>60°C) system. While travertines around the world have been the subject of a substantial body of literature, only one other study has reported the Ca isotope fractionation that occurs between travertine carbonates and fluid. In this work, δ13C, δ44Ca, Sr/Ca and Mg/Ca ratios in bulk carbonates and fluids are measured. Although the Sr/Ca ratios in the bulk solid are much higher than expected based on experimental studies, I argue that they may not be anomalous for thermogene travertines. The spatially evolving δ13C and Sr/Ca composition of the fluid and solid indicates decreasing aragonite content down-conduit, suggesting that the Ca isotope fractionation will reflect the relative change in calcite and aragonite fractions. Although aragonite is expected to produce a larger magnitude fractionation than calcite, we do not observe any trend with distance from the inlet corresponding to the systematically changing mineral fractions. This observation can be explained by ongoing recrystallization of carbonate in the porous conduit walls, demonstrating the complex nature of multi-mineral travertine deposits and the impact of diagenetic alteration on Ca isotope systems in nature.

Finally, in Chapter 4, I probe calcite growth at the smallest spatial and temporal scales studied in this dissertation. I present results from an experimental study in which calcite was precipitated from solutions of varying Mg/Ca ratios with a constant saturation index (SI) under controlled conditions. The aim of this project was to determine the effect of Mg on calcite growth as reflected in the Ca isotope fractionation between solid and solution. Mg is thought to primarily inhibit calcite precipitation via a kink blocking mechanism. However, the change in Ca isotope fractionation with increasing Mg/Ca and decreasing precipitation rate at constant SI demonstrates that in addition to kink blocking, Mg also impedes growth via incorporation inhibition. This additional mechanism alters the relative Ca attachment and detachment fluxes at the crystal surface, resulting in the observed fractionation. This finding has important implications for the interpretation of Ca isotope fractionation in both marine inorganic and biomineralized calcite precipitating from Mg-bearing seawater and sedimentary pore fluids.