Theoretical Stable Isotope Geochemistry of Alkaline-Earth Elements

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

Recent advances in mass-spectrometric techniques and instrumentation have enabled accurate stable isotope measurements of many heavy elements. In order to realize the full potential of these new measurements, it is important to develop a thorough understanding of the basic mechanisms causing heavy-element isotopic fractionation through careful theoretical studies and laboratory experiments. Here, a systematic theoretical exploration of the equilibrium stable isotope geochemistry of the alkaline-earth elements (magnesium, calcium, strontium, and barium) is proposed. All of the elements in the proposed field of study lack systematic theoretical characterization, and the role of equilibrium isotopic fractionations in producing observed isotope abundance variations is not known. These elements are logical choices because of their relevance to geology and low-temperature geochemistry, because major uncertainties exist regarding the underlying causes of observed fractionations, and because of the relative ease with which they can be modeled. Studies will focus on materials that are subjects of current and likely future isotope measurements, including carbonates, silicates, and aqueous species. The alkaline-earth elements all have relatively simple electronic structures and a single dominant oxidation state, making them ideal test cases for determining the effects of coordination number and hydration on isotopic fractionations. Because of their simple electronic structures, these elements are amenable to many theoretical treatments, particularly ab initio modeling of crystals using density functional perturbation theory (DFPT). Ab initio computational chemistry and empirical force-fields will be used to estimate the vibrational frequencies of isotopically substituted crystals, molecules, and solvated clusters, providing the necessary input data for quantum-statistical calculations of equilibrium stable isotope fractionation factors. Materials with well known vibrational spectra, like MgO and CaO, will also be modeled to check whether the calculated vibrational properties are reasonable and accurate. Vibrational models generated in this project will also become the nucleus of an animated mineral vibration database for use in mineralogy instruction. A successful research program devoted to these elements will provide a basic geochemical framework for designing and interpreting stable isotope measurements in a wide variety of natural samples. NSF Merit Review Criteria: Intellectual Merit: The proposed research will be of broad interest in studies of the paleoclimate records preserved in mineral precipitates, in searching for robust biosignatures in minerals and biological molecules, in tracing alteration processes in the seafloor and their contributions to global geochemical cycles in the mantle and subduction zones, and in meteoritic studies of the origin of the solar system. The proposed research is also relevant to basic studies in stable-isotope geochemistry, because so little is known about the natural isotopic fractionations of elements heavier than sulfur, and in particular for very heavy elements like barium and strontium. The project will also adapt ab initio techniques from computational chemistry and materials science that will be broadly applicable to theoretical studies of the stable isotope geochemistry of other main-group elements like silicon, sulfur and lithium. The PI is highly qualified to perform the proposed research, having previously modeled stable isotope fractionations of iron, chlorine, and chromium in molecules, crystals and aqueous complexes. Broader Impacts: This project will advance graduate education by supporting thesis research. Undergraduate education will also be enhanced through hands-on summer research opportunities. The proposed work involves the adaptation of modeling techniques from computational chemistry and materials science, and will partially support a computer modeling laboratory, improving the infrastructure for research and interdisciplinary cross-fertilization. The online database of mineral vibrations is intended to advance mineralogical research and education, particularly in the areas of infrared and Raman spectroscopy, thermodynamics and heat transfer, and displacive phase transformations.