Theoretical Stable Isotope Geochemistry of Alkaline-Earth Elements

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Recent advances in mass-spectrometric techniques and instrumentation have enabledaccurate stable isotope measurements of many heavy elements. In order to realize the fullpotential of these new measurements, it is important to develop a thorough understanding of thebasic mechanisms causing heavy-element isotopic fractionation through careful theoreticalstudies and laboratory experiments. Here, a systematic theoretical exploration of the equilibriumstable isotope geochemistry of the alkaline-earth elements (magnesium, calcium, strontium, andbarium) is proposed. All of the elements in the proposed field of study lack systematic theoreticalcharacterization, and the role of equilibrium isotopic fractionations in producing observed isotopeabundance variations is not known. These elements are logical choices because of their relevanceto geology and low-temperature geochemistry, because major uncertainties exist regarding theunderlying causes of observed fractionations, and because of the relative ease with which theycan be modeled. Studies will focus on materials that are subjects of current and likely futureisotope measurements, including carbonates, silicates, and aqueous species.The alkaline-earth elements all have relatively simple electronic structures and a singledominant oxidation state, making them ideal test cases for determining the effects of coordinationnumber and hydration on isotopic fractionations. Because of their simple electronic structures,these elements are amenable to many theoretical treatments, particularly ab initio modeling ofcrystals using density functional perturbation theory (DFPT). Ab initio computational chemistryand empirical force-fields will be used to estimate the vibrational frequencies of isotopicallysubstituted crystals, molecules, and solvated clusters, providing the necessary input data forquantum-statistical calculations of equilibrium stable isotope fractionation factors. Materials withwell known vibrational spectra, like MgO and CaO, will also be modeled to check whether thecalculated vibrational properties are reasonable and accurate. Vibrational models generated in thisproject will also become the nucleus of an animated mineral vibration database for use inmineralogy instruction. A successful research program devoted to these elements will provide abasic geochemical framework for designing and interpreting stable isotope measurements in awide variety of natural samples.NSF Merit Review Criteria:Intellectual Merit: The proposed research will be of broad interest in studies of thepaleoclimate records preserved in mineral precipitates, in searching for robust biosignatures inminerals and biological molecules, in tracing alteration processes in the seafloor and theircontributions to global geochemical cycles in the mantle and subduction zones, and in meteoriticstudies of the origin of the solar system. The proposed research is also relevant to basic studies instable-isotope geochemistry, because so little is known about the natural isotopic fractionations ofelements 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 materialsscience that will be broadly applicable to theoretical studies of the stable isotope geochemistry ofother main-group elements like silicon, sulfur and lithium. The PI is highly qualified to performthe 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 thesisresearch. Undergraduate education will also be enhanced through hands-on summer researchopportunities. The proposed work involves the adaptation of modeling techniques fromcomputational chemistry and materials science, and will partially support a computer modelinglaboratory, improving the infrastructure for research and interdisciplinary cross-fertilization. Theonline database of mineral vibrations is intended to advance mineralogical research andeducation, particularly in the areas of infrared and Raman spectroscopy, thermodynamics andheat transfer, and displacive phase transformations.