Theoretical Stable Isotope Geochemistry of Alkaline-Earth Elements

质谱学技术和仪器的最新进展使许多重元素的准确稳定同位素测量成为可能。为了实现这些新测量的全部潜力，重要的是通过仔细的理论研究和实验室实验来彻底了解引起重元素同位素分馏的基本机制。本文对碱土元素(镁、钙、锶、钡)的平衡稳定同位素地球化学进行了系统的理论探讨。拟议研究领域中的所有元素都缺乏系统的理论描述，平衡同位素分馏在产生观测到的同位素丰度变化中的作用尚不清楚。这些元素是合乎逻辑的选择，因为它们与地质学和低温地球化学有关，因为关于观察到的分馏的根本原因存在重大不确定性，并且因为它们可以相对容易地进行建模。研究将集中在当前和未来可能成为同位素测量对象的材料上，包括碳酸盐、硅酸盐和水物种。碱土元素都具有相对简单的电子结构和单一的主导氧化态，这使它们成为确定配位数和水合作用对同位素分馏影响的理想测试案例。由于它们简单的电子结构，这些元素可以进行许多理论处理，特别是用密度泛函微扰理论(DFPT)对晶体进行从头算模拟。从头计算化学和经验力场将被用来估计同位素取代的晶体、分子和溶剂化团簇的振动频率，为平衡稳定同位素分馏因子的量子统计计算提供必要的输入数据。具有已知振动光谱的材料，如氧化镁和氧化钙，也将被模拟，以检查计算的振动性质是否合理和准确。在这个项目中产生的振动模型也将成为用于矿物学教学的动画矿物振动数据库的核心。致力于这些元素的成功研究计划将为设计和解释各种自然样品的稳定同位素测量提供基本的地球化学框架。美国国家科学基金会审查标准：智力价值：拟议的研究将在研究矿物沉淀中保存的古气候记录、寻找矿物和生物分子中的可靠生物特征、追踪海底的变化过程及其对地幔和俯冲带全球地球化学循环的贡献以及太阳系起源的陨石学研究中具有广泛的兴趣。这项拟议的研究也与基础研究不稳定同位素地球化学有关，因为人们对比硫更重的元素的自然同位素分馏知之甚少，特别是对钡和锶等非常重的元素。该项目还将采用计算化学和材料科学的从头计算技术，这些技术将广泛适用于硅、硫和锂等其他主族元素的稳定同位素地球化学的理论研究。PI非常有资格执行拟议的研究，以前曾对分子、晶体和水合物中铁、氯和铬的稳定同位素分馏进行建模。广泛的影响：该项目将通过支持该研究来促进研究生教育。本科教育也将通过实践暑期研究机会得到加强。拟议的工作涉及采用计算化学和材料科学的建模技术，并将部分支持一个计算机建模实验室，改善研究和跨学科交叉培养的基础设施。矿物振动在线数据库旨在促进矿物学研究和教育，特别是在红外和拉曼光谱、热力学和热传递以及置换相变领域。

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.