Theoretical Stable Isotope Geochemistry of Alkaline-Earth Elements

碱土元素的理论稳定同位素地球化学

• 批准号: 0345433
• 负责人: Edwin Schauble
• 金额: $ 24.8万
• 依托单位: University of California-Los Angeles
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-08-01 至 2008-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0345433&HistoricalAwards=false
• 关键词: Theoretical; Stable; Isotope; Geochemistry; Alkaline

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.

质谱技术和仪器的最新进展使许多重元素的稳定同位素测量变得准确。为了充分发挥这些新测量方法的潜力，通过仔细的理论研究和实验室实验，对引起重元素同位素分馏的基本机制有一个全面的了解是很重要的。本文对碱土元素（镁、钙、锶、钡）的平衡不稳定同位素地球化学进行了系统的理论探索。所提出的研究领域的所有元素都缺乏系统的理论表征，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，因为观测到的分选的潜在原因存在很大的不确定性，因为它们可以相对容易地建模。研究将集中在当前和未来可能进行同位素测量的物质上，包括碳酸盐、硅酸盐和含水物质。碱土元素都具有相对简单的电子结构和单一的优势氧化态，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。由于其简单的电子结构，这些元素适用于许多理论处理，特别是使用密度泛函微扰理论（DFPT）从头开始建模晶体。从头算计算化学和经验力场将用于估计同位素取代晶体，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。具有众所周知的振动谱的材料，如氧化镁和氧化钙，也将被建模，以检查计算的振动特性是否合理和准确。在这个项目中产生的振动模型也将成为矿物学教学中使用的动画矿物振动数据库的核心。一个成功的对这些元素的研究项目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。该研究将对保存在矿物沉淀中的古气候记录的研究、寻找矿物和生物分子中强有力的生物特征、追踪海底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研究具有广泛的兴趣。拟议的研究也与不稳定同位素地球化学的基础研究有关，因为对比硫重的元素的自然同位素分馏所知甚少，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中的从头算技术，这些技术将广泛适用于硅、硫和锂等其他主要族元素的稳定同位素地球化学理论研究。PI非常有资格进行拟议的研究，之前已经模拟了铁、氯和铬在分子、晶体和水络合物中的稳定同位素分馏。更广泛的影响：该项目将通过支持论文研究来推进研究生教育。本科教育也将通过暑期实践研究机会得到加强。拟议的工作包括适应计算化学和材料科学的建模技术，并将部分支持计算机建模实验室，改善研究和跨学科交叉施肥的基础设施。矿物振动的在线数据库旨在推进矿物学的研究和教育，特别是在红外和拉曼光谱，热力学和传热，置换相变等领域。