Experimental Investigation of Non-Traditional Stable Isotope Fractionation in Geological Materials

组成岩石的不同元素的稳定同位素(即不经历放射性衰变的同位素)被用作地球质量再分配的示踪剂。本研究计划开发一种新的方法来准确测量许多重要成岩元素的稳定同位素比值，以提高我们对几个地质过程的认识。作为研究计划的一部分，它将确定使用铁和镁的同位素作为这些重要元素在不同地质储集层之间移动的示踪剂所需的基本参数。在过去的几十年里，地质材料中稳定同位素分馏的研究主要是研究较轻元素碳、氢、氧、氮和硫的同位素(其中，只有氧是主要的成岩元素)，而多收集器电感耦合等离子体质谱(MC-ICPMS)的出现使地球科学家能够研究铁、镁和硅等主要成岩元素的同位素分布。与此同时，计算化学的进步使人们有能力对具有地质意义的物质之间的同位素分配进行定量预测。在这项工作中，研究人员概述了一项利用这两项进展的研究计划。最终目标是确定铁和镁的同位素在具有地质意义的矿物中的平衡分布，并在实验室将这些分布与这些主要成岩元素在自然系统中的稳定同位素分布进行比较。研究人员打算通过实验来验证铁(Fe)和镁(Mg)同位素在矿物相中分馏的理论预测。铁和镁被选为研究的重点，因为这些成岩元素在许多地球化学和生物地球化学循环中具有重要意义。这项研究由三个相关方面组成：1)预测铁和镁同位素在不同矿物之间的分配方式；2)实施所谓的三同位素实验，旨在确定同位素在感兴趣的矿物中的平衡分配；以及3)高精度测量实验产品中的稳定同位素比率。这些实验包括让感兴趣的矿物在加热的“活塞筒”中承受高温和压力，直到达到同位素平衡。三同位素技术是用一种特定的同位素对样品进行人工浓缩(“添加”)，可以对实验产品达到平衡的程度进行定量评估。这个项目将利用地质科学中的几个子学科来加深我们对元素如何在地球深处移动的理解。一个好处是，与该项目相关的博士生接受了非常不寻常的跨学科培训。不同地球科学家群体成员之间的交流预计将对该领域产生持久的积极影响。该项目通过将这类群体(妇女、非裔美国人)的成员纳入研究团队来促进未被充分代表的群体参与科学。

Stable isotopes (i.e., isotopes that do not undergo radioactive decay) of different elements comprising rocks are used as tracers of redistribution of mass in Earth. The present study plans to exploit a new methodology for measuring accurately the stable isotope ratios of many important rock-forming elements to improve our knowledge of several geological processes. As part of the research plan, it is to establish the fundamental parameters necessary to use the isotopes of iron and magnesium as tracers of the movement of these important elements between different geological reservoirs. Whereas in past decades investigations of stable isotope fractionation in geological materials meant mainly studies of the isotopes of the lighter elements carbon, hydrogen, oxygen, nitrogen, and sulfur (of these, only oxygen is a major rock-forming element), the advent of multiple-collector inductively-coupled plasma-source mass spectrometry (MC-ICPMS) has allowed geoscientists to examine the distribution of isotopes of major rock-forming elements such as iron, magnesium, and silicon. At the same time, advances in computational chemistry have afforded the capacity to make quantitative predictions for the partitioning of isotopes between materials of geological interest. In this work, the investigators outline a research program that takes advantage of both of these advances. The ultimate goal is to determine equilibrium distribution of the isotopes of iron and magnesium in mineral materials of geological significance, and to compare these distributions in the laboratory to the distributions of the stable isotopes of these major rock-forming elements in natural systems. The investigators intend to test theoretical predictions of iron (Fe) and magnesium (Mg) isotope fractionation among mineral phases experimentally. Iron and Mg were selected as the focus of the study because of the importance of these rock-forming elements in many geochemical and biogeochemical cycles. The research is composed of three related facets: 1) prediction of the ways that Fe and Mg isotopes partition themselves between different minerals; 2) implementation of so-called three-isotope experiments designed to determine equilibrium isotope partitioning of isotopes among minerals of interest; and 3) high-precision measurement of stable isotope ratios in the experimental products. The experiments involve subjecting minerals of interest to high temperatures and pressures in a heated "piston cylinder" until isotopic equilibrium is achieved. The three isotope technique, by which samples are artificially enriched ("spiked") with a particular isotope, permits quantitative assessment of the degree to which the experimental products achieved equilibrium.This project will take advantage of several sub-disciplines in the geological sciences to further our understanding of how elements move deep within Earth. A benefit is the highly unusual interdisciplinary training afforded a Ph.D. student associated with the project. Communication between members of the disparate groups of geoscientists is expected to have a lasting positive impact on the field. The project promotes participation of underrepresented groups in the sciences by including members of such groups (women, African American) on the research team.