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

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布的示踪剂。 本研究计划开发一种新的方法来精确测量许多重要的成岩元素的稳定同位素比值，以提高我们对几种地质作用的认识。 作为研究计划的一部分，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层之间运动的示踪剂所需的基本参数。 过去几十年来，对地质材料稳定同位素分馏的研究主要是对较轻元素碳、氢、氧、氮和硫的同位素的研究（其中，只有氧是主要的岩石形成元素），多接收电感耦合等离子体源质谱仪出现（MC-ICPMS）使地球科学家能够检查主要成岩元素（如铁、镁和硅）的同位素分布。 与此同时，计算化学的进步使人们有能力对同位素在具有地质意义的材料之间的分配进行定量预测。 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。 最终目标是确定具有地质意义的矿物材料中铁和镁同位素的平衡分布，并将这些分布在实验室中与这些主要造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测试矿物相之间铁（Fe）和镁（Mg）同位素分馏的理论预测。 铁和镁是成岩元素中重要的地球化学和地球化学循环，因此被选为研究的重点。 该研究由三个相关方面组成：1）预测Fe和Mg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，旨在确定感兴趣的矿物之间同位素的平衡同位素分配; 3）高精度测量实验产品中的稳定同位素比值。 这些实验涉及将感兴趣的矿物置于加热的“活塞缸”中，直到达到同位素平衡。 三种同位素技术，通过这种技术，样品被人为地加浓（“掺加”）某种同位素，从而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分支学科，进一步了解元素如何在地球深处移动。 一个好处是，博士学位提供了极不寻常的跨学科培训。与项目相关的学生。 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。 该项目通过吸收代表性不足的群体成员（妇女、非裔美国人）参加研究小组，促进这些群体参与科学。

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.