Collaborative Research: Experimental Determination of Abiologic and Inorganic Iron and Oxygen Isotope Fractionation

过渡金属的同位素变化在过去一两年中引起了人们的注意，这在很大程度上是因为这些元素可能通过生物过程进行同位素分馏。到目前为止，在实验室中只对铁进行了明确的生物产生的同位素分异，这导致了铁同位素可能是一种有用的“生物特征”的提议，可用于追踪生物在现代和古代环境中的作用，并可能有助于理解地球或其他行星上生命的起源和进化。然而，由于显著的铁同位素变化（在56Fe/54Fe中3-4 / mil[/]）只在低温环境中的流体、岩石和矿物中发现，无机或生物铁同位素分馏可能至少部分解释了自然样品中测量到的铁同位素组成的范围。无机或生物铁同位素分馏的支持来自实验数据和理论计算，涉及矿物和流体系统。来自uw Madison小组关于沉积岩中的氧化物和碳酸盐矿物的数据，以及矿物-流体和铁(II)-铁（III）水系统的初步实验结果表明，在一些（但不是全部）系统中，无机铁同位素的分值为1-3 /，与少数情况下的理论计算结果相似，但也指出了实验与理论之间的重大差异。拟议的研究涉及一套广泛的实验确定铁同位素分馏因子共存的水铁物种之间，矿物和流体之间，以及共存的矿物之间。在所有提出的实验中，将通过使用“三同位素方法”或使用富集同位素示踪剂来区分动力学和平衡同位素分馏。拟议研究的一个阶段将侧重于确定共存的Fe（II）和Fe（III）配合物之间的平衡Fe同位素分异，作为温度，离子强度，pH和配体化学的函数，包括水，氯和氰化物配合物，以及混合配合物。计划工作的另一个阶段将包括确定T-P范围内200-600摄氏度和1-20千巴的矿物-矿物和矿物-流体同位素分馏因子；在许多情况下，这项工作将涉及从同一运行产品中确定O和Fe同位素分馏因子，以便比较这两种元素的相对同位素交换率。考虑到这些矿物在低温岩石记录中的重要性，最初的目标是确定菱铁矿、磁铁矿、赤铁矿和流体之间的同位素分馏，但我们的工作可能会扩展到其他矿物（如硫化物），这取决于结果。最终，天然矿物的O和Fe同位素联合分析可以为同位素平衡的实现和它们沉淀的流体来源提供重要的交叉检查。提案中概述了一些实验策略，这些策略旨在解决已知的实验问题，但其目的是足够灵活，以便根据初步结果尝试不同的方法。计划实验方案可行性的一个重要组成部分是非常高的精度，现在可以使用威斯康星大学麦迪逊分校的新仪器来实现，在那里，铁同位素组成可以在非常小（~ 100-300 ng）的铁量上，以56Fe/54Fe的比例测定真实样品的外部精度为±0.05 /。考虑到铁同位素地球化学在解决跨越多个学科的问题方面的巨大潜力，在铁同位素地球化学领域向前发展之前，铁同位素分馏因子的实验确定被认为是必不可少的。

Johnson and BeardEAR-0106614Isotopic variations of transition metals have attracted attention in the last year or two, in large part because such elements may be isotopically fractionated by biological processes. So far, clear biologically produced isotopic fractionation has been measured in the laboratory only for Fe, and this led to the proposal that Fe isotopes may be a useful "biosignature" that may be applied to tracing the role of organisms in modern and ancient environments, and may help in understanding the origin and evolution of life on Earth or other planetary bodies. However, because significant Fe isotope variations (3-4 per mil [/] in 56Fe/54Fe) are found only in fluids, rocks, and minerals from low temperature environments, it is possible that inorganic or abiologic Fe isotope fractionation may at least in part explain the range of Fe isotope compositions measured for natural samples. Support for inorganic or abiologic Fe isotope fractionation comes from experimental data and theoretical calculations, involving both mineral and fluid systems. Data from the U.W. Madison group on oxide and carbonate minerals from sedimentary rocks, as well as initial experimental results from mineral-fluid and Fe(II)-Fe(III) aqueous systems, indicate inorganic Fe isotope fractionations on the order of 1-3 / for some (but not all) systems, similar to those calculated from theory for a few cases, but also pointing to significant discrepancies between experiments and theory.The proposed research involves an extensive set of experimental determinations of Fe isotope fractionation factors between coexisting aqueous Fe species, between minerals and fluids, and between coexisting minerals. In all of the proposed experiments, distinction between kinetic and equilibrium isotope fractionations will be accomplished through use of the "three isotope method" or use of enriched isotope tracers. One phase of the proposed research will focus on determining the equilibrium Fe isotope fractionation between coexisting Fe(II) and Fe(III) complexes as a function of temperature, ionic strength, pH, and ligand chemistry, including aquo, chloro, and cyanide complexes, as well as mixed complexes. Another phase of the planned work will involve determining mineral-mineral and mineral-fluid isotope fractionation factors over the T-P range 200-600 degrees C and 1-20 kbar; in many cases, this work will involve determination of both O and Fe isotope fractionation factors from the same run products so that the relative isotopic exchange rates of these two elements may be compared. Initially, the goal will be to determine isotopic fractionations between siderite, magnetite, hematite, and fluid, given the importance of these minerals in the low-temperature rock record, but our work may be extended to other minerals (such as sulfides) depending upon the results. Ultimately, combined O and Fe isotope analysis of natural minerals may provide important cross-checks of attainment of isotopic equilibrium and the sources of fluids from which they precipitated. A number of experimental strategies are outlined in the proposal, which are aimed at addressing known experimental issues, but are intended to be flexible enough so that different approaches may be tried depending upon initial results. An important component to the viability of the planned experimental program is the very high precision that is now attainable using new instrumentation at U.W. Madison, where Fe isotope compositions may be determined on real samples to an external precision of plus or minus0.05 / in 56Fe/54Fe ratios on very small (~ 100-300 ng) quantities of Fe. Given the great potential that Fe isotope geochemistry has for addressing problems that cut across several disciplines, the proposed experimental determinations of Fe isotope fractionation factors are viewed as essential before the field of Fe isotope geochemistry can move forward.