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

BratermanEAR-0106611在过去一两年里，过渡金属的同位素变化引起了人们的注意，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，在实验室中只测量到了铁的明显的生物产生的同位素分馏，这导致了这样的建议，即铁同位素可能是一个有用的“生物标志”，可以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体上生命的起源和演化。然而，由于只在低温环境中的流体、岩石和矿物中发现了显著的铁同位素变化(56Fe/54Fe中每毫升3-4个/mil[/])，因此无机或非生物铁同位素分馏可能至少部分解释了天然样品的铁同位素组成范围。无机或非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。D.W.Madison小组关于沉积岩中氧化物和碳酸盐矿物的数据，以及矿物-流体和Fe(II)-Fe(III)水相体系的初步实验结果表明，对于某些(但不是所有)体系，无机Fe同位素分馏在1-3量级，与少数情况下的理论计算结果相似，但也表明实验与理论之间存在显著差异。拟议的研究涉及一系列关于共存水相Fe物种之间、矿物与流体之间、共存矿物之间的Fe同位素分馏系数的实验测定。在所有拟议的实验中，将通过使用“三种同位素方法”或使用浓缩的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集中于确定共存的Fe(II)和Fe(III)络合物之间的平衡Fe同位素分馏作为温度、离子强度、pH和配体化学的函数，包括水、氯和氰化物络合物以及混合络合物。计划工作的另一个阶段将涉及在200-600摄氏度和1-20kbar的温度-压力范围内测定矿物-矿物和矿物-流体同位素分馏系数；在许多情况下，这项工作将涉及从同一生产产品中测定O和Fe同位素分馏系数，以便可以比较这两种元素的相对同位素交换率。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，考虑到这些矿物在低温岩石记录中的重要性，但我们的工作可能会根据结果扩展到其他矿物(如硫化物)。最终，天然矿物的氧和铁同位素联合分析可能会对同位素平衡的实现和它们沉淀的流体来源提供重要的相互核对。提案中概述了一些实验战略，这些战略旨在解决已知的实验问题，但其目的是足够灵活，以便可以根据初步结果尝试不同的方法。计划中的实验计划的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获得非常高的精度，在该仪器中，可以对非常少量(~100-300 ng)的铁的实际样品进行测定，外部精度为正负0.05/56Fe/54Fe比值。鉴于铁同位素地球化学在解决跨越几个学科的问题方面具有巨大的潜力，在铁同位素地球化学领域取得进展之前，拟议的铁同位素分馏系数的实验测定被认为是必不可少的。

BratermanEAR-0106611Isotopic 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 minus 0.05 / 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.