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

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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.