HR: 0800h
AN: PP31B-1538    [Abstracts]
TI: Mechanisms of Iron Isotope Fractionation During Electrodeposition
AU: * Shahar, A
EM: ashahar@ess.ucla.edu
AF: University of California Los Angeles, Earth and Space Sciences Department, Los Angeles, CA 90095 United States
AU: Kavner, A
EM: akavner@ucla.edu
AF: University of California Los Angeles, Earth and Space Sciences Department, Los Angeles, CA 90095 United States
AU: Kavner, A
EM: akavner@ucla.edu
AF: Institute for Geophysics and Planetary Physics, UCLA, Los Angeles, Ca 90095 United States
AU: Young, E
EM: edyoung@ucla.edu
AF: University of California Los Angeles, Earth and Space Sciences Department, Los Angeles, CA 90095 United States
AU: Young, E
EM: edyoung@ucla.edu
AF: Institute for Geophysics and Planetary Physics, UCLA, Los Angeles, Ca 90095 United States
AB: Variations in Fe isotope ratios provide important information about the redox chemistry of geochemical and biogeochemical systems. Interpretation of Fe stable isotope signatures requires an in-depth understanding of the physical mechanisms underlying fractionation of ⁵⁷Fe, ⁵⁶Fe, and ⁵⁴ Fe. Several studies have shown a connection between electrochemical reactions and Fe isotope fractionation, including a relationship between Fe isotope fractionation and applied voltage in an Fe electrodeposition reaction (Kavner et al., Geochim. Cosmochim. Acta, 69, 2971-2979, 2005). Results of this previous study suggested that electron transfer may be isotope selective. Here we present results from a follow-up study designed to ascertain the individual roles of both charge transfer and mass transport in fractionating Fe isotopes at an electrode, and to test specific predictions arising from an evaluation of the isotope specificity of charge transfer. We performed a series of Fe electrodeposition experiments using an Autolab PGStat30 composed of glassy carbon working electrodes and an Ag/AgCl reference electrode. Several parameters were varied from experiment to experiment, including the concentration of the FeCl₂-HCl plating bath (0.025 to 2M), the total coulombic charge passed in the electroplating experiment (20 to 80 Coulombs), and the plating potential (-0.9 to - 2.0V). The isotopic compositions of the initial Fe in solution and the plated Fe were obtained using a ThermoelectronNeptune MC-ICPMS operated at a mass resolving power of ~ 14000. Instrumental mass fractionation was compensated for using sample/standard comparisons and results were referenced to the IRMM-014 Fe isotope standard. The outcome was a plot of fractionation versus voltage. The result is in basic agreement with an activated complex mechanism for isotope fractionation during charge transfer. Preliminary data show that the slope of fractionation versus voltage changes sign as the plating solution is diluted. For a 2M FeCl₂ solution, the electroplated Fe becomes increasingly enriched in lighter isotopes as the potential increases from -0.9 to -2.0 V. Conversely, for plating from a 0.025M FeCl₂ solution, the electroplated Fe becomes progressively enriched in heavier isotopes as the potential increases from -1.25 to -1.75V. However, the wide range of fractionations observed (the y-intercept) within the same concentration of solution (δ⁵⁶Fe values between 0 and -4.0 ‰) suggest that other mechanisms, such as mass transport, may also contribute to the observed fractionations.
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4870 Stable isotopes (0454, 1041)
SC: Paleoceanography and Paleoclimatology [PP]
MN: Fall Meeting 2005