Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.h51c0810b&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #H51C-0810
Mathematics
Logic
1030 Geochemical Cycles (0330), 1041 Stable Isotope Geochemistry (0454, 4870), 1094 Instruments And Techniques
Scientific paper
Redox processes have played a pivotal role in shaping Earth's interior and surface and making life possible. A record of this evolution is found within stable isotope signatures arising from chemical redox changes occurring in our continents, oceans and atmosphere over time. Experimental and theoretical studies of redox- related transition metal isotope fractionation provide a physical basis to understand how isotopes are fractionated under natural conditions, relating geochemical signatures to earth processes from which they arise. Here we present experimental evidence that charge transfer processes drive the fractionation of stable isotopes of Fe and Zn, and the magnitude of fractionation can be tuned as a function of redox potential and other physical variables. To quantitatively evaluate isotopic signatures of redox processes, we have conducted electrochemical experiments measuring the fractionation of Fe and Zn isotopes during their electrodeposition from aqueous solution: M2+ + 2e- = M(s). The electrochemical cell consisted of anodic and cathodic half- cells separated by a salt bridge, and connected to a potentiostat which applied an adjustable constant voltage (vs. Ag/AgCl). Metallic Fe and Zn plated on glassy carbon electrodes was recovered in acid for analysis of the stable isotope composition on a Thermo-Finnigan Neptune MC-ICP-MS. Results are reported as a large delta difference between the isotopic composition (56Fe/54Fe and 66Zn/64Zn) of plated metal relative to the stock solution. The results show some clear trends; fractionation is a function of applied voltage (overpotential: η = E- E0 Volts), ranging from Δ56Fe ~ -4 to -0.9 ‰, and Δ66Zn ~ - 5.5 to -4 ‰ at η = -0.5 to -1.25 V and -0.1 to -0.5 V, respectively. Temperature affects fractionation in a counter-intuitive manner, with fractionation increasing with increasing temperature (at η = -1.0 V: Δ56Fe ~ -1.25 ‰ at 0°C and -1.62 ‰ at 35°C). The results can be explained in terms of two end-member processes: an electrochemically controlled regime which creates larger fractionations and a mass-transport/diffusion limited regime where smaller fractionations are expected [Rodushkin et al., 2004, Anal. Chem. 76: 2148]. To test whether charge transfer processes are responsible for the larger fractionations observed a series of experiments were run using a rotating disc electrode. In these experiments mass-transport properties near the electrode surface are controlled by the electrode's rotation rate (ω). Results show that at low η = -0.5 V a constant fractionation factor (Δ56Fe ~ -3.75 ‰) is obtained at ω > 2000 rpm. Our research has shown that electrochemical experiments can be used to separately study charge transfer and mass-transport processes that affect stable isotope fractionation. Charge transfer reactions produce a large fractionation of Fe and Zn isotopes in comparison to mass-transport/diffusion controlled processes. It is intriguing to note that the range in δ56Fe observed in these experimental systems nearly encompasses those seen in the geologic record [Anbar and Rouxel, 2007, Annu. Rev. Earth Planet. Sci. 35: 717]. Further studies may help link these isotopic signatures to past redox conditions and processes.
Black Jay R.
John Sajeev
Kavner Abby
Young Edward D.
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