Thomas Mueller^a,b,*, E. Bruce Watson^a, Dustin Trail^a,c, Michael Wiedenbeck^d, James Van Orman^e and Erik H. Hauri^f

^aNew York Center for Astrobiology, Rensselaer Polytechnic Institute, 110 8[th] Street, Troy, NY 12180, USA
^bInstitut für Geologie, Mineralogie & Geophysik, Ruhr-Universität Bochum, D-44801 Bochum, Germany
^cDepartment of Earth and Environmental Sciences, University of Rochester, Rochester NY, 14627.
^dHelmholtz Zentrum Potsdam, GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany
^eDept. of Earth, Environmental and Planetary Sciences, Case Western University, Cleveland, USA
^fCarnegie Institution of Washington, Washington, DC 20015, USA

Carbon is an abundant element of planets and meteorites whose isotopes provide unique insights into both organic and inorganic geochemical processes. The identities of carbonaceous phases and their textural and isotopic characters shed light on dynamical processes in modern Earth systems and the evolution of the early solar system. In meteorites and their parent bodies, reduced carbon is often associated with Fe-Ni alloys, so knowledge of the mechanisms that fractionate C isotopes in such phases is crucial for deciphering the isotopic record of planetary materials. Here we present the results of a diffusion-couple experiment in which cylinders of polycrystalline Fe containing 11,500 and 150 μg/g of C were juxtaposed at 1273K and 1.5 GPa for a duration of 36 min. Diffusion profiles of total C concentration and ¹³C/¹²C were measured by secondary ion mass spectrometry (SIMS). The elemental diffusivity extracted from the data is ∼3.0 × 10^-11 m²s^-1, where ¹³C/¹²C was observed to change significantly along the diffusion profile, reflecting a higher diffusivity of ¹²C relative to ¹³C. The maximum isotopic fractionation along the diffusion profile is ∼30-40‰. The relative diffusivities (D) of the carbon isotopes can be related to their masses (M) by D_13C/D_12C = (M_12C/M_13C)^β; the exponent β calculated from our data has a value of 0.225±0.025. Similarly high β values for diffusion of other elements in metals have been taken as an indication of interstitial diffusion, so our results are consistent with C diffusion in Fe by an interstitial mechanism. The high β-value reported here means that significant fractionation of carbon isotopes in nature may arise via diffusion in Fe(-Ni) metal, which is an abundant component of planetary interiors and meteorites.

Reference
Mueller T, Watson EB, Trail D, Wiedenbeck M, Orman JV and Hauri EH (in press) Diffusive fractionation of carbon isotopes in γ-Fe: experiment, models and implications for early solar system processes. Geochimica et Cosmochimica Acta
[doi:10.1002/2013JE004426]
Copyright Elsevier

Link to Article