Diffusion-driven kinetic isotope effect of Fe and Ni during formation of the Widmanstätten pattern

Abstract

Iron meteorites show resolvable Fe and Ni isotopic fractionation between taenite and kamacite. For Toluca (IAB), the isotopic fractionations between the two phases are around +0.1 ppm/amu for Fe and -0.4 ppm/amu for Ni. These variations may be due to i) equilibrium fractionation, ii) differences in the diffusivities of the different isotopes, or iii) a combination of both processes. A computer algorithm was developed in order to follow the growth of kamacite out of taenite during the formation of the Widmanstätten pattern as well as calculate the fractionation of Fe and Ni isotopes for a set of cooling rates ranging from 25 to 500 degrees C/Myr. Using a relative difference in diffusion coefficients of adjacent isotopes of 4 ppm/amu for Fe and Ni (Beta = 0.25), the observations made in Toluca can be reproduced for a cooling rate of 50 degrees C/Myr. This value agrees with earlier cooling rate estimates based on Ni concentration profiles. This supports the idea that the fractionation measured for Fe and Ni in iron meteorites is driven by differences in diffusivities of isotopes. It also supports the validity of the value of 0.25 adopted for Beta for diffusion of Fe and Ni in Fe-Ni alloy in the temperature range of 400-700 degrees C.