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

Abstract

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‰/amu for Fe and −0.4‰/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 °C/Myr. Using a relative difference in diffusion coefficients of adjacent isotopes of 4‰/amu for Fe and Ni (β = 0.25), the observations made in Toluca can be reproduced for a cooling rate of 50 °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 β for diffusion of Fe and Ni in Fe-Ni alloy in the temperature range of 400-700 °C.

Experimental determination of equilibrium nickel isotope fractionation between metal and silicate from 500°C to 950°C

“…Error bars on individual D 62 Ni values are ±2se. Dauphas, 2007), and thus consistent with the interpretation that kinetic processes perturbed equilibrium in N20.…”

Magnesium and iron isotopes in 2.7 Ga Alexo komatiites: Mantle signatures, no evidence for Soret diffusion, and identification of diffusive transport in zoned olivine

“…However, this is partially compensated by the fact that the relative mass difference between 24 Mg and 26 Mg (8.3%) is a factor of 2 larger than that between 54 Fe and 56 Fe (3.7%). This is analogous to what was discussed in the context of Fe-Ni interdiffusion during formation of the Widmanstätten pattern in iron meteorites (Dauphas, 2007;Cook et al, 2007). The isotopic measurements made on olivine separated from M712 are qualitatively consistent with isotopic fractionation due to diffusive exchange of Mg and Fe in olivine (Fe shows large fractionation and is isotopically light while Mg shows no significant fractionation).…”

“…If it is associated with different partitioning of ferrous and ferric iron in olivine Dauphas et al, 2009a), this could potentially be used to estimate the oxygen fugacity of komatiite magmas during their crystallization. Kinetic isotope effects associated with diffusion-limited growth of spinifex crystals may also be present, in which case Fe and Mg isotopic fractionation could be used as crystal growth speedometers (Dauphas, 2007;Watson and Mü ller, 2009). In deeper parts of the flow, the expected timescale for cooling would have been sufficiently long that Fe and Mg could have diffused in and out of olivine (Arndt, 1986), which could have fractionated isotopes (Richter et al, 2008(Richter et al, , 2009a.…”

Mass‐dependent fractionation of nickel isotopes in meteoritic metal

“…Dauphas and Rouxel (2006) showed that phase growth in a diffusion-limited regime can create diffusive kinetic isotope fractionation between two phases and control the expression of equilibrium isotope fractionation between the co-existing phases. More recently, Dauphas (2007) developed a numerical simulation to model the formation of the Widmanstätten pattern during cooling of Fe-Ni alloy and to calculate the associated fractionation of Ni and Fe isotopes. During formation of the Widmanstätten pattern, nucleation of kamacite occurs in the solid state and its growth is controlled by diffusion (Wood 1964;Goldstein and Ogilvie 1965).…”