69th Annual Meteoritical Society Meeting (2006) 5263.pdf

KINETIC FRACTIONATION OF NICKEL AND IRON BETWEEN KAMACITE AND TAENITE: INSIGHTS INTO COOLING RATES OF IRON METEORITES. B. Bourdon1, G. Quitté1 and A.N. Halliday2. 1IGMR, ETH Zurich, Switzerland. E-mail:[email protected]. 2Dept. Earth Sciences, Oxford University, UK.

Introduction: Fractionation of stable isotopes provides new insights into the differentiation processes of iron meteorites; in particular, stable isotopes are generally sensitive to crystalliza- tion, cooling and phase exsolution. Most iron meteorites are characterized by an intergrowth of two Fe-Ni alloys, Ni-poor kamacite and Ni-rich taenite. Here, we present Ni isotopic data for a whole set of iron meteorites from different groups, for sul- phide inclusions and for coexisting taenite and kamacite. Results: It has been previously shown that iron meteorites display no anomaly in radiogenic 60Ni [1]: all isotopes can there- fore be used for studying mass dependent fractionation. All bulk metal phases show enrichment in heavy isotopes relative to the terrestrial standard, in good agreement with data reported by [2]. Variations of 0.3‰ per mass unit are observed. The range in Ni isotope composition is much wider for sulphides than for metals (2‰ per mass unit). In all magmatic iron meteorites, the sulphide yields a heavier Ni isotopic composition than the metal or a com- position similar to the metal. We also observed that the Ni-poor kamacite is isotopically heavier than the Ni-rich taenite. Opposite results are obtained for Fe isotopes [3, 4]. Interpretation and Model: During cooling, the growth of kamacite exsolved from taenite depends on the diffusion rate of Ni from kamacite to taenite. The rate limiting parameter is in fact the Ni diffusion coefficient in taenite and the light Ni isotope is enriched in the taenite. We modeled this process over the tem- perature range 700-350°C, with a temperature-dependent diffu- sion coefficient and a moving boundary between kamacite and taenite phases. Concentration profiles were calculated in the tae- nite for two Ni isotopes and the isotopic fractionation factor was then inferred. Input parameters for the model - such as the diffu- sion coefficient as a function of the Ni and P concentration, or the relative diffusion coefficients for two isotopes in the solid metal - are poorly constrained. However, we demonstrated that the precise cooling rate of the meteorite can be deduced from the isotopic fractionation measured between kamacite and taenite, once the diffusion coefficients are better determined. This kinetic process can also explain the Fe isotopic data: Fe isotopes are fractionated with an opposite sign relative to Ni because of the reverse diffusion fluxes. Such a diffusion-related mass dependent isotopic fractionation may also be responsible for the Ni frac- tionation between metal and sulphides. References: [1] Quitté G. et al. 2006. Earth and Planetary Science Letters 242:16-25. [2] Moynier F. et al. 2005. Abstract #1593.36th Lunar & Planetary Science Conference. [3] Poitrasson F. et al. 2005. Earth and Planetary Science Letters 234: 151-164 [4] Williams H. et al. 2006. Earth and Planetary Science Letters, in revision.