HIGH PRECISION IRON ISOTOPIC ANALYZES OF METEORITES AND TERRESTRIAL ROCKS: Fe DISTRIBUTION AND MASS FRACTIONATION LAWS

Introduction: High precision iron isotopic analyzes have proven useful for investigating the distribution of Fe in the protoplanetary disk (Fe is a shortlived isotope that decays into Ni with t1/2=1.49 My) [1]. This stems from the fact that Fe, the most neutron-rich stable isotope of iron is produced together with Fe in AGB-stars and supernovae by neutron capture reactions. The lighter isotopes, Fe, Fe, and Fe are produced by other processes. For instance, in core-collapse supernovae, they are synthesized in more internal regions by nuclear statistical equilibrium. Heterogeneous distribution or late injection of Fe would therefore be accompanied by isotopic anomalies at Fe in bulk chondrites. Because of its very low abundance in nature (0.282 %), very few measurements of Fe have been published thus far [1-4]. Völkening and Papanastassiou [2] demonstrated that large Fe excesses, up to +290 ε, were present in FUN inclusions. More recently, Dauphas et al. [1] reported high precision measurements of Fe (±0.3 ε) in several groups of meteorites and found isotopic compositions identical to terrestrial. Here, we extend the database of high precision Fe measurements to more meteorites and terrestrial samples. High precision measurements can also be useful to investigate the slope of mass fractionation laws, which could convey clues on the mechanism responsible for such fractionation. Young et al. [5] showed that resolvable differences in slopes could be measured for Mg isotopes. We have also tested whether such differences could be measured for Fe by analyzing samples showing significant degrees of mass-dependent fractionations but formed in very different conditions (e.g., a granite and a banded-iron formation). Separation of Fe for isotopic analysis: The method that is routinely used at the University of Chicago has been described in a recent publication [6]. It however does not separate Fe from Cu and we have developed a more selective method that efficiently purifies Fe. It relies on a 10.5 cm long Teflon column (∅= 0.62 cm) filled with 3 mL of AG1-X8 resin. Iron is fixed on the column in >8 M HCl. The elutant containing Ni is saved for future isotopic analysis. Approximately 8 resin volumes (25 mL) of 4 M HCl is then passed through the column to eliminate Cu. Iron is finally eluted with 8 mL of 0.4 M HCl. The column separation procedure is repeated 3 times. Elution curves are shown in Fig. 1. Several geostandards as well as pure IRMM-014 were passed through this sequence of anion exchange chromatography and the measured iron isotopic compositions are indistinguishable from those reported in the literature [7]. This shows that no isotopic fractionation is introduced on the column and it also confirms that previous measurements carried out with a more rapid chemistry are accurate. Isotopic analyses were performed on a Neptune MC-ICPMS in high resolution mode to separate argide interferences from Fe isotopes. On some samples, the level of Ni interference on Fe was significant (up to 170 ε) but this can be very accurately corrected for by analyzing standards doped with Ni to correct for the fact that βNi≠ βFe. The measurements were replicated 20 times by standard bracketing and the uncertainties correspond to the 95 % confidence interval of the average.