Iron Isotope Fractionation at High Temperatures

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Most observed Fe isotope variations are believed to result from low temperature processes, (e.g., Beard et al., 1999; Anbar et al., 2000; Zhu et al., 2000). At high temperatures, the magnitude of equilibrium fractionation factors decreases. Beard et al. (2003) observed no variation between igneous rocks beyond their precision of ± 0.1 ppm (2SD), respectively. In contrast, Zhu et al. (2002), Poitrasson et al. (2002) and Williams et al. (2002) reported Fe isotope variations in the sub-per mil level between various mineral phases in the mantle or different igneous rocks from the Earth and other planetary bodies. The work of Zhu et al. (2002), Poitrasson et al. (2002) and Williams et al. (2002) employed the use of desolvation nebulization to minimize ArO^+, ArOH+ and ArN^+ interferences, and sample-standard bracketing assuming identical instrument mass bias between samples and standards. We are assessing this question using a high mass resolution MC-ICPMS with the technique of Weyer et al., submitted. With this technique molecular interferences are adequately resolved. Samples can be measured with both wet or dry plasma conditions to assess potential matrix effects. Increased precision for rock samples is achieved using Cu for mass bias correction. An external precision of 0.1 ppm (2SD) can be achieved for δ56Fe. In preliminary measurements of Fe from mineral separates from different terrestrial igneous rocks, mesosiderites and pallasites all samples fall within a range of ± 0.2 ppm, with slightly positive (0.1-0.2 ppm) δ56Fe compared to IRMM014 in average. Further results and interpretations will be presented. References: Anbar, A.D., et al., (2000) Science 288, 126. Beard, B.L., et al. (1999) Science 285, 1889. Beard, B.L. et al. (2003) Chem. Geol. , special issue, in press. Poitrasson, F., et al., (2002) Geochim. Cosmochim. Acta Suppl. 66, A608. Weyer, S. and Schwieters, J. B. (2002) submitted to Int. J. Mass Spectr. Williams, H., et al., (2002) Geochim. Cosmochim. Acta Suppl. 66, A838. Zhu X. K., et al., (2002) Earth Planet. Sci. Let. 6220, 1.

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