Search for Mass-independent Molybdenum Isotope Anomalies in Iron

Introduction: Primitive meteorites contain components such as presolar grains and some refractory inclusions that are isotopically different from the average solar system. These isotope anomalies are witness of an initial isotopic heterogeneity of the solar nebula. In spite of this initial isotopic heterogeneity, the isotope composition of larger planetary bodies in the solar system seems to be more or less uniform for most elements , suggesting efficient and large-scale mixing of the solar nebula prior to accretion of planetary bodies. However, Mo isotope studies of meteoritic and planetary materials reveal differences in the Mo isotope composition among different solar system objects. The various Mo isotopes are produced by different nucleosynthetic processes (92 Mo and 94 Mo are almost exclusively produced by the p-process, 96 Mo is a pure s-process and 100 Mo a pure r-process nuclide, 95 Mo, 97 Mo and 98 Mo are produced in the sand r-process) and it was thus suggested that the Mo isotope variations reflect incomplete mixing of gas and dust in the solar nebula. However, other Mo isotope studies could not find resolvable Mo isotope variations among different meteorite parent bodies and the reasons for these discrepancies are yet unclear. We developed new analytical techniques for precise Mo isotope measurements using MC-ICP-MS and present here our first results obtained for magmatic iron meteorites. Samples and analytical techniques: Samples investigated for this study are from the iron meteorite groups IIAB (North Chile, Negrillos), IIIAB (Cape York, Henbury) and IVA (Duell Hill). Samples were cleaned with abrasives, ultrasonicated in ethanol, and leached in dilute HNO 3. Then the iron meteorites were completely digested in 6M HCl. A small aliquot for the determination of Mo concentrations by isotope dilution was taken at this stage. The remaining solution was dried and Mo was separated from the matrix elements and interfering Ru and Zr by a combination of anion and cation exchange chromatographic methods. The separation procedure reduced the Ru/Mo ratio from initially ~1 to <10-5. Mo yields are >85% for all samples , as determined by isotope dilution measurements on a small aliquot of the purified Mo.