Isotopic studies of Mg, Fe, Mo, Ru and W in Fremdlinge from Allende refractory inclusions

Abstract

The isotopic abundances of Mg, Fe, Mo, Ru and W in Fremdlinge and a refractory metal nugget in coarse-grained, Ca-Al-rich inclusions from the Allende meteorite have been determined using the PANURGE ion microprobe. These data represent the first measurement of the isotopic composition of Fremdlinge. No evidence for large, non-mass-dependent deviations (≳1%) from normal solar isotopic abundances was found for any of the elements measured. The only hint, of a nonlinear isotopic anomaly is for Mo in Fremdling 1 of Allende inclusion 5171; the measured ^(100)Mo/^(98)Mo ratio lies slightly more than two standard deviations above the value measured in standards. Isotopic variations due to mass-dependent fractionation are suggested for Mo in two Fremdlinge, Zelda of Egg 6 and Fremdling 1 of 5171, and for Ru in a refractory metal nugget in the FUN inclusion Cl. Mo is enriched in the lighter isotopes by ~ 4‰/amu and ~ 9‰/amu, respectively, while Ru is enriched in the heavier isotopes by ~ 4‰/amu. Despite their unusual mineralogy and extreme enrichment in refractory siderophile elements, Fremdlinge do not contain an enhanced component from a single nucleosynthetic source, but appear to contain the normal solar system mixture of є-, s-, and r-process material. While the possibility that some Fremdlinge may have formed under presolar conditions cannot be excluded, we believe that the chemical, petrologic and isotopic data are most plausibly interpreted in terms of the origin of Fremdlinge within the solar nebula.

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