UNDERSTANDING VARIOUS CONTRIBUTIONS TO THE CHROMIUM ISOTOPIC COMPOSITION OF METEORITES, AND THEIR IMPLICATIONS FOR Mn-Cr CHRONOLOGY

Introduction: Previous studies have shown that 54 Cr is heterogeneously distributed in Solar System materials, including chondrites, differentiated stony and iron meteorites [1]. Each meteorite group has its characteristic 54 Cr signature. Even though the 54 Cr anomalies are widely thought to be nucleosynthetic in origin, the nucleosynthetic process responsible for its production and the cause of the heterogeneous distribution of these nucleosynthetic products remain mysterious. No correlation with other iron-group elements has yet been found, except possibly with 46 Ti and 50 Ti [2]. The carrier phases of the 54 Cr anomalies have never been isolated or identified. Even though CAIs in car-bonaceous chondrites (CC) have high 54 Cr [3,4], the low abundance of CAIs in CCs and their low Cr concentration means that they cannot account for the anomaly observed in the bulk meteorites. Step-wise acid dissolution experiments have revealed that there are both 54 Cr-poor and 54 Cr-rich carrier phases [5,6]. The former is soluble in relatively weak acid and the latter is more acid-resistant. In our recent work, we attempted to concentrate 54 Cr-rich phases in insoluble organic matter (IOM) rich residues through CsF digestion of the sample, the same technique used to concentrate pre-solar grains. Large positive 54 Cr anomalies up to 200 ε were found in the hot HCl leachates of the IOM of relatively primitive CCs [7]. IOM of primitive ordinary chondrites (OC) also show positive 54 Cr anomalies up to 40 ε in their leachates, in contrast to the small negative 54 Cr anomalies (~-0.4 ε) in the bulk rocks. Another concern is whether the existence of 54 Cr anomalies compromise the use of the short-lived chro-nometer 53 Mn-53 Cr. Correlated excesses in 53 Cr and 54 Cr have been found for CCs [8]. The HCl-leachates of IOMs of chondrites, however, show very small negative 53 Cr anomalies of 2 ε or less in spite of large 54 Cr excesses [7] suggesting that the 54 Cr anomalies do not significantly affect 53 Cr abundances. The Cr isotopic compositions of some meteorites, or fractions thereof, also may have been affected by spallation of target elements (e.g., Fe) during their exposure to ga-lactic cosmic rays. Spallation can produce both 53 Cr and 54 Cr, and further complicate the Cr systematics in meteorites. In this study, we have reexamined the Cr isotope systematics in various types of bulk meteorites. We also took a closer look …