ABSTRACT

I report here on a comprehensive ion microprobe study of Mn-Cr systematics of six pallasites. Four pallasites show excesses of radiogenic ⁵³Cr (⁵³Cr*), which correlate well with the Mn/Cr ratios. For these pallasites, the inferred initial (⁵³Mn/⁵⁵Mn)_o ratios are ~1 × 10^-5, which indicates that pallasites formed very early in the solar system. Eagle Station and Imilac show hints of ⁵³Cr*, but the excesses are not resolved from terrestrial standards. Comparisons of Mn-Cr results with those of Pd-Ag indicate that these two short-lived radionuclide chronometers are self-consistent in pallasites. Inferred (⁵³Mn/⁵⁵Mn)_o ratios in this study are generally higher than those previously observed by TIMS (1-3 × 10^-6). It is not clear what mechanism caused such a discrepancy. Nevertheless, it should be noted that ion probe depends on the zoning profiles to give high Mn/Cr ratios at the edges of the olivine crystals. Thus, it dates the closure time of diffusive fractionation of Mn and Cr in pallasite olivines.