A comparative petrologic study of iron meteorites with 107Ag* anomalies

We have made a petrographic study of the iron meteorites: Derrick Peak (IIB), Cape York (IIIA), Grant (IIIB), Gibeon (IVA) and Santa Clara (IVB). Each of these meteorites has been reported to contain excess 107Ag from the decay of now extinct 107Pd. Isotopic data on metal and sulfide phases for groups IIB and IIIAB iron meteorites have been explained using conventional interpretations which show a correlation between 107Ag/109Ag and Pd/Ag while data on sulfide nodules in meteorites from groups IVA and IVB are very complex. We have studied the extent to which shock and thermal events may have caused the redistribution of Ag and disturbed the original relationship between 107Ag* and Pd, particularly in sulfide-rich regions. The petrography of the iron meteorites studies is consistent with the Pd-Ag isotopic data. The textures observed in the metal and sulfide in groups IIA and IIIAB were found to be indicative of shock but not extensive thermal metamorphism. This petrographic study has found that the complexity of Pd-Ag isotopic data is related to the shock and thermal histories of the iron meteorites. However, no self-consistent explanation has been found to explain the Pd-Ag results observed for the nodules. The mineral phase(s) in which Ag resides in both the metal and sulfide phases is as yet undetermined, though late-forming Cu-precipitates and chalcopyrite found in IIIAB irons are possible getters of volatiles (Ag, Au) and may be host to 107Ag*. (author)