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

Abstract

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 studied 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. The contacts between metal and sulfide are sharp and the sulfide and metal have remained discrete homogeneous phases. These meteorites yield isochrons between metal and sulfide and it is concluded that no significant Ag redistribution occurred after their formation. The petrography of group IVA and IVB meteorites reveals a history involving extensive shock and thermal metamorphism. Contacts between metal and sulfide nodules are irregular and the two phases are intimately mixed. In Gibeon, unequilibrated α 2-martensitic structures in the metal matrix suggest that some reheating affected entire individual fragments of this meteorite. While bulk samples of the metal phases of the IVA and IVB meteorites appear well behaved with regard to Pd-Ag systematics, the sulfide-rich regions show extreme isotopic heterogeneities and do not yield well-defined isochrons. The Pd-Ag data reflect complicated postformation Ag transport. 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 ∗.