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 ^(107)Ag from the decay of now extinct ^(107)Pd. 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 ^(107)Ag/^(109)Ag 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 ^(107)Ag∗ 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 ^(107)Ag∗.
Additional Information
© 1986 Pergamon Journals Ltd. Received April 22, 1985; accepted in revised form June 18, 1986. The samples studied here were obtained through the courtesy of Brian Mason, Carleton Moore and Vagn Buchwald. Their aid in providing these materials is greatly appreciated. The thorough and penetrating reviews by Vagn Buchwald and A. Kracher are greatly appreciated. The associate editor provided an extra goal for improvement with the most thorough review of all. We apologize for originally placing too much confidence in the metallurgical skills of both primitive and modern man (including the authors). Special thanks to Chris Price and Priscilla Piano who helped in the preparation of this manuscript. This work was supported by a grant from the National Aeronautics and Space Administration (NAG 9-43). Editorial handling: R. BrettAdditional details
- Eprint ID
- 42117
- Resolver ID
- CaltechAUTHORS:20131029-142330017
- NAG 9-43
- NASA
- Created
-
2013-10-29Created from EPrint's datestamp field
- Updated
-
2021-11-10Created from EPrint's last_modified field
- Other Numbering System Name
- Lunatic Asylum Lab
- Other Numbering System Identifier
- 473