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Problems with Re-Os isochron determinationsWe have developed closed-system techniques for the equilibration of Os isotopes and for the calibration of Re and Os tracers using high-purity metals. The results show reproducibility of +/- 0.5% for calibrations of Os using different chunks of the same high-purity Os metal produced by vacuum arc melting. On a Re-Os evolution diagram the analyses of Negrillos and of Tocopilla are displaced along horizontal lines and are not consistent with a well-defined isochron. We conclude that is unclear whether whole-rock samples of irons of individual classes of iron meteorites (including the IIA class) define isochrons.In light of these observations, one must consider the evidence for phases in which Re and Os may become redistributed. We note that the IIA irons are composed of large or single kamacite crystals and do not contain taenite. This may hinder redistribution of Re and Os and may allow the IIA irons to define isochrons in preference to other magmatic iron classes. However, the IIA irons also include ubiquitous rhabdites, up to 20 mm in length, and troilite-daubreelite nodules. It has been observed that rhabdites are extremely rich in Pt-group elements. In addition to these considerations, Negrillos and Tocopilla were found in nitrate deposits and show evidence of corrosion. Therefore, it is possible that different samples of these meteorites have been differentially weathered, although our samples appeared fresh. We conclude that it is important to identify the phases in which Re and the PGEs are located and to address in detail the evidence for Re and Os remobilization.
Document ID
19950042210
Acquisition Source
Legacy CDMS
Document Type
Reprint (Version printed in journal)
Authors
Papanastassiou, D. A.
(California Institute of Technology, Pasadena, CA US, United States)
Ngo, H. H.
(California Institute of Technology, Pasadena, CA US, United States)
Wasserburg, G. J.
(California Institute of Technology, Pasadena, CA US, United States)
Date Acquired
August 16, 2013
Publication Date
July 1, 1994
Publication Information
Publication: Meteoritics
Volume: 29
Issue: 4
ISSN: 0026-1114
Subject Category
Lunar And Planetary Exploration
Accession Number
95A73809
Funding Number(s)
CONTRACT_GRANT: NAGW-3337
Distribution Limits
Public
Copyright
Other

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