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Published August 1996 | public
Journal Article

Precise Re-Os determinations and systematics of iron meteorites


The Re-Os system for samples of FeNi, sulphide, and phosphide from iron meteorites was investigated. Techniques were developed which yield reproducible analyses for Re/Os at the 2‰ level and which permit complete isotopic exchange between sample and tracer, as is necessary for concentration measurements of Re and Os by isotope dilution. High precision osmium and rhenium isotope data have been obtained using negative ion thermal ionization, with ionization efficiencies of up to 10% for Os and 20% for Re, both for normals and for Re and Os extracted from the samples. Replicate analyses of Re/Os are in good agreement, within ±2.5‰. The results show a well defined correlation line on a ^(187)Re-^(187)Os evolution diagram for iron meteorites from groups IAB, IIAB, IIIAB, IVA, and IVB, all taken together. This correlation line yields a slope of 0.07863 ± 0.00031 (2σ) and initial ^(187)Os/^(188)Os = 0.09560 ∓ 0.00018 (2σ). If the individual groups of iron meteorites for which there is sufficient dispersion in Re/Os are considered, data on the IIAB and on the IVA irons appear to indicate a difference in age of 60 ± 45 Ma, with the IVA group being older-This age difference is qualitatively the same as obtained for Pd-Ag data but is larger. Sulphides from two IAB iron meteorites show extremely low concentrations of Re and Os and indicate that Re and Os are not partitioned into this phase during planetary differentiation. There is evidence for recent element remobilization or contamination, corresponding to relative enrichment of Re or loss of Os in the sulphides. Schreibersites contain small but significant amounts of Re and Os, with high Re/Os relative to the metal phases and with ^(187)Os/^(188)Os much more radiogenic than in the metal. Model ages for the schreibersites are relatively young (4.3-3.5 AE) and indicate that the schreibersites were open-systems for Re-Os at least 0.5-1 AE after the original formation of the iron meteorites, It now appears possible to use metal-schreibersite pairs to determine internal isochrons. Based on the schreibersite model ages, the cooling rates for the two IAB meteorites are estimated to be ~ 1°C/Ma, more than an order of magnitude lower than the most recently determined metallographic cooling rates for IAB irons (Herpfer et al., 1994).

Additional Information

© 1996 Elsevier Science Lid. Received 13 September 1995. Accepted 26 March 1996. We want to acknowledge the extensive contributions of R. A. Creaser to the early steps of the development of the analytical techniques. We thank W. R. Kelly for communicating his caution against explosions for the Carius tube technique. R. S. Clarke, Jr. and J. I. Goldstein have shared their insight and experience in schreibersite formation. We thank R. S. Clarke, Jr. who has identified and provided us with suitable samples of massive schreibersite, and E. J. Olsen and C. B. Moore for additional iron meteorite samples. J. H. Chen shared his knowledge of iron meteorites and their chemistry. H. H. Ngo provided the calibrations of the Re and Os tracers and applied his analytical skills to the measurement of additional schreibersites. With well-chosen words, M. Roy-Barman shared his experience with Os micro-distillation and A. D. Anbar contributed in the improvement of the Re analyses. Reviews by J. W. Morgan and H. Haack and detailed comments by D. W. Mittlefehldt are appreciated. This work was supported by NASA Grant NAGW-3337. This is Division Contribution No. 5543 (899).

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