Re-Os systematics in chondrites and the fractionation of the platinum group elements in the early solar system

• Published in: Geochimica et cosmochimica acta 1998-10, Vol.62 (19), p.3379-3392
• Main Authors: Chen, J.H, Papanastassiou, D.A, Wasserburg, G.J
• Format: Article
• Language: English
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/S0016-7037(98)00238-5

We have investigated the Re-Os system for samples of whole rock, metal, and sulfide from ordinary chondrites. Using closed-system analytical techniques, we found complete exchange between sample and tracer isotopes for silicate-containing samples and obtained precise and reliable Re-Os concentration measurements. Results on two Group IVA iron meteorites and on a silicate-rich iron (Steinbach, IVA-AN) are consistent with the IVA-IVB isochron and support the previous observation that IVA-IVB irons may be slightly older than IIAB irons. Data on whole-rock fragments and metal-rich separates from the St. Séverin chondrite (LL6) show a large range in 187Re/ 188Os and in 187Os/ 188Os, which makes possible, in principle, the determination of a Re-Os internal isochron on a chondrite, for the first time. This Re-Os fractionation may be due to partial melting of FeNiS and macroscopic redistribution of metal and sulfide. The St. Séverin data show a good correlation line on a 187Re- 187Os evolution diagram. If this is considered to represent an internal isochron, it gives an age T = 4.68 ± 0.15 AE [λ( 187Re) = 1.64 × 10 −11 a −1] and an initial ( 187Os/ 188Os) 0 = 0.0953 ± 0.0013. This age is in agreement with but slightly older than the more precise 187Re- 187Os age for the IIAB irons as well as for irons from other groups (T = 4.61 ± 0.01 AE). A St. Séverin sulfide nodule has very low Re and Os concentrations and shows a young Re-Os model age (2.3 AE), indicating recent element remobilization. Whole rock and metal-rich separates of H-Group chondrites (H3 to H6) yield restricted ranges in 187Re/ 188Os (0.42–0.47) and 187Os/ 188Os (0.128–0.133). There is a systematic difference between Re/Os in the metal extracted from a chondrite and the bulk chondrite. This shows that there is a small but significant Re-Os fractionation within subsystems contained in the chondrites. From whole rock samples of H Group chondrites we calculate a mean 187Re/ 188Os = 0.423 ± 0.007 and 187Os/ 188Os = 0.12863 ± 0.00046, which may characterize the evolution of an average chondritic reservoir for Re-Os. The ordinary chondrite data plot close to the IIAB isochron, although the deviations found are larger than found for the irons. The Re-Os chronometer in iron meteorites is apparently controlled by the Re-Os fractionation due to fractional crystallization of liquid metal. Re-Os ages of iron meteorites give the time of crystallization of metal segregations and cores of early planetary bodies. In