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

We have investigated the Re-Os system for samples of whole rock, metal, and sulfide from ordinary chondrites. Using closed-system analytical technique s, we found complete exchange between sample and tracer isotopes for silica te-containing samples and obtained precise and reliable Re-Os concentration measurements. Results on two Group IVA iron meteorites and on a silicate-r ich iron (Steinbach, IVA-AN) are consistent with the IVA-IVB isochron and s upport the previous observation that IVA-IVB irons may be slightly older th an IIAB irons. Data on whole-rock fragments and metal-rich separates from t he St. Severin chondrite (LL6) show a large range in Re-187/Os-188 and in O s-187/Os-188, which makes possible, in principle, the determination of a Re -Os internal isochron on a chondrite, for the first time. This Re-Os fracti onation may be due to partial melting of FeNiS and macroscopic redistributi on of metal and sulfide. The St. Severin data show a good correlation line on a Re-187-Os-187 evolution diagram. If this is considered to represent an internal isochron, it gives an age T = 4.68 +/- 0.15 AE [lambda(Re-187) = 1.64 x 10(-11) a(-1)] and an initial (Os-187/Os-188)(O) = 0.0953 +/- 0.0013 . This age is in agreement with but slightly older than the more precise Re -187-Os-187 age for the IIAB irons as well as for irons from other groups ( T = 4.61 +/- 0.01 AE). A St. Severin sulfide nodule has very low Re and Os concentrations and shows a young Re-Os model age (2.3 AE), indicating recen t element remobilization. Whole rock and metal-rich separates of H-Group ch ondrites (H3 to H6) yield restricted ranges in Re-187/Os-188 (0.42-0.47) an d Os-187/Os-188 (0.128-0.133). There is a systematic difference between Re/ Os in the metal extracted from a chondrite and the bulk chondrite. This sho ws that there is a small but significant Re-Os fractionation within subsyst ems contained in the chondrites. From whole rock samples of H Group chondri tes we calculate a mean Re-187/Os-188 = 0.423 +/- 0.007 and Os-187/Os-188 = 0.12863 +/- 0.00046, which may characterize the evolution of an average ch ondritic 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 b y the Re-Os fractionation due to fractional crystallization of liquid metal . Re-Os ages of iron meteorites give the time of crystallization of metal s egregations and cores of early planetary bodies. In contrast, the behavior in ordinary chondrites, while also dominated by the metal phases, must refl ect fractionation and transport on a local macroscopic scale within the cho ndrites between the metal phases after aggregation, due to partial melting of FeNiS or represent variable Re-Os fractionation of the metal phases prio r to the accretion of the chondrites. However, for St. Severin, we attribut e the major Re-Os fractionation to early heating of the meteorite, above th e Fe-FeS eutectic. We do not consider that the Re-Os fractionation observed in other chondrites is due to the redistribution of Re and Os during chond rite metamorphism (including shock) but it may plausibly represent earlier stages of Re-Os fractionation for the different FeNi metal constituents pri or to accretion. Copyright (C) 1998 Elsevier Science Ltd.