PRECISE RE-OS DETERMINATIONS AND SYSTEMATICS OF IRON-METEORITES

The Re-Os system for samples of FeNi, sulphide, and phosphide from iro n meteorites was investigated. Techniques were developed which yield r eproducible analyses for Re/Os at the 2 parts per thousand level and w hich permit complete isotopic exchange between sample and tracer, as i s necessary for concentration measurements of Re and Os by isotope dil ution. High precision osmium and rhenium isotope data have been obtain ed 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 O s extracted from the samples, Replicate analyses of Re/Os are in good agreement, within +/-2.5 parts per thousand. The results show a well d efined correlation line on a Re-187-Os-187 evolution diagram for iron meteorites from groups IAB, IIAB, IIIAB, IVA, and IVB, all taken toget her. This correlation line yields a slope of 0.07863 +/- 0.00031 (2 si gma) and initial Os-187/Os-188 = 0.09560 -/+ 0.00018 (2 sigma). If the individual groups of iron meteorites for which there is sufficient di spersion in Re/Os are considered, data on the IIAB and on the IVA iron s 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 ob tained for Pd-Ag data but is larger. Sulphides from two IAB iron meteo rites show extremely low concentrations of Re and Os and indicate that Re and Os are not partitioned into this phase during planetary differ entiation. There is evidence for recent element remobilization or cont amination, corresponding to relative enrichment of Re or loss of Os in the sulphides. Schreibersites contain small but significant amounts o f Re and Os, with high Re/Os relative to the metal phases and with Os- 187/Os-188 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 th e original formation of the iron meteorites, It now appears possible t o use metal-schreibersite pairs to determine internal isochrons. Based on the schreibersite model ages, the cooling rates for the two IAB me teorites are estimated to be similar to 1 degrees C/Ma, more than an o rder of magnitude lower than the most recently determined metallograph ic cooling rates for IAB irons (Herpfer et al., 1994).