Rhenium-osmium isotope systematics in meteorites I: Magmatic iron meteorite groups IIAB and IIIAB

Using resonance ionization mass spectrometry (RIMS), Re and Os abundances were determined by isotope dilution (ID) and ¹⁸⁷Os/¹⁸⁶Os ratios measured in nineteen iron meteorites: eight from group IIAB, ten from group IIIAB, and Treysa (IIIB anomalous). Abundances range from 1.4 to 4800 ppb Re, and from 13 to 65000 ppb Os, and generally agree well with previous ID and neutron activation (NAA) results. The Re and Os data suggest that abundance trends in these iron groups may be entirely explained by fractional crystallization. Addition of late-formed metal to produce ReOs variation in the B subgroups is not essential but cannot be excluded. Whole-rock isochrons for the IIAB and IIIAB groups are statistically indistinguishable. Pooled data yield an initial ¹⁸⁷Os/¹⁸⁶Os of 0.794 ± 0.010, with a slope of (7.92 ± 0.20) × 10⁻² corresponding to a magmatic iron meteorite age of 4.65 ± 0.11 Ga (using a decay constant of 1.64 × 10⁻¹¹ a⁻¹). Given the errors in the slope and half life, this age does not differ significantly from the canonical chondrite age of 4.56 Ga, but could be as young as 4.46 Ga.