Pt-Re-Os systematics of group IIAB and IIIAB iron meteorites

The Pt-Re-Os isotopic and elemental systematics of 13 group IIAB and 23 group IIIAB iron meteorites are examined. As has been noted previously for iron meteorite groups and experimental systems, solid metal-liquid metal bulk distribution coefficients (D values) for both IIAB and IIIAB systems show D_Os>D_Re>>D_Pt>1 during the initial stages of core crystallization. Assuming closed-system crystallization, the latter stages of crystallization for each core are generally characterized by D_Pt>D_Re>D_Os. The processes governing the concentrations of these elements are much more complex in the IIIAB core relative to the IIAB core. Several crystallization models utilizing different starting parameters and bulk distribution coefficients are considered for the Re-Os pair. Each model has flaws, but in general, the results suggest that the concentrations of these elements were dominated by equilibrium crystallization and subsequent interactions between solid metal and both equilibrium and evolved melts. Late additions of primitive metal to either core were likely minor or nonexistent.The ¹⁸⁷Re-¹⁸⁷Os systematics of the IIAB and IIIAB groups are consistent with generally closed-system behavior for both elements since the first several tens of Ma of the formation of the solar system, consistent with short-lived chronometers. The Re-Os isochron ages for the complete suites of IIAB and IIIAB irons are 4530 ± 50 Ma and 4517 ± 32 Ma, respectively, and are similar to previously reported Re-Os ages for the lower-Ni endmembers of these two groups. Both isochrons are consistent with, but do not require crystallization of the entire groups within 10-30 Ma of the initiation of crystallization.The first high-precision ¹⁹⁰Pt-¹⁸⁶Os isochrons for IIAB and IIIAB irons are presented. The Pt-Os isochron ages for the IIAB and IIIAB irons, calculated using the current best estimate of the λ for ¹⁹⁰Pt, are 4323 ± 80 Ma and 4325 ± 26 Ma respectively. The Re-Os and Pt-Os ages do not overlap within the uncertainties. The younger apparent ages recorded by the Pt-Os system likely reflect error in the ¹⁹⁰Pt decay constant. The slope from the Pt-Os isochron is combined with the age from the Re-Os isochron for the IIIAB irons to calculate a revised λ of 1.415 × 10⁻¹² a⁻¹ for ¹⁹⁰Pt, although additional study of this decay constant is still needed.