RHENIUM-OSMIUM CONCENTRATION AND ISOTOPE SYSTEMATICS IN GROUP IIAB IRON-METEORITES

Rhenium and osmium abundances, and osmium isotopic compositions were measured by negative thermal ionization mass spectrometry in thirty samples, including replicates, of five IIA and eight IIB iron meteorites. Concentrations in IIA irons range from 4800 ppb Re and 66000 ppb Os (Negrillos) to 160 ppb Re and 800 ppb Os (Lombard). In the IIB subgroup, concentrations vary from 28 ppb Re and 180 ppb Os (Navajo) down to 0.8 ppb Re and 9 ppb Os (Sao Juliao de Moreira and Santa Luzia). Log plots of Os vs. Re abundances for IIA and IIB irons describe straight lines that approximately converge on Lombard, which has the lowest Re and Os abundances and highest Re-187/Os-188 measured in a IIA iron to date. The linear IIA trend may be exactly reproduced by fractional crystallization with constant k(Re) and k(Os) but is not well fitted using variable partition coefficients. The IIB iron trend, however, cannot be entirely explained by simple fractional crystallization. One explanation is that small amounts of Re and Os were added to the asteroid core during the final stages of crystallization. Another possibility is that diffusional enrichment of Os may have occurred in samples most depleted in Re and Os. The combined Re-Os isotopic data for IIA irons give the following results: slope = 0.07803 +/- 0.00076; intercept = 0.09609 +/- 0.00045; age = 4584 +/- 43 Ma (neglecting the uncertainty in the decay constant of +/-3%). Four IIB iron meteorites (Mount Joy, Central Missouri, DRPA 78009, Santa Luzia) also plot within the analytical uncertainty of the IIA isochron. These results are consistent with rapid (probably <50 Ma) core segregation, differentiation, and crystallization in the IIAB parent. Several IIB irons (Navajo, Sandia Mountains, Smithsonian Iron, and perhaps Sao Juliao de Moreira) he beyond analytical uncertainty above the IIA iron isochron, averaging 8 +/- 2 parts per thousand higher in Os-187/Os-188. These irons may have crystallized significantly after the IIA irons and Mount Joy, but only if the Re-187/Os-188 Of the melt was greater than or equal to 2.2. There is no evidence for a IIA iron crystallizing in equilibrium with a melt having such a high ratio. Alternatively, the osmium isotopic systematics of these irons may have been slightly disturbed long after crystallization at ca. 3.3 Ga ago.