RE, OS AND IR IN ANTARCTIC UNEQUILIBRATED ORDINARY CHONDRITES AND IMPLICATIONS FOR THE SOLAR-SYSTEM ABUNDANCE OF RE

The concentrations of Re, Os and Ir in Antarctic unequilibrated ordinary chondrites (UOC) were determined by radiochemical neutron activation analysis. Large fractionations among these elements were found in Antarctic UOC. This is quite different from non-Antarctic UOC and equilibrated ordinary chondrites (EOC). Average Re/Os and Re/Ir ratios in ordinary chondrites are 25-30 % higher than those in C1 chondrites and 10-15 % higher than those in C2 and C3 chondrites. In contrast, the Os/Ir ratio is not variable among these chondrite groups. It is therefore suggested that Re is fractionated among ordinary and carbonaceous chondrites. Atom abundances of Os and Ir relative to 10(6) Si atoms of H chondrites are nearly equal to those of C1 chondrites, but their Re value is 30 % higher than the C1 value. Considering that the currently accepted value for solar system abundance of Re (0.517 atom relative to 10(6) Si atoms) [Anders and Grevesse, 1989] is the lowest value among (Si-normalized) Re abundances in C1, C2, C3 and H chondrites, its value should be changed to a larger value. If C2 chondrites are adopted as the solar system standard, the solar system abundance of Re is changed to 0.0589, with those of Os and Ir being 0.740 and 0.685, respectively, at the same time.