| Research Abstract |
The concentrations of ReOs 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.Average Re/Os and Re/Ir rations 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 cho
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ndrites 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, respecitively, at the same time.
Inductively coupled plasma mass spectormetry (ICP-MS) was successfully applied to the bulk sample of Allende, Jilin, Modoc, Saint-Severin and Atlanta for the determination of rare earth elements (REE) (Y and 14 lanthanoids), Th and U.The results of ICP-MS showed a good agreement with recommended values and their reproducibilities were high enough to discuss the detailed abundances of lanthanoids and actinoids in chondritic meteorites. For the Allende reference sample issued by the Smithsonian Institution, a positive anomaly of Tm, a fractionation between light REE and heavy REE and a high Th/U ratio were observed in the CI-normalized abundances of REE,Th and U.These features are common for group II inclusions in Allende, suggesting that the abundances of refractory lithophiles in Allende are somewhat influenced by those in a specific constituent. For the other chondritic meteorites, the zigzag change was commonly observed in the heavy REE region of their CI-normalized abundance patterns. It is suggested that such a zigzag pattern is attributable to erratically high abundances of mono-isotopic REE (Tb, Ho and Tm) for CI values. Less
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