ABSTRACT

In order to understand the metamorphic history of the parent bodies of unequilibrated ordinary chondrites (UOC's), we compare REE distributions in the HCI/HNO₃ (acid)-residues of seven Antarctic UOC' s with those of equilibrated ordinary chondrites (EOC's). REE parameters designated to characterize the abundance pattern of REE's—normalized La abundance, normalized La/Lu abundance ratio and the degree of Eu anomaly—were found to be highly variable in the acid-residues of UOC's, suggesting that REE's were redistributed on UOC parent bodies. In contrast, the REE parameters are fairly constant in the acid-residues of EOC's, suggesting that REE distribution attained equilibrium on EOC parent bodies. The REE parameters for the acid-residues of UOC's are poorly correlated with the In, Zn and Se contents in the whole rock samples analyzed in this study, but appear to correlate with carbon content. This implies that REE parameters can be used as indices for the subclassification of UOC's. From the viewpoint of REE distribution, ALH 77011 (and its paired ALH 78038) appears to be one of the least metamorphosed UOC's.