Light rare earth element enrichments in ureilites: A detailed ion microprobe study

This paper explores the possible origin of the light rare earth element (LR EE) enrichments observed in some ureilites, a question that has both petrog enetic and chronologic implications for this group of achondritic meteorite s. Rare earth element and other selected elemental abundances were measured in situ in 14 thin sections representing 11 different ureilites. The spati al microdistributions of REEs in C-rich matrix areas of the three ureilites with the most striking V-shaped whole-rock REE patterns (Kenna, Goalpara, and Novo Urei) were investigated using the ion imaging capability of the io n microprobe. All olivines and clinopyroxenes measured have LREE-depleted patterns with l ittle variation in REE abundances, despite large differences in their major element compositions from ureilite to ureilite. Furthermore, we searched f or but did not find any minor mineral phases that carry LREEs. The only exc eption is one Ti-rich area (similar to 20 mu m) in Lewis Cliff (LEW) 85400 with a major element composition similar to that of titanite; REE abundance s in this area are high, ranging from La approximate to 400 x CI to Lu appr oximate to 40 x CI. In contrast, all ion microprobe analyses of C-rich matr ix in Kenna, Goalpara, and Novo Urei revealed large LREE enrichments. In ad dition, C-rich matrix areas in the three polymict ureilites, Elephant Morai ne (EET) 83309, EET 87720, and North Haig, which have less pronounced V-sha ped whole-rock REE patterns, show smaller but distinct LREE-enrichments. Th e C-rich matrix in Antarctic ureilites tends to have much lower LREE concen trations than the matrix in non-Antarctic ureilites. There is no obvious as sociation of the LREEs with other major or minor elements in the C-rich are as. Ion images further show that the LREE enrichments are homogeneously dis tributed on a microscale in most C-rich matrix areas of Kenna, Goalpara, an d Novo Urei. These observations suggest that the LREEs in ureilites most pr obably are absorbed on the surface of fine-grained amorphous graphite in th e C-rich matrix. It is unlikely that the LREE enrichments are due to shock melts or are the products of metasomatism on the ureilite parent body. We f avor LREE introduction by terrestrial contamination.