Microdistributions and petrogenetic implications of rare earth elements inpolymict ureilites

Polymict ureilites contain various mineral and lithic clasts not observed i n monomict ureilites, including plagioclase, enstatite, feldspathic melt cl asts and dark inclusions. This paper investigates the microdistributions an d petrogenetic implications of rare earth elements (REES) in three polymict ureilites (Elephant Moraine (EET) 83309, EET 87720 and North Haig), focusi ng particularly on the mineral and lithic clasts not found in monomict urei lites. As in monomict ureilites, olivine and pyroxene are the major heavy (H)REE c arriers in polymict ureilites. They have light (L)REE-depleted patterns wit h little variation in REE abundances, despite large differences in major el ement compositions. The textural and REE characteristics of feldspathic melt clasts in the thre e polymict ureilites indicate that they are most likely shocked melt that s ampled the basaltic components associated with ureilites on their parent bo dy. Simple REE modeling shows that the most common melt clasts in polymict ureilites can be produced by 20-30% partial melting of chondritic material, leaving behind a ureilitic residue. The plagioclase clasts, as well as som e of the high-Ca pyroxene grains, probably represent plagioclase-pyroxene r ock types on the ureilite parent body. However, the variety of REE patterns in both plagioclase and melt clasts cannot be the result of a single igneo us differentiation event. Multiple processes, probably including shock melt ing and different sources, are required to account for all the REE characte ristics observed in lithic and mineral clasts. The C-rich matrix in polymict ureilites is LREE-enriched, like that in mono mict ureilites. The occurrence of Cc anomalies in C-rich matrix, dark inclu sions and the presence of the hydration product, iddingsite, imply signific ant terrestrial weathering. A search for Mg-26 excesses, from the radioactive decay of Al-26, in the po lymict ureilite EET 83309 was negative.