| Summary: | We measured the contents and isotopic compositions for C, N, and noble gases in the diamond fractions separated in a heavy liquid (ρ = 2.9 g/cm3) from a sample enriched with diamond from the Novo Urei ureilite. The results show that the concentrations of nitrogen and noble gases in the diamond fraction isolated from the supernatant (the fraction is named DNU-1) are more than a factor of 1.5 higher than those in the diamond fraction from the residue (DNU-2). This difference is probably caused by smaller sizes of grains and (or) clusters of smaller grains as well as by larger defectiveness of the crystal lattice of the diamond in the DNU-1 fraction as compared to DNU-2. Both fractions are similar in the isotopic composition of C and N and in the ratios of trapped chemical elements. The results obtained and the published data concerning C, N, and noble gases in different fractions of other ureilites allow us to conclude the following. (1) The ureilite diamond was most likely formed from graphite and the fine-grained crystalline (or semiamorphous) carbonaceous phase as a result of shock transformation in the parent bodies. (2) The negative result in the search for the isotopically light component of nitrogen (δ 15 N is about –100) in the Antarctic unshocked ureilite ALH 78019 (Rai et al. , 2002), which introduced serious difficulties for explaining the origin of the ureilite diamond in the parent bodies during the impact, is most likely caused by the absorption of atmospheric nitrogen by the carbonaceous material in the processes of terrestrial weathering. (3) The source of light nitrogen (δ 15 N~–100) in the ureilite diamond was probably the presolar diamond in the initial carbonaceous material of the ureilite parent bodies, because the impurity elements, including nitrogen (δ 15 N < –350), in this diamond could be trapped in the magmatic processes by the carbonaceous material, which became a precursor of the ureilite diamond in the shock event.
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