Multiple nitrogen isotopic components coexisting in ureilites

Nitrogen and noble gas isotopic compositions and C abundance of ureilites were analyzed using a stepwise combustion technique. Four Antarctic ureilites, ALHA77257, Asuka 881931, Yamato 791538 and Yamato 790981 were analyzed. Multiple N isotopic components were observed in these ureilites. The delta(15)N values of these N components ranged from +160 to -120 parts per thousand. The minimum delta(15)N values of typically -120 parts per thousand were observed at combustion temperatures at 700-900 degrees C where large amounts of C were released. A heavy N component was observed in only two ureilites, ALHA77257 and Asuka 881931. Silicate-enriched fractions and C-concentrated fractions were prepared for these two ureilites. We conclude that both the light N and the heavy N are trapped in the carbonaceous vein minerals. The lack of correlation between the NIC ratio and the Ar-36/C ratio suggests that the primary carrier phase of the light N does not correspond to that of the planetary noble gases. We consider that the isotopically heavy N, which was observed in this study, is related to the heavy N observed among polymict ureilites. Small amounts (<0.5 ppm) of light N with the minimum delta(15)N value of-120 parts per thousand were observed among the silicate fractions at the highest combustion temperature of 1200 degrees C, although the exact carrier phase of this light N is not known. We consider that the currently observed ureilites were produced by injection of several volatile-rich objects into volatile-poor ureilitic silicates.