MULTIPLE NITROGEN ISOTOPIC COMPONENTS COEXISTING IN UREILITES

Nitrogen and noble gas isotopic compositions and C abundance of ureili tes were analyzed using a stepwise combustion technique. Four Antarcti c ureilites, ALHA77257, Asuka 881931, Yamato 791538 and Yamato 790981 were analyzed. Multiple N isotopic components were observed in these u reilites. The delta(15)N values of these N components ranged from +160 to -120 parts per thousand. The minimum delta(15)N values of typicall y -120 parts per thousand were observed at combustion temperatures at 700-900 degrees C where large amounts of C were released. A heavy N co mponent was observed in only two ureilites, ALHA77257 and Asuka 881931 . Silicate-enriched fractions and C-concentrated fractions were prepar ed 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 cor relation 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 t he planetary noble gases. We consider that the isotopically heavy N, w hich was observed in this study, is related to the heavy N observed am ong polymict ureilites. Small amounts (<0.5 ppm) of light N with the m inimum delta(15)N value of-120 parts per thousand were observed among the silicate fractions at the highest combustion temperature of 1200 d egrees C, although the exact carrier phase of this light N is not know n. We consider that the currently observed ureilites were produced by injection of several volatile-rich objects into volatile-poor ureiliti c silicates.