NITROGEN ISOTOPIC COMPOSITION OF CK CHONDRITES

Nitrogen abundances and isotopic compositions of four CK chondrites (ALH85002, EET92002, Yamato6903 and Karoonda) were measured by a stepped-combustion method. Neon and Ar were also measured for the same samples. Two types of isotopicaly light N were observed. One of them is labile N released at low temperatures (similar to 300 degrees C). This N is observed only in ALH85002. The other N is extracted at high temperatures (900-1200 degrees C) from all CK chondrites; although, the isotopic compositions are somewhat variable. There is a fair correlation between the excess N-15 values and the abundance of trapped Ar-36 for the high-temperature component, suggesting presolar origin of these species. The light N (delta(15)N = -106.8 parts per thousand) observed in Karoonda is one of the lightest N components ever reported for bulk chondrites.