NITROGEN ISOTOPES IN BULK ORDINARY CHONDRITES

Abstract

Nitrogen isotopic compositions of twenty-one ordinary chondrites were measured using a stepwise combustion technique. We observed a good correlation between the abundances of cosmogenic 15N and cosmogenic 38Ar. We obtained a relationship between the abundances of these two cosmogenic nuclides as follows:15Nc(atoms/g) = (16.0 +/- 3.7).38Arc(atoms/g).-12 g-15N/g.myr, assuming the cosmogenic 38Ar production rate given by Eugster (1988). This value is consistent with the production rate on the lunar surface estimated by Becker et al. (1976). Isotopic ratios of trapped nitrogen in ordinary chondrites seem to be heterogeneous, among both unequilibrated and equilibrated meteorites. Heterogeneity among the latter are well observed in this study. The isotopic ratios of the trapped components seem to differ by more than 110%%, ranging from -18%%, observed in Guarena (H6), to +95%%, observed in Jilin (H5). A lack of correlation between nitrogen abundances and isotopic ratios indicates that differences in nitrogen isotopic ratios are not produced by mass-dependent isotopic fractionation. An inverse correlation between nitrogen abundances and the degree of thermal metamorphism is observed among most of the unequilibrated chondrites. Nitrogen abundances decrease from 8-25 p.p.m. to <1 p.p.m. with increasing petrographic subtype from 3.4-3.9. The nitrogen abundance in equilibrated chondrites is generally higher than the amount expected by extending the trend observed among unequilibrated ones. This is probably because contaminating nitrogen masked the trend at small amounts of trapped nitrogen.