Isotopic signatures and origin of nitrogen in IIE and IVA iron meteorites

Nitrogen concentrations and isotopic signatures have been determined in gro ups IIE and IVA iron meteorites. Contrary to assumptions made in the litera ture, the present data show that spallation components significantly modify the N signatures of the metal. All N-15 data are corrected for cosmic-ray produced spallation components using Ne-21 concentrations measured in aliqu ots. A production-rate ratio Ne-21/N-15 = 0.80 is obtained, which can relia bly be used for this correction, since it is not sensitive to shielding dif ferences. The trapped N signatures in group IVA irons fall into two subgrou ps IVA (-26) with delta(15)N = -26 +/- 2 parts per thousand and TVA (-6) wi th delta(15)N = -6 +/- 1.4 parts per thousand, respectively. Only the latte r is close to values reported for metal of L-chondrites. Group IIE irons al so define two distinct subgroups IIE (Y) with delta(15)N = -7.5 +/- 1.5 par ts per thousand and IIE (O) with delta(15)N = -2.3 +/- 1 parts per thousand . Therefore, the earlier proposed subdivision of IIE irons into "young" and "old" subgroups is substantiated by the different trapped N signatures. Wi th regard to a possible relationship with H-chondrites as suggested by oxyg en isotopes, only the young IIE subgroup overlaps the range of signatures r eported in H-chondrite metal. Seymchan has a distinctly lighter signature ( delta(15)N = -54 parts per thousand), consistent with its reclassification as an ungrouped iron. We observe no correlation of either nitrogen concentr ations or isotopic signatures with abundances of Ga, Ge, Tr or Ni. The dist inct N components in the so-called magmatic group TVA constrain the thermal history of the parent body, as these signatures need to be reconciled with the magmatic history. Implications for the origin of nitrogen components a re discussed. Some of the spread in N isotopic data in the literature appar ently is due to inclusions. Copyright (C) 2000 Elsevier Science Ltd.