Nitrogen components in IAB/IIICD iron meteorites

Abstract

Isotopic variations have been reported for many elements in iron meteorites, with distinct N signatures found in the metal and graphite of IAB irons. In this study, a dozen IAB/IIICD iron meteorites (see Table 1 for new classifications) were analyzed by stepwise pyrolysis to resolve nitrogen components. Although isotopic heterogeneity has been presumed to be lost in thermally processed parent objects, the high-resolution nitrogen isotopic data indicate otherwise. At least one reservoir has a light nitrogen signature, delta-15N = -(74 +/- 2) ppm, at 900 degrees C to 1000 degrees C, with a possible second, even lighter, reservoir in Copiapo (delta-15N is less than or equal to -82 ppm). These releases are consistent with metal nitride decomposition or low-temperature metal phase changes. Heavier nitrogen reservoires are observed in steps less than or equal to 700 degrees C and at 1200 degrees C to 1400 degrees C. The latter release has a delta-15N signature with a limit of greater than or equal to -16 ppm. Xenon isotopic signatures are sensitive indicators for the presence of inclusions because of the very low abundances of Xe in metal. The combined high-temperature release shows 131Xe and 129Xe excesses to be consistent with shifts expected for Te(n,gamma) reaction in troilite by epithermal neutrons, but there are also possible alterations in the isotopic ratios likely due to extinct 129I and cosmic-ray spallation. The IAB/IIICD iron data imply that at least one light N component survived the formation processes of iron parent objects which only partially exchanged nitrogen between phases. Preservation of separate N reservoirs conflicts with neither the model of impact-heating effects for these meteorites nor reported age differences between metal and silicates.