IngentaConnect Nitrogen components in IAB/IIICD iron meteorites

Authors: Ponganis, K.V.; Marti, K.

Source: Meteoritics & Planetary Science, Volume 42, Issue 3, Pages 307-474 (March 2007) , pp. 331-346(16)

Publisher: Arizona Board of Regents (University of Arizona)

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, δ¹⁵N = −(74 ± 2)‰, at 900 °C to 1000 °C, with a possible second, even lighter, reservoir in Copiapo (δ¹⁵N ≤ −82‰). These releases are consistent with metal nitride decomposition or low-temperature metal phase changes. Heavier nitrogen reservoirs are observed in steps ≤700 °C and at 1200 °C to 1400 °C. The latter release has a δ¹⁵N signature with a limit of ≥−16‰. 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,γ) 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.