Nitrogen components in IAB/IIICD iron meteorites

Abstract

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, δ 15 N = −(74 ± 2)‰, at 900 °C to 1000 °C, with a possible second, even lighter, reservoir in Copiapo (δ 15 N ≤ −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 δ 15 N 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 131 Xe and 129 Xe 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 129 I 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.

A very early origin of isotopically distinct nitrogen in inner Solar System protoplanets

“…We compiled abundances and isotopic ratios of N in iron meteorites reported in literature (Supplementary Table 1, Extended Data Fig. 1) 19,20,[36][37][38][39][40][41][42][43][44][45] . N abundances and isotopic ratios have…”

“…For each iron meteorite group,  15 N falls in a narrow range, while N contents may vary over two orders of magnitude. Lack of any relationship between  15 N values and N abundances within any given group of iron meteorites argues against significant mass-dependent fractionation of N isotopes via volatility-related losses during planetary processing 19,20,43 .  15 N values for a given group of iron meteorites represents the mean of the measurements calculated from the data in Supplementary Table 1.…”

Noble gases and nitrogen in Raghunathpura (IIAB) and Nyaung (IIIAB) iron meteorites

“…In the case of nonmagmatic irons (IAB and IIICD), the N isotopic heterogeneity observed was explained as due to intact/partial survival of light N from presolar grains from the parent objects (prior to metal/silicate segregation) since its incorporation into metal (Ponganis and Marti ). The observed preservation of organic matter in processed meteorites (Busemann et al.…”

A comprehensive study of noble gases and nitrogen in “Hypatia”, a diamond-rich pebble from SW Egypt

“…5), C-rich samples of iron meteorites appear to contain nitrogen with quite heterogeneous δ 15 N values. For example, isotopically very light nitrogen with δ 15 N < -82 ‰ has been reported in Copiapo (IAB) (Ponganis and Marti, 2007). Notably, radiogenic 129 Xe as well as trapped noble gases similar to Q have also been found in graphite nodules of Canyon Diablo (IAB) (Matsuda et al, 2005).…”