Origin of Nitrogen Isotopic Variations in the Rocky Bodies of the Solar System

Abstract

Noncarbonaceous (NC; inner solar system) meteorites have lower ¹⁵N/¹⁴N ratios than carbonaceous (CC; outer solar system) meteorites. Whether this is evidence of a primordial heterogeneity of N reservoirs in the protosolar disk remains unclear. In this study, I consider the N isotopic compositions of meteorite (chondrite, achondrite, and iron meteorite) parent bodies as a function of their growth zones. Despite the ¹⁵N/¹⁴N ratios of CC meteorites being generally higher than NC meteorites, there is a substantial overlap between them. Late-stage mixing of isotopically distinct reservoirs cannot explain this overlap. ¹⁵N/¹⁴N ratios of meteorites, independent of the growth zones, are correlated with the accretion ages of their parent bodies. A common correlation of the ¹⁵N/¹⁴N ratios of NC and CC chondrites with their peak metamorphic temperatures suggests that N isotopic compositions of meteorites were likely set by a universal time-dependent process—thermal evolution of their parent bodies by radiogenic heating. Therefore, heterogeneous N isotopic compositions of meteorites do not allude to isotopically heterogeneous primitive N reservoirs in the protosolar disk. Rather, it is likely that the N isotopic compositions of meteorites are a direct reflection of a differential response of labile ¹⁵N-rich and refractory ¹⁵N-poor components in common organic precursors to variable degrees of parent body processing. Consequently, the isotopic ratios of N, and other highly volatile elements like C and H, in meteorites do not reflect the isotopic compositions of primitive volatile reservoirs in the protosolar disk and thus cannot be used independently to cosmolocate volatile reservoirs in the disk.