Tracing early disk substructures and accretionary relationships through nitrogen isotopes in iron meteoritesOPEN ACCESS
Evelyn Füri, Fridolin Spitzer, Julie Gamblin, Christoph Burkhardt, Béatrice Luais, Julien Boulliung, Laurent Zimmermann
Geochimica et Cosmochimica Acta
Available online 2 June 2026
“Iron meteorites provide key insights into the formation conditions of planetesimals in the early protoplanetary disk. To investigate the sources of nitrogen (N) in non-carbonaceous (NC) and carbonaceous (CC) iron meteorite parent bodies—and to trace the spatiotemporal N isotopic heterogeneity in the inner and outer disk—we present combined N–Ne–Ar isotopic data for 14 irons from the under-studied IIAB-IIG and IID groups, and 14 ungrouped irons of either NC or CC heritage. These measurements, obtained via CO2 laser heating and multi-collection noble gas mass spectrometry, allow us to exclude contributions from solar gases and cosmogenic 15N. New data for two N-rich IIG irons confirm a genetic link with the IIAB group, supporting the view that NC magmatic iron meteorite groups are characterized by low δ15N values (–90 to –80 ‰). These results suggest that NC iron meteorite parent bodies accreted a smaller proportion of 15N-rich (organic) carriers than later-formed chondritic bodies, possibly due to thermal processing at the tar line. Two NC ungrouped magmatic irons have markedly higher δ15N values (–2.0 ‰ and + 9.1 ‰); however, their Ge-depleted compositions raise uncertainty as to whether these signatures reflect primary, parent-body-specific characteristics or result from secondary volatile loss. In contrast, CC magmatic irons, including the IID group and CC ungrouped irons, predominantly record positive δ15N values. Among these, the N isotopic compositions of three CC ungrouped irons—New Baltimore (+104 ‰), Hammond (+112 ‰), and La Caille (+206 ‰)—resemble those of IIC irons and Renazzo-type (CR) carbonaceous chondrites, reinforcing their genetic link to the IIC/CR reservoir, where ammonia ice was likely an important N carrier. This implies that an isotopically CR-like, ammonia‑rich reservoir was established in the outer protoplanetary disk during the earliest stages of its evolution and persisted for several million years. Taken together, these findings indicate that the primary N isotopic composition of differentiated and undifferentiated planetesimals was governed by the local mixture of isotopically distinct N-bearing phases, itself shaped by evolving condensation and evaporation fronts in the early protoplanetary disk”
