H-N budgets and isotopic signatures of Oued Chebeika 002 and CI reservoir heterogeneity

We report the hydrogen (H) and nitrogen (N) elemental and isotopic compositions of the recently recovered CI chondrite Oued Chebeika 002 (OC 002) and compare them with published data for other CI meteorites and CI-like samples returned from the asteroids Ryugu and Bennu. When measured using the same protocol (vacuum degassing at 120 °C for 48 hr), OC 002 exhibits similar H concentrations but lighter isotopic compositions than other CIs and Bennu samples. Nitrogen data likewise indicate that the parent body of OC 002 accreted isotopically lighter nitrogen. Compilation of available data reveals pronounced H-N isotopic heterogeneities among CI-like materials, even when analysed under comparable conditions. These observations suggest that CI chondrites are intrinsically heterogeneous and that a single representative CI isotopic composition cannot be defined. This variability likely reflects the late accretion history of CIs and the incorporation of organic grains and ammonia ice that experienced diverse thermal histories in the protosolar disk. Such intrinsic heterogeneity strongly affects mass balance estimates of CI contributions to Earth’s surface volatiles, potentially varying by more than a factor of two depending on the CI reference used.