The H, C and N abundances and isotopic compositions of samples with outer Solar System heritage: asteroid Bennu and the Tarda and Tagish Lake meteorites
Dionysis I. Foustoukos, Conel M. O’D. Alexander, George D. Cody, Jennifer C. Stern, Steven M. Bates, Yoshihiro Furukawa, Toshiki Koga, Daniel P. Glavin, Jason P. Dworkin, Jessica J. Barnes, Ann N. Nguyen, Harold C. Connolly Jr., Dante S. Lauretta
Geochimica et Cosmochimica Acta
In Press, Journal Pre-proof, Available online 27 May 2026
“We present an integrated analysis of the bulk elemental and isotopic compositions of H, C, and N in samples of diverse types returned from near-Earth asteroid (101995) Bennu collected by the OSIRIS-REx spacecraft (including homogenized aggregate mixtures and splits from individual angular, hummocky, and mottled particles). We also conducted elemental, isotopic and Raman spectroscopic analyses of extracted insoluble organic matter (IOM). In addition, we quantified the fraction of total C residing in carbonate minerals and measured their C and O isotopic compositions within the aggregate samples. Aggregate samples and extracted IOM from the carbonaceous chondrites Murchison and Tarda, as well as homogenized samples from the Tagish Lake (11L) meteorite, underwent parallel analyses for comparison with the Bennu materials.
In the Bennu samples, the bulk H, C and N concentrations are 0.76–1.02 wt%, 3.54–4.70 wt% and 0.145–0.250 wt%, respectively. The bulk δ13C and δ15N of the samples ranges from −5.5 ‰ to 7.2 ‰ and from 43.6 ‰ to 109.5 ‰, respectively. The δD values for the homogenized aggregate, and the angular and hummocky particles ranged from 244 ‰ to 351 ‰. The observed δD variability among the subsamples of a mottled particle (350 ‰ to 641 ‰) may be attributed to evaporative processes. The C in carbonate minerals accounts for ∼ 0.6 wt% of the aggregate sample with δ13C of 72 ‰ and δ18O of 30 ‰. Five IOM residues extracted from Bennu aggregate material exhibit an average H content of 4.0 ± 0.2 wt%, C content of 68.0 ± 2.5 wt% and N content of 2.9 ± 0.1 wt%. The corresponding average isotopic compositions are δD = 1111 ± 30 ‰, δ13C = −15.7 ± 1.0 ‰ and δ15N = 47.2 ± 0.8 ‰. The elemental and isotopic compositions of H, C and N in the bulk Bennu and IOM samples generally support a genetic relationship between Bennu, Ryugu and other primitive carbonaceous chondrites, including the CIs, CMs, and the C2-ung Tagish Lake and Tarda. Based on the measured N/C and H/C atomic ratios, Bennu’s IOM may have undergone a low degree of thermal alteration similar to that experienced by CIs, CMs, Tagish Lake 5b and Tarda. This alteration could have occurred in an aqueous environment, enabling the evolution of N/C and H/C atomic ratios to approximate those found in type-2 carbonaceous chondrites. Similar conclusions are supported for the evolution of the IOM δD assuming H exchange between IOM and H2O under hydrothermal conditions. Furthermore, Raman vibrational spectra suggest that the Bennu IOM, akin to Murchison and Tarda IOM, is characterized by a higher degree of disorder than in heated CMs. Laboratory analyses of H, C and N in the returned Bennu samples confirm the major compositional inferences made from spacecraft-based spectra and reconcile Bennu’s spectral heterogeneity with true lithologic variability, demonstrating that the asteroid’s surface diversity reflects genuine compositional/mineralogical heterogeneity rather than observational artifacts.”
