¹Lisa Maria Eckart, ¹Henner Busemann, ¹Daniela Krietsch, ¹Cornelia Mertens, ¹Colin Maden, ²¹Conel M. O’D. Alexander, ^3,4Kevin Righter
Geochimica et Cosmocimica Acta )(in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2025.04.021]
¹Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse 25, 8092 Zurich, Switzerland
²Carnegie Institution for Science, 5251 Broad Branch Rd NW, Washington, DC 20015, United States
³NASA Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, United States
⁴Department of Earth and Environmental Sciences, University of Rochester, 227 Hutchison Hall, Rochester, NY 14627, United States
Copyright Elsevier

Carbonaceous chondrites of the Ornans type (COs) include some of the most primitive meteorites known to date, yet most of them show evidence of having experienced mild degrees of thermal alteration in their parent asteroid. Previous studies on aqueously altered CM, CR, and CY chondrites have shown that the noble gases trapped in various components with distinct susceptibility to alteration can be used to assess the extent of parent body processing. In this study, we investigated the noble gas compositions of 51 CO chondrites ranging from petrologic type 3.0 to 3.8, three suspected Mighei-type chondrites (CMs; MIL 090073, DOM 10121, DOM 10299) initially classified as COs, and DOM 10900 with intermediate properties between CMs and COs. The COs show a noble gas mixture typical for carbonaceous chondrites, deriving from primordial carriers such as presolar grains, phase Q, and the carrier of the Ar-rich/V component, which has been observed in anhydrous chondrites, and occasionally air. Additionally, the newly identified W component could be present, which is highly susceptible to water. Combining our results with CO noble gas data from the literature, we show that the 20Netr/132Xecorr ratios correlate best (decrease) with the degree of thermal alteration, likely related to the abundance of presolar diamonds, and may thus serve as tool to subclassify thermally altered chondrites. Based on its 20Netr/132Xecorr ratio, DOM 10847 (paired with DOM 08006) is the most primitive CO, followed by NWA 13464 and Y-74135. The 20Netr/132Xecorr subclassification tool, however, may not be applicable for intergroup comparisons as the stability of the responsible carriers are sensitive to the chemical environment of the parent body. The abundances of heavy noble gases in bulk CO samples are much higher compared to CO etch residues (remaining after demineralization of a bulk meteorite) from the literature, indicating that other carrier(s) than insoluble organic matter must contribute significantly to the heavy noble gas inventory, which is/are susceptible to thermal alteration. DaG 331 was subclassified in this work to be a CO3.1 using the method by Grossmann and Brearley (2005) and the trend line defined by Davidson et al. (2019).
The COs show a wide range in cosmic ray exposure (CRE) ages between 0.17 ± 0.05 Ma (Isna) and 78 Ma (maximum age determined for Dominion Range [DOM] 18286 with an uncertainty of < 10 %), although the majority of CRE ages are > 10 Ma. DOM 18286 has the highest CRE age reported to date for a carbonaceous chondrite. We did not find any age clusters hinting at a major impact event, nor a correlation between CRE ages and the petrologic types. Strikingly, none of the 63 COs analyzed for their noble gases (including literature) contains solar wind, indicating that this group stems from below the regolith surface layer. The COs and CMs show similar matrix-corrected primordial noble gas abundances, suggesting that they accreted their volatiles from a common reservoir.

¹Nathan Asset, ¹Marc Chaussidon, ²Christian Koeberl, ³Johan Villeneuve, ⁴François Robert
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Access [https://doi.org/10.1016/j.gca.2025.05.011]
¹Université Paris-Cité, Institut de Physique du Globe de Paris, CNRS, F-75005 Paris, France
²Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria
³Université de Lorraine, CNRS, CRPG, UMR 7358, F-54000 Nancy, France
⁴Institut Origine et Evolution, Muséum National d’Histoire Naturelle, Sorbonne Université, IMPMC-UMR 7590 CNRS, 75005Paris, France
Copyright Elsevier

During the world’s first nuclear explosion, in 1945, glassy melts called “trinitites”, mostly derived from the sands at the surface of the test site, formed and were deposited at or near the hypocenter. The processes of formation of this fallout remain unclear. Here, we show how the oxygen and silicon isotopic compositions of three trinitites allow to refine their formation scenario. The three samples are typical of trinitites, being composed of various crystalline phases (feldspars, quartz, and calcite) and of glassy phases divided into three chemical groups (CaMgFe, alkali, silica) that are mixed in various proportions in the three samples. The three samples show a large range of oxygen and silicon isotopic variations (−10.9 ± 0.6 ‰ <δ30Si < 4.2 ± 0.6 ‰, and 2.3 ± 0.4 < δ18O < 24.2 ± 0.5 ‰). At variance with the Hiroshima fallout deposits, no oxygen mass-independent isotopic fractionation was found in the three trinitites. The chemical and isotopic compositions of the chemical groups reveal that they result from different processes: the silica phases are molten fragments of the site material, while the CaMgFe and alkali phases are produced by the mixing of condensates and molten site material. Models show that the observed silicon isotopic variations resulted from Rayleigh distillation during condensation of the gaseous species injected into the cloud, while the variability in composition of the site materials also played an important role for controlling the oxygen isotopic compositions. From these observations, a general scenario, beginning with the vaporization of the site surface, producing a depression, is proposed. The vaporized material condensed and grew by agglomeration with other condensates and liquid materials. These agglomerates rained on the surface and quenched, forming the trinitites. This scenario is different from the formation of the Hiroshima glasses but shows some similarities to the tektites formation.