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DOE OSTI2026

Triple oxygen and hydrogen stable isotope composition of hydroxyl water in Orgueil (CI-type) and Tagish Lake (C2-type) carbonaceous chondrites

The primitive carbonaceous chondrites are of interest to cosmochemical science because they contain relatively large amounts of ‘water’ (H 2 O and/or OH – ) within phyllosilicate minerals. This water is evidence for the accretion of ices by their parent planetesimals, and thus represents an archive of the isotopic compositions of H 2 O in the protoplanetary environments. Here, in this study, we used thermogravimetry-enabled laser spectroscopy (TGA-IRIS) analyses of the Orgueil and Tagish Lake meteorites to make δ 2 H, δ 18 O, and Δ′ 17 O measurements of the H 2 O and OH – contained in the different hydrous minerals that comprise each meteorite. In Orgueil, we measured mass-weighted averages of OH – in the saponite and serpentine phyllosilicate matrix to be δ 2 H = 192 ‰, δ 18 O = 1.5 ‰, which are unquestionably of extraterrestrial origin with Δ′ 17 O = 1.0 ‰. For Tagish Lake, analogous values of OH– in the saponite and serpentine phyllosilicate matrix are δ 2 H = 704 ‰, δ 18 O = 11.3 ‰, and are similarly unambiguously extraterrestrial with Δ′ 17 O = 0.82 ‰. We estimate that the parent H 2 O involved in aqueous alteration of Orgueil had δ 18 O value ≥ +23 ‰. In Orgueil, we interpret the phyllosilicate petrographic relationships, and the δ 18 O values of OH – in saponite and serpentine to indicate that saponite formed first, at a lower temperature by 35 to 53 °C than serpentine. This suggests that the Orgueil parent body experienced increasing temperature during the phase of active aqueous alteration (prograde) which set the δ 18 O OH values of the serpentine and saponite. In the case of Tagish Lake, serpentine formed at a lower temperature by 32 to 60 °C than saponite, for the simplest case with constant δ 18 O H2O values. If serpentine formed first, followed by saponite formation at 32 to 60 °C °C higher temperature, this suggests that Tagish Lake sample TL1 underwent prograde aqueous alteration as the parent body heated up. We find evidence that the parent H 2 O for Orgueil, Tagish Lake sample TL1, and Murchison (based on data from a previous study) may have had Δ′ 17 O values of >0.64 ‰.

Aqueous alteration↗

DOE OSTI2023

Interaction between clay minerals and organics in asteroid Ryugu

The Hayabusa 2 spacecraft brought back to Earth grains of the carbonaceous asteroid Ryugu. Such grains represent the pristine state of CI chondritic materials and have been preserved from exposure to Earth’s atmosphere. Here, we show evidence of the presence of organics trapped within the interlayer space of smectite layers in Ryugu grains. No such organics are found in the Orgueil CI meteorite. We put forward that the organics within the interlayer space of smectite in Orgueil CI meteorite were lost during their oxidation on Earth. Also, we propose that the presence of organics within the interlayers space of smectite might be responsible for the possible NH infrared signature observed in Ryugu particles and potentially to a few large C-type asteroids including Ceres

58 GEOSCIENCES↗

DOE OSTI2023

The iron oxidation state of Ryugu samples

The Hayabusa2 mission sampled Ryugu, an asteroid that did not suffer extensive thermal metamorphism, and returned rocks to the Earth with no significant air exposure. It therefore offers a unique opportunity to study the redox state of carbonaceous Cb-type asteroids and evaluate the overall redox state of the most primitive rocks of the solar system. An analytical framework was developed to investigate the iron mineralogy and valence state in extraterrestrial material at the micron scale by combining x-ray diffraction, conventional Mössbauer (MS), and nuclear forward scattering (NFS) spectroscopies. An array of standard minerals was analyzed and cross-calibrated between MS and NFS. Then, MS and NFS spectra on three Ryugu grains were collected at the bulk and the micron scales. In Ryugu samples, iron is essentially accommodated in magnetite, clay minerals (serpentine–smectite), and sulfides. Only a single set of Mössbauer parameters was necessary to account for the entire variability observed in MS and NFS spectra, at all spatial scales investigated. These parameters therefore make up a fully consistent iron mineralogical model for the Ryugu samples. As far as MS and NFS spectroscopies are concerned, Ryugu grains are overall similar to each other and share most of their mineralogical features with CI-type chondrites. In detail however, no ferrihydrite is found in Ryugu particles even at the very sensitive scale of Mössbauer spectroscopy. The typical Fe 3+ /Fe tot of clay minerals is much lower than typical redox ratios measured in CI chondrites (Fe 3+ /Fe tot = 85%–90%). Furthermore, magnetite from Ryugu is stoichiometric with no significant maghemite component, whereas up to 12% of maghemite was previously identified in the Orgueil's so-called magnetite. These differences suggest that most CI meteorites suffered terrestrial alteration and that the preterrestrial composition of these carbon-rich samples was less oxidized than previously measured. However, it is not clear yet whether or not the parent bodies of CI chondrites were as reduced as Ryugu. Finally, the high spatial resolution of NFS allows to disentangle the redox state and the crystal chemistry of iron accommodated in serpentine and smectite. The most likely polytype of serpentine is lizardite, containing <35% of Fe 3+ , a fraction of which being tetrahedrally coordinated. Smectite is more oxidized (Fe 3+ /Fe tot > 65%) and mainly contains octahedral ferric iron. In conclusion, this finding implies that these clays formed from highly alkaline fluids and the spatial variability highlighted here may suggest a temporal evolution or a spatial variability of the nature of this fluid.

58 GEOSCIENCES↗

DOE OSTI2025

Physical properties, internal structure, and the three‐dimensional petrography of CI chondrites

physical properties and the nature of their breccation, we investigated nine samples of the Ivuna and Orgueil CI chondrites ranging in size from 1 mm to 4 cm in approximate diameter. The combined mass of unique material investigated in this work is 113 g. For our investigations, we use ideal gas pycnometry, 3-D laser scanning, x-ray computed microtomography (μCT), and accompanying digital data extraction techniques. We found that the bulk density of the samples ranged from 1.61 to 2.10 g cm −3 . Larger samples tend to have a lower bulk density. Grain density (ranging from 2.44 to 2.55 g cm −3 ) is significantly less variable than the bulk density in our samples and the quantity of porosity (ranging from 14.6% to 33.8%) is the dominant factor in determining the bulk density of CI chondrite material. Our μCT results show that the visible porosity across all sizes of our CI chondrite samples is in the form of cracks, but these cracks can account for less than two-thirds of the porosity in the CI chondrites. Other porosity is not visible, even at μCT resolutions of 2.7 μm voxel edge −1 and we conclude that it is sub-micron in nature. It is not clear if the cracks seen in our samples are indigenous to the chondrites or are a result of terrestrial processes. We also find that the CI chondrites are excellent examples of the fractal-like nature of brecciation, where clasts can be observed at all scales we imaged. The breccias are composed of sub-equant-shaped and sub-rounded-textured clasts like melt-free impact breccias on other solar system bodies. From our μCT volume and digital data extraction, we determine that the Ivuna CI chondrite breccia is organized: the mostly sub-equant clasts within our ~2 cm chunk of Ivuna have a mean diameter of 1.33 mm and their aligned longest axes define a lineation structure. We speculate that the lineation was imparted after fragmentation of the clasts by slight shear on the parent asteroid which could be the result of seismic-related granular flow or mild non-axial impact-related compaction. These data will help to place returned asteroidal material from asteroids 162173 Ryugu and 101955 Bennu and the CI chondrites into a mutual geological context.

CI chondrite↗