¹Kana Amano (>10)
Geochimica et Cosmochimica Acta (in Press), Open Source Link to Article [DOI: 10.1016/j.gca.2026.09.011]
¹Muséum National d’Histoire Naturelle (MNHN), Institut de Minéralogie, Physique des Matériaux et Cosmochimie (IMPMC), Sorbonne Université, UMR CNRS 7590, 75005 Paris, France
Copyright Elsevier

Recent findings from sample-return missions targeting carbonaceous asteroids suggest that CI-like (Carbonaceous Ivuna-type) small bodies are more common in the solar system than previously thought. However, the general redox characteristics and variations among CI materials remain largely unexplored due to their limited availability on Earth. Moreover, CI meteorites have likely undergone irreversible terrestrial oxidation, resulting in modifications of the iron speciation and the redox states. In this context, the iron mineralogy of samples from the carbonaceous asteroid Bennu allows us to reassess the redox conditions of CI materials during its formation. We performed X-ray diffraction and Mössbauer spectroscopy of two Bennu particles from the hummocky group, as well as Oued Chebeika 002 CI meteorite. We compare their iron speciation to that of other CI and CI-like materials, including samples from the asteroid Ryugu, Orgueil CI meteorite, and Oued Chebeika 002 CI meteorite. The results show that Bennu particles contain substantial amounts of Fe sulfides (pyrrhotite, mackinawite, and pentlandite), along with Fe oxide (magnetite) and Fe-bearing (Mg-rich) phyllosilicates, similar to Ryugu samples. Cryogenic Mössbauer analysis indicates no significant contribution from ferric oxyhydroxides (i.e., ferrihydrite) in Bennu and Oued Chebeika 002 samples, confirming the previous electron microscopy observations of those samples. These observations imply that Fe³⁺ forming ferrihydrite observed in historical CI meteorites (e.g., Orgueil, Alais) results from long-term terrestrial oxidation of Fe²⁺ in sulfides, especially submicron-sized ones. This further strengthens the view that the actual CI materials and their formation environments are less oxidizing than previously estimated from old CI meteorite falls. The Fe³⁺/(Fe³⁺+Fe²⁺) ratios in phyllosilicates in the hummocky particles range from 50 to 70%, indicating local heterogeneity within individual particles. Whereas similar values have been reported for Bennu samples (determined by X-ray Absorption Near-Edge Structure spectroscopy), Ryugu samples, and Oued Chebeika 002 samples (i.e., ∼50–65% of Fe³⁺/(Fe³⁺+Fe²⁺)_{phyllosilicates}), there may be slightly Fe³⁺-rich sub-regions (close to ∼ 70%) in our Bennu particles. Furthermore, we found two distinct mineral assemblages among CI and CI-like materials: (i) relatively well-crystallized phyllosilicates without mackinawite but with pyrrhotite, and (ii) disordered phyllosilicates coexisting with mackinawite, a metastable Fe sulfide. The latter finding is consistent with recent studies of mackinawite in Bennu and Ryugu samples, which suggest its formation at the early stages of CI alteration. Together with previous studies of Bennu, Ryugu, and Orgueil samples, our results suggest that different phyllosilicate assemblages preserve distinct Fe valence states, potentially recording redox conditions associated with different stages of formation. The CI materials have conventionally been regarded as relatively equilibrated and homogeneous, however, our results demonstrate the potential of combining clay mineral crystallinity and Fe valence states to unravel formation histories at a finer scale.

¹Damanveer S. Grewal, ²Zhongtian Zhang
Geochimica et Cosmochimica Acta (in Press), Link to Article [DOI: 10.1016/j.gca.2026.08.036]
¹Department of Earth and Planetary Sciences, Yale University, New Haven, CT 06511, USA
²Department of Geosciences, Princeton University, Princeton, NJ 08544, USA
Copyright Elsevier

Water-rock reactions during aqueous alteration were critical in establishing the redox state of early Solar System planetesimals. The Urey-Craig (UC) diagram, which plots “reduced Fe” (metallic Fe + sulfide Fe) against “oxidized Fe” (silicate- and oxide-bound Fe), is the canonical framework for interpreting redox variations among chondrites. Historical CI chondrites, assumed to be metal-free with their sulfur residing in sulfate, were assigned no “reduced Fe” and long regarded as the oxidized endmember. However, the finding that the returned Ryugu and Bennu samples, along with fresh CI material, host sulfides rather than sulfates, revises their position in the UC diagram, placing them closer to the more reduced CR, L, and LL chondrites. This shift has been interpreted as evidence that CIs are more reduced, challenging their long-standing role as the oxidized endmember.

This interpretation, however, conflates two distinct Fe reservoirs. Under the low-Eh conditions of planetesimal alteration, metallic Fe oxidizes readily whereas sulfides persist, so only the non-sulfide fraction of bulk Fe is accessible to water-driven oxidation. Because bulk S content increases with matrix fraction, the proportion of non-sulfide Fe decreases, making matrix-rich CIs the group with the smallest “oxidizable” Fe reservoir. By quantifying the mean valence state and oxygen associated with the non-sulfide Fe, we show that CI chondrites, although they appear reduced in the revised UC diagram, record pervasive oxidation of the non-sulfide Fe pool, comparable to or greater than that of the most altered CM lithologies. This convergence occurs despite mineralogically distinct alteration pathways on the CM and CI parent bodies, consistent with the endpoint Fe valence being set by the redox potential of the alteration fluid rather than by specific reaction pathways. The lower total O uptake by Fe in CIs reflects not reduced conditions but a smaller initial reservoir of “oxidizable” Fe, itself limited by high bulk S. We introduce a Fe⁰-FeS-Fe_ox (metallic Fe-sulfide Fe-oxidized Fe) ternary framework that reconciles extensive oxidation in CIs with their misleading UC position and provides a more accurate basis for tracing aqueous redox pathways in carbonaceous chondrite parent bodies.

¹N. Meunier-Mili, ¹M.-A. Kaczmarek, ²M. Bystricky
Geochimica et Cosmochimica Acta (in Press), Open Source Link to Article [DOI: 10.1016/j.gca.2026.08.025]
¹Géosciences Environnement Toulouse (GET), CNRS – CNES – IRD – Université de Toulouse, Observatoire Midi Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France
²Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS – CNES – Université de Toulouse, Observatoire Midi Pyrénées, 14 avenue E. Belin, 31400 Toulouse, France
Copyright Elsevier

Chassignite meteorites are martian achondrites that display a magmatic cumulate texture and are mainly composed of olivine with a small amount of poikilitic pyroxene. This study presents a detailed petrological and microstructural analysis using electron backscatter diffraction of all Chassignite meteorites documented up to now, Chassigny, NWA 2737 and NWA 8694, in order to decipher early magmatic, deformation and shock processes. The new results reveal that olivine crystals in Chassignite meteorites initially formed through synneusis, a magmatic process where individual crystals accumulate and aggregate together in specific orientations in the earlier stages of consolidation. Many olivine crystals stick together along (100) crystalline faces as evidenced by the significant concentration of misorientation [100] axes at grain boundaries for all three samples. Synneusis was probably interrupted and progressively replaced by crystal settling until complete crystallisation was achieved. Olivine crystallographic preferred orientations indicate slip on 100 and olivine subgrain boundaries reveal high densities of geometrically necessary dislocations of the 100 type. The 100 slip system is active at high temperatures and low stresses in the Earth’s upper mantle and is often observed in plastically deformed peridotites, though it is not necessarily dominant in terrestrial olivine cumulative rocks. Furthermore, pyroxene crystallographic preferred orientations are consistent with those of olivine ([100] olivine axes parallel to pyroxene [001] axes), supporting the hypothesis of minor plastic deformation of olivine and pyroxene at high temperatures and low stresses. These observations are consistent with possible crystallization and emplacement of the Chassignite magma in the shallow Martian crust. Finally the strong shock events recorded by these meteorites is supported by significant internal deformation of olivine grains, concentration of misorientation axes along their [100] axes, and the activation of the 001 slip system leading to formation of olivine subgrain boundaries. Altogether these results are good indicators to decipher primary planetary processes.

¹Kang Liu, ¹Li Zhang, ¹Hesan Wu, ¹Wan-Cai Li, ^1,2Huaiwei Ni
Geochimica et Cosmochimica Acta (in Press), Link to Article [DOI: 10.1016/j.gca.2026.08.028]
¹State Key Laboratory of Lithospheric and Environmental Coevolution, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
²Deep Space Exploration Laboratory, Hefei 230088, China
Copyright Elsevier

Lunar volcanic glass beads preserve records of sulfur evolution in primitive lunar magmas and provide important constraints on the sulfur inventory of the lunar interior. Reconstruction of their pre-eruptive sulfur contents requires sulfur diffusivity data for lunar melts, which have remained largely unavailable. Here we experimentally determined sulfur diffusivities in three synthetic lunar basaltic melts, spanning compositions from Apollo green (low TiO₂, 0.52 wt%) and orange (intermediate TiO₂, 8.58 wt%) to red (high TiO₂, 13.93 wt%), with SiO₂ decreasing from 45.35 to 35.92 wt% and MgO from 16.00 to 11.00 wt%, using diffusion-couple experiments at 0.5 GPa and 1674–1876 K in a piston cylinder apparatus. Sulfur diffusivity increases systematically from green to orange to red glass compositions, with NBO/T rising from 1.76 to 2.49 to 2.82, respectively. At 1673 K, sulfur diffusivity in lunar melts is 6–13 times higher than that in terrestrial basaltic melts (NBO/T = 0.61–0.79), primarily owing to the more depolymerized structure of lunar melts. Combining these results with published experimental data, we develop a general parameterization for sulfur diffusivity in anhydrous lunar and terrestrial silicate melts under reduced conditions as a function of temperature (T) and melt composition (X_Si+Al, expressed by the combined mole fraction of Si + Al), which reproduces nearly all available experimental data within a factor of 2. Applying our new sulfur diffusivity data to Apollo green and orange glass beads yields revised pre-eruptive sulfur concentrations of 282–311 and 331 μg/g, corresponding to mantle source sulfur abundances of 9–28 and 27–63 μg/g, respectively. These estimates differ substantially from values previously inferred for Apollo and Chang’e mare basalts, indicating significant sulfur heterogeneity within the lunar interior, likely inherited from lunar magma ocean differentiation and subsequent mantle overturn.