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.

Cosmochemistry Papers will have a short Summer Break from 28.08.2026 to 14.09.2026.

¹Ziqing Li, ²Mengfan Zhang, ¹Bo Zhang, ³Yuqi Qian, ⁴Tao Long, ⁴Xiaochao Che, ¹Ao Su, ⁵James W. Head
Journal of Geophysical Research: Planets, 131, e2026JE009898 Open Source Link to Article [DOI: 10.1029/2026JE009898]
¹Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing, China
²School of Earth and Space Sciences and Institute of Energy, Peking University, Beijing, China
³Department of Earth and Planetary Sciences, NWU-HKU Joint Center of Earth and Planetary Sciences, The University of Hong Kong, Hong Kong, China
⁴Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China
⁵Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA
Published by arrangement with John Wiley & Sons

The cause of the observed mare basalt asymmetry between the lunar nearside and farside remains a long-lasting conundrum. In this study, we characterized the petrology and geochemistry of the Chang’e-6 low-Ti basalts from the lunar farside, performed petrological modeling of major elements and conducted Monte Carlo simulation of trace elements, for Chang’e-6 and Chang’e-5 basalts. The results from multiple approaches indicate that the young Chang’e-6 (2.8 Ga) and Chang’e-5 (2.0 Ga) basalts both originated from shallow, depleted ilmenite-bearing cumulate (IBC). Based on remote sensing and thermodynamic constraints, we estimated the mantle source ilmenite abundance for young shallow-source basalts (<3.0 Ga) globally. The modeling results indicate that the mantle sources of these basalts on the nearside generally contains ∼16–32 wt% ilmenite, especially beneath the PKT, whereas the mantle sources of these basalts beneath the farside SPA basin only has ∼13–24 wt% ilmenite. The phase equilibrium calculations show that the nearside mantle sources with higher ilmenite abundance leads to a significant decrease in melting point and tends to produce more melt compared to farside mantle sources. Therefore, asymmetric IBCs may play a key role in accounting for asymmetric nearside/farside volcanism.

¹F. Alberquilla (>10)
Journal of Geophysical Research: Planets, 131, e2026JE009742 Open Source Link to Article [DOI: 10.1029/2026JE009742]
¹University of the Basque Country (EHU), Leioa, Spain
Published by arrangement with John Wiley & Sons

Volcanic tubes provide stable environments where unique mineralogical assemblages can form and be preserved, making them valuable terrestrial analogs for Martian studies. This work investigates tree-like structures within two lava tubes on La Palma Island (Canary Islands, Spain) through a comprehensive geochemical and mineralogical characterization. Samples were analyzed using X-ray diffraction (XRD), micro Energy Dispersive X-Ray fluorescence (µ-EDXRF), Raman microscopy, Scanning Electron Microscopy Energy-Dispersive X-ray Spectroscopy (SEM-EDX), Short-Wave infrared (SWIR) hyperspectral imaging, and multivariate analysis Principal Component Analysis and Multivariate Curve Resolution-Alternating Least Squares (PCA and MCR-ALS) to identify primary mineral phases and assess potential organic matter preservation. The results reveal that these structures are predominantly composed of hydrated silica (opal-A), with secondary phyllosilicates, carbonates, sulfates, and iron oxides. Organic signatures were identified as spectral features consistent with microbial pigments such as β-carotene within the opal matrix, suggesting the potential encapsulation and preservation during silica precipitation. Hyperspectral analyses revealed spectral features consistent with the presence of opal-A, calcite, and nontronite, highlighting complex mineral associations. The comparison with CRISM observations from Jezero Crater revealed similarities in mineral assemblages and suggests that comparable alteration pathways may have affected basaltic substrates, with hydrated silica formation followed by carbonate precipitation under evolving fluid conditions. These findings underscore the relevance of terrestrial lava tubes as analogs for Martian volcanic environments and emphasize the potential of hydrated silica deposits to preserve biosignatures in extraterrestrial contexts.

¹Alessandro Pisello (>10)
Journal of Geophysical Research: Planets 131, e2025JE009371 Open Source Link to Article [DOI: 10.1029/2025JE009371]
¹Department of Physics and Geology, University of Perugia, Perugia, Italy
Published by arrangement with John Wiley & Sons

Understanding how the glass/crystal ratio influences the spectral response of volcanic rocks is crucial for interpreting planetary remote sensing data. Here, four mafic rocks simulating a possible Martian composition were synthesized with identical bulk chemistry but different mineralogical assemblages, from fully amorphous to ∼70 wt.% crystal content, to investigate how crystal content affects Visible and Near-Infrared (VNIR) reflectance spectra. Bi-directional VNIR reflectance was collected at room temperature across a range of incidence (0°, 30°, 60°) and emergence (−70° to +70°) angles. The diagnostic absorptions of the two most abundant phases, pyroxene and glass, are not distinguishable in the spectra and can even reproduce the spectral fingerprint of olivine, absent from the samples; only minor iron oxides (magnetite and hematite, ∼1–7 wt.%) produce clearly identifiable absorptions. Instead, spectral slope emerges as the primary proxy for the glass/crystal ratio: NIR/VNIR slope decreases systematically with increasing crystal content, driven jointly by the incorporation of iron oxides and the loss of the positive-slope contribution of residual glass. Principal Component Analysis and k-means clustering independently confirm this control, identifying three clusters that map onto the glass/crystal ratio. This decoupling indicates that the crystal content of a mafic terrain cannot be inferred from pyroxene- or glass-related absorptions alone, but rather from spectral slope and iron-oxide features. These results experimentally support previous hypotheses linking the spectral diversity of Martian mafic terrains to their degree of crystal content and oxidation state and highlight variable glass abundance as an under-considered contributor to the spectral interpretation of the Martian surface.

^1,2,3Scott M. Perl, ^2,4Aaron J. Celestian, ⁴Frank A. Corsetti
Journal of Geophysical Research: Planets 131, e2026JE009730 Link to Article [DOI: 10.1029/2026JE009730]
¹Earth, Planetary, and Space Sciences, University of California Los Angeles, Los Angeles, CA, USA
²Mineral Sciences, Los Angeles Natural History Museum, Los Angeles, CA, USA
³Blue Marble Space Institute of Science, Seattle, WA, USA
⁴Department of Earth Sciences, University of Southern California, Los Angeles, CA, USA
Published by arrangement with John Wiley & Sons

Evaporite minerals can capture and entomb organic matter within their intercrystalline and intracrystalline structure because they precipitate relatively quickly (nomenclature adopted from Schopf et al. (2012), https://doi.org/10.1089/ast.2012.0827). Thus, evaporite minerals constitute a target for biosignature investigation on Earth and Mars, where evaporitic deposits are known to exist. However, little is known about the process of organic preservation and detection in evaporites, or the stability of such molecules when exposed to significant UV radiation as would be present on the surface of Mars. Here, we investigate the incorporation of β-carotene into halite (NaCl) by growing halite in the lab in the presence of know concentrations of β-carotene and examining the resultant precipitated crystals and fluid inclusions via Raman spectroscopy. Following brine evaporation, the experimental β-carotene-containing halite was exposed to UV-C to simulate conditions on the Martian surface. Results reveal that β-carotene has a strong Raman signature that remains intact even when entombed in halite. In particular, fluid inclusions within the halite displayed particularly strong β-carotene Raman signatures after UV-C exposure. Little change was observed even after several days of UV-C delivery. Our results reveal that complex organic molecules like β-carotene should be preserved well in halite (especially in fluid inclusions) and that halite does provide protection from organic matter degradation from UV-C radiation. Thus, evaporites constitute a good target for the search for biomarkers on Mars. These findings will allow for proper criteria for the discovery of any potential physical biosignature and chemical biomarker that would be on active ocean worlds (Europa, Enceladus) and for future Mars subsurface drilling missions.

¹S. Desikamani, ²L.D. Peterson, ¹M.E. Newcombe, ³C.M.O’D. Alexander, ³J. Wang, ¹R.D. Ash, ⁴S.G. Nielsen, ¹P.M. Piccoli, ³E.S. Bullock
Geochimica et Cosmochimica Acta, (in Press) Link to Article [DOI: 10.1016/j.gca.2026.08.024]
¹University of Maryland, College Park, MD 20742, USA
²Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
³Carnegie Institution for Science, Washington DC 20015, USA
⁴Centre de Recherches Pétrographiques et Géochimiques (CRPG – CNRS), Nancy 54501, France
Copyright Elsevier

Nucleosynthetic isotope signatures indicate that the Earth is predominantly made from inner solar system non-carbonaceous (NC) materials. However, a major uncertainty in models of water addition to the proto-Earth is the extent to which H (calculated in this study as µg/g H₂O, but present as H-bearing species) in nominally anhydrous minerals (NAMs) from NC materials could contribute to the bulk Earth water budget. The preserved water concentration of NC meteorite NAMs may also shape our understanding of processes occurring in the solar protoplanetary disk (e.g., implantation of H on the surfaces of NAM grains) and in planetesimals (e.g., metamorphism in chondrite parent bodies). Reported water contents for NAMs from the ordinary chondrites (OCs), the dominant NC material falling to Earth today, range between 10⁰-10⁴ µg/g H₂O. In order to better constrain the potential contribution of ordinary chondrites to the Earth’s water budget we have measured water concentrations in NAMs from eight equilibrated OCs, six of which have not previously been investigated for water, and two of which (Chelyabinsk and Bensour) were previously measured in other laboratories. We find that olivine and low-Ca pyroxene from equilibrated OCs contain less than ∼ 10 µg/g H₂O. Based on these measurements, the water content of the NAM fraction of equilibrated OCs is < 10 µg/g H₂O (a factor of ∼ 60–120 lower than prior estimates). Combining these constraints of equilibrated OC NAM water contents with published measurements of NAMs and glassy mesostases from unequilibrated OCs, we estimate that NAMs and glass in OC parent bodies could have delivered no more than ∼ 0.2 ocean masses of water to Earth (∼1% of an assumed total water budget of 18 ocean masses). Additional water could have been delivered from phyllosilicates and organics in the most primitive OC material that is not considered in our modeling. The difference in NAM water concentrations obtained here relative to some prior studies may be rooted in analytical artifacts associated with their nanoscale secondary ion mass spectrometric measurements.