¹K. Righter et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Articlel [https://doi.org/10.1111/maps.70182]
¹Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York, USA
Published by arrangement with John Wiley & Sons

Examination of 10 Bennu aggregate particles has revealed the presence of many phases which taken together can provide constraints on the oxygen fugacity (fO2) of Bennu samples. Phyllosilicates (saponite and serpentine), carbonates, oxides (magnetite, chromite), sulfides (pyrrhotite, pentlandite), phosphate (hydroxyapatite, Na-Mg-phosphate), and phosphides (schreibersite, andreyivanovite) all occur in most Bennu particles. The Bennu samples have experienced a high degree of aqueous alteration leaving only <1% of the original mineralogy unaltered. However, both the precursor anhydrous and alteration phases can present different constraints on fO2. Precursor phases include olivine, pyroxene, spinel, hibonite, chromite, phosphide, very rare Fe-Ni metal, apatite, and possibly MgS and MnS, all of which are typically <25 μm in size. Alteration phases include phyllosilicates, carbonates, magnetite, sulfides, sulfates, phosphates, chlorides, and fluorides. Detailed calculations of fO2 rely on having quantitative electron microprobe analyses of the phases involved in equilibria amongst both the precursor and alteration phases. In general, the absence of Fe-Ni metal, coupled with the stability of the Fe3O4 component in chromite, places a lower limit on the fO2. Concomitantly, the absence of Fe sulfates places an upper limit on fO2. Altogether, the textures, mineral compositions, and calculations suggest that some components in the Bennu samples (chondrules, inclusions) may have originally equilibrated at fO2 well below the iron-wüstite buffer but then experienced higher fO2 near or higher than the fayalite-magnetite-quartz (FMQ) buffer during aqueous alteration that produced coarser grained oxidized assemblages.

¹A. C. Singleton,¹G. R. Osinski
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70184]
¹Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

The ~28 km Kamestastin (Mistastin) Lake impact structure is a relatively well-preserved and well-exposed complex impact structure. The central uplift of this structure is accessible as two islands in the middle of Kamestastin Lake. We present an updated, detailed geological map and description of Horseshoe and Bullseye islands that provides increased accuracy and detail of the target rock outcrop and contact locations. In addition, we document six occurrences of impact melt-poor breccia dikes and one occurrence of impact melt rock on Horseshoe Island for the first time. The impact melt rock outcrop is proposed to be a remnant of a veneer of impact melt on the original central peak, and the impact melt-bearing breccia dikes to have had a dynamic emplacement mechanism. We also carried out the first detailed, systematic shock study of the central uplift. Planar deformation features in quartz and diaplectic feldspar glass suggest local peak shock pressure of up to 45 GPa. These shock pressures are higher than the peak pressures recorded in the central uplifts of similarly sized impact structures. We suggest that this difference is due to the minimal erosion of the central uplift at the Kamestastin Lake impact structure.

¹Glenn J. MacPherson, ²Kazuhide Nagashima, ²Alexander N. Krot, ³Noriko T. Kita, ^3,4Takayuki Ushikubo, ²Elena Dobrică
Geochimica et Cosmochimica Acta (in Press) Link to Article [10.1016/j.gca.2026.06.023]
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington DC 20560, the United States of America
²University of Hawai‘i at Mānoa, Honolulu, HI 96822, the United States of America
³WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, the United States of America
⁴Kochi Institute for Core Sample Research, JAMSTEC, Nankoku, Kochi 783-8502 Japan
Copyright Elsevier

Using secondary ion mass-spectrometry, we analyzed the Al/Mg isotopic compositions of cleanly resolved Wark-Lovering (WL) rim layers on six calcium-aluminum-rich inclusions (CAIs) of different petrologic types from the Vigarano CV3 chondrite. In five cases, the inferred initial ²⁶Al/²⁷Al ratios [(²⁶Al/²⁷Al)₀] are within analytical error of the ratios of the host inclusions and of each other, and in the sixth case the value for the rim sequence is suspect and not deemed reliable. The results for the five reliable CAIs mean that the maximum age difference between the rim sequences and their host inclusions is at most 10⁵ years and possibly less. Petrologic observations show that the effects of the rim-forming process were not limited to the rims themselves but extended well into the inclusion interiors. The fact that the (²⁶Al/²⁷Al)₀ of WL rims are within analytical error of each other raises the possibility that all WL rim sequences formed during a single nebula-wide event, but the fact that the inclusions are diverse petrologically makes this unlikely. We propose instead that the diverse rims formed during multiple nebular reheating events after the initial formation of the inclusions, leading to surface melting, volatilization, and recondensation. Our isotopic data for some rim phases, especially forsterite and diopside, tend to be isotopically lighter (lower δ²⁵Mg) than their host inclusion interiors, suggesting that they formed predominantly by recondensation.

^1,2Longye Du, ^1,3Xiaoxiao Yu, ⁴Yiliang Li, ¹Xiao Wu, ¹Lin Gong, ¹Gangjian Wei, ¹Qiang Wang, ^1,5Mang Lin
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [10.1016/j.gca.2026.06.019]
¹State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
²University of Chinese Academy of Sciences, Beijing 100049, China
³College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
⁴Department of Earth and Planetary Sciences, the University of Hong Kong, Hong Kong Special Administrative Region of China
⁵Center for Advanced Planetary Science (CAPS), Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
Copyright Elsevier

A primary objective of Mars sample return missions is the detection and characterization of potential biosignatures. Sulfates, among the most abundant hydrated minerals on Mars, represent promising targets for such investigation. Terrestrial studies have shown that gypsum can preserve life-associated organic matter and that its δ34S signatures provide key evidence for microbial sulfate reduction as early as ∼ 3.5 billion years ago. This study analyzes quadruple sulfur (δ34S, Δ33S, Δ36S) and triple oxygen (δ18O, Δ́17O) isotopes in gypsum from three strong evaporation basins on the Qinghai-Tibet Plateau, where sulfate deposition is extensive but surface environments are only marginally habitable due to severe limitations in water, soil development, and nutrient availability. We focus on the Qaidam Basin as a Mars analog environment and place it in context through comparison with a higher-elevation region (Shuanghu) and a magmatic rock-dominated region (Yushu). Together with isotopic mixing models, an open-system steady-state sulfur cycle model, and triple-oxygen isotope oxidation pathway calculations, our quadruple sulfur and triple oxygen isotope measurements reveal that the studied gypsum deposits preserve distinct isotopic biosignatures. In particular, the isotopic compositions of the samples reflect microbial sulfur disproportionation and cryptic sulfur cycling, both of which extend beyond widespread microbial sulfate reduction, demonstrating how sulfur can be internally recycled and energetically utilized to sustain life in Mars-analog environments. This integrated isotopic approach significantly advances our capability to decipher sulfur cycling in extreme terrestrial environments, including Mars analog systems, and provides critical methodology for interpreting potential biosignatures in returned Martian samples.

^1,2Yankun Di, ¹Magdalena H. Huyskens, ²Qing-Zhu Yin, ^1,3Yuri Amelin
Geochimica et Cosmoschimica Acta (in Press) Open Access Link to Article [10.1016/j.gca.2026.06.017]
¹Research School of Earth Sciences, Australian National University, Acton, ACT 2601, Australia
²Department of Earth and Planetary Sciences, University of California Davis, Davis, CA 95616, USA
³State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
Copyright Elsevier

Compared to the Earth and inner Solar System planets, calcium–aluminium-rich inclusions (CAIs) possess nucleosynthetic Sr isotope anomalies manifested as elevated ⁸⁴Sr/⁸⁶Sr after internal normalisation using ⁸⁸Sr/⁸⁶Sr. These anomalies can be generated by heterogeneous incorporation of s-, r-, or p-process nucleosynthesis components. Accurately distinguishing between the enrichment or depletion in those components is critical for correctly understanding the timing of planetary volatile depletion, as they predict very different anomalies in ⁸⁷Sr/⁸⁶Sr and lead to disparate interpretations of ⁸⁷Rb–⁸⁷Sr chronology. Here, we constrain the origin of Sr isotope anomalies in the original Mason and Taylor (1982) set of Allende CAIs by examining their nucleosynthetic Sr and Nd isotope systems. Most CAIs analysed exhibit positive μ⁸⁴Sr and negative μ^145,148,150Nd anomalies (μ-values are defined as part-per-million deviations of isotopic ratios relative to terrestrial standards) in agreement with previous studies, but we also detected significant isotope heterogeneities among them, including discovery of CAIs with Sr and Nd isotope anomalies in directions opposite to the majority. The Nd isotope heterogeneity among CAIs is predominantly consistent with variations in the abundance of the s-process component, with a minor but clearly resolved p-process deficit on μ¹⁴²Nd_corr (μ¹⁴²Nd_corr is μ¹⁴²Nd corrected for ¹⁴⁶Sm decay). The less steep μ¹⁴²Nd_corr vs. μ¹⁴⁸Nd slope defined by the CAIs compared to that predicted by stellar models supports the recent suggestion that the accessible Earth has a small radiogenic excess in μ¹⁴²Nd relative to chondrites. Correlated Sr and Nd isotope anomalies in the CAIs suggest that (1) they formed from at least two isotopically distinct reservoirs, one with and the other without p-process Sr excesses relative to Earth, (2) the majority of CAIs formed in the p-process-Sr-enriched reservoir with additional s-process excesses, and (3) variations in r-process Sr and Nd are not observed among the CAIs. The s-process-induced ⁸⁷Sr/⁸⁶Sr anomalies in CAIs (relative to the inner Solar System) predicted based on Nd isotopes are below the typical measurement precision, negating the need for nucleosynthetic correction on CAIs’ ⁸⁷Sr/⁸⁶Sr in chronological interpretations

^1,2Yankun Di, ²Magdalena H. Huyskens, ²Qing-Zhu Yin, ^1,3Yuri Amelin
Geochimcia et Cosmochimica Acta (in Press) Open Access Link to Article [10.1016/j.gca.2026.06.017]
¹Research School of Earth Sciences, Australian National University, Acton, ACT 2601, Australia
²Department of Earth and Planetary Sciences, University of California Davis, Davis, CA 95616, USA
³State Key Laboratory of Deep Earth Processes and Resources, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
Copyright Elsevier

Compared to the Earth and inner Solar System planets, calcium–aluminium-rich inclusions (CAIs) possess nucleosynthetic Sr isotope anomalies manifested as elevated ⁸⁴Sr/⁸⁶Sr after internal normalisation using ⁸⁸Sr/⁸⁶Sr. These anomalies can be generated by heterogeneous incorporation of s-, r-, or p-process nucleosynthesis components. Accurately distinguishing between the enrichment or depletion in those components is critical for correctly understanding the timing of planetary volatile depletion, as they predict very different anomalies in ⁸⁷Sr/⁸⁶Sr and lead to disparate interpretations of ⁸⁷Rb–⁸⁷Sr chronology. Here, we constrain the origin of Sr isotope anomalies in the original Mason and Taylor (1982) set of Allende CAIs by examining their nucleosynthetic Sr and Nd isotope systems. Most CAIs analysed exhibit positive μ⁸⁴Sr and negative μ^145,148,150Nd anomalies (μ-values are defined as part-per-million deviations of isotopic ratios relative to terrestrial standards) in agreement with previous studies, but we also detected significant isotope heterogeneities among them, including discovery of CAIs with Sr and Nd isotope anomalies in directions opposite to the majority. The Nd isotope heterogeneity among CAIs is predominantly consistent with variations in the abundance of the s-process component, with a minor but clearly resolved p-process deficit on μ¹⁴²Nd_corr (μ¹⁴²Nd_corr is μ¹⁴²Nd corrected for ¹⁴⁶Sm decay). The less steep μ¹⁴²Nd_corr vs. μ¹⁴⁸Nd slope defined by the CAIs compared to that predicted by stellar models supports the recent suggestion that the accessible Earth has a small radiogenic excess in μ¹⁴²Nd relative to chondrites. Correlated Sr and Nd isotope anomalies in the CAIs suggest that (1) they formed from at least two isotopically distinct reservoirs, one with and the other without p-process Sr excesses relative to Earth, (2) the majority of CAIs formed in the p-process-Sr-enriched reservoir with additional s-process excesses, and (3) variations in r-process Sr and Nd are not observed among the CAIs. The s-process-induced ⁸⁷Sr/⁸⁶Sr anomalies in CAIs (relative to the inner Solar System) predicted based on Nd isotopes are below the typical measurement precision, negating the need for nucleosynthetic correction on CAIs’ ⁸⁷Sr/⁸⁶Sr in chronological interpretations

¹Pipasa Layak, ^1,2Nachiketa Rai, ³Kuljeet Kaur Marhas, ^4,5Hilary Downes
Geochemistry (Chemie der Erde) (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2026.126426]
¹Department of Earth Sciences, Indian Institute of Technology Roorkee, 247667, India
²Centre for Space Science and Technology, Indian Institute of Technology Roorkee, 247667, India
³Planetary Science Division, Physical Research Laboratory, Ahmedabad, 380009, India
⁴School of Natural Sciences, Birkbeck University of London, Malet Street, London, WC1E 7HX, UK
⁵Natural History Museum, Cromwell Road, London, SW7 5BD, UK
Copyright Elsevier

This study models the compositional evolution of a chondritic starting material representative of the Mesosiderite Parent Body (MSPB) under relevant pressure-temperature-redox conditions (1 bar, 1800–500 °C, fO2 = IW + 1.8), focusing exclusively on the evolution of the silicate portion of the system and not on the origin or evolution of the metallic component. The modelling framework assumes crystallization within a silicate magma ocean, and explores crystallization pathways involving varying degrees of equilibrium crystallization (EC) and fractional crystallization (FC). In addition, we present new mineral-chemistry, and phase data from two mesosiderite specimens, Estherville and Mincy.
Modelling results indicate that mesosiderite silicate mineralogy can be derived from a chondritic composition through an efficient three-stage cooling sequence: 40–50% EC, followed by FC down to 1395 °C, and then final EC of the remaining melt to 914 °C, at which point crystallization is complete. The predicted modal abundances—69 wt% pyroxenes, 26 wt% plagioclase, 1.9 wt% tridymite, and 1.6 wt% whitlockite—closely match the observed proportions in Estherville and Mincy. In both meteorites, pyroxenes and tridymites serve as robust geothermometers, stable across 870–1470 °C. The strong agreement between modelled Mg#, Fe#, density, and fO2 with published mesosiderite values further supports a chondritic starting composition of the MSPB.
The model suggests that the MSPB mantle consisted of olivine-orthopyroxene cumulates (dunitic in character), while the lower crust was dominated by pigeonite and hypersthene, forming a pyroxenitic lithology. The upper crust was enriched in plagioclase and pyroxene, reflecting a basaltic composition. Following differentiation, the MSPB likely underwent a collisional encounter with a differentiated impactor, leading to excavation of its silicate crust, followed by brecciation, remelting, and clast metamorphism. These processes ultimately produced mesosiderite meteorites as composite breccias of MSPB-derived silicates intermixed with metallic phases contributed by the impacting body.

¹Dian Ji, ¹Rajdeep Dasgupta
Earth and Planetary Science Letters 690, 120133 Link to Article [https://doi.org/10.1016/j.epsl.2026.120133]
¹Department of Earth, Environmental and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USA
Copyright Elsevier

As the youngest returned mare basalt to date, deciphering the petrogenesis of Chang’e-5 (CE-5) basalts could assist us in understanding the late-stage lunar mantle evolution. However, the lithology of the CE-5 mantle is still controversial. Here, we present a joint study of high pressure-temperature major element phase equilibria using laboratory experiments and trace element modeling, and show that garnet is present in the source of young mare basalts on the Moon. Our high-pressure-temperature experiments approaching the multiple saturation point of the putative parental melt compositions of young CE-5 basalt show garnet and clinopyroxene coexisting with the quenched silicate melt. Trace element modeling also confirms that small amounts of garnet in the mantle source are required to reconcile the rare-earth element pattern of the CE-5 basalts, regardless of whether the CE-5 basalts were the direct mantle-melting products or produced through extensive fractional crystallization of a primitive basalt. The existence of garnet in the source either implies that lunar mantle overturn played a crucial role in contributing to the lunar mantle heterogeneity, creating a garnet-clinopyroxenitic mantle source of these young mare basalts that survived the subsequent mantle evolution, or a late-stage process introduced fertile material into the deep mantle, and such geodynamic processes within the lunar interior may have continued until 2 Ga.

¹Varun Manilal, ¹Kyusei Tsuno, ²Hideharu Kuwahara, ³Axel Wittmann, ⁴Anne Pommier, ⁵Christy Till, ¹Damanveer S. Grewal
Geochimica et Cosmochimica Acta (in Press) Link to Article [10.1016/j.gca.2026.06.018]
¹Department of Earth and Planetary Sciences, Yale University, New Haven, CT 06511, USA
²Geodynamics Research Center, Ehime University, Ehime 790-8577, Japan
³Eyring Materials Center, Arizona State University, Tempe, AZ 85281, USA
⁴Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA
⁵School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA
Copyright Elsevier

Percolation is a leading mechanism for metal-silicate segregation during the earliest stages of planetesimal differentiation triggered by 26Al-heating. The efficacy of percolative core formation strongly depends on the compositions of metallic melts and co‑existing silicate mineralogies. We report results from 21 high-pressure–temperature (P = 0.5–2 GPa; T = 1350 °C) experimental charges designed to simulate percolative core formation conditions in oxidized planetesimals. These experiments equilibrated olivine aggregates having variable FeO contents (0–30 wt%) with Fe-O-S metallic melts generated from two starting compositions (eutectic-like Fe1.46S and stoichiometric FeS). Our results quantify how pressure, FeO content in olivine, and metal composition affect oxygen solubility and dihedral angles in Fe-O-S alloys. Both O solubility and dihedral angles are largely insensitive to pressure over planetesimal conditions (P < 2 GPa) but are strongly dependent on olivine FeO content and metallic composition. For eutectic systems, dihedral angles decrease from 80° to 44° for olivine containing 1 wt% and 20 wt% FeO, respectively, crossing the critical interconnection threshold near 10 wt% FeO. For FeS systems, dihedral angles decrease more gradually (67° at 5 wt% FeO to 53° at 20 wt% FeO), consistent with the lower solubility of O in FeS-rich melts and a higher FeO threshold for connectivity. We developed a Langmuir competitive adsorption framework to explain these observations: S and O compete for interfacial binding sites at the olivine-metal boundary, with their relative coverage controlled by the activities of FeO and FeS. This predictive framework explains why O solubility is systematically lower in FeS-rich melts than in eutectic melts at the same fO2 and demonstrates that both S and O act synergistically as surface-active elements that reduce dihedral angles and interfacial energies, with O exerting a stronger effect. Applying these results to differentiation of oxidized planetesimals (fO2 > IW; based on the compositions of oxidized chondrites), we find that the first-formed metallic melts would have formed interconnected networks at extremely low melt fractions and were capable of forming cores prior to widespread silicate melting in their parent bodies. Using the experimentally defined connectivity and O-solubility thresholds, we model core compositions in oxidized planetesimals as combinations of eutectic and FeS-rich Fe-O-S melts. The resulting metallic cores are predicted to be consistently O-bearing, ranging from ∼3–4 wt% O in CM-/CI-like bodies to ∼5–6 wt% O in CV-ox- and RC-like bodies. These predicted O contents are first-order estimates, as both the higher experimental temperature (1350 °C vs. ∼988–1200 °C during percolative core formation in planetesimals) and the absence of Ni in our metallic melts would likely overestimate O solubility in natural systems.

¹Ralph E. Milliken et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70179]
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, Rhode Island, USA
Published by arrangement with John Wiley & Sons

Near-Earth rubble-pile asteroids Bennu and Ryugu are part of the carbonaceous taxonomic complex (C-complex), and samples returned from both bodies resemble the most aqueously altered carbonaceous chondrites. However, telescopic and spacecraft visible–near infrared (VIS–NIR) reflectance spectra of Ryugu exhibit a red (positive) spectral slope, whereas Bennu has a blue (negative) spectral slope characteristic of the rare B-type subclass of asteroids. The asteroid spectra also suggest different levels of hydration, with Ryugu dominated by OH and Bennu containing spectral evidence of more H2O. To understand what causes these differences, we acquired VIS–NIR reflectance data (~0.3–5 μm) from a variety of Bennu samples over spatial scales of 100 μm to several millimeters. No single sample reproduces the average spectral properties of Bennu, but by evaluating samples of different petrology and physical states—groups of particles, isolated particles, and larger stones—we demonstrate that primary composition, and highly hydrated Mg-rich phosphate in particular, plays a strong role in controlling the spectral slope and average hydration absorption strength of Bennu materials. Bennu and Ryugu may be dominated by different lithologies originating from different regions of a common planetesimal, thus explaining their different spectral evolution. The spectral characteristics of B-type asteroids, particularly those with blue slopes at near-infrared wavelengths and broad hydration features at ~3 μm, may indicate the presence of Mg phosphate and thus a history of complex fluid–rock interactions relevant to prebiotic chemistry.