^1,2Jing Li,^1,3Lixin Gu,¹Xu Tang,^1,2Xiaoying Liu,¹Sen Hu,^1,2Yangting Lin
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116082]
¹Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
²University of Chinese Academy of Sciences, Beijing, China
³Institutional Center for Shared Technologies and Facilities, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Copyright Elsevier

Meteorite impact and solar wind irradiation are two major processes of space weathering on the Moon, but their effects on lunar soils remain poorly understood. We report the first discovery of native copper and FeCo alloy from a Chang’e-5 lunar basaltic clast. The native copper grains (~0.1–2 μm) coexist unexceptionally with thin Cu-sulfide (digenite) layers (~50–100 nm) that cover troilite, the associated iron whiskers and adjacent pyroxene and ilmenite. The FeCo alloy (~100–200 nm) was associated with the iron metal on troilite. Both Cu and Co are probably exogenous since they are not detected in the troilite. These observations suggest that the digenite layer, native copper and FeCo alloy condensed from an impact-induced vapor with a high Cu/S ratio and Co-bearing composition. Our discoveries shed light on complicated space weathering processes on the Moon.

^1,2G.L. Dalla Pria,^1,3O. Sohier,¹C. Scirè,¹R.G. Urso,¹G.A. Baratta,¹M.E. Palumbo
Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2024.116077]
¹INAF-Osservatorio Astrofisico di Catania, Via Santa Sofia 78, Catania, 95123, Italy
²Luleå University of Technology, Laboratorievägen 14, Luleå, 97187, Sweden
³Université de Versailles Saint-Quentin-en-Yvelines, 55 avenue de Paris, Versaille, 78035, France
Copyright Elsevier

Volatile organic molecules and a complex organic refractory material were detected on the Moon and on lunar samples. The Moon’s surface is exposed to a continuous flux of solar UV photons and fast ions, e.g. galactic cosmic rays (GCRs), solar wind (SW), and solar energetic particles (SEPs), that modify the physical and chemical properties of surface materials, thus challenging the survival of organic compounds. With this in mind, the aim of this work is to estimate the lifetime of organic compounds on the Moon’s surface under processing by energetic particles. We performed laboratory experiments to measure the destruction cross section of selected organic compounds, namely methane (CH4), formamide (NH2CHO), and an organic refractory residue, under simulated Moon conditions. Volatile species were deposited at low temperature (17 – 18 K) and irradiated with energetic ions (200 keV) in an ultra-high vacuum chamber. The organic refractory residue was produced after warming up of a CO:CH4 ice mixture irradiated with 200 keV H+ at 18 K. All the samples were analyzed in situ by infrared transmission spectroscopy. We found that destruction cross sections are strongly affected (up to one order of magnitude) by the dilution of a given organic in an inert matrix. Among the selected samples, organic refractory residues are the most resistant to radiation. We estimated the lifetime of organic compounds on the surface of the Moon by calculating the dose rate due to GCRs and SEPs at the Moon’s orbit and by using the experimental cross section values. Taking into account impact gardening, we also estimated the fraction of surviving organic material as a function of depth. Our results are compatible with the detection of CH4 in the LCROSS eject plume originating from layers deeper than about 0.7 m at the Moon’s South Pole and with the identification of complex organic material in lunar samples collected by Apollo 17 mission.

^1,2S. A. Eckley,²R. A. Ketcham,³Y. Liu,^4,5,6,7J. Gross,⁷F. M. McCubbin
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14165]
¹Jacobs—JETS, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, Texas, USA
²Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas, USA
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
⁴Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
⁵Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
⁶Lunar and Planetary Institute, Houston, Texas, USA
⁷NASA Johnson Space Center, Houston, Texas, USA
Published by arrangement with John Wiley & Sons

Shergottites are mafic to ultramafic igneous rocks that represent the majority of known Martian meteorites. They are subdivided into gabbroic, poikilitic, basaltic, and olivine–phyric categories based on differences in mineralogy and textures. Their geologic contexts are unknown, so analyses of crystal sizes and preferred orientations have commonly been used to infer where shergottites solidified. Such environments range from subsurface cumulates to shallow intrusives to extrusive lava flows, which all have contrasting implications for interactions with crustal material, cooling histories, and potential in situ exposure at the surface. In this study, we present a novel three-dimensional (3-D) approach to better understand the solidification environments of these samples and improve our knowledge of shergottites’ geologic contexts. Shape preferred orientations of most phases and crystal size distributions of late-forming minerals were measured in 3-D using X-ray computed tomography (CT) on eight shergottites representing the gabbroic, poikilitic, basaltic, and olivine–phyric categories. Our analyses show that highly anisotropic, rod-like pyroxene crystals are strongly foliated in the gabbroic samples but have a weaker foliation and a mild lineation in the basaltic sample, indicating a directional flow component in the latter. Star volume distribution analyses revealed that most phases (maskelynite, pyroxene, olivine, and oxides/sulfides) preserve a foliated texture with variable strengths, and that the phases within individual samples are strongly to moderately aligned with respect to one another. In combination with relative cooling rates during the final stages of crystallization determined from interstitial oxide/sulfide crystal size distribution analyses, these results indicate that the olivine–phyric samples were emplaced as shallow intrusives (e.g., dikes/sills) and that the gabbroic, poikilitic, and basaltic samples were emplaced in deeper subsurface environments.

¹Tatsuhiro Michikami,²Axel Hagermann,^3,4,5Akira Tsuchiyama,¹Yushi Otsuka,⁶Michihiko Nakamura,⁶Satoshi Okumura,⁷Harumasa Kano,³Junya Matsuno,⁸Sunao Hasegawa
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116068]
¹Faculty of Engineering, Kindai University, Hiroshima Campus, 1 Takaya Umenobe, Higashi-Hiroshima, Hiroshima 739-2116, Japan
²Luleå University of Technology, Space Campus, Kiruna 981 28, Sweden
³Research Organization of Science and Technology, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga 525-8577, Japan
⁴CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), 511 Kehua Street, , Tianhe District, Guangzhou 510640, Wushan, China
⁵CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
⁶Department of Earth Science, Graduate School of Science, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan
⁷The Tohoku University Museum, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai, Miyagi 980-8578, Japan
⁸Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-8510, Japan
Copyright Elsevier

The surfaces of sub-kilometer-sized asteroids directly explored by spacecraft, such as Itokawa, Ryugu and Bennu, are covered with blocks and/or regolith particles, whose shapes are considered clues to understanding their formation and evolution on the asteroid’s surface. Ryugu particles returned by the Hayabusa2 mission are likely fragments resulting from impacts because their shapes resemble impact fragments from laboratory experiments. However, there is a lack of laboratory impact experiments examining the shapes of fragments in carbonaceous chondrites, thought to originate from carbonaceous asteroids such as Ryugu and Bennu. The measured sizes of Ryugu particles are in the mm and sub-mm range, similar to the sizes of chondrules. Also, carbonaceous chondrites are generally structurally weaker than the basalts and granites often used in previous laboratory impact experiments. Differences in the strength of the chondrules and matrix might affect the overall strength of the meteorite. In this study, as a first step towards a better understanding of impact fragment shapes in carbonaceous chondrites, we conducted impact experiments on the carbonaceous meteorite Allende (CV3). A spherical alumina projectile with 1.0 mm and a glass projectile with 0.80 mm in diameter were fired into 1–2 cm-sized Allende targets at nominal impact velocities of 2.0 and 4.0 km/s, respectively. To investigate the correlation between the chondrules (typically sub-mm in size) and the shapes of fine fragments, we measured the shape distributions of sub-mm impact fragments using X-ray microtomography. We observed several impact fracture surfaces along the chondrule boundaries. In addition, these fragments tended to be rounder than fragments from previous impact experiments. However, because the total number of these fragments is relatively small, the fragments were found to have the same overall shape distribution as previous laboratory impact fragments, Itokawa particles and Ryugu particles. This may imply that impact fragment shapes are independent of the bulk material strength. These findings will be useful for understanding the formation process of regolith layers on asteroid surfaces, Itokawa particles, Ryugu particles, and Bennu particles.

¹Xue Su,¹Youxue Zhang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.04.002]
¹Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA
Copyright Elsevier

The H2O concentration and H2O/Ce ratio in olivine-hosted melt inclusions are high in lunar pyroclastic sample 74,220 (H2O up to 1410 ppmw; H2O/Ce up to 77) but lower (H2O 10 to 430 ppmw; H2O/Ce 0.3 to 9.4) in all other lunar samples studied before this work. The difference in H2O concentration and in H2O/Ce ratio is absent for other volatile elements (F, S, and Cl) in melt inclusions in 74,220 and other lunar samples. Because H2O (or H) is a critical volatile component with significant ramifications on the origin and evolution of the Moon, it is important to understand what causes such a large gap in H2O/Ce ratio between 74,220 and other lunar samples. Two explanations have been advanced. One is that volcanic product in sample 74,220 has the highest cooling rate and thus best preserved H2O in melt inclusions compared to melt inclusions in other samples. The other explanation is that sample 74,220 comes from a localized heterogeneity enriched in some volatiles. To distinguish these two possibilities, here we present new data from two rapidly cooled lunar samples with glassy melt inclusions: olivine-hosted melt inclusions (OHMIs) in 79,135 regolith breccia (unknown cooling rate but with glassy MIs similar in texture with those in 74220), and pyroxene-hosted melt inclusions (PHMIs) in 15,597 pigeonite basalts (known high cooling rate, second only to 74,220 and 15421). In addition, we also investigated new OHMIs in sample 74220. If the gap is due to the difference in cooling rates, samples with cooling rates between those of 74,220 and other studied lunar samples should have preserved intermediate H2O concentrations and H2O/Ce ratios. Our results show that melt inclusions in 79,135 and 15,597 contain high H2O concentrations (up to 969 ppmw in 79,135 and up to 793 ppmw in 15597) and high H2O/Ce ratios (up to 21 in 79,135 and up to 13 in 15997), bridging the big gap in H2O/Ce ratio among 74,220 and other lunar samples. Combined with literature data, we confirm that H2O/Ce ratios of different lunar samples are positively correlated to the cooling rates and independent of the type of mare basalts. We hence reinforce the interpretation that the lunar sample with the highest cooling rate best represents pre-eruptive volatiles in lunar basalts due to the least degassing. Based on Ce concentration in the primitive lunar mantle, we estimate that H2O concentration in the primitive lunar mantle (meaning bulk silicate Moon) is 121 ± 15 ppmw. Our new data also further constrain F/P, S/Dy and Cl/Ba ratios in lunar basalts and the lunar mantle. Estimated F, P, and S concentrations in the lunar primitive mantle are 4.4 ± 1.1 ppmw, 22 ± 8 ppmw, and
ppmw, respectively.

^1,2Evan W. O’Neal,¹A.M. Ostwald,¹A. Udry,^3,4J. Gross,⁴M. Righter,⁵T.J. Lapen,⁶J. Darling,⁷G.H. Howarth,¹R. Johnsen,⁵D.R. McQuaig
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.03.016]
¹Department of Geoscience, University of Nevada Las Vegas, Las Vegas NV 89154, USA
²Jacobs Technology, NASA Johnson Space Center (JSC), Houston, TX 77058, USA
³Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA
⁴Department of Earth and Planetary History, American Museum of Natural History, New York, NY 10024, USA
⁵Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX 77204, USA
⁶School of the Environment, Geography and Geosciences, University of Portsmouth, Portsmouth PO1 3QL, UK
⁷Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa
Copyright Elsevier

Martian poikilitic shergottites are cumulate rocks that can help advance the understanding of magmatic evolution from near the base of the crust (∼10 kbar) to near-surface conditions. Through a comprehensive petrographic and geochemical study, we aim to better understand poikilitic shergottite formation and the evolution in the martian interior. A suite of poikilitic shergottites including Northwest Africa (NWA) 7755, NWA 11043, NWA 11065, NWA 10618 and Alan Hills (ALHA) 77,005 were investigated for their major, minor, and trace element compositions of olivine-hosted melt inclusions (MI). The MI occur within both the early-evolutional stage textural and late-evolution stage textural domains in olivine. Major element compositions of MI indicate fractional crystallization between the early and late-crystallizing domains. Calculated parental melt compositions from these MI data yielded results that also petrogenetically link the poikilitic shergottites with the olivine-phyric shergottite subgroup. Trace element compositions of MI show that the later-crystallizing MI could have undergone open-system processes, such as fluid exsolution. Lutetium-Hf and Sm-Nd isotopic analyses were performed on NWA 7755 and NWA 11043 to constrain their age and source isotopic compositions. Northwest Africa 7755 shows a 176Lu/177Hf crystallization age of 223 ± 46 Ma, which fits into the expected range for enriched shergottites of ∼165 Ma to 225 Ma. A similar crystallization age and 176Lu/177Hf and 147Sm/144Nd source composition of NWA 7755 to the other enriched shergottites suggests that this specimen likely shares a long-lived geochemical source with these samples that has lasted for at least 60 Ma. Northwest Africa 11043 shows scatter throughout the Lu-Hf and Sm-Nd isotopic data, suggesting that this sample is not in isotopic equilibrium. This sample was possibly inherited from high-temperature processes, such as incomplete magmas mixing from a similar, but distinct, source. We conducted in situ U-Th-Pb isotope analyses of Ca-phosphate minerals for NWA 11043 and found an unreliable crystallization age of 59.2 ± 138.4 Ma: phosphates are likely recording the period of shock metamorphism related to the ejection event. Consistent crystallization ages and magmatic histories support previous work that suggest there is a common magmatic system on Mars that is responsible for the formation of enriched shergottites.

^1,2,3Akira Tsuchiyama et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.03.032]
¹Research Organization of Science and Technology, Ritsumeikan University, Shiga 525-8577, Japan
²Chinese Academy of Sciences (CAS) Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, CAS, Guangzhou 510640, China
³CAS Center for Excellence in Deep Earth Science, Guangzhou 510640, China
Copyright Elsevier

Samples collected from the surface/subsurface of C-type asteroid 162,173 Ryugu by the Hayabusa2 mission were nondestructively analyzed in three dimensions (3D). Seventy-three small particles (approximately 10–180 µm in size) were observed using X-ray nanotomography, with an effective spatial resolution of approximately 200 nm. Detailed descriptions of these samples in terms of mineralogy, petrology, and variations among particles were reported. The 57 most common particles consisted of a phyllosilicate matrix containing mineral grains, mainly magnetite, pyrrhotite, dolomite and apatite. The remaining particles were mostly monomineralic particles (pyrrhotite, dolomite, breunnerite, apatite, and Mg-Na phosphate) with two unique particles (calcite in a Al2Si2O5(OH)4 matrix, and CaCO3, phyllosilicate, and tochilinite-chronstedtite inclusions in a carbonaceous material matrix). The results confirmed that the samples correspond to Ivuna-type carbonaceous chondrites (CI chondrites) or related materials. Many small inclusions of voids and carbonaceous materials were detected in pyrrhotite, dolomite, breunnerite, and apatite. However, no fluid inclusions were observed, except for those in pyrrhotite that have already been reported. Magnetite exhibited a wide variety of morphologies, from irregular shapes (spherulites, framboids, plaquettes, and whiskers) to euhedral shapes (equants, rods, and cubes), along with transitional shapes. In contrast, the other minerals exhibit predominantly euhedral shapes (pyrrhotite: pseudo-hexagonal plates, dolomite: flattened rhombohedrons, breunnerite: largely flattened rhombohedrons, and apatite: hexagonal prisms) or aggregates of faceted crystals, except for Mg-Na phosphate. The matrices were heterogeneous with variable phyllosilicate particle sizes, Mg/Fe ratios, density (1.7 ± 0.2 g/cm3), nanoporosities (36 ± 9 %), and abundances of nanograins of Fe(-Ni) sulfides. The macroporosity of the particles was estimated as 12 ± 4 %.

The observed textural relationships among the minerals suggest a precipitation sequence of: magnetite (spherulite → plaquette/framboid → rod/equant) → pyrrhotite (pentlandite → pyrrhotite) → apatite → dolomite → breunnerite → coarse phyllosilicates. Fe-bearing olivine (or low-Ca pyroxene) might have precipitated later than dolomite, indicating a high Mg activity in the aqueous solution. This precipitation sequence corresponds to a transition from irregular crystal forms (as seen in some magnetite) to regular forms of euhedral crystals (observed in some magnetite and other minerals). Based on the precipitation sequence and mineral morphologies, together with previously reported observations, a model for aqueous alteration in the Ryugu parent body was proposed as follows: CO2-H2O ice, amorphous silicates (GEMS-like material), and some minerals (mostly metal, sulfides, and anhydrous silicates) accumulated to form the parent body of Ryugu. Amorphous silicates and Fe-Ni metal quickly dissolved into the melted ice to form a highly supersaturated aqueous solution. Poorly-crystalized phyllosilicate and spherulitic magnetite precipitated first, followed by plaquette/framboidal magnetites with decreasing degree of supersaturation due to precipitation. Pseudo-hexagonal pyrrhotite plates were formed by dissolution and reprecipitation under relatively low supersaturation. Subsequently, apatite, dolomite, and breunnerite precipitated in this order in response to decreasing supersaturation.

^1,2Jie-Jun Jing,^3,4Ben-Xun Su,⁵Jasper Berndt,²Hideharu Kuwahara,¹Wim van Westrenen
Geochimica et Cosmochimica Acta (inPress) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.03.028]
¹Department of Earth Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands
²Geodynamics Research Center, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
³Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, 100029, Beijing, China
⁴University of Chinese Academy of Sciences, 100049, Beijing, China
⁵Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Correnstraße 24, D48149 Münster, Germany
Copyright Elsevier

Fifteen experiments at 1 atm pressure and 1400 °C have been conducted to determine partition coefficients between olivine and silicate melt (
) of the first-row transition elements (FRTEs, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn), Ga and Ge in the system FeO-CaO-MgO-Al2O3-SiO2 (FCMAS). Bulk iron contents are varied between 0 and 10 wt% FeO, and oxygen fugacity ranges from 2 log units below the iron-wüstite buffer (IW-2) to 2 log units above the quartz-fayalite-magnetite buffer (QFM + 2), covering a range of igneous processes involving olivine in terrestrial and lunar conditions. Results show that multi-valent Fe and V are redox-sensitive and more incompatible at oxidizing conditions, consistent with previous studies. The moderately volatile elements (Cu, Zn, Ga and Ge) become more volatile at reducing conditions. No correlation between partition coefficients and oxygen fugacity is observed for other multi-valent (Ti, Cr, Mn) and for homo-valent elements (Sc, Co and Ni). Mostshow no sensitivity to bulk system iron contents, but
is significantly higher in our experiments compared toderived from olivine-melt inclusion pairs in lunar samples with much higher FeO contents.
values are nearly constant at a range of oxygen fugacities above the IW buffer, but abruptly decrease when the system is very reducing (below the IW buffer). As a result,
ratios that are constant (∼0.3) at or above the IW buffer increase significantly (0.72–0.99) at IW-2. Using the newly derived partition coefficients, we re-assess two aspects of lunar basalt generation. First, we conclude that the Cr-rich nature of the olivines in lunar basalts compared to terrestrial basalts must be attributed to the Cr-nature of cumulate mantle source of lunar basalts, linked to the early crystallization of Cr-poor minerals olivine and orthopyroxene in the lunar magma ocean resulting in a shallow Cr-rich cumulates. Second, the higher Co/Ni ratios in olivine in high-titanium lunar basalts compared to olivine in low-titanium lunar basalts suggest the former were formed at more reducing conditions (below the IW buffer).

¹Zaicong Wang et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.03.029]
¹State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
Copyright Elsevier

The Moon’s mare volcanism predominantly occurs within the Procellarum KREEP Terrane (PKT), which is widely thought to be associated with KREEP components within the lunar interior. The Chang’e-5 (CE-5) mission sampled a young (2 Ga) mare basalt Em4/P58 unit of northern Oceanus Procellarum. The geochemistry of the CE-5 mare basalt enables assessment of mantle source compositions which are essential to understand the thermo-chemical mechanism for prolonged volcanism during secular cooling of the Moon. Geochemical compositions of the CE-5 bulk soil, breccias, and basalt clasts from various depths within the drill core consistently display high concentrations of incompatible trace elements (ITE: ∼ 0.3 × high-K KREEP; ∼ 5 μg/g Th) with KREEP-like inter-element ratios, for example for La/Sm, Nb/Ta, and Zr/Y. Exotic impact ejecta, extensive magma differentiation (<70 % fractional crystallization) and significant assimilation of KREEP materials during magma transit and eruption cannot account for the ITE contents and ratios or radiogenic isotope compositions (e.g., εNdinitial of + 8 to + 9 and εHfinitial of + 40 to + 46) of the CE-5 basalts; instead, partial melting of their mantle source played a dominant role. The Chang’e-5 basalt is a chemically evolved low-Ti mare basalt (Mg# of ∼ 34) with enriched KREEP-like ITE compositions but high long-term time-integrated Sm/Nd and Lu/Hf ratio, which represents a hitherto unsampled type of mare basalt. It formed by melting of an augite-rich mantle source (late-stage magma ocean cumulates containing > 30–60 % augite, and little or no ilmenite), with a small amount of late-stage interstitial melt that resembles KREEP (∼1–1.5 modal %, equivalent to 0.2–0.3 μg/g Th). The voluminous mare basalts making up the Em4/P58 unit (>1500 km3) provide compelling evidence for large-scale, ITE enriched young mare magmatism within Oceanus Procellarum. In combination with remote sensing data and with the unique Th-rich Apollo 12 basalt fragment 12032,366–18 (impact ejecta likely from Oceanus Procellarum), this implies that significant portions of the FeO- and Th-rich mare regions of the western PKT may also have formed in a similar way.

¹Takazo Shibuya,^2,3Yasuhito Sekine,⁴Sakiko Kikuchi,^5,6,2Hiroyuki Kurokawa,³Keisuke Fukushi,⁷Tomoki Nakamura,⁸Sei-ichiro Watanabe
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.03.022]
¹Super-cutting-edge Grand and Advanced Research (SUGAR) Program, Institute for Extra-cutting-edge Science and Technology Avant-garde Research (X-star), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Yokosuka 237-0061, Japan
²Earth-Life Science Institute (ELSI), Tokyo Institute of Technology, Meguro, Tokyo 152-8550, Japan
³Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
⁴Kochi Institute for Core Sample Research, Institute for Extra-cutting-edge Science and Technology Avant-garde Research (X-star), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Nankoku, Kochi 783-8502, Japan
⁵Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo 153-8902, Japan
⁶Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan
⁷Department of Earth Sciences, Tohoku University, Sendai 980-8578, Japan
⁸Department of Earth and Environmental Sciences, Nagoya University, Nagoya 464-8601, Japan
Copyright Elsevier

Parent bodies of carbonaceous chondrites that initially contained metallic iron potentially exert strong reduction power during aqueous alteration to generate molecular hydrogen in excess of hydrogen solubility in water-rich fluids. The surplus hydrogen escapes from the system, which is subsequently supplied to overlying regions in planetesimals. Based on this concept, we conducted chemical equilibrium modeling of the aqueous alteration and simulated gaseous H2 migration within the icy planetesimal that has a melted mantle and an icy shell during the early stages of radiogenic heating. In the chemical equilibrium modeling, we simulated the aqueous alteration of chondritic rocks at 0–350 °C and a water/rock mass ratio of 0.2–10 with initial CO2 contents of 0–10 mol% in the fluid. The results showed that the mineral assemblage and solution composition change with the temperature, water/rock mass ratio, and initial fluid composition. The reproduced mineral paragenesis and abundance well explain those of carbonaceous chondrites. Furthermore, it was revealed that the initial H2 fugacity of the system influences not only the stability of minerals and solution compositions, but also the preservation potential of organic molecules. Indeed, within these parameter spaces, the modeling results account for the organic/inorganic carbon-rich alterations reported for the Tagish Lake meteorite, Ceres, and Ryugu. Simulations of gaseous H2 migration in a planetesimal revealed that gaseous H2 in the deep interior can be transported to the interface with an icy shell even if the permeability is low. Moreover, it is highly possible that an H2-rich layer would have been widely formed just below the icy shell. Therefore, it is expected that H2-rich regions beneath the ice layer in planetesimals have substantial potential for the synthesis and preservation of organic molecules. These results imply that the alteration of carbonaceous chondrite parent bodies and C-complex asteroids is characterized by not only the type of parent bodies (e.g., formation age and distance from the Sun) but also the locations within their parent bodies.