^1,2Daisuke Nakashima,^3,4Takaaki Noguchi,^2,5Takayuki Ushikubo,^6,7Makoto Kimura,²Noriko Kita
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.04.011]
¹Department of Earth Science, Tohoku University, Aoba, Sendai, Miyagi 980-8578, Japan
²Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
³Faculty of Arts and Science, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
⁴Division of Earth and Planetary Sciences, Kyoto University, Kyoto 606-8502, Japan
⁵Kochi Institute for Core Sample Research, JAMSTEC, Monobe-otsu 200, Nankoku, Kochi 783-8502, Japan
⁶Faculty of Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan
⁷National Institute of Polar Research, Tokyo 190-8518, Japan
Copyright Elsevier

Oxygen isotope ratios and elemental compositions of porphyritic chondrules and olivine and pyroxene fragments in the Asuka-881020 CH chondrite were analyzed. The oxygen isotope ratios inside individual porphyritic chondrules are homogeneous within the uncertainty, except for relict grains of olivine and low-Ca pyroxene that have distinct oxygen isotope ratios. The average oxygen isotope ratios of the individual chondrules plot along and above the primitive chondrule mineral (PCM) line with Δ17O (=δ17O – 0.52 × δ18O) values from −4.7 ‰ to +4.1 ‰. The olivine and pyroxene fragments, which have Δ17O values ranging from −2.1 ‰ to +3.2 ‰, are likely to be fragments of the porphyritic chondrules.

Unlike the non-porphyritic chondrules in CH and CB chondrites and chondrules in other carbonaceous chondrites, the type I and II chondrules do not show a systematic difference in the Δ17O values. Furthermore, the Δ17O values of the type I chondrules increase from −4.7 ‰ to +4.1 ‰ with increasing Mg# (=molar [MgO]/[MgO + FeO] × 100) from 96 to 99. We argue that the positive Δ17O-Mg# trend is explained by an addition of 16O-poor carbon-rich organics as a reducing agent to the relatively 16O-rich precursor silicate, which is a new environment for chondrule formation. This hypothesis is supported by the two lines of evidence observed in the present study. (1) The chondrules and fragments with higher Δ17O values show larger deviations from the PCM line towards low δ18O, suggesting oxygen isotope mass fractionation between the chondrule melt and CO or CO2. (2) Olivine phenocrysts in the chondrules with high Δ17O values contain Ni-poor Fe-metal particles surrounded by silica-rich glass, which may be reduction products during the chondrule formation. Thus, it is suggested that the porphyritic chondrules in CH and CB chondrites have different origins from chondrules in any other chondrite types, even from the non-porphyritic chondrules in CH and CB chondrites.

¹Randy G. Hopkins,¹John. G. Spray,²Erin L. Walton
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14168]
¹Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada
²Department of Physical Sciences, MacEwan University, Edmonton, Alberta, Canada
Published by arrangement with John Wiley & Sons

Thermodynamic modeling has been applied to determine pressure–temperature–time conditions leading to shock vein formation during the passage of a natural shock wave generated by hypervelocity impact. The approach is novel in considering both shock front and rarefaction pressures, as well as simultaneously forming and cooling the shock veins via two-dimensional steady-state conduction. Model results are tested using shock veins developed in granitic rocks that constitute the central uplift of the Steen River impact structure in Canada. Here, two variants of majoritic garnet were generated in different settings: (1) along the margins of shock veins due to pargasite and biotite breakdown (accompanied by maskelynite formation), and (2) within the originally molten shock vein matrix as newly grown crystals. We determine that during shock vein formation, the shock front pressure and wave width at the reconstructed sample location were 18 GPa and 830 m, respectively, with a dwell time of 160 ms. Intra-vein melting at 2150°C was attained within 1 μs. Melt cooled to the solidus in 150 ms following shock front passage. Majoritic garnet formation was facilitated by the high temperatures realized within the veins as a result of frictional melting that accompanied shock loading. The calculated pressure–temperature–time (P–T–t) path provides constraints on the formation conditions of majoritic garnet at Steen River. The model results independently support previously determined P–T conditions based on mineral stability fields. The vein margin garnets (35–39 mole% majorite) and maskelynite formed first under higher P–T conditions for a longer duration (36 ms). The matrix garnets (11–22 mole% majorite) crystallized from melt under lower P–T conditions and for a shorter duration (22 ms). Our results indicate that shock pressure alone should not be used as a basis for shock classification. Instead, the interplay between pressure and temperature with time and the duration of shock immersion (dwell) must be considered.

^1,2T. Michalik,¹A. Maturilli,³E. A. Cloutis,¹K. Stephan,⁴R. Milke,¹K.-D. Matz,⁴R. Jaumann,²L. Hecht,⁵H. Hiesinger,¹K. A. Otto
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14156]
¹Institut für Planetenforschung, Deutsches Zentrum für Luft- und Raumfahrt e.V., Berlin, Germany
²Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Museum für Naturkunde, Berlin, Germany
³Department of Geography, University of Winnipeg, Winnipeg, Manitoba, Canada
⁴Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany
⁵Institut für Planetologie, Westfälische Wilhelms-Universität, Münster, Germany
Published by arrangement with John Wiley & Sons

Pitted impact deposits on Vesta show higher reflectance and pyroxene absorption band strengths compared to their immediate surroundings and other typical Vestan materials. We investigated whether heating to different temperatures for different durations of Vestan regolith analog materials can reproduce these spectral characteristics using mixtures of HEDs, the carbonaceous chondrite Murchison, and terrestrial analogs. We find no consistent spectral trend due merely to temperature increases, but observed that the interiors of many heated samples show both higher reflectance and pyroxene band I strength than their heated surfaces. With electron probe microanalysis, we additionally observe the formation of hematite, which could account for the higher reflectance. The presence of hematite indicates oxidation occurring in the sample interiors. In combination with heat, this might cause the increase of pyroxene band strengths through migration of iron cations. The effect grows larger with increasing temperature and duration, although temperature appears to play the more dominant role. A higher proportion of Murchison or the terrestrial carbonaceous chondrite analog within our mixtures also appears to facilitate the onset of oxidation. Our observations suggest that both the introduction of exogenic material on Vesta as well as the heating from impacts were necessary to enable the process (possibly oxidation) causing the observed spectral changes.

¹Noriko T. Kita et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14163]
¹WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, Madison, Wisconsin, USA
Published by arrangement with John Wiley & Sons

Oxygen 3-isotope ratios of magnetite and carbonates in aqueously altered carbonaceous chondrites provide important clues to understanding the evolution of the fluid in the asteroidal parent bodies. We conducted oxygen 3-isotope analyses of magnetite, dolomite, and breunnerite in two sections of asteroid Ryugu returned samples, A0058 and C0002, using a secondary ion mass spectrometer (SIMS). Magnetite was analyzed by using a lower primary ion energy that reduced instrumental biases due to the crystal orientation effect. We found two groups of magnetite data identified from the SIMS pit morphologies: (1) higher δ18O (from 3‰ to 7‰) and ∆17O (~2‰) with porous SIMS pits mostly from spherulitic magnetite, and (2) lower δ18O (~ −3‰) and variable ∆17O (0‰–2‰) mostly from euhedral magnetite. Dolomite and breunnerite analyses were conducted using multi-collection Faraday cup detectors with precisions ≤0.3‰. The instrumental bias correction was applied based on carbonate compositions in two ways, using Fe and (Fe + Mn) contents, respectively, because Ryugu dolomite contains higher amounts of Mn than the terrestrial standard. Results of dolomite and breunnerite analyses show a narrow range of ∆17O; 0.0‰–0.3‰ for dolomite in A0058 and 0.2‰–0.8‰ for dolomite and breunnerite in C0002. The majority of breunnerite, including large ≥100 μm grains, show systematically lower δ18O (~21‰) than dolomite (25‰–30‰ and 23‰–27‰ depending on the instrumental bias corrections). The equilibrium temperatures between magnetite and dolomite from the coarse-grained lithology in A0058 are calculated to be 51 ± 11°C and 78 ± 14°C, depending on the instrumental bias correction scheme for dolomite; a reliable temperature estimate would require a Mn-bearing dolomite standard to evaluate the instrumental bias corrections, which is not currently available. These results indicate that the oxygen isotope ratios of aqueous fluids in the Ryugu parent asteroid were isotopically heterogeneous, either spatially, or temporary. Initial water ice accreted to the Ryugu parent body might have ∆17O > 2‰ that was melted and interacted with anhydrous solids with the initial ∆17O < 0‰. In the early stage of aqueous alteration, spherulitic magnetite and calcite formed from aqueous fluid with ∆17O ~ 2‰ that was produced by isotope exchange between water (∆17O > 2‰) and anhydrous solids (∆17O < 0‰). Dolomite and breunnerite, along with some magnetite, formed at the later stage of aqueous alteration under higher water-to-rock ratios where the oxygen isotope ratios were nearly at equilibrium between fluid and solid phases. Including literature data, δ18O of carbonates decreased in the order calcite, dolomite, and breunnerite, suggesting that the temperature of alteration might have increased with the degree of aqueous alteration.

¹Andreas Morlok,²Alexander Sehlke,¹Aleksandra Stojic,³Alan Whittington,¹Iris Weber,¹Maximilian P. Reitze,¹Harald Hiesinger,⁴Joern Helbert
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116078]
¹Institut für Planetologie, Wilhelmstr.10, 48149 Münster, Germany
²NASA Ames Research Center, Moffett Field, CA 94035, USA
³Department of Earth and Planetary Sciences, The University of Texas at San Antonio, USA
⁴Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
Copyright Elsevier

We studied a series of hermean lava analogs in the mid-infrared (2.5 μm–18 μm) to provide characteristic spectra for enstatite basalt, the Northern Volcanic Plains and Na-rich Northern Volcanic Plains. Our aim is to provide spectra for the interpretation of the data expected from Mercury from the MERTIS (MErcury Radiometer and Thermal Infrared Spectrometer) instrument on the ESA/JAXA BepiColombo mission.

Bulk powder spectra show bands of glass with a dominating broad Si-O-Si stretching feature around 10 μm. Crystalline components are mainly enstatite and forsterite with Reststrahlen Bands (RBs) around 9.3 μm, 9.6–9.9 μm, 10.0 μm, and 10.3–10.7 μm. Increasing intensity of crystalline features in the spectra reflect the increase in the crystallites in glass with decreasing temperature of equilibration and quenching. Micro-FTIR data allowed to extract spectral of individual components and glass. The position of the Christiansen Feature (CF) has only a weak correlation with the degree of crystallinity.

Correlations are observed between the Christiansen Feature (CF) and the bulk SiO₂ content of the materials, as does the correlation of this feature with the compositional index SCFM = SiO₂/(SiO₂ + CaO + FeO + MgO) on an atomic basis. This study also confirms the correlation line of glass-rich, irradiated Mercury analogs in these systems (Weber et al.,2023), indicating a similar spectral response of the glass rich materials expected for the surface of Mercury. The position of the strongest silicate main band (MB) compared to the SiO₂ content, confirms a trend for samples formed in experiments simulating high velocity impacts fall on a different trend line than analog samples formed in magmatic processes.

A comparison of the results to an Earth-based hermean surface spectrum showed similarities to spectra obtained for NVP samples.

¹A.R. Poppe,²I. Garrick-Bethell,³S. Fatemi,⁴C. Grava
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2024.116079]
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA 94720, USA
²Department of Earth and Planetary Sciences, Univ. of California Santa Cruz, Santa Cruz, CA, USA
³Department of Physics, Umeå University, Umeå, Sweden
⁴Southwest Research Institute, San Antonio, TX, USA
Copyright Elsevier

The ratio of 40Ar/36Ar trapped within lunar grains, commonly known as the lunar antiquity indicator, is an important semi-empirical method for dating the time at which lunar samples were exposed to the solar wind. The behavior of the antiquity indicator is governed by the relative implantation fluxes of solar wind-derived 36Ar ions and indigenously sourced lunar exospheric 40Ar ions. Previous explanations for the behavior of the antiquity indicator have assumed constancy in both the solar wind ion precipitation and exospheric ion recycling fluxes; however, the presence of a lunar paleomagnetosphere likely invalidates these assumptions. Furthermore, most astrophysical models of stellar evolution suggest that the solar wind flux should have been significantly higher in the past, which would also affect the behavior of the antiquity indicator. Here, we use numerical simulations to explore the behavior of solar wind 36Ar ions and lunar exospheric 40Ar ions in the presence of lunar paleomagnetic fields of varying strengths. We find that paleomagnetic fields suppress the solar wind 36Ar flux by up to an order-of-magnitude while slightly enhancing the recycling flux of lunar exospheric 40Ar ions. We also find that at an epoch of
2 Gya, the suppression of solar wind 36Ar access to the lunar surface by a lunar paleomagnetosphere is
somewhat fortuitously
nearly equally balanced by the expected increase in the upstream solar wind flux. These counterbalancing effects suggest that the lunar paleomagnetosphere played a critical role in preserving the correlation between the antiquity indicator and the radioactive decay profile of indigenous lunar 40K. Thus, a key implication of these findings is that the accuracy of the 40Ar/36Ar indicator for any lunar sample may be strongly influenced by the poorly constrained history of the lunar magnetic field.

^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.