¹Zack Gainsforth et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14161]
¹Space Sciences Laboratory, University of California at Berkeley, Berkeley, California, USA
Published by arrangement with John Wiley & Sons

We analyzed an asteroid Ryugu sample returned to Earth by JAXA’s Hayabusa2 mission using nanoIR, SEM, and TEM microscopy. We identified multiple distinct carbon reservoirs within the phyllosilicate matrix and demonstrate infrared spectral affinities for some of the carbon to insoluble organic matter (IOM). TEM studies of Ryugu samples have allowed us to better understand the interrelationship between the crystallographic orientations of phyllosilicates and the secondary minerals such as carbonate, sulfide, and apatite. Transport of elements provides a unifying theme for understanding these interrelationships.

¹Luke Daly et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14164]
¹School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK
²Australian Centre for Microscopy and Microanalysis, The University of Sydney, Sydney, New South Wales, Australia
³Department of Materials, University of Oxford, Oxford, UK
Published by arrangfement with John Wiley & Sons

The Mighei-like carbonaceous (CM) chondrites have been altered to various extents by water–rock reactions on their parent asteroid(s). This aqueous processing has destroyed much of the primary mineralogy of these meteorites, and the degree of alteration is highly heterogeneous at both the macroscale and nanoscale. Many CM meteorites are also heavily brecciated juxtaposing clasts with different alteration histories. Here we present results from the fine-grained team consortium study of the Winchcombe meteorite, a recent CM chondrite fall that is a breccia and contains eight discrete lithologies that span a range of petrologic subtypes (CM2.0–2.6) that are suspended in a cataclastic matrix. Coordinated multitechnique, multiscale analyses of this breccia reveal substantial heterogeneity in the extent of alteration, even in highly aqueously processed lithologies. Some lithologies exhibit the full range and can comprise nearly unaltered coarse-grained primary components that are found directly alongside other coarse-grained components that have experienced complete pseudomorphic replacement by secondary minerals. The preservation of the complete alteration sequence and pseudomorph textures showing tochilinite–cronstedtite intergrowths are replacing carbonates suggest that CMs may be initially more carbonate rich than previously thought. This heterogeneity in aqueous alteration extent is likely due to a combination of microscale variability in permeability and water/rock ratio generating local microenvironments as has been established previously. Nevertheless, some of the disequilibrium mineral assemblages observed, such as hydrous minerals juxtaposed with surviving phases that are typically more fluid susceptible, can only be reconciled by multiple generations of alteration, disruption, and reaccretion of the CM parent body at the grain scale.

^1,2Neeraja S. Chinchalkar,²David T. King Jr,²Willis E. Hames
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14169]
¹Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
²Department of Geosciences, Beard Eaves Memorial Coliseum, Auburn University, Auburn, Alabama, USA
Published by arrangement with John Wiley & Sons

Wetumpka impact structure is a Late Cretaceous, marine-target impact crater of about 5 km diameter. The apparent crater rim is mostly made of crystalline local basement, and the apparent crater floor consists of a mixed sediments of target lithology. These sediments are the provenance of the crater-filling impactite sands, overlying trans-crater slide unit, and the capping polymict impact breccia deposit, often referred to by previous workers as “central polymict breccia.” The unit has been known to contain elongated mega-clasts of up to tens of meters in size. This study attempted to understand the mode of emplacement of this polymict breccia, which occurs in some places on the apparent crater floor and resembles a polymict proximal ejecta deposit. This work also reports the first documentation of rare, potential impact spherules in the polymict impact breccia, interpreted to be a part of distal ejecta. The presence of large, decimeter-sized clasts in the breccia can be best explained by the movement of overturned rim flap forming part of proximal ejecta from the crater rim to the apparent crater floor during early modification stage of impact cratering. Our work highlights the bimodal clast size distribution of the polymict breccia, and so we propose that the term “mega-clast-bearing impact breccia” be used for this unit. We attribute a generally steep orientation of the decameter sized clasts to primary imbrication during emplacement. The emplacement of this breccia is interpreted as associated with the ejecta emplacement process that occurred before the return of marine resurge.

¹Bruno Daniel Leite Mendes,¹Agnes Kontny,²Katarzyna Dudzisz,³Franziska D. H. Wilke
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14170]
¹Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, Germany
²Institute of Geophysics, Polish Academy of Sciences, Warsaw, Poland
³Helmholtz Centre Potsdam – Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, Potsdam, Germany
Published by arrangement with John Wiley & Sons

Large-scale impact events are some of the most catastrophic and instantaneous geological processes in nature, and leave in their wake conspicuous geological structures with characteristic magnetic anomalies. Despite magnetic anomalies in craters being well-documented, their relationship with the magnetic mineral composition of the target and impactites is not always straightforward. Furthermore, the influence of impact shock and post-impact events in the magnetism of natural craters remains elusive. In the Ries crater, Germany, the negative magnetic anomalies are attributed to a reverse polarity remanent magnetization in the impact-melt bearing lithologies. We report new chemical, rock-, and mineral-magnetic data from the shocked basement and impactites, from surface samples, NR73 and SUBO-18 boreholes, and explore how temperature and hydrothermalism may influence the magnetic mineralogy in the crater. We identified shocked, pure magnetite in the basement, and low-cation substituted magnetite in the impactites as the main magnetic carriers. The shocked basement is demagnetized but remains largely unaltered by post-impact hydrothermalism, while the impactites show weak magnetization and are extensively altered by neutral-to-reducing post-impact hydrothermalism. We suggest that the magnetic mineralogy of the demagnetized uplifted basement may contribute significantly to the magnetic anomaly variation, in line with recent findings from the Chicxulub peak-ring.

^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.