¹Kevin Righter,²Alexander P. Holmwood
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13465]
¹NASA Johnson Space Center, 2101 NASA Parkway, Houston, Texas, 77058 USA
²Department of Geosciences, Hamilton College, Clinton, New York, 13323 USA
Published by arrangement with John Wiley & Sons

The Allan Hills 76005 polymict eucrite pairing group consists of 15 paired masses recovered during six different field seasons in the Transantarctic Mountains. Although this group has been well studied in general, most of the meteorites contain a significant portion of dark clasts that have not been well characterized. The Dawn mission to Vesta discovered dark materials that provide insight into its evolution. The ALH dark clasts are thus of great interest to understanding the evolution of Vesta. Here, 45 different dark clasts from 15 different thin sections from the pairing group are characterized in detail to better understand their nature and origin. Five different textural types of dark clasts are recognized among this group—skeletal, vitrophyric, pilotaxitic, fan spherulitic, and troilite‐silica‐plagioclase‐rich clasts with aphyric or blobby textures. Mineralogy of the clasts is dominated by plagioclase and pyroxene, with minor troilite, silica, ilmenite, chromite, and rare Fe‐Ni metal. All of the textures can be produced by rapid cooling rates on the order of 60–2500°C h⁻¹. Bulk compositions of the clasts are demonstrably eucritic, and not chondritic, howarditic, or diogenitic. The combination of mineralogy, composition, and textures strongly suggests that the dark clasts are eucritic impact melts. Several craters on Vesta have associated orange deposits that have been proposed as impact melt breccias. The ALH pairing group may thus represent material that originated near Oppia or Octavia craters.

¹Noriyuki Kawasaki,¹Sohei Wadaa,²Changkun Park,³Naoya Sakamoto,^1,3,4Hisayoshi Yurimoto
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.045]
¹Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, Republic of Korea
³Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
⁴Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
Copyright: Elsevier

Fine-grained Ca-Al-rich inclusions (FGIs) in CV chondrites are suggested to be condensates formed directly from the solar nebular gas. Al−Mg mineral isochrons of seven FGIs from reduced CV chondrites Efremovka, Vigarano, Thiel Mountains 07007, and Northwest Africa 8613 were obtained via in situ Al–Mg isotope measurements using secondary ion mass spectrometry. The slopes of the mineral isochrons for seven FGIs exhibit statistically significant variations in initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, ranging from (5.19 ± 0.17) to (3.35 ± 0.21) × 10⁻⁵, which correspond to a relative age spread of 0.44 ± 0.07 Myr. Inferred upper limit of (²⁶Al/²⁷Al)₀ for the FGIs is identical to the Solar System (²⁶Al/²⁷Al)₀ of ∼5.2 × 10⁻⁵as determined by whole-rock Al–Mg isochron studies for CAIs in CV chondrites. The intercepts of the mineral isochrons, the initial ²⁶Mg/²⁴Mg ratios the FGIs formed with, are consistent with Mg-isotope evolution path of a solar-composition nebular gas. The observed variations in (²⁶Al/²⁷Al)₀ for FGIs are essentially similar to those (∼5.2 to ∼4.2 × 10⁻⁵) for coarse-grained, igneous CAIs of CV chondrites that are formed by melting and solidification. If ²⁶Al was distributed homogeneously in the forming region, then our data indicate that thermal processes of condensation and melting for CAI formation occurred contemporaneously and continued for at least ∼0.4 Myr at the very beginning of the Solar System. Alternatively, the observed variations in (²⁶Al/²⁷Al)₀ also indicate the possibility of heterogeneous distributions of ²⁶Al in the forming region, corresponding to a range of over at least 3.4 × 10^–5 < (²⁶Al/²⁷Al)₀ < 5.2 × 10^–5.

¹Connor D. Hilton,¹Richard D. Ash,¹Philip M. Piccoli,²David A. Kring,³Timothy J. McCoy,¹Richard J. Walker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13464]
¹Department of Geology, University of Maryland, College Park, Maryland, 20742 USA
²Lunar and Planetary Institute, USRA, Houston, Texas, 77058 USA
¹Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, District of Columbia, 20560 USA
Published by arrangement with John Wiley & Sons

Previous studies attributed the origin of metal veins penetrating graphite nodules in the Canyon Diablo IAB main group iron meteorite to condensation from vapor or melting of host metal. Abundances of 16 siderophile elements measured in kamacite within vein and host meteorite are most consistent with an origin by melting of the host metal followed by fractional crystallization of the liquid. The presence of the veins within graphite nodules may be explained by impact, as peak shock temperatures, and thus the most likely areas to undergo metal melting are at metal–graphite interfaces. The origin of the veins is constrained by Re‐Os chronometry to have occurred early (>4 Ga) in solar system history.

¹Noriyuki Kawasaki,¹Sohei Wada,² Changkun Park,³Naoya Sakamoto,^1,3,4Hisayoshi Yurimoto
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.03.045]
¹Department of Natural History Sciences, Hokkaido University, Sapporo 060-0810, Japan
²Division of Earth-System Sciences, Korea Polar Research Institute, Incheon 21990, Republic of Korea
³Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, Sapporo 001-0021, Japan
⁴Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara 252-5210, Japan
Copyright Elsevier

Fine-grained Ca-Al-rich inclusions (FGIs) in CV chondrites are suggested to be condensates formed directly from the solar nebular gas. Al−Mg mineral isochrons of seven FGIs from reduced CV chondrites Efremovka, Vigarano, Thiel Mountains 07007, and Northwest Africa 8613 were obtained via in situ Al–Mg isotope measurements using secondary ion mass spectrometry. The slopes of the mineral isochrons for seven FGIs exhibit statistically significant variations in initial ²⁶Al/²⁷Al ratios, (²⁶Al/²⁷Al)₀, ranging from (5.19 ± 0.17) to (3.35 ± 0.21) × 10⁻⁵, which correspond to a relative age spread of 0.44 ± 0.07 Myr. Inferred upper limit of (²⁶Al/²⁷Al)₀for the FGIs is identical to the Solar System (²⁶Al/²⁷Al)₀ of ∼5.2 × 10⁻⁵ as determined by whole-rock Al–Mg isochron studies for CAIs in CV chondrites. The intercepts of the mineral isochrons, the initial ²⁶Mg/²⁴Mg ratios the FGIs formed with, are consistent with Mg-isotope evolution path of a solar-composition nebular gas. The observed variations in (²⁶Al/²⁷Al)₀ for FGIs are essentially similar to those (∼5.2 to ∼4.2 × 10⁻⁵) for coarse-grained, igneous CAIs of CV chondrites that are formed by melting and solidification. If ²⁶Al was distributed homogeneously in the forming region, then our data indicate that thermal processes of condensation and melting for CAI formation occurred contemporaneously and continued for at least ∼0.4 Myr at the very beginning of the Solar System. Alternatively, the observed variations in (²⁶Al/²⁷Al)₀ also indicate the possibility of heterogeneous distributions of ²⁶Al in the forming region, corresponding to a range of over at least 3.4 × 10^–5 < (²⁶Al/²⁷Al)₀ < 5.2 × 10^–5.

^1,2Julia Semprich,¹Justin Filiberto,¹Allan H.Treiman
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113779]
¹Lunar and Planetary Institute, USRA, 3600 Bay Area Bld., Houston, TX 77058, USA
²Astrobiology OU, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
Copyright Elsevier

Rock alteration processes on Venus are still poorly understood due to the limited geochemical data on surface rocks and uncertainties in atmospheric compositions. We use phase equilibria calculations to constrain mineral stabilities at Venus surface conditions for different rock and gas compositions resulting in the chemical system SiO₂-TiO₂-Al₂O₃-FeO-MgO-CaO-Na₂O-K₂O with C-O-H-S gas at varying O₂ and S₂fugacities. While the low concentrations of H₂O in the present-day atmosphere result in conditions, under which anhydrous mineral assemblages dominate, higher amounts of water, possibly during an earlier stage in Venus’ history, could have resulted in the formation of amphibole and biotite. Even in a sulfur-free atmosphere, carbonates would be stable only in alkali-rich basalts. The presence of SO₂ in the atmosphere, however, causes the formation of anhydrite. The stabilities of iron oxides and sulfides are highly sensitive to gas fugacities (i.e., the composition of the atmosphere), as well as temperature. While the modeled magnetite-hematite transition is located close to conditions relevant for planetary radius, the assemblage of anhydrite + hematite ± pyrite may be stable at higher elevations if a similar range of fO₂ as at the lowlands is assumed. Therefore, our model agrees with pyrite as the proposed cause of the high radar backscatter observed at high elevations in the northern highlands.

^1,2B. D. Byron,^1,2K. D. Retherford,²T. K. Greathouse,²D. Wyrick,³J. T. S. Cahill,⁴A. R. Hendrix,^1,2U. Raut,³K. E. Mandt,³B. W. Denevi
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006269]
¹Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA
²Space Science and Engineering Department, Southwest Research Institute, San Antonio, TX, USA
³The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
⁴Planetary Science Institute, Tucson, AZ, USA
Published by arrangement with John Wiley & Son

Using data from the Lunar Reconnaissance Orbiter (LRO) Lyman Alpha Mapping Project (LAMP), we investigate the spectral properties of rayed craters in the far‐ultraviolet (FUV). Because LAMP is sensitive to the uppermost layer of the lunar surface and regolith grains, it is ideal for characterizing regolith maturity and space weathering products such as submicroscopic iron. We find that crater rays from a survey of the largest Copernican‐age craters have high Off‐band (155–190 nm)/On‐band (130–155 nm) albedo (Off/On) LAMP product ratios, consistent with immature regolith and low amounts of submicroscopic iron. The Off/On ratio of the highlands crater rays decreases linearly over time (0.095 per 100 My), and we use this trend to estimate the age of Jackson crater (~152 My). Some large young highlands craters (e.g., Tycho, Jackson, Giordano Bruno, and Necho) display lower ratio halos around the crater cavity, at regions where previous studies have suggested abundant impact melt exists. The lower Off/On ratio is likely due to the increased glass component of the regolith at these highlands regions, which would act to increase absorption at Off‐band wavelengths. We also find that ejecta blankets from large maria craters (e.g., Copernicus and Aristillus) have a similar Off/On ratio to the mature background highlands. This supports previous findings that determined that the rays from these craters are composed of highlands material excavated from beneath the maria and subsequently weathered to maturity.

¹A. J. Brown,²C. E. Viviano,³T. A. Goudge
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2019JE006011]
¹Plancius Research, Severna Park, MD, USA
²Johns Hopkins Applied Physics Laboratory, Laurel, MD, USA
³Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX, USAPublished by arrangement with John Wiley & Sons

A well‐preserved, ancient delta deposit, in combination with ample exposures of carbonate outcrops, makes Jezero Crater in Nili Fossae a compelling astrobiological site. We use Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) observations to characterize the surface mineralogy of the crater and surrounding watershed. Previous studies have documented the occurrence of olivine and carbonates in the Nili Fossae region. We focus on correlations between these two well‐studied lithologies in the Jezero crater watershed. We map the position and shape of the olivine 1 μm absorption band and find that carbonates are found in association with olivine which displays a 1 μm band shifted to long wavelengths. We then use Thermal Emission Imaging Spectrometer (THEMIS) coverage of Nili Fossae and perform tests to investigate whether the long wavelength shifted (redshifted) olivine signature is correlated with high thermal inertia outcrops. We find that there is no consistent correlation between thermal inertia and the unique olivine signature. We discuss a range of formation scenarios for the olivine and carbonate associations, including the possibility that these lithologies are products of serpentinization reactions on early Mars. These lithologies provide an opportunity for deepening our understanding of early Mars and, given their antiquity, may provide a framework to study the timing of valley networks and the thermal history of the Martian crust and interior from the early Noachian to today.

¹Dougal J. Ritson,^2,3Stephen J. Mojzsis,¹John. D. Sutherland
Nature Geoscience (in Press) Link to Article [DOI https://doi.org/10.1038/s41561-020-0556-7]
¹MRC Laboratory of Molecular Biology, Cambridge Biomedical Campus, Cambridge, UK
²Department of Geological Sciences, University of Colorado, Boulder, CO, USA
Stephen J. Mojzsis
³Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Budapest, Hungary
Stephen J. Mojzsis

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¹Erick J. Cano,¹Zachary D. Sharp,²Charles K. Shearer
Nature Geoscience (in Press) Link to Article [DOI https://doi.org/10.1038/s41561-020-0550-0]
¹Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, USA
²Institute of Meteoritics, University of New Mexico, Albuquerque, NM, USA

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¹Shima Pilehvar,²Marlies Arnhof,³Ramón Pamies,⁴Luca Valentini,¹Anna-Lena Kjøniksen
Journal of Cleaner Production 247, 119177 Link to Article [https://doi.org/10.1016/j.jclepro.2019.119177]
¹Faculty of Engineering, Østfold University College, P.O. Box 700, 1757, Halden, Norway
²Advanced Concepts Team, ESA European Space Research and Technology Centre, Keplerlaan 1, TEC-SF, 2201AZ, Noordwijk, Netherlands
³Department of Materials Engineering and Manufacturing, Technical University of Cartagena, Cartagena, Murcia, Spain
⁴Department of Geosciences, University of Padua, 35131, Padua, Italy

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