¹J.Gattacceca et al. (>10)
Earth & Plantetary Science Letters 569, 117049 Link to Article [https://doi.org/10.1016/j.epsl.2021.117049]
¹CNRS, Aix Marseille Univ, IRD, INRAE, CEREGE, 13545 Aix-en-Provence, France
Copyright Elsevier

Glassy ejecta are associated to a limited number of impact craters, and yet hold key information about hypervelocity impact processes. Here we report on the discovery of a ∼650 km2 impact glass strewnfield in the Central Depression of the Atacama Desert. These cm-sized splash-form objects, that we refer to as atacamaites, are essentially composed of a dacitic glass formed by high-temperature melting of local magmatic rocks, with the addition of a variable iron meteorite contamination, 5 wt.% on average. The most likely nature for the impactor is the IIAB iron group. The fission-track plateau method, on two samples, yielded a mean formation age of Ma. No associated impact crater has been discovered so far, suggesting it may be a relatively small, km-sized crater. The glassy nature, aerodynamic shapes, elevated formation temperature, and low water content are reminiscent of tektites. However, their small size, heterogeneity, oxidation state, significant contamination by the impactor, and likely more proximal provenance distinguish them from tektites. Atacamaites have no equivalent among the few known terrestrial ejected impact glasses, and increase the intriguing diversity of such products that we propose to name “tektoids”.

^1,2Guillaume Florin,¹Béatrice Luais,^2,3Olivier Alard,²Tracy Rushmer
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13698]
¹CNRS, CRPG, Université de Lorraine, Nancy, F-5400 France
²Department of Earth and Planetary Sciences, Macquarie University, Sydney, New South Wales, 2109 Australia
³Géosciences Montpellier, UMR 5243, CNRS & Université Montpellier, Montpellier, 34095 France
Published by arrangement with John Wiley & Sons

We analyzed the highly siderophile element (HSE) contents and bulk Ge isotopic compositions of large metal grains in the CB chondrites Bencubbin (CBa), Gujba (CBa), and HaH 237 (CBb). Our results suggest that the large grains were formed by the aggregation of smaller condensed grains, and the two Benccubinite groups are distinguishable based on their bulk metal δ74/70Ge mass-dependent isotopic values of 0.99 ± 0.30‰ (CBa) and −0.65 ± 0.10‰ (CBb). Based on our observations of these three samples, the isotopic compositions of metal in CBa chondrites are best explained by condensation at slow cooling rates in the center of an impact plume, whereas the metal in CBb chondrites formed under fast cooling rates along the plume edges. We also analyzed the Ge contents and isotopic compositions of the core, intermediate, and rim fractions of two Gujba metal grains, which were separated by sequential digestion. These results show a gradual decrease in δ74/70Ge and [Ge] from core to rim. We suggest that these δ74Ge zonations result from near-equilibrium condensation and evaporation processes in a heterogeneous plume. We propose a model for their formation in which (1) small grains (to become grain cores) condensed at equilibrium; (2) these grains were transported to a warmer region of the plume where they reached temperatures lower than that of Fe-Ni condensation, but high enough for the rapid evaporation of Ge; (3) Ge evaporation followed by slow cooling enriched the grains in heavy Ge isotopes and the surrounding gas in light Ge isotopes; and (4) equilibrium recondensation of metal from the gas and around the small grains formed the light Ge isotopic zonations observed in grain rims.

¹K. D. Burgess,¹R. M. Stroud
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13692]
¹U.S. Naval Research Laboratory, Washington, District of Columbia, 20375 USA
Published by arrangement with John Wiley & Sons

The return of samples from S-type asteroid 25143 Itokawa have enabled significant improvements in our understanding of space weathering on asteroids and the link between S-type asteroids and ordinary chondrites. We report on three new particles, providing details on space weathering of adjacent grains within a particle and several different phases. The features we observe are consistent with formation via irradiation from the solar wind, as opposed to micrometeoroid bombardment. We also see differences in the degree of weathering for grains collected from the two different touchdown locations on the asteroid. Continued analysis of new grains from Itokawa allow us to draw a more complete picture of the variety of space weathering features and the processes that lead to their formation on Itokawa and other airless bodies.

^1,2Giovanni Pratesi,¹Vanni Moggi Cecchi,³Richard C. Greenwood,³Ian A. Franchi,³Samantha J. Hammond,⁴Mario Di Martino,^4,5Dario Barghini,⁵Carla Taricco,⁶Albino Carbognani,⁴Daniele Gardiol
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13695]
¹Dipartimento di Scienze della Terra, Università degli Studi di Firenze, Via G. La Pira 4, Florence, 50121 Italy
²INAF—Istituto di Astrofisica e Planetologia Spaziali, Via Fosso del Cavaliere 100, Rome, 00133 Italy
³Planetary and Space Sciences, The Open University, Milton Keynes, MK7 6AA UK
⁴INAF—Osservatorio Astrofisico di Torino, Via Osservatorio 20, Turin, 10025 Italy
⁵Dipartimento di Fisica, Università degli Studi di Torino, Via P. Giuria 1, Pino Torinese, 10125 Italy
⁶INAF—Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, Bologna, 40129 Italy
Published by arrangement with John Wiley & Sons

The Cavezzo meteorite, which fell on January 1, 2020, is the first meteorite detected and recovered by the Italian PRISMA Fireball Network. Two specimens, weighing 3.12 g (specimen 1) and 52.19 g (specimen 2), were collected 3 days after the bolide was observed, thanks to an effective media campaign that encouraged the involvement of local people. The two specimens of this meteorite have not only completely different lithological characteristics but also a different geochemistry and oxygen isotopic composition as well. Specimen 1 is anomalous both for the textural–structural features, varying seamlessly from chondritic to “achondritic,” and a very unusual modal mineralogy—such as the relatively high amount of olivine (63.1 vol%), plagioclase (18.2 vol%), high-Ca pyroxene (10.3 vol%), and chlorapatite (2.1 vol%); and the unusually low content of low-Ca pyroxene (5.8 vol%), metal (0.1 vol%), and troilite (much lesser than 0.1 vol%)—although the compositional values for olivine (Fa 24.24 mol%) and low-Ca pyroxene (Fs 20.41 mol%) appear to be similar to those of the L chondrite group. Conversely, in specimen 2, not only the texture and the crystal chemistry but also the modal mineralogy (low-Ca pyroxene much more abundant than high-Ca pyroxene and occurrence of metal and sulfides) look like those of an ordinary L chondrite. The differences between the two specimens are also confirmed by geochemistry. The oxygen isotope composition of specimen 1 plots at the boundary between the H and L groups (δ17O‰ 3.250; δ18O‰ 4.736; Δ17O‰ 0.788) whereas specimen 2 plots at the boundary of the L and LL fields (δ17O‰ 3.737; δ18O‰ 4.957; Δ17O‰ 1.159). The bulk chemistry shows a different content of many minor and trace elements (including rare earth elements), such as a strong depletion of siderophile and chalcophile elements in specimen 1. The two specimens then do not contain fragments of each other, thus preventing us from classifying this “double face” meteorite as an ordinary chondrite breccia. In detail, specimen 1 can be considered a “xenolith” in which chondritic structure and igneous texture coexist without discontinuity, and therefore, it represents a previously unsampled portion of the L parent body. In summary, these findings support the classification of Cavezzo as an L5 anomalous chondrite.

¹Antara Sen et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13699]
¹Department of Physics and Astronomy, Ithaca College, Ithaca, New York, 14850 USA
Published by arrangement with John Wiley & Sons

Data returned by the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer) spacecraft have shown that asteroid (101955) Bennu has a globally low-albedo surface covered in boulders with diverse texture, color, and albedo properties, and an aqueously altered composition dominated by phyllosilicates. To test whether Bennu’s color and albedo diversity could be caused by texture and/or composition variations, we performed a laboratory-based study using simple two-component mixtures (called “mudpies”) of the phyllosilicate saponite and carbon-rich opaques. Each mudpie is prepared in four different textures: fine powder, coarse particles, sanded slab, and textured rock. We find that a sanded slab made from 90% saponite and 10% lampblack is a good analog for Bennu, and the color and albedo changes due to texture variations are substantial. At 550 nm, texture changes alone can create up to 36% brightness contrast, and in color measured as a 473 nm/847 nm ratio, texture changes can provide up to 18% color contrast. In comparison, Bennu shows approximately 25% albedo and <1% color contrasts from boulder type to boulder type. These findings suggest that if texture contributes to color on Bennu, the texture variations are typically more subtle than what we simulated in the laboratory. According to our study, the color and albedo properties of different boulder types on Bennu are consistent with different concentrations of carbon-rich opaques (and possibly consistent with variations in carbonate concentration). The variations within each boulder group are consistent with textural differences.

¹Lionel G.Vacher,¹Ryan C.Ogliore,²Clive Jones,²Nan Liu,²David A.Fike
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.06.026]
¹Department of Physics, Washington University in St. Louis, St. Louis, MO, USA
²Department of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, MO, USA
Copyright Elsevier

The Sun’s astrophysical birth environment affected the formation and composition of the Solar System. Primitive meteorites display mass-independent oxygen isotope anomalies that were likely caused by ultraviolet (UV) photochemistry of CO gas-phase molecules, either (i) in the outer solar nebula by light from the young Sun or (ii) in the parent molecular cloud by light from nearby stars. However, measurements of oxygen isotopes alone cannot unambiguously constrain the UV spectrum of the source responsible for the photochemistry. Sulfur, with four stable isotopes, can be used as a more direct probe of the astrophysical environment of mass-independent photochemistry. Here, we report the in situ isotopic analysis of paired oxygen and sulfur isotope systematics in cosmic symplectite (COS), magnetite-pentlandite intergrowths, in the primitive ungrouped carbonaceous chondrite Acfer 094. We show that COS grains contain mass-independent sulfur isotope anomalies (weighted means of Δ33S = +3.84 ± 0.72‰ and Δ36S = −6.05 ± 2.25‰, 2SE) consistent with H2S photochemistry by UV from massive O and B stars close to the Solar System’s parent molecular cloud, and inconsistent with UV from the protosun. The presence of coupled mass-independent sulfur and oxygen (Δ17O = 86 ± 6‰, 2SE) isotope anomalies in COS imply that these anomalies originated in the same astrophysical environment. We propose that this environment is the photodissociation region (PDR) of the Solar System’s parent molecular cloud, where nearby massive stars irradiated the edge of the cloud. We conclude that the Sun’s stellar neighbors, likely O and B stars in a massive-star-forming region, affected the composition of the Solar System’s primordial building blocks.

¹Anna Barbaro,¹Maria Chiara Domeneghetti,^2,3Konstantin D.Litasov,⁴Ludovic Ferrière,⁴Lidia Pittarello,⁵Oliver Christ,⁵Sofia Lorenzon,¹Matteo Alvaro,^5,6Fabrizio Nestola
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2021.06.022]
¹Dipartimento di Scienze della Terra e dell’Ambiente, Università degli Studi di Pavia, Via Ferrata 1, I-27100, Pavia, Italy
²Vereshchagin Institute for High Pressure Physics RAS, Troitsk, Moscow, 108840, Russia
³Fersman Mineralogical Museum RAS, Moscow, 115162, Russia
⁴Natural History Museum, Department of Mineralogy and Petrography, Burgring 7, 1010 Vienna, Austria
⁵Dipartimento di Geoscienze, Università degli Studi di Padova, Via G. Gradenigo 6, I-35131 Padova, Italy
⁶Geoscience Institute, Goethe-University Frankfurt, Altenhöferalee 1, 60323 Frankfurt, Germany
Copyright Elsevier

The occurrence of shock-induced diamonds in ureilite meteorites is common and is used to constrain the history of the ureilite parent bodies. We have investigated a fragment of the Kenna ureilite by micro-X-ray diffraction, micro-Raman spectroscopy and scanning electron microscopy to characterize its carbon phases. In addition to olivine and pigeonite, within the carbon-bearing areas, we identified microdiamonds (up to about 10 μm in size), nanographite and magnetite. The shock features observed in the silicate minerals and the presence of microdiamonds and nanographite indicate that Kenna underwent a shock event with a peak pressure of at least 15 GPa. Temperatures estimated using a graphite geothermometer are close to 1180 °C. Thus, Kenna is a medium-shocked ureilite, yet it contains microdiamonds, which are typically found in highly shocked carbon-bearing meteorites, instead of the more common nanodiamonds. This can be explained by a relatively long shock event duration (in the order of 4-5 seconds) and/or by the catalytic effect of Fe-Ni alloys known to favour the crystallization of diamonds. For the first time in a ureilite, carletonmooreite with formula Ni3Si and grain size near 4-7 nm, was found. The presence of nanocrystalline carletonmooreite provides further evidence to support the hypothesis of the catalytic involvement of Fe-Ni bearing phases into the growth process of diamond from graphite during shock events in the ureilite parent body, enabling the formation of micrometer-sized diamond crystals.

^1,2Maggie McAdam,^2,3Annika Gustafsson
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114592]
¹NASA Ames Research Center, United States of America
²Northern Arizona University, United States of America
³Lowell Observatory, United States of America
Copyright Elsevier

We present rotationally resolved spectroscopy of asteroid (93) Minerva using the Lowell Discovery Telescope with the Near-Infrared High Throughput Spectrograph (NIHTS). We obtained spectroscopy over ~34% of the asteroid’s rotation period. Minerva has been shown to be spectrally similar to primitive carbonaceous chondrites (e.g., McAdam et al., 2018, Icarus 306, 32–49) indicating it has amorphous materials on its surface. The extent to which these materials appear over Minerva’s surface could provide constraints on the asteroid’s formation time and/or directly relate the asteroid to a chemical group of carbonaceous chondrite meteorites. Parent asteroids are thought to preserve primitive meteorites in either an outer shell of material or by avoiding parent body processing (e.g., accreting after the peak heat flux of 26Al or before the introduction of exogenous 26Al to the Solar System). These two scenarios are expected to have different properties: the no processing scenario produces an asteroid with a compositionally homogenous surface and interior while the outer shell scenario would have compositionally distinct surface and interior. Over the observed region, we report that Minerva’s surface appears to have amorphous materials, potentially indicating a homogeneous surface. However, more data are needed to determine Minerva’s compositional uniformity and which formation scenario is most appropriate.

¹S.M.Ferrone et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114579]
¹LESIA-Observatoire de Paris, Université PSL, CNRS, Université de Paris, Sorbonne Université, Paris, France
²Department of Physics and Astronomy, Ithaca College, Ithaca, NY, USA
Copyright Elsevier

The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS) onboard the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) spacecraft detected ~3.4-μm absorption features indicative of carbonates and organics on near-Earth asteroid (101955) Bennu. We apply a Kolmogorov-Smirnov similarity test to OVIRS spectra of Bennu and laboratory spectra of minerals to categorize 3.4-μm features observed on Bennu as representing either carbonates or organics. Among the 15,585 spectra acquired by OVIRS during high-resolution (4 to 9 m/spectrum footprint) reconnaissance observations of select locations on Bennu’s surface, we find 544 spectral matches with carbonates and 245 spectral matches with organics (total of 789 high-confidence spectral matches). We map the locations of these matches and characterize features of Bennu’s surface using corresponding image data. Image data are used to quantitatively characterize the albedo within each spectrometer footprint. We find no apparent relationships between spectral classification and surface morphological expression, and we find no correlation between carbon species classification and other spectral properties such as slope or band depth. This suggests either that carbonates and organics are ubiquitous across the surface of Bennu, independent of surface features (consistent with findings from laboratory studies of carbonaceous chondrites), or that the observations do not have the spatial resolution required to resolve differences. However, we find more organic spectral matches at certain locations, including the site from which the OSIRIS-REx mission collected a sample, than at others. Higher concentrations of organics may be explained if these materials have been more recently exposed to surface alteration processes, perhaps by recent crater formation.

¹Chiho Sugimoto et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114591]
¹The University of Tokyo, Tokyo 113-0033, Japan
Copyright Elsevier

Many small boulders with reflectance values higher than 1.5 times the average reflectance have been found on the near-Earth asteroid 162,173 Ryugu. Based on their visible wavelength spectral differences, Tatsumi et al. (2021, Nature Astronomy, 5, doi:doi:10.1038/s41550-020-1179-z) defined two bright boulder classes: C-type and S-type. These two classifications of bright boulders have different size distributions and spectral trends. In this study, we measured the spectra of 79 bright boulders and investigated their detailed spectral properties. Analyses obtained a number of important results. First, S-type bright boulders on Ryugu have spectra that are similar to those found for two different ordinary chondrites with different initial spectra that have been experimentally space weathered the same way. This suggests that there may be two populations of S-type bright boulders on Ryugu, perhaps originating from two different impactors that hit Ryugu’s parent body. Second, the model space-weathering ages of meter-size S-type bright boulders, based on spectral change rates derived in previous experimentally irradiated ordinary chondrites, are 105–106 years, which is consistent with the crater retention age (<106 years) of the ~1-m deep surface layer on Ryugu. This agreement strongly suggests that Ryugu’s surface is extremely young, implying that the samples acquired from Ryugu’s surface should be fresh. Third, the lack of a serpentine absorption in the S-type clast embedded in one of the large brecciated boulders indicates that fragmentation and cementation that created the breccias occurred after the termination of aqueous alteration. Fourth, C-type bright boulders exhibit a continuous spectral trend similar to the heating track of low-albedo carbonaceous chondrites, such as CM and CI. Other processes, such as space weathering and grain size effects, cannot primarily account for their spectral variation. Furthermore, the distribution of the spectra of general dark boulders, which constitute >99.9% of Ryugu’s volume, is located along the trend line in slope/UV-index diagram that is occupied by C-type bright boulders. These results indicate that thermal metamorphism might be the dominant cause for the spectral variety among the C-type bright boulders on Ryugu and that general boulders on Ryugu may have experienced thermal metamorphism under a much narrower range of conditions than the C-type bright boulders. This supports the hypothesis that Ryugu’s parent body experienced uniform heating due to radiogenic energy rather than impact heating.