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1. Investigating the response of biotite to impact metamorphism: Examples from the Steen River impact structure, Canada

   https://doi.org/10.1111/maps.13011  

   Walton, E. L. ; Sharp, T. G. ; Hu, J. ; Tschauner, O. ( November 2017 , Meteoritics & Planetary Science)

   Impact metamorphic effects from quartz and feldspar and to a lesser extent olivine and pyroxene have been studied in detail. Comparatively, studies documenting shock effects in other minerals, such as double chain inosilicates, phyllosilicates, carbonates, and sulfates, are lacking. In this study, we investigate impact metamorphism recorded in crystalline basement rocks from the Steen River impact structure (SRIS), a 25 km diameter complex crater inNWAlberta, Canada. An array of advanced analytical techniques was used to characterize the breakdown of biotite in two distinct settings: along the margins of localized regions of shock melting and within granitic target rocks entrained as clasts in a breccia. In response to elevated temperature gradients along shock vein margins, biotite transformed at high pressure to an almandine‐Ca/Fe majorite‐rich garnet with a density of 4.2 g cm⁻³. The shock‐produced garnets are poikilitic, with oxide and silicate glass inclusions. Areas interstitial to garnets are vesiculated, in support of models for the formation of shock veins via oscillatory slip, with deformation continuing during pressure release. Biotite within granitic clasts entrained within the hot breccia matrix thermally decomposed at ambient pressure to produce a fine‐grained mineral assemblage of orthopyroxene + sanidine + titanomagnetite. These minerals are aligned to the (001) cleavage plane of the original crystal. In this and previous work, the transformation of an inosilicate (pargasite) and a phyllosilicate (biotite) to form garnet, an easily identifiable, robust mineral, has been documented. We contend that in deeply eroded astroblemes, high‐pressure minerals that form within or in the environs of shock veins may serve as one of the possibly few surviving indicators of impact metamorphism.

    

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2. Sierra Gorda 009: A new member of the metal‐rich G chondrites grouplet

   https://doi.org/10.1111/maps.13546  

   Ivanova, Marina A. ; Lorenz, Cyril A. ; Humayun, Munir ; Corrigan, Catherine M. ; Ludwig, Thomas ; Trieloff, Mario ; Righter, Kevin ; Franchi, Ian A. ; Verchovsky, Alexander B. ; Korochantseva, Ekaterina V. ; et al ( August 2020 , Meteoritics & Planetary Science)

   We investigated the metal‐rich chondrite Sierra Gorda (SG) 009, a member of the new G chondrite grouplet (also including NWA 5492, GRO 95551). G chondrites contain 23% metal, very reduced silicates, and rare oxidized mineral phases (Mg‐chromite, FeO‐rich pyroxene). G chondrites are not related to CH‐CB chondrites, based on bulk O, C, and N isotopic compositions, mineralogy, and geochemistry. G chondrites have no fine‐grained matrix or matrix lumps enclosing hydrated material typical for CH‐CB chondrites. G chondrites’ average metal compositions are similar to H chondrites. Siderophile and lithophile geochemistry indicates sulfidization and fractionation of the SG 009 metal and silicates, unlike NWA 5492 and GRO 95551. The G chondrites have average O isotopic compositions Δ¹⁷O>0‰ ranging between bulk enstatite (E) and ordinary (O) chondrites. An Al‐rich chondrule from SG 009 has Δ¹⁷O>0‰ indicating some heterogeneity in oxygen isotopic composition of G chondrite components. SG 009’s bulk carbon and nitrogen isotopic compositions correspond to E and O chondrites. Neon isotopic composition reflects a mixture of cosmogenic and solar components, and cosmic ray exposure age of SG 009 is typical for O, E, and R chondrites. G chondrites are closely related to O, E, and R chondrites and may represent a unique metal‐rich parent asteroid containing primitive and fractionated material from the inner solar system. Oxidizing and reducing conditions during SG 009 formation may be connected with a chemical microenvironment and possibly could indicate that G chondrites may have formed by a planetesimal collision resulting in the lack of matrix.

    

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3. Low-speed Impacts into Ice–Dust Granular Mixtures

   https://doi.org/10.3847/PSJ/ac779a  

   Brisset, Julie ; Cox, Christopher ; Metzger, Jessica ; Miletich, Thomas ; Mohammed, Nadia ; Rascon, Allison ; Forczyk, Laura ; Dove, Adrienne ; Colwell, Joshua ( July 2022 , The Planetary Science Journal)

   We present the results of a series of laboratory low-speed impacts (< 4 m s⁻¹) of centimeter-sized spherical projectiles into simulated dry and icy regolith samples. The target material was comprised of JSC-1 (Johnson Space Center) lunar simulant grains in the size range 100–250μm, mixed with similar-sized water ice grains. Impacts were performed under vacuum, either at room temperature for JSC-1 samples or at cryogenic temperatures (<150 K) for icy mixtures. We measured the ejecta masses from a collection plate and impact crater dimensions from post-impact crater photographs. We find that both the ejecta masses and crater diameters followed trends predicted by established scaling laws, albeit with different fitting parameters, and we were able to fit a strength regimeπscaling to our measured crater diameters. The water ice in our target material took two forms: grains mixed with the regolith grains and frost from air condensation coating regolith grains. In both cases, the presence of water ice in the sample led to lower ejected masses and smaller crater sizes. In addition, our measured crater sizes were several orders of magnitude larger than expected for impacts into solid rock or water ice. Using our measured scaling parameters, we applied our findings to a planetary context for the study of secondary craters on icy moons, as well as eroding collisions occurring in Saturn’s rings. We found that the deviation of our measurements from solid targets and from commonly used scaling parameters allowed us to reconcile our measurements with the models in both cases.

    

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4. Swelling as a stabilizing mechanism in irradiated thin films: III. Effect on critical angle in a composite model

   https://doi.org/10.1088/1361-648X/acd31b  

   Evans, Tyler ; Norris, Scott ( May 2023 , Journal of Physics: Condensed Matter)

   Ion-beam irradiation of an amorphizable material such as Si or Ge may lead to spontaneous pattern formation, rather than flat surfaces, for irradiation beyond some critical angle against the surface normal. It is observed experimentally that this critical angle varies according to many factors, including beam energy, ion species and target material. However, many theoretical analyses predict a critical angle${\theta }_{\mathrm{c}}$of 45^∘independent of energy, ion and target, disagreeing with experiment. Previous work on this topic has suggested that isotropic swelling due to ion-irradiation may act as a stabilization mechanism, potentially offering a theoretical explanation for the elevated value of${\theta }_{\mathrm{c}}$in Ge compared to Si for the same projectiles. In the present work, we consider a composite model of stress-free strain and isotropic swelling with a generalized treatment of stress modification along idealized ion tracks. We obtain a highly-general linear stability result with a careful treatment of arbitrary spatial variation functions for each of the stress-free strain-rate tensor, a source of deviatoric stress modification, and isotropic swelling, a source of isotropic stress. Comparison with experimental stress measurements suggests that the presence of angle-independent isotropic stress may not be a strong influence on${\theta }_{\mathrm{c}}$for the 250 eV Ar${}^{+}\to$Si system. At the same time, plausible parameter values suggest that the swelling mechanism may, indeed, be important for irradiated Ge. As secondary results, we show the unexpected importance for${\theta }_{\mathrm{c}}$of the relationship between free and amorphous-crystalline interfaces in the thin film model. We also show that under simple idealizations used elsewhere, spatial variation of stress may not contribute to${\theta }_{\mathrm{c}}$selection. These findings prompt modeling refinements which will be the focus of future work.

    

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5. Organic matter from the Chicxulub crater exacerbated the K–Pg impact winter

   https://doi.org/10.1073/pnas.2004596117  

   Lyons, Shelby L. ; Karp, Allison T. ; Bralower, Timothy J. ; Grice, Kliti ; Schaefer, Bettina ; Gulick, Sean P. S. ; Morgan, Joanna V. ; Freeman, Katherine H. ( September 2020 , Proceedings of the National Academy of Sciences)

   An asteroid impact in the Yucatán Peninsula set off a sequence of events that led to the Cretaceous–Paleogene (K–Pg) mass extinction of 76% species, including the nonavian dinosaurs. The impact hit a carbonate platform and released sulfate aerosols and dust into Earth’s upper atmosphere, which cooled and darkened the planet—a scenario known as an impact winter. Organic burn markers are observed in K–Pg boundary records globally, but their source is debated. If some were derived from sedimentary carbon, and not solely wildfires, it implies soot from the target rock also contributed to the impact winter. Characteristics of polycyclic aromatic hydrocarbons (PAHs) in the Chicxulub crater sediments and at two deep ocean sites indicate a fossil carbon source that experienced rapid heating, consistent with organic matter ejected during the formation of the crater. Furthermore, PAH size distributions proximal and distal to the crater indicate the ejected carbon was dispersed globally by atmospheric processes. Molecular and charcoal evidence indicates wildfires were also present but more delayed and protracted and likely played a less acute role in biotic extinctions than previously suggested. Based on stratigraphy near the crater, between 7.5 × 10¹⁴and 2.5 × 10¹⁵g of black carbon was released from the target and ejected into the atmosphere, where it circulated the globe within a few hours. This carbon, together with sulfate aerosols and dust, initiated an impact winter and global darkening that curtailed photosynthesis and is widely considered to have caused the K–Pg mass extinction.

    

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