Gavin G. Kenny^a, Martin Schmieder^b,c, Martin J. Whitehouse^a, Alexander A.Nemchin^a,d, Luiz F. G. Morales^e Elmar Buchner^f,g, Jeremy J.Bellucci^a, Joshua F. Snape^a
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1021/j.gca.2018.11.012]
^aDepartment of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
^bLunar and Planetary Institute – USRA, 3600 Bay Area Boulevard, Houston TX 77058, USA
^cNASA – Solar System Exploration Research Virtual Institute (SSERVI)
^dDepartment of Applied Geology, Curtin University, Perth, WA 6845, Australia
^eScientific Center for Optical and Electron Microscopy (ScopeM), HPT D 9, Auguste-Piccard-Hof 1, 8093 Zürich, Switzerland
^fHNU Neu-Ulm University of Applied Sciences, Wileystraße 1, 89231 Neu-Ulm, Germany
^gInstitut für Mineralogie und Kristallchemie, Universität Stuttgart, Azenbergstraße 18, 70174 Stuttgart, Germany
Copyright Elsevier

Accurate and precise dating of terrestrial impact craters is a critical requirement for correlating impacts with events such as mass extinctions. A number of isotopic systems have been applied to impact chronology but it is important to understand what an age actually represents and, thus, if it accurately represents the ‘true’ impact age and is suitable for use in correlation. Here we report imaging, microstructural characterisation and high spatial resolution ion microprobe U-Pb analysis of shocked zircon from the approximately 23 km-in-diameter Lappajärvi impact structure, Finland, for which a well-established ⁴⁰Ar/³⁹Ar framework exists. Microstructural analysis identified two distinct styles of shock recrystallisation: (i) granular zircon that displays multiple domains of similarly oriented neoblasts, some of which are interpreted to be the product of reversion from the high-pressure ZrSiO₄ polymorph, reidite, and (ii) granular zircon composed entirely of similarly oriented neoblasts. Only the former gave concordant U-Pb data interpreted to record the age of the impact. The U-Pb ‘concordia age’ reported here, 77.85 ± 0.78 Ma (1.0 %; MSWD = 0.60; probability = 0.87; n = 8; 2σ; full external uncertainty), is resolvable from the previously published ‘best estimate’ ⁴⁰Ar/³⁹Ar age for impact melt rock (76.20 ± 0.29 Ma) and ⁴⁰Ar/³⁹Ar K-feldspar ages as young as 75.11 ± 0.36 Ma, and is therefore interpreted to more accurately reflect the age of the impact event. The resolvable disparity between the zircon U-Pb and the ⁴⁰Ar/³⁹Ar data indicates that even the oldest statistically robust ⁴⁰Ar/³⁹Ar ages obtained at medium- and large-sized impact craters may not accurately record the timing of an impact event at a kyr level. The offset between the U-Pb and ⁴⁰Ar/³⁹Ar data is interpreted to be, at least in part, a result of the zircon data recording a higher isotopic closure temperature, and the younger ⁴⁰Ar/³⁹Ar ages recording the progressive cooling of different domains of the impact structure. The Lappajärvi impact structure is the first Phanerozoic impact structure dated by U-Pb analysis of shock-recrystallised zircon to better than, or equal to, 1.0 % uncertainty. This further demonstrates that well-characterised granular zircon grains are likely to have wide utility in the accurate and precise dating of terrestrial impact events.

Axel WITTMANN¹, Randy L. KOROTEV², Bradley L. JOLLIFF², Kunihiko NISHIIZUMI³, A. J. Timothy JULL⁴, Marc W. CAFFEE^5,6, Michael ZANETTI⁷, and Anthony J. IRVING⁸
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13218]
¹Eyring Materials Center, Arizona State University, 901 S. Palm Walk, PSA 213, Tempe, Arizona 85287–1704, USA
²Department of Earth and Planetary Sciences and the McDonnell Center for the Space Sciences, Washington University inSt. Louis, One Brookings Drive, St. Louis, Missouri 63130, USA
³Space Sciences Laboratory, University of California Berkeley, Berkeley, California 94720–7450, USA
⁴Department of Geosciences, University of Arizona, 1040 East Fourth St., Tucson, Arizona 85721–0077, USA
⁵Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette,Indiana 47907–2036, USA
⁶Department of Earth, Atmospheric, and Planetary Sciences, Purdue University, 525 Northwestern Avenue, West Lafayette,Indiana 47907–2036, USA
⁷Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street N, London, Ontario N6A 5B7, Canada
⁸Department of Earth and Space Sciences, University of Washington, 4000 15thAvenue NE, Seattle, Washington 98195, USA
Published by arrangement with John Wiley & Sons

Oued Awlitis 001 is a highly feldspathic, moderately equilibrated, clast‐rich, poikilitic impact melt rock lunar meteorite that was recovered in 2014. Its poikilitic texture formed due to moderately slow cooling, which judging from textures of rocks in melt sheets of terrestrial impact structures, is observed in impact melt volumes at least 100 m thick. Such coherent impact melt volumes occur in lunar craters larger than ~50 km in diameter. The composition of Oued Awlitis 001 points toward a crustal origin distant from incompatible‐element‐rich regions. Comparison of the bulk composition of Oued Awlitis 001 with Lunar Prospector 5° γ‐ray spectrometer data indicates a limited region of matches on the lunar farside. After its initial formation in an impact crater larger than ~50 km in diameter, Oued Awlitis 001 was excavated from a depth greater than ~50 m. The cosmogenic nuclide inventory of Oued Awlitis 001 records ejection from the Moon 0.3 Ma ago from a depth of at least 4 m and little mass loss due to ablation during its passage through Earth’s atmosphere. The terrestrial residence time must have been very short, probably less than a few hundred years; its exact determination was precluded by a high concentration of solar cosmic ray‐produced ¹⁴C. If the impact that excavated Oued Awlitis 001 also launched it, this event likely produced an impact crater >10 km in diameter. Using petrologic constraints and Lunar Reconnaissance Orbiter Camera and Diviner data, we test Giordano Bruno and Pierazzo as possible launch craters for Oued Awlitis 001.

Yanxia Xie¹, Luis C. Ho^1,2, Aigen Li³, and Jinyi Shangguan^1,2
Astrophysical Journal 867, 91 Link to Article [DOI: 10.3847/1538-4357/aae2b0]
¹Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China
²Department of Astronomy, School of Physics, Peking University, Beijing 100871, People’s Republic of China
³Department of Physics and Astronomy, University of Missouri, Columbia, MO 65211, USA

Interstellar dust spans a wide range in size distribution, ranging from ultrasmall grains of a few Ångströms to micrometer-size grains. While the presence of nanometer-size dust grains in the Galactic interstellar medium was speculated six decades ago and was previously suggested based on early infrared observations, systematic and direct analysis of their properties over a wide range of environments has been lacking. Here we report the detection of nanometer-size dust grains that appear to be universally present in a wide variety of astronomical environments, from Galactic high-latitude clouds to nearby star-forming galaxies and galaxies with low levels of nuclear activity. The prevalence of such a grain population is revealed conclusively as prominent mid-infrared continuum emission at λ  10 μm seen in the Spitzer/Infrared Spectrograph data, characterized by temperatures of ~300–400 K that are significantly higher than the equilibrium temperatures of common, submicron-size grains in typical galactic environments. We propose that the optimal carriers of this pervasive, featureless hot dust component are very small carbonaceous (e.g., graphite) grains of nanometer size that are transiently heated by single-photon absorption. This grain population accounts for ~1.4% of the total infrared emission at ~5–3000 μm and ~0.4% of the total interstellar dust mass.

¹Kozyrev, A.S et al. (>10)
Physics of Atomic Nuclei 81, 527-539 Link to Article [DOI: 10.1134/S1063778818040099]
¹Space Research Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997, Russian Federation

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G. J. Archer^a,b, R. J. Walker^a, J. Tino^a, T. Blackburn^c, T. S. Kruijer^b,d, J. L. Hellmann^b
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.11.012]
^aDepartment of Geology, University of Maryland, College Park, MD 20742, USA
^bInstitut für Planetologie, University of Münster, Münster 48149, Germany
^eEarth and Planetary Sciences, University of California, Santa Cruz, SantacCruz, CA 95064, USA
^dNuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, CA 94550, USA
Copyright Elsevier

The abundances of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Pd), as well as ¹⁸⁷Re-¹⁸⁷Os and ¹⁸²Hf-¹⁸²W isotopic systematics were determined for separated metal, slightly magnetic, and nonmagnetic fractions from seven H4 to H6 ordinary chondrites. The HSE are too abundant in nonmagnetic fractions to reflect metal-silicate equilibration. The disequilibrium was likely a primary feature, as ¹⁸⁷Re-¹⁸⁷Os data indicate only minor open-system behavior of the HSE in the slightly and non-magnetic fractions. ¹⁸²Hf-¹⁸²W data for slightly magnetic and nonmagnetic fractions define precise isochrons for most meteorites that range from 5.2±1.6 Ma to 15.2±1.0 Ma after calcium aluminum inclusion (CAI) formation. By contrast, ¹⁸²W model ages for the metal fractions are typically 2 to 5 Ma older than the slope-derived isochron ages for their respective, slightly magnetic and nonmagnetic fractions, with model ages ranging from 1.4±0.8 Ma to 12.6±0.9 Ma after CAI formation. This indicates that the W present in the silicates and oxides was not fully equilibrated with the metal when diffusive transport among components ceased, consistent with the HSE data. Further, the W isotopic compositions of size-sorted metal fractions from some of the H chondrites also differ, indicating disequilibrium among some metal grains. The chemical/isotopic disequilibrium of siderophile elements among H chondrite components is likely the result of inefficient diffusion of siderophile elements from silicates and oxides to some metal and/or localized equilibration as H chondrites cooled towards their respective Hf-W closure temperatures. The tendency of ¹⁸²Hf-¹⁸²W isochron ages to young from H5 to H6 chondrites may indicate derivation of these meteorites from a slowly cooled, undisturbed, concentrically-zoned parent body, consistent with models that have been commonly invoked for H chondrites. Overlap of isochron ages for H4 and H5 chondrites, by contrast, appear to be more consistent with shallow impact disruption models.

The W isotopic composition of metal from one CR chondrite was examined to compare with H chondrite metals. In contrast to the H chondrites, the CR chondrite metal is characterized by an enrichment in ¹⁸³W that is consistent with nucleosynthetic s-process depletion. Once corrected for the correlative nucleosynthetic effect on ¹⁸²W, the ¹⁸²W model age for this meteorite of 7.0 ± 3.6 Ma is within the range of model ages of most metal fractions from H chondrites. The metal is therefore too young to be a direct nebular condensate, as proposed by some prior studies.

Susann SIEGERT^1,2 and Lutz HECHT^1,2
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13210]
¹Museum f€ur Naturkunde, Leibniz-Institut f€ur Evolutions- und Biodiversitätsforschung, Invalidenstraße 43,10115, Berlin, Germany
²Freie Universität Berlin, Institut für Geologische Wissenschaften, Malteserstraße 74-100, 12249, Berlin, Germany
Published by arrangement with John Wiley & Sons

Impact melt‐bearing clastic deposits (suevites) are one of the most important records of the impact cratering process. A deeper understanding of their composition and formation is therefore essential. This study focuses on impact melt particles in suevite at Ries, Germany. Textures and chemical evidence indicate that the suevite contains three melt types that originate from different shock levels in the target. The most abundant melt type (“melt type 1”) represents well‐mixed whole‐rock melting of crystalline basement and includes incompletely mixed mafic melt schlieren (“melt type 1 mafic”). Polymineralic melt type 2 comprises mixes between monomineralic melt types 3 and melt type 1. Melt types 2 and 3 are located within melt type 1 as small patches or schlieren but also isolated within the suevite matrix. The main melt type 1 is heterogeneous with respect to trace elements, varying geographically around the crater: in the western sector, it has lower values in trace elements, e.g., Ba, Zr, Th, and Ce, than in the eastern sector. The west–east zoning likely reflects the heterogeneous nature of crystalline basement target rocks with lower trace element contents, e.g., Ba, Zr, Th, and Ce, in the west compared to the east. The chemical zoning pattern of suevite melt type 1 indicates that mixing during ejection and emplacement occurred only on a local (hundreds of meters) scale. The incomplete larger scale mixing indicated by the preservation of these local chemical signatures, and schlieren corroborate the assumption that mixing, ejection, and quenching were very rapid, short‐lived processes.

Tre’Shunda James^1,2 and Renyu Hu^1,3
Astrophysical Journal 867, 17 Link to Article [DOI: 10.3847/1538-4357/aae2bb]
¹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA
²Occidental College, Los Angeles, CA 90041, USA
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA

Atmospheric chemistry models have shown that molecular oxygen can build up in CO₂-dominated atmospheres on potentially habitable exoplanets without input of life. Existing models typically assume a surface pressure of 1 bar. Here we present model scenarios of CO₂-dominated atmospheres with the surface pressure ranging from 0.1 to 10 bars, while keeping the surface temperature at 288 K. We use a one-dimensional photochemistry model to calculate the abundance of O₂ and other key species, for outgassing rates ranging from a Venus-like volcanic activity up to 20 times Earth-like activity. The model maintains the redox balance of the atmosphere and the ocean, and includes the pressure dependency of outgassing on the surface pressure. Our calculations show that the surface pressure is a controlling parameter in the photochemical stability and oxygen buildup of CO₂-dominated atmospheres. The mixing ratio of O₂ monotonically decreases as the surface pressure increases at very high outgassing rates, whereas it increases as the surface pressure increases at lower-than-Earth outgassing rates. Abiotic O₂ can only build up to the detectable level, defined as 10⁻³ in volume mixing ratio, in 10-bar atmospheres with the Venus-like volcanic activity rate and the reduced outgassing rate of H₂ due to the high surface pressure. Our results support the search for biological activities and habitability via atmospheric O₂ on terrestrial planets in the habitable zone of Sun-like stars.

¹Krivovichev, V.G.,²Charykova, M.V., ^3,4Krivovichev, S.V.
European Journal of Mineralogy 30, 219-230 Link to Article [DOI: 10.1127/ejm/2018/0030-2699]
¹Department of Mineralogy, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russian Federation
²Department of Geochemistry, Institute of Earth Sciences, St. Petersburg State University, University Emb. 7/9, St. Petersburg, 199034, Russian Federation
³Department of Crystallography, Institute of Earth Sciences, St. Petersburg State University, St. Petersburg, 199034, Russian Federation

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Åke V.Rosén^a, Jonas Pape^b, Beda A. Hofmann^a,b Edwin Gnos^c Marcel Guillong^d
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2018.11.016]
^aInstitute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, Switzerland
^bNatural History Museum Bern, Bernastrasse 15, 3005 Bern, Switzerland
^cNatural History Museum of Geneva, 1, Route de Malagnou, 1208 Geneva, Switzerland
^dInstitute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse 25, 8092 Zurich, Switzerland
Copyright Elsevier

Ureilites are the second largest group of achondrite meteorites but consensus is still lacking on the nature of their precursors, melting processes, and the genetic relationship between monomict ureilites and brecciated ureilites. The recently found ureilite Ramlat as Sahmah 517 is of special interest in this context. This meteorite lacks shock features in its primary silicates and belongs to a rare augite- and chromite-bearing subset of the monomict ferroan ureilites. It hosts abundant intergranular glass veinlets speckled with pyroxene and metal globules. Detailed petrographic investigations show that the Si-Al rich glass represents quenched anatectic melt that was present prior to formation of the reduced olivine rims by incomplete low-pressure equilibration (smelting) of carbon and silicates. The melt facilitated smelting which, along with rapid crystallization of secondary pyroxene, modified the originally trachyandesitic melt. Melt-silicate equilibrium preceding these events is constrained by modelling using MELTS and the first reported in-situ measurements of LREE-enriched glass that is largely complementary to the depleted mafic silicates in monomict ureilites. The inferred major element composition of the partial melt that formed in RaS 517 is similar to that of trachyandesite in Almahata Sitta but RaS 517 lacks phosphates which are abundant in the Almahata Sitta trachyandesite and in alkali-rich feldspathic clasts in polymict ureilites. The LREE-depletion in the dominant monomict ferroan ureilite population can be explained by the formation of melt fractions similar to the glass in RaS 517 after initial rapid melting of phosphates. These finds provide evidence for a genetic relationship between ferroan ureilites and lithologies similar to the Almahata Sitta trachyandesite and further suggest that these ureilites formed by partial melting of P- and alkali-rich precursors with trace element concentrations similar to equilibrated ordinary chondrites. Quenched Si-Al rich glass also occurs in magnesian ureilites but has lower concentrations of alkalis and LREE-depleted trace element signatures which can reflect more depleted compositions at the onset of partial melting. The evidence presented here favors a scenario in which the primary ureilite differentiation was driven by gradual heating from radioactive decay with resulting temperatures (>1100 °C) being maintained until disruption of the ureilite parent asteroid.

R. L. PALMA^1,2, R. O. PEPIN², A. J. WESTPHAL³, E.F€URI⁴, D. J. SCHLUTTER², Z.S.GAINSFORTH³, and D. R. FRANK⁵
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13189]
¹Department of Physics and Astronomy, Minnesota State University, Mankato, Minnesota 56001, USA
²School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
³Space Sciences Laboratory, University of California, Berkeley, California 94720–7450, USA
⁴Centre de Recherches Petrographiques et Geochimiques, CNRS-UL, 54501 Vandoeuvre-les-Nancy Cedex, France
⁵Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, Hawai’i 96822, USA
Published by arrangement with John Wiley & Sons

Helium and neon distributions are reported for a variety of Stardust comet 81P/Wild 2 samples, including particle tracks and terminal particles, cell surface and subsurface slices from the comet coma and interstellar particle collection trays, and numerous small aerogel blocks extracted from comet cells C2044 and C2086. Discussions and conclusions in several abstracts published during the course of the investigation are included, along with the relevant data. Measured isotope ratios span a broad range, implying a similar range for noble gas carriers in the Wild 2 coma. The meteoritic phase Q‐²⁰Ne/²²Ne ratio was observed in several samples. Some of these, and others, exhibit ²¹Ne excesses too large for attribution to spallation by galactic cosmic ray irradiation, suggesting exposure to a solar proton flux greatly enhanced above current levels in an early near‐Sun environment. Still others display evidence for a solar wind component, particularly one C2086 block with large abundances of isotopically solar‐like helium and neon. Eighty‐nine small aerogel samples were cut from depths up to several millimeters below the cell C2044 surface and several millimeters away from the axis of major track T41. A fraction of these yielded measurable and variable helium and neon abundances and isotope ratios, although none contained visible tracks or carrier particle fragments and their locations were beyond estimated penetration ranges for small particles or ions incident on the cell surface, or for lateral ejecta from T41. Finding plausible emplacement mechanisms and sources for these gases is a significant challenge raised by this study.