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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