^1,2,3Marianne M. Mader, ^1,2,4Gordon R. Osinski
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13173]
¹Centre for Planetary Science and Exploration, University of Western OntarioLondon, Ontario, N6A 5B7, Canada
²Department of Earth Sciences, University of Western OntarioLondon, Ontario, Canada
³Centre for Earth & Space, Royal Ontario MuseumToronto, Ontario, Canada
⁴Department of Physics and Astronomy, University of Western OntarioLondon, Ontario, Canada
Published by arrangement with John Wiley & Sons

The Mistastin Lake impact structure is an intermediate‐size (~28 km apparent crater diameter), complex crater formed ~36 Myr. The original crater has been differentially eroded; however, a subdued terraced rim and distinct central uplift are still observed and impactites are well exposed in three dimensions. The inner portion of the structure is covered by Mistastin Lake and the surrounding area is locally covered by soil/glacial deposits and vegetation. The crystalline target rocks of the Mistastin Lake region are dominated by anorthosite, granodiorite, and quartz monzonite. Previous studies of the Mistastin Lake impactites have primarily focussed on the impact melt rocks. This study further evaluates the entire suite of impactite rocks in terms of their location within the crater structure and emplacement mechanisms. Locally, allochthonous impactite units including impact melt and various types of breccias are distributed around the lake in the terraced rim and are interpreted as proximal ejecta deposits. A multistage model for the origin and emplacement of impact melt rocks and the formation of impact ejecta is proposed for the Mistastin Lake impact structure based on a synthesis of the field and petrographic observations. This model involves the generation of a continuous ballistic ejecta blanket during the excavation stage, followed by the emplacement of melt‐rich, ground‐hugging flows during the terminal stages of crater excavation and the modification stage of crater formation. Impact melt‐bearing breccias—also termed “suevite” at other sites—are present in several distinct settings within the Mistastin Lake structure and likely have more than one formation mechanism.

¹Ellinor Martin, ¹Birger Schmitz, ²Hans‐Peter Schönlaub
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13174]
¹Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
²Center for Geosciences, Austrian Academy of Sciences, Vienna, Austria
Published by arrangement with John Wiley & Sons

We present the first reconstruction of the micrometeorite flux to Earth in the Silurian Period. We searched 321 kg of condensed, marine limestone from the Late Silurian Cellon section, southern Austria, for refractory chrome‐spinel grains from micrometeorites that fell on the ancient sea floor. A total of 155 extraterrestrial spinel grains (10 grains >63 μm, and 145 in the 32–63 μm fraction) were recovered. For comparison, we searched 102 kg of similar limestone from the mid‐Ordovician Komstad Formation in southern Sweden. This limestone formed within ~1 Ma after the breakup of the L chondrite parent body (LCPB) in the asteroid belt. In the sample we found 444 extraterrestrial spinel grains in the >63 μm fraction, and estimate a content of at least 7000 such grains in the 32–63 μm fraction. Our results show that in the late Silurian, ~40 Ma after the LCPB, the flux of ordinary equilibrated chondrites has decreased by two orders of magnitude, almost down to background levels. Among the ordinary chondrites, the dominance of L‐chondritic micrometeorites has waned off significantly, from >99% in the post‐LCPB mid‐Ordovician to ~60% in the Late Silurian, with ~30% H‐, and ~10% LL‐chondritic grains. In the Late Silurian, primitive achondrite abundances are similar to today’s value, contrasting to the much higher abundances observed in pre‐LCPB mid‐Ordovician sediments.

^1,2Poorna Srinivasan, ³Daniel R. Dunlap, ^1,2Carl B. Agee, ³Meenakshi Wadhwa, ⁴Daniel Coleff, ^1,2Karen Ziegler, ⁵Ryan Zeigler, ⁵Francis M. McCubbin
Nature Communications 9, 3036 Link to Article [https://doi.org/10.1038/s41467-018-05501-0]
¹Institute of Meteoritics, University of New Mexico, Albuquerque, NM, 87131, USA
²Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM, 87131, USA
³Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, AZ, 85287, USA
⁴Jacobs Technology, NASA Johnson Space Center, Mail Code XI3, 2101 NASA Parkway, Houston, TX, 77058, USA
⁵NASA Johnson Space Center, Mail Code XI2, 2101 NASA Parkway, Houston, TX, 77058, USA

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