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Determining the impactor of the Ordovician Lockne crater: Oxygen and neon isotopes in chromite versus sedimentary PGE signatures

Author:
  • Birger Schmitz
  • Philipp R. Heck
  • Carl Alwmark
  • Noriko T. Kita
  • Matthias Meier
  • Bernhard Peucker-Ehrenbrink
  • Takayuki Ushikubo
  • John W. Valley
Publishing year: 2011
Language: English
Pages: 149-155
Publication/Series: Earth and Planetary Science Letters
Volume: 306
Issue: 3-4
Document type: Journal article
Publisher: Elsevier

Abstract english

Abundant chromite grains with L-chondritic composition in the resurge deposits of the Lockne impact crater (458 Myr old: dia. similar to 10 km) in Sweden have been inferred to represent relict fragments of an impactor from the break-up of the L-chondrite parent body at 470 Ma. This view has been challenged based on It/Cr and platinum group element (PGE) patterns of the same resurge deposits, and a reinterpretation of the origin of the chromite grains. An impactor of the non-magmatic iron meteorite type was proposed instead. Here we show that single-grain oxygen and noble-gas isotope analyses of the chromite grains from the resurge deposits further support an origin from an L-chondritic asteroid. We also present PGE analyses and Ir/Cr ratios for fossil L-chondritic meteorites found in mid-Ordovician marine limestone in Sweden. The L-chondritic origin has been confirmed by several independent methods, including major element and oxygen isotopic analyses of chromite. Although the meteorites show the same order-of-magnitude PGE and Cr concentrations as recent L chondrites, the elements have been redistributed to the extent that it is problematic to establish the original meteorite type from these proxies. Different PGE data processing approaches can lead to highly variable results, as also shown here for the Lockne resurge deposits. We conclude that the Lockne crater was formed by an L-chondritic impactor, and that considerable care must be taken when inferring projectile type from PGEs in sedimentary ejecta deposits. (C) 2011 Elsevier B.V. All rights reserved.

Keywords

  • Geology
  • extraterrestrial chromite
  • impact projectile
  • asteroid impact
  • oxygen
  • isotopes
  • platinum group elements
  • impact crater

Other

Published
  • ISSN: 1385-013X
Carl Alwmark
E-mail: carl [dot] alwmark [at] geol [dot] lu [dot] se

Senior lecturer

Lithosphere and Biosphere Science

+46 46 222 78 71

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Sölvegatan 12, Lund

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