Meteoritics & Planetary Science 40, Nr 4, 591–607 (2005) Abstract available online at http://meteoritics.org Laser argon dating of melt breccias from the Siljan impact structure, Sweden: Implications for a possible relationship to Late Devonian extinction events Wolf U. REIMOLD1*, Simon P. KELLEY2, Sarah C. SHERLOCK2, Herbert HENKEL3, and Christian KOEBERL4 1Impact Cratering Research Group, School of Geosciences, University of the Witwatersrand, Private Bag 3, P. O. Wits 2050, Johannesburg, South Africa 2Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK 3Department of Land and Water Resources Engineering, Division of Engineering Geology and Geophysics, Royal Institute of Technology, Teknikringen 72, SE 100-44 Stockholm, Sweden 4Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria *Corresponding author. E-mail:
[email protected] (Received 12 July 2004; revision accepted 08 February 2005) Abstract–In earlier studies, the 65–75 km diameter Siljan impact structure in Sweden has been linked to the Late Devonian mass extinction event. The Siljan impact event has previously been dated by K- Ar and Ar-Ar chronology at 342–368 Ma, with the commonly quoted age being 362.7 ± 2.2 Ma (2 σ, recalculated using currently accepted decay constants). Until recently, the accepted age for the Frasnian/Famennian boundary and associated extinction event was 364 Ma, which is within error limits of this earlier Siljan age. Here we report new Ar-Ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from Siljan (interpreted to be impact melt breccia). The analytical results show some scatter, which is greater in samples with more extensive alteration; these samples generally yield younger ages.