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. The two samples with the least alteration yield the most reproducible weighted mean ages: one yielded a laser spot age of 377.2 ± 2.5 Ma (95% confidence limits) and the other yielded both a laser spot age of 376.1 ± 2.8 Ma (95% confidence limits) and a laser stepped heating plateau age over 70.6% 39Ar release of 377.5 ± 2.4 Ma (2 σ). Our conservative estimate for the age of Siljan is 377 ± 2 Ma (95% confidence limits), which is significantly different from both the previously accepted age for the Frasnian/Famennian (F/F) boundary and the previously quoted age of Siljan. However, the age of the F/F boundary has recently been revised to 374.5 ± 2.6 Ma by the International Commission for Stratigraphy, which is, within error, the same as our new age. However, the currently available age data are not proof that there was a connection between the Siljan impact event and the F/F boundary extinction. This new result highlights the dual problems of dating meteorite impacts where fine-grained melt rocks are often all that can be isotopically dated, and constraining the absolute age of biostratigraphic boundaries, which can only be constrained by age extrapolation. Further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times, perhaps resulting in major extinction events in Earth’s biostratigraphic record. INTRODUCTION known impact structure in Europe. Its diameter was originally estimated at 52 km (Grieve 1988), but Von Dalwigk and The Siljan impact structure (Fredriksson and Wickman Kenkmann (1999) and Kenkmann and Von Dalwigk (2000) 1963; Wickman et al. 1963; Svensson 1971, 1973; Rondot made a case for a larger diameter of at least 65 km on the basis 1975; BodÈn and Eriksson 1988; Juhlin and Pedersen 1987; of structural geological considerations and by applying the Kenkmann and von Dalwigk 2000; Henkel and Aaro 2005) is empirical morphometric scaling laws provided by Therriault located in the Dalarna region of south-central Sweden, et al. (1997). In contrast, Henkel and Aaro (2005) observe a centered at 61°02′N/14°52′E (Fig. 1). Siljan is the largest 75 km wide current topographic expression and estimate that 591 © The Meteoritical Society, 2005. Printed in USA. 592 W. U. Reimold et al. Fig. 1. The geology of the Siljan impact structure. The inset shows the location of Siljan in Scandinavia. the pre-erosional crater diameter could have been as much as Consequently, very little material that could be used to obtain 85 km. a reliable age for this impact structure has been produced. The evidence for impact at Siljan includes the presence Only “allochthonous breccia, small and narrow breccia dikes, of shatter cones and planar deformation features (PDFs) in and float of melt breccia” were reported by Rondot (1975). quartz. Exposure is poor in much of the Siljan structure, and Svensson (1971, 1973) suggested a post-Silurian age for the bona fide impact melt rock has, to date, remained elusive. impact event. Laser Ar dating of melt breccias from Siljan 593 In the 1980s, Siljan experienced an “impact exploration intriguing part of the Late Devonian, during which several boom” in the wake of Gold’s proposal (reviewed in Gold and important events occurred (geological time scale of Harland Soter 1980; Gold 1987, 1988) that the structure could provide et al. 1989). In this scheme, the Devonian was placed between access to significant mantle-derived hydrocarbon resources 417 and 354 Ma, with the Givetian stage between 380 and that might have infiltrated into the impact-deformed basement 370 Ma, the Frasnian from 370–364 Ma, and the Famennian of the structure. Accordingly, Siljan was extensively and from 364 to 354 Ma. It should also be noted that Ellwood deeply drilled (BodÈn and Eriksson 1988), but no economic et al. (2003) suggested the presence of evidence for impact at potential could be established. Some impact-related the Eifelian/Givetian stage boundary of the mid-Devonian, a hydrothermal Pb-Zn mineralization does, however, occur, and suggestion that has remained controversial (Racki and has been mined locally (e.g., Reimold et al. 2005 and Koeberl 2004). As reviewed by Sandberg et al. (2002), the references therein; Hode et al. 2002). Late Frasnian mass extinction occured just prior to 364 Ma Bottomley et al. (1978) referred to an outcrop with a (within 20,000 years), representing a major extinction event “small dikelet [of melt breccia] at shatter cone locality 3 of that decimated most groups of marine organisms. This event Svensson (1973).” Two samples from this site were described has been associated with alleged impact evidence, including a as containing 30–25% inclusions, predominantly quartz that weak iridium enrichment found at the Frasnian/Famennian occasionally shows shock deformation, but also with clasts of boundary in southern China (Wang et al. 1991) and in a cross- feldspar and brecciated granite with incipient recrystallization boundary section in the state of New York (Over et al. 1997). and rare inclusions of sandstone. Melt matrix was In addition, the presence of so-called “microtektite-like glass” microcrystalline and granular. Interstitial devitrified glass did at a locality in Belgium was reported by Claeys and Casier also occur. One sample yielded a humped spectrum with a (1994) and discussed by Sandberg et al. (1988). The Siljan maximum age of 380 Ma but no plateau. The second yielded impact structure was proposed by Claeys and Casier as the a similar pattern but with a three-step plateau comprising 92% possible source for these Belgian microtektites. Furthermore, of release, but only because the analytical error on the middle the Amˆnau catastrophic event of central Germany has also step was 4.5% (around 10 times the two adjacent plateau been tentatively linked with impact (Sandberg et al. 2000, steps). Re-analysis of the Bottomley et al. (1978) data using 2002), although no bona fide impact evidence has been ISOPLOT (Ludwig 1999) yields an age of 358.3 ± 4.8 Ma reported for this event. The Amˆnau event was placed at the (2 σ). In addition, the final age quoted by Bottomley et al. Givetian/Frasnian boundary at 370 Ma. The (1978) was quoted at the 1 σ level and was not the plateau age biochronologically dated Alamo impact breccia of southern but an integrated total fusion age, in effect a K-Ar age. It is Nevada (e.g., Warme et al. 2002) occurred in the early well documented that fine-grained whole rock samples Frasnian punctata zone at about 367 Ma (Sandberg and showing younger ages in the highest temperature release Warme 1993; Sandberg and Morrow 1998; Sandberg et al. result from 39Ar recoil (McDougall and Harrison 1999) and 2002). The latter authors proposed links between this Alamo are more likely obtained from altered samples. Ar-Ar impact breccia and the Siljan and Flynn Creek impact analyses presented below demonstrate that the hydrothermal structures and suggested that these events could have resulted alteration around Siljan has led to alteration of many melt from a comet shower. Such a link is, however, highly unlikely, samples and it is likely that the age of 362.7 ± 2.2 Ma as the geometry of the distribution of the Alamo breccia rather significantly underestimates the true age of the impact at indicates a nearby source crater in Nevada. A further mass Siljan. In fact, a later publication by Bottomley et al. (1990) extinction close to the Devonian/Carboniferous boundary was quotes an age of 368 ± 1.1 Ma, apparently from the same placed at 357 Ma. dataset, although no explanation was given for the difference. Recently, the International Commission on Stratigraphy A K-Ar age of 349 Ma for “shock melt” from another (ICS) proposed a revised geological time scale (Gradstein locality was cited by Åberg and Bollmark (1985); Juhlin et al. et al. 2004; Ogg 2004; Gradstein and Ogg 2004), resulting in a (1991) cite a 40Ar–39Ar date for a “granitic pseudotachylite” shift of the ages for the Givetian, Frasnian, and Famennian of 359 ± 4 Ma, as well as two K-Ar ages for “doleritic stages of the Devonian period from 370–380 to 385.3–392, pseudotachylite” of 342 ± 3 Ma and 349 ± 2 Ma (whereby it 364–370 to 374.5–385.3, and 359.2–374.5 Ma (errors at 2.5– is assumed that these breccias would have been formed by the 2.7 Ma), respectively.