Geochimica et Cosmochimica Acta, Vol. 67, No. 20, pp. 3889–3903, 2003 Copyright © 2003 Elsevier Ltd Pergamon Printed in the USA. All rights reserved 0016-7037/03 $30.00 ϩ .00 doi:10.1016/S0016-7037(03)00213-8 Geochemistry of carbonaceous impactites from the Gardnos impact structure, Norway 1, 2 1 1 1 3 3 I. GILMOUR, *B.M.FRENCH, I. A. FRANCHI, J. I. ABBOTT, R. M. HOUGH, J. NEWTON, and C. KOEBERL 1Planetary and Space Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, UK 2Smithsonian Institution, Washington, DC 20560, USA 3Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria (Received September 17, 2001; revised 14 October 2002; accepted in revised form October 14, 2002) Abstract—The Gardnos impact structure in southern Norway is one of only two known impact structures (among ϳ175) whose impactites contain significant amounts (typically 0.2–1.0 wt.%) of carbon, or 5 to 10 times the amount present in the target rocks; Sudbury, Canada is the other. This study extends a previous investigation of the geochemistry and petrology of Gardnos impactites (French et al., 1997) with additional sampling and a detailed investigation of the nature and possible origin of the carbonaceous material present. Two principal carbon components have been identified in Gardnos impactites: (1) impact-produced diamonds, 0.5 to 1 ␮m in size, with a cubic crystal structure, predominantly hexagonal morphologies with platey layers and an estimated concentration of Ͻ 0.19 ppm in Gardnos suevites and (2) graphitic carbon ranging from poorly ordered to moderately crystalline. Geochemical data suggests that there are no suitable target rocks that could provide a single source for the carbon in Gardnos impactites. However, Raman spectroscopy, stable isotope analysis and transmission electron microscopy of the impact diamonds and graphitic carbon suggests that there were at least two episodes of C emplacement in Gardnos impactites: an impact-related incorporation and shock transformation of graphitic material from target rocks followed by later mobilization of C, possibly during postimpact cooling or later regional metamorphism. Copyright © 2003 Elsevier Ltd 1. INTRODUCTION present overlying the basement rocks at the impact site, was suggested as a possible carbon source by French et al. (1997). The Gardnos impact structure (Dons and Naterstad, 1992; The original study (French et al., 1997) was based on a suite French et al., 1997) is located in southern Norway (60°40ЈN, of samples collected in 1994. During the summer of 1996, a 9°00ЈE), ϳ125 km northwest of Oslo, Norway. The structure, larger sample suite was collected at Gardnos to expand the originally ϳ5 km in diameter, has been deeply eroded. It lacks available suite of known basement (target) rocks and impactites any distinctive circular form and is now represented by out- and to investigate in more detail the source of the carbon in the crops of impact-produced breccias, both lithic and melt-bear- impactites. Gardnos samples were collected both from outcrop ing. Its impact origin has been convincingly established from and from a tunnel dump along the Dokkelvi River, which had three lines of evidence (French et al., 1997): (1) the presence of provided fresh samples of a wide range of basement rocks and melt-bearing breccias containing granitic rock fragments with impactites (French et al., 1997). In addition, samples of two definite Planar Deformation Features (PDFs) in quartz and black shale units, which might originally have been present feldspar; (2) the demonstration, using chemical mixing models, among the Gardnos target rocks, were collected from the clos- that the breccia compositions could be produced by mixing the est exposures to Gardnos. The Cambrian Alum Shale (Anders- exposed target rocks; (3) an extraterrestrial signature from the son et al., 1985; Thickpenny and Leggett, 1987; Bharati et al., projectile in the breccias, established using osmium-isotope 1995) was sampled from near Bjørgo, ϳ50 km ENE of Gard- analyses. The age of the structure is poorly constrained between nos, and the Proterozoic Biri Shale (Bjørlykke et al., 1976; a metamorphic age for the basement target rocks (ϳ900 Ma) Tucker, 1983; Vidal and Nystuen, 1990) from the west shore of and the Caledonian orogeny at ϳ385 Ma (Grier et al., 1999). Lake Mjøsa, ϳ100 km E of Gardnos. A major unresolved problem about the Gardnos structure is This paper presents results of geochemical studies on the the unusually high content of carbon in its impact-produced new group of samples, describes detailed studies on the carbo- rocks. Gardnos is one of only two known impact structures naceous material in the Gardnos impactites and related rocks, (among ϳ175) whose impactites contain significant amounts and evaluates the role of black shales in general, and the Biri (typically 0.2–1.0 wt.%) of carbon, or 5 to 10 times the amount and Alum Shales in particular, as sources for the carbonaceous present in the target rocks (French et al., 1997); Sudbury, material in the Gardnos impactites. Canada is the other (French, 1968; Bunch et al., 1999; Hey- mann et al., 1999). Values of ␦13C for the Gardnos carbon range from Ϫ28 to Ϫ32‰ (French et al., 1997), strongly 2. SAMPLE DESCRIPTIONS suggesting an origin from biogenically derived carbon. The carbonaceous Proterozoic Biri Shale, which could have been Based on hand-specimen and petrographic examinations, no new lithologic types not described earlier (French et al., 1997) were identified at Gardnos among either the target rocks or the * Author to whom correspondence should be addressed various types of impactites. The new samples (see Appendix) ([email protected]). can be included in the earlier nomenclature, and the earlier 3889 3890 I. Gilmour et al. petrographic descriptions can be applied to the current suite of 3. EXPERIMENTAL Gardnos samples (French et al., 1997, pp. 877–883). 3.1. XRF and INAA Three basement (target) lithologies have been distinguished (French et al., 1997): (1) a variable suite of granitic gneisses Major element oxides and some trace elements (Rb, Sr, Y, Zr, Nb, Ͼ Co, Ni, Cu, Zn, V, Cr and Ba) concentrations were determined on that form the majority ( 75–80 area %) of the outcrop area; (2) powdered samples by X-ray fluorescence (XRF) spectrometry at the amphibolite, generally present as crosscutting dikes in the Department of Geology, University of Witwatersrand, Johannesburg, granitic gneisses; (3) a coarse-grained metamorphic ortho- South Africa. Details on procedures, precision and accuracy are de- quartzite, which is white and massive outside the structure and scribed by (Reimold et al., 1994). Other trace elements (Sc, Cr, Co, Ni, becomes black and highly fractured within it. In addition, a Zn, As, Se, Br, Rb, Sr, Zr, Sb, Cs, Ba, Hf, Ta, W, Ir, Au, Th and U) and the REEs were determined using Instrumental neutron activation anal- wide range of impactites are exposed within the structure. The ysis (INAA). These analyses were carried out at the Institute of Geo- term impactite here designates all rocks, both coherent and chemistry, University of Vienna, Austria. Details on analytical proce- fragmental, produced by the action of shock waves (Stoeffler dures (including standard data) are given in (Koeberl, 1993). and Grieve, 1994). Impactites at Gardnos are: (1) shocked quartzite, which is black and highly fractured within the struc- 3.2. Isolation and Identification of Carbon Components ture (French et al., 1997, pp. 877–879); (2) lithic breccias: (a) Powdered samples (ϳ4 g) were partially demineralised using pro- the well-known subcrater “Gardnos Breccia” (Broch, 1945), cedures described previously (Gilmour et al., 1992; Hough et al., 1997) which consists of fragments of white granitic gneiss in a with the exception that microwave assisted dissolution was used for the pulverized black matrix; (b) a “black-matrix breccia” (French initial removal of silicates. The acid-dissolved residues were sub- sampled and treated with chromic acid (6 mol/L at 70°C) to remove et al., 1997, p. 881), which is similar to the “Gardnos Breccia,” amorphous and organic carbon. Aliquots of this residue were then but contains fewer fragments and a higher percentage of gen- treated with fuming perchloric acid at 110°C to remove crystalline erally darker matrix; (3) melt-bearing breccias: (a) suevite, in graphite. The residues were examined using a transmission electron which fragments of melt and basement rock occur in the clastic microscope (TEM), a JEOL 2000FX operated at 200 kV, and equipped matrix; (b) melt-matrix breccias (or impact-melt breccias) in with an EDS system for chemical characterization. The TEM was also to obtain selected-area electron diffraction (SAED) patterns to confirm which crystallized melt forms a matrix to rock and mineral the identity of carbon components. clasts (French et al., 1997). Samples of two carbon-bearing black shale lithologies now 3.3. Carbon Abundance and Isotopic Measurements exposed in the region around Gardnos, the Alum Shale (Cam- brian) and the Biri Shale (Proterozoic), were also sampled to Whole-rock carbon abundance and isotopic composition were deter- mined on powdered samples treated with dilute HCl to remove any examine their suitability as sources for the carbon, on the carbonate components. Triplicate analyses were performed using an assumption that one or both of these units might have been elemental analyser coupled to a stable isotope mass spectrometer present at the site of the Gardnos structure at the time of the (Micromass Optima operated in continuous flow mode with a Carlo impact event (see French et al., 1997, pp. 898–899). Erba elemental analyser). Acid residue samples were also analysed using stepped combustion, incremental heating in oxygen, on a high The Alum Shale was collected from an outcrop near Bjørgo, sensitivity static vacuum mass spectrometer to determine the carbon ϳ50 km ENE of Gardnos, where it occurs as a crumpled and content and isotopic composition of components within the residue.