Meteoritics & Planetary Science 40, Nr 12, 1777–1787 (2005) Abstract available online at http://meteoritics.org Re-evaluating the age of the Haughton impact event Sarah C. SHERLOCK1*, Simon P. KELLEY1, John PARNELL2, Paul GREEN3, Pascal LEE4, Gordon R. OSINSKI5, and Charles S. COCKELL6 1Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK 2Department of Geology and Petroleum Geology, College of Physical Sciences, University of Aberdeen, Meston Building, King’s College, Aberdeen, AB24 3UE, UK 3Geotrack International, 37 Melville Road, Brunswick West, Victoria 3055, Australia 4Mars Institute, SETI Institute and NASA Ames Research Center, MS 245-3, Moffett Field, California 94035–1000, USA 5Canadian Space Agency, 6767 Route de l’Aeroport, Saint-Hubert, QC J3Y 8Y9, Canada 6Centre for Earth, Planetary, Space and Astronomical Research (CEPSAR), Planetary and Space Sciences Research Institute, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK *Corresponding author. E-mail: [email protected] (Received 29 October 2004; revision accepted 07 November 2005) Abstract–We have re-evaluated the published age information for the Haughton impact structure, which was believed to have formed ∼23 Ma ago during the Miocene age, and report new Ar/Ar laser probe data from shocked basement clasts. This reveals an Eocene age, which is at odds with the published Miocene stratigraphic, apatite fission track and Ar/Ar data; we discuss our new data within this context. We have found that the age of the Haughton impact structure is ∼39 Ma, which has implications for both crater recolonization models and post-impact hydrothermal activity. Future work on the relationship between flora and fauna within the crater, and others at high latitude, may resolve this paradox. INTRODUCTION target rocks are potassium-bearing and undergo melting, potassium partitions into the melt, such that the resulting Establishing the ages of terrestrial impacts is critically melts are also potassic and thus particularly suitable materials important for our understanding of both the terrestrial for the Ar/Ar dating method. cratering record and post-impact processes, such as impact- Ar/Ar dating has proved to be a very powerful tool in induced hydrothermal activity and intracrater sedimentary dating rare and altered material to reveal ages for terrestrial infilling. Crater dating can be achieved in a number of ways. meteorite impacts (e.g., Kunk et al. 1989; Reimold et al. 1990, Bracketing the age of the impact event using the age of the 1992; Koeberl et al. 1993; Spray et al. 1995; Kelley and Spray target rocks and the stratigraphic age of the youngest 1997; Thompson et al. 1998; Kelley and Gurov 2002). The intracrater sediments is often the only possible method where Ar/Ar method can be accomplished by measurement on craters are buried (e.g., Mjølnir Crater, Barents Sea) single aliquots of sample, which is much smaller than is (Gudlaugsson 1993) or poorly preserved, or where suitable required for K-Ar dating and is inherently more precise. The isotopic dating material no longer exists. More direct methods advantage of this is that it can be used on very heterogeneous include the stratigraphic bracketing or isotopic dating of distal material and can discriminate between alteration, mixing of ejecta (Krogh et al. 1993; Laurenzi et al. 2003; Walkden et al. different argon reservoirs, and reservoirs that are partly 2003), but one of the most direct methods is to date impact outgassed. Friction melts resulting from crater collapse after melt glasses in shocked basement materials and meteorite impacts are a complex mix of melt, target-rock pseudotachylites by Ar/Ar (Spray et al. 1995; Kelley and clasts, and refractory target-rock minerals; the same is true of Spray 1997; Kelley and Gurov 2002). Shock melt that forms shock-melt veins. Samples are also adversely affected by during the contact and compression stage of an impact event post-formational processes, such as alteration or argon loss and pseudotachylite that forms both during the compression resulting from a post-impact hydrothermal system, or tectonic and crater modification stages are both “new” materials that burial. These result in an age younger than the impact. As a are generated during the impact cratering process. Where consequence of these problems, many attempts to date 1777 © The Meteoritical Society, 2005. Printed in USA. 1778 S. C. Sherlock et al. impact-related samples have been only partially successful. existence of a post-impact crater lake (Omar et al. 1987; Two Ar-Ar dating techniques have been applied to impact Hickey et al. 1988; Whitlock and Dawson 1990). melts: step heating and laser probe spot dating. The former is The first age estimate for the Haughton impact event was a bulk-sample method; samples of up to 200 mg may be ∼15 to 25 Ma, with a quoted age of 20 ± 5 Ma, based on heated incrementally in a vacuum furnace to temperatures of macro- and micro-paleontological observations within the ∼1100° C, or much smaller samples of ∼1–30 mg may be crater-fill sediments (Frisch and Thorsteinsson 1978). This heated using a focused infrared laser, which circumvents early Miocene age was viewed as both the age of the impact problems of argon blank from hot furnace walls. The and the age of the fossil assemblage, the implication being advantage of the step heating method is that it can that the crater lake formed immediately post-impact. The discriminate against contamination where the contributions imprecise nature of this age was largely borne out by the from clasts and alteration products are small. Where the sizes absence of index fossils and the extreme geographic isolation of target-rock clasts and patches of alteration are much larger, at high latitude a large distance from the nearest correlative the laser probe spot-dating technique can discriminate assemblages. between contaminating clasts and alteration to extract melt The first attempt to constrain the absolute age of the ages. This technique has been successfully used to analyze impact event and fossil assemblage was through fission-track friction melts from fault zones where the resolution of the analysis of apatite grains from shocked basement clasts laser probe, which is ∼50 µm, is such that clasts of host-rock (Omar et al. 1987). The clasts were recovered from the and host-rock minerals can be avoided. The disadvantage of surface of impact melt breccia slopes within the crater and this approach is that the precision is often lower due to the yielded an age of 22.4 ± 1.4 Ma (Omar et al. 1987). much smaller gas fractions being analyzed. Following this, the widely cited Ar/Ar age for Haughton This study compares previously published furnace step- was obtained by step heating a granitic, impact melt-bearing, heating analyses with new laser probe step-heating and spot- basement clast (Jessberger 1988). The 23.4 ± 1.0 Ma age is dating analyses from shocked basement clasts from the within error of the apatite fission-track age, and has since Haughton impact structure, Devon Island, Canadian High been widely cited as the absolute age for the Haughton impact Arctic. A combination of step heating and spot dating event and post-impact lacustrine deposition. However, on highlights the presence of two argon reservoirs: biotite and closer scrutiny, the final age of 23.4 ± 1.0 Ma is difficult to quartz/feldspar. On this basis, it is possible to discuss all of reconcile with the Ar/Ar data of Jessberger (1988). Plotting the data from Haughton; it is also apparent that the previously the original Ar/Ar data as a step-heating spectrum using derived age of 23.4 ± 1.0 Ma for Haughton (Jessberger 1988) Isoplot (Ludwig 2003), the data yield a staircase spectrum and is difficult to reconcile with the complete data set, while an not a plateau (Fig. 2a). Crucially, the youngest age, Eocene age is in keeping with a re-evaluation of existing and corresponding to the lowest temperature step, is 41.9 ± new data. 0.8 Ma. A conventional interpretation of this spectrum would evoke the mixing of at least two Ar reservoirs from a material Geological Setting and Previous Age Constraints of the of ∼40 Ma with a contaminant refractory reservoir of ∼180 to Haughton Impact Structure 200 Ma. Referring to the material described by Jessberger (1988)—“a strongly shocked, banded biotite gneiss The Haughton impact structure is located on Devon containing highly vesiculated alkali feldspar and plagioclase Island in the Canadian Arctic Archipelago (Fig. 1). It formed glass, diaplectic quartz and plagioclase glass, and completely in a ∼1880 m series of Lower Paleozoic sedimentary rocks decomposed biotite, which is either opaque or forms schlieren dominated by carbonate facies, overlying a Precambrian of brownish glass”—it is possible that the two end-member crystalline basement (Frisch and Thorsteinsson 1978; reservoirs correspond to the glass phases outgassing at low Robertson and Sweeney 1983; Osinski et al. 2005a). The and medium temperatures (yielding the younger ages), crater is filled with carbonate-rich polymict impact melt probably glassy biotite schlieren followed by refractory breccia (Osinski and Spray 2001, 2003; Osinski et al. 2005b), diaplectic quartz and mixed feldspar glass, and any more which contains clasts of basement gneisses and granites, weakly shocked minerals outgassing at higher temperatures 37 39 indicating a depth of penetration in excess of 2 km (Redeker (yielding the older ages). In addition, low initial ArCa/ ArK and Stˆffler 1988). Scattered remnants of post-impact ratios rise in higher-temperature release, confirming a higher Tertiary lacustrine sediments, the Haughton Formation, contribution from a high-calcium/low-potassium material unconformably overlie the impact melt breccias. This is a such as feldspar and plagioclase glass in the later stratigraphic unit of interbedded dolomite-rich, poorly sorted release steps.