Meteoritics & Planetary Science 40, Nr 12, 1777–1787 (2005) Abstract available online at http://meteoritics.org

Re-evaluating the age of the Haughton

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 , 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. 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- 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 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, , 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 . This is a such as feldspar and plagioclase glass in the later stratigraphic unit of interbedded dolomite-rich, poorly sorted release steps. The final age reached by Jessberger (1988) of silt, fine sand, and mud that was originally described as 23.4 ± 1.0 Ma is an intercept age from an inverse isochron “sedimentary fill” or “lake fill” by Frisch and Thorsteinsson correlation diagram, using heating steps 2, 3, and 4, which has (1978). The presence of remnant lacustrine crater-fill a 40Ar/36Ar intercept of 714 ± 26 (Fig. 2b). An extraneous sediments, including fossil freshwater fish, implied the 40Ar reservoir is frequently detected as a contaminating Re-evaluating the age of the Haughton impact crater 1779

Fig. 1. The location and simplified geology of the Haughton impact. component in strongly heterogeneous rocks such as ANALYTICAL PROCEDURES pseudotachylite (M¸ller et al. 2001; Kohut and Sherlock AND MATERIAL ANALYZED 2003), but is almost universally recognized within a step- heating spectrum by bringing about elevated ages in both the All of the samples used in our study were collected low- and high-temperature release steps. The spectrum in during the 2004 summer field campaign of the Haughton- Fig. 2a has elevated yet realistic ages in the high-temperature Project. We applied two different analytical methods: steps but not in the low-temperature steps, making it less laser probe spot dating and laser probe step heating. Laser likely that the sample is significantly contaminated with probe spot dating was performed on polished thick sections excess 40Ar. (300 µm thick) of shock metamorphosed basement clasts. Stephan and Jessberger (1992) compared the Ar/Ar 5 mm × 5 mm sections were selected for irradiation. Samples furnace step-heating data from experimentally and naturally 5-1 and 5-2 (Fig. 3a) are from a single melt-dominated clast shocked mineral separates. The natural samples were from the containing melt veins of <1 mm in width. The melt veins Haughton structure. In naturally shocked biotite and those preserve flow-banding and vesicles, as well as entrained from Haughton, it was noted that argon retentivity decreases refractory host-rock mineral grains up to 200 µm in diameter with the degree of . Proterozoic (Figs. 3b and 3c). Host-rock minerals within the clasts are basement distal to the Haughton structure yielded a highly shattered and predominantly diaplectic glasses of Proterozoic Ar/Ar age (∼1.7 Ga) (Stephan and Jessberger unknown composition. The in situ laser spot analyses were 1992), while heavily shocked samples yielded significantly performed using a Spectron Lasers SL902 CW Nd-YAG 1064 younger ages. Two individual biotite samples were described nm laser with a manual shutter. Individual analyses were as having “well-defined” plateau ages, with ages of 32.2 ± carried out by firing the laser in 30 msec shots, commonly one 0.6 Ma and 34.9 ± 1.8 Ma, respectively, although Stephan and shot per analysis, resulting in pits ∼75 µm in diameter. Laser Jessberger (1992) considered these to be meaningless ages probe step heating was performed on individual fragments since they are significantly older than the earlier age of 23 Ma (<1 mm) of shocked basement clasts; gases were released age. Quartz-feldspar separates were also analyzed in this incrementally at increasingly higher laser power using the study. The resulting spectra were disturbed and not same laser probe as used in the spot dating experiments. considered to be meaningful (Stephan and Jessberger 1992). All samples were ultrasonically cleaned alternately in These results will be discussed in greater detail in later deionized water and methanol prior to packaging in sections. aluminium foil. Additional washing in dilute HCl was 1780 S. C. Sherlock et al.

gas mass spectrometer, where 40Ar to 36Ar are measured. Analyses were corrected for blanks measured either side of two consecutive samples analyses, 37Ar decay and neutron- induced interference reactions using the correction factors (39Ar/37Ar)Ca = 0.00065 ± 0.0000033, (36Ar/37Ar)Ca = 0.000264 ± 0.0000013, and (40Ar/39Ar)K = 0.0085 ± 0.0; the mass discrimination value used was 283.

RESULTS AND DISCUSSION

Spot-Dating Experiments

Ages of individual analyses from samples 5-1 and 5-2 range from 185.1 ± 1.8 to 30.0 ± 0.4 Ma (Table 1). Inter- sample age variations are notable; for sample 5-1, the ages vary from 185.1 ± 3.0 to 36.6 ± 0.2 Ma, while for sample 5-2, the ages vary from 51.5 ± 0.6 to 30.0 ± 0.4 Ma. Distinct isotopic composition-related differences between the two 37 39 samples are observed in terms of age versus ArCa/ ArK (Fig. 4a). All but one of the data points from sample 5-2 form 37 39 a low ArCa/ ArK cluster (0.003–0.030), while data points 37 39 from sample 5-1 are widely scattered, with ArCa/ ArK values ranging from 0.01 to 0.485 (Fig. 4a; Table 1).

Step-Heating Experiments

Chips from four individual shocked granitic basement clasts were step heated. The samples displayed unusual characteristics on coupling with the laser that are normally only observed during analysis of volatile-rich volcanic glasses. The rock chips inflated to twice their original size at laser powers that were too low to couple with biotite. Subsequent heating steps, initially coupling with biotite, caused further expansion of the chip; at higher laser powers, there was coupling with feldspar/quartz, expansion ceased, Fig. 2. Ar-Ar data of Jessberger (1988) for the generally accepted age and the feldspar/quartz melted to a glass sphere. The for the Haughton impact presented as: a) a step-heating spectrum, expansion of the samples indicates the presence of a high with a dashed line representing the ∼23 Ma age, and the shaded steps volatile component. Increased gettering might have improved are those used to plot derive the intercept age; and b) inverse isochron the analyses, but it is likely that the glasses all contain high correlation diagram with data points 2, 3, and 4 plotted, and the derived intercept age of 23.2 ± 3.2 Ma. Both diagrams are plotted concentrations of CO2 since the partly melted clasts lie in a using Isoplot (Ludwig 2003). matrix of carbonate and carbonate melts (Osinski and Spray 2001). High volatile contents can lead to irreproducible required for the rock chips used for laser probe step heating isotope analyses, particularly at mass 36. We monitored because they were coated in carbonate; this was repeated masses indicating hydrocarbon (mass 41), and chlorine (mass three times prior to a final wash in deionized water. All of the 35) during all of the analyses and detected increases during samples were irradiated in the McMaster reactor (Canada); early steps of each of the stepped heating analyses. Volatile those used for spot dating were irradiated for 50 hr and those concentrations were sufficiently high that the peaks interfered used for step heating were irradiated for 25 hr. In both cases, with isotope analyses even though we used high mass neutron flux was monitored using GA1550 biotite standard resolution (∼300), but we were able to use the variation of 35 (98.79 ± 0.96 Ma) (Renne et al. 1998), giving a J value of and 41 in the blanks to correct sample analyses (e.g., 0.01175 ± 0.000059 (spot dating) and 0.00639 ± 0.000032 McConville et al. 1988). The resulting argon isotope data are (step heating). During all of the analyses, the unwanted gas of poorer precision than the spot analyses as a result of the species are removed from released gases by two Zr-Al getters corrections, and do not yield data of sufficient quality to for 5 min prior to automatic inlet into an MAP 215-50 noble calculate reliable ages. Figure 5 illustrates the corrected Re-evaluating the age of the Haughton impact crater 1781

Fig. 3. a) A photograph of sample 5 (thin section). This is the conjugate piece to the irradiated polished slab used for laser probe dating. Boxes 1 and 2 represent the two individual pieces selected for irradiation, corresponding to analyses 5-1 and 5-2. b) An SEM image of melt vein in A1. c) An SEM image of melt vein in A2. release spectra which, although compromised by the (Fig. 6a), and it seems likely that these features are the result interferences, illustrate the form of the release spectra. All of severe 39Ar-recoil. Stephan and Jessberger (1992) also release spectra display ages of ∼60–80 Ma over the first 40– analyzed a strongly /feldspar separate 60% of release, followed by a staircase increase to reach ages (HD7192), the resulting spectrum (Fig. 6b) is described as in the range 120–240 Ma and in the later release steps the heavily disturbed, providing only a lower limit for the time of spectra commonly show age decrease. This release pattern is crystallization. The release spectrum shows negative ages in very similar to those seen in previous analyses of partially the first 19% 39Ar release then ages increase from ∼15 Ma to melted basement rocks from the Haughton impact (Jessberger ∼35 Ma from 19% to 94% of the release spectrum. At high 1988). temperature, the ages jump to ∼150 Ma and peak at ∼444 Ma before dropping back to ∼22 Ma (Fig. 6b). It is apparent from Discussion of Haughton Geochronology Fig. 6c that the feldspar is the most refractory mineral in the whole rock and is contributing argon only at the highest Combining the Ar/Ar data of Jessberger (1988), Stephan temperatures. The zero initial ages and young ages at high and Jessberger (1992) and this study afford valuable insight temperature clearly indicate severe 39Ar recoil effects. into the complexity of argon reservoirs in shocked basement Shocked quartz commonly contains fluid inclusions, and minerals and melt veins in the Haughton impact structure. Haughton target rocks were carbonate sediments overlying Stephan and Jessberger (1992) analyzed two strongly crystalline basement. Osinski and Spray (2001) show that shocked biotite separates by furnace step heating. Both are carbonate was mobilized during the impact and thus it is described as yielding well-defined plateau ages: 32.2 ± highly probable that quartz/feldspar from Haughton contain 0.6 Ma (sample HD7192) (Fig. 6a) and 34.9 ± 1.8 Ma (DIG9) inclusions of CO2, argon, and water. The experiments of (Fig. 6a). The light grey steps are those used by Stephan and Stephan and Jessberger (1992) performed on both biotite and Jessberger (1992) to calculate plateau ages. In order to quartz/feldspar separates are illuminating in that they compare ages from all three studies, those shown in Figs. 2, demonstrate two argon reservoirs in the shocked basement 6, and 7 have been calculated using ISOPLOT 3 (Ludwig clasts with distinct argon-isotope properties: biotite with 2003). HD7192 yields a plateau age of 32.0 ± 1.8 Ma (1σ, younger ages, disturbed high-temperature steps with 39Ar- MSWD = 3.6), based on six consecutive steps and 77.7% of loss, and quartz/feldspar that is dominated by feldspar with a 39 37 39 the Ar. Sample DIG9 does not yield a plateau age, but range of ages (higher ArCa/ ArK ratios) in the high shows a humped release spectrum. Both samples exhibit temperature steps of the release spectrum and affected by lower ages at both low and high temperature release steps fluid inclusions in the highest temperature release steps. 1782 S. C. Sherlock et al.

Table 1. Laser spot fusion argon isotope and age data. Age Sample Analysis type 40Ar/39Ar 38Ar/39Ar 37Ar/39Ar 36Ar/39Ar 39Ar 40Ar*/39Ar (Ma) +/− 5-2 Pocket 1.89 0.010 0.016 0.002 0.093 1.43 30.0 0.4 5 2 Pocket 2.31 0.011 0.030 0.002 0.062 1.63 34.1 1.5 5-2 Pocket 2.64 0.010 0.005 0.003 0.032 1.66 34.8 1.0 5-2 Pocket 2.54 0.011 0.006 0.003 0.035 1.73 36.4 2.7 5-1 Veinlet 3.20 0.011 0.030 0.005 0.067 1.74 36.6 0.2 5-2 Pocket 1.94 0.012 0.004 0.001 0.065 1.76 36.9 1.4 5-2 Pocket 1.79 0.009 0.003 0.000 0.144 1.79 37.6 0.7 5-2 Pocket 1.92 0.009 0.003 0.000 0.174 1.80 37.7 0.4 5-2 Pocket 2.10 0.010 0.009 0.001 0.061 1.81 37.9 0.4 5-1 Veinlet 3.79 0.013 0.251 0.006 0.069 1.89 39.6 1.7 5-2 Pocket 2.05 0.009 0.007 0.000 0.041 1.91 40.0 0.5 5-2 Pocket 2.30 0.008 0.004 0.001 0.086 1.92 40.3 0.8 5-2 Pocket 2.75 0.011 0.005 0.003 0.060 1.99 41.7 0.6 5-1 Veinlet 3.74 0.012 0.010 0.006 0.069 2.01 42.0 1.1 5-2 Pocket 2.45 0.010 0.003 0.001 0.073 2.08 43.7 0.3 5-1 Veinlet 3.55 0.008 0.023 0.004 0.020 2.35 49.1 1.0 5-1 Veinlet 5.78 0.012 0.485 0.011 0.043 2.40 50.2 2.0 5-1 Veinlet 5.49 0.010 0.133 0.010 0.009 2.41 50.4 7.1 5-2 Pocket 4.35 0.010 0.081 0.006 0.080 2.46 51.5 0.6 5-1 Veinlet 5.06 0.011 0.019 0.008 0.038 2.58 53.9 0.6 5-1 Veinlet 4.17 0.010 0.018 0.005 0.039 2.64 55.1 2.3 5-1 Veinlet 4.54 0.010 0.264 0.006 0.084 2.67 55.8 0.9 5-1 Veinlet 4.70 0.012 0.016 0.007 0.060 2.71 56.7 1.2 5-1 Veinlet 6.25 0.012 0.303 0.011 0.057 2.91 60.7 1.1 5-1 Veinlet 4.86 0.013 0.213 0.006 0.051 3.07 63.9 0.4 5-1 Veinlet 6.49 0.012 0.422 0.011 0.059 3.11 64.8 2.3 5-1 Veinlet 5.06 0.009 0.048 0.006 0.072 3.16 65.9 1.8 5-1 Veinlet 5.97 0.010 0.016 0.009 0.082 3.22 67.0 1.2 5-1 Veinlet 5.57 0.013 0.375 0.008 0.075 3.26 67.8 1.2 5-1 Veinlet 7.84 0.013 0.046 0.015 0.042 3.36 69.8 2.1 5-1 Veinlet 5.05 0.011 0.166 0.005 0.064 3.64 75.6 1.0 5-1 Veinlet 6.04 0.013 0.057 0.007 0.027 4.10 84.9 3.1 5-1 Veinlet 6.21 0.013 0.054 0.007 0.032 4.21 87.0 2.7 5-1 Veinlet 5.35 0.011 0.050 0.004 0.056 4.21 87.1 1.4 5-1 Veinlet 10.29 0.015 0.153 0.019 0.049 4.60 95.0 1.3 5-1 Veinlet 8.64 0.013 0.090 0.013 0.054 4.83 99.5 2.2 5-1 Veinlet 7.66 0.010 0.014 0.005 0.093 6.22 127.2 0.9 5-1 Veinlet 12.10 0.009 0.420 0.010 0.020 9.20 185.1 3.0 Analyses in bold are used to calculate the weighted mean age using Isoplot (Ludwig 2003).

In whole rock samples also containing melt glasses, there The shocked basement clasts from Haughton appear to will be a mix of these two reservoirs in varying proportions. In preserve a complex mix of shocked biotite, quartz and fact, the form of the release spectra from this study and from feldspar, and melt veins laden with refractory clasts that is Jessberger (1988) clearly demonstrates this to be the case (Fig. rich in a volatile component most likely to be water, CO2, and 6d). In all samples, low-temperature release is dominated by argon. Dating biotite mineral separates may be the most melt and shocked biotite and feldspar, at high temperature, fruitful avenue, provided that the suspected 39Ar recoil release from both feldspar and quartz dominates, yet all problem can be understood. Laser probe spot dating is an release spectra show signs of recoiled 39Ar re-distribution. alternative that may circumvent some of these problems, as it 37 39 This is corroborated by the ArCa/ ArK release pattern is able to position the laser within melt veins. Samples with for samples analyzed in this study (Fig. 6e), which closely melt veins appear to be rare in the Haughton structure. Of mirrors the release pattern for Stephan and Jessberger’s more than 30 individual basement clasts, only one contained quartz/feldspar separate. The release pattern for the sample pristine melt veins, while a second was less well-preserved, analyzed in Jessberger (1988) is also similar (Fig. 6f), with melt veins appearing green/brown and strongly altered, 37 39 although the ArCa/ ArK ratio increases at much lower presumably by the well-documented post-impact temperatures. hydrothermal system (Osinski et al. 2001, 2005c). The in situ Re-evaluating the age of the Haughton impact crater 1783

37 39 Fig. 4. ArCa/ ArK versus age for samples 5-1 and 5-2.

Fig. 5. Step-heating spectra for four individual basement clasts; the age information from these are arbitrary given the dominant volatile component within the basement material. What is important to note is the form of the spectra; each is dominated by relatively low “ages” in the low-temperature steps, rising to their maxima in the highest temperature steps. laser probe spot-dating experiments performed on melt veins melts (Sherlock and Hetzel 2001; M¸ller et al. 2001; Kelley in splits 5-1 and 5-2 from the melt-bearing basement clast and Gurov 2002). In this study, the laser probe spot data are provided the opportunity to extract age information at a entirely consistent with the analysis of a strongly higher resolution. Comparing the bulk sampling analytical heterogeneous shock melt, which is strongly zoned (Fig. 3b) method by furnace step heating with in situ laser probe and laden with refractory mineral grains (quartz and feldspar analysis for Ar/Ar dating has been performed on strongly in sample 5-1 and 5-2) (Figs. 3b and 3c). The wide age heterogeneous materials such as shock or friction-derived variation in the laser probe spot data reflect this, but in view 1784 S. C. Sherlock et al.

Fig. 6. Comparative step-heating spectra from the three studies: a) biotite separates (Stephan and Jessberger 1991); b) and c) the quartz/ feldspar separate (Stephan and Jessberger 1991); d) whole-rock basement clasts, with dark grey showing this study and light grey showing Jessberger (1988); e) whole-rock this study; and f) whole-rock Jessberger (1988). of the information provided by Stephan and Jessberger (1992) refractory feldspar clasts are present in the melt veins, they and the step-heating data in this study it is possible to separate are volumetrically small compared with the melt material, out the data points that are highly contaminated by refractory which is also apparent from Fig. 3. 37 39 clasts using argon isotope variations. The ArCa/ ArK variation in the laser probe spot data (Fig. 4a) and individual How Old Is the Haughton Impact Structure? steps from the step-heating data (Fig. 4b) strongly indicate that the known reservoirs dominate the spot ages. In both data The 23.4 ± 1.0 Ma Ar/Ar age for Haughton (Jessberger 37 39 sets, the age increases with ArCa/ ArK. That these values in 1988) was considered robust since it corroborated the earlier Fig. 4a are an order of magnitude lower suggests that where 22.4 ± 1.4 Ma fission track age of Omar et al (1987). Scrutiny Re-evaluating the age of the Haughton impact crater 1785 of these earlier fission track data and our attempts to reproduce them suggests that there are some anomalies in the data. In the study of Omar et al. (1987), just 21 grains were recovered from 40 kg of sample; however, in our attempt to reproduce the findings of the study, we were unable to detect any suitable grains. The data in Omar et al (1987) was insufficiently reported to allow for a detailed assessment. However, the Ns/Ns ratios are twice the ρs.ρi ratios in both runs, rather than being equal as we would normally expect, and it is unclear which ratio is most appropriate for the age calculation. Furthermore, the ρi values quoted in their Table 1 suggest that the apatites contain over 150 ppm uranium if the neutron fluences are correct, whereas 10 ppm is a more typical value. In summary, the data reported by Omar et al. (1987) contain a number of anomalies that raise doubts concerning the validity of the resulting ages. Due to the lack of suitable apatites in our samples, we have been unable to Fig. 7. Inverse isochron correlation diagram with an intercept age of reproduce these data. 41.7 ± 2.6 Ma calculated using Isoplot (Ludwig 2003). Comparing the step-heating data of Jessberger (1988), 37 39 Stephan and Jessberger (1991), and this study to the laser Those individual analyses with the lowest ArCa/ ArK probe spot dating of this study affords insight into the ratios, and as such are the most potassium-rich analyses, are complexity of the argon reservoirs in the shocked basement in the best estimate for the age of the Haughton impact structure. the Haughton impact structure. Notably, the biotite, quartz/ These are the data points that are the least contaminated by feldspar, and melt-vein components contribute differently to any refractory quartz/feldspar reservoir, although this is still a the overall whole rock. In the study of Stephan and Jessberger somewhat arbitrary selection of data points based on the best (1991), one of two biotites yielded a plateau age, while both available laser probe resolution. Plotting the fourteen data displayed 39Ar-redistribution by recoil. The quartz/feldspar points on an inverse isochron correlation diagram to establish separate yielded a distinctive release spectrum, dominated by the presence, or not, of extraneous 40Ar (Fig. 7) shows that the low ages but with a marked contribution from the feldspar, line of best fit has an 40Ar/36Ar intercept of 254 ± 38, which is and concomitant high ages at high temperatures. The whole- within error of the atmospheric value (295.5). This suggests rock experiment of Jessberger (1988) and those in this study either that there is no extraneous 40Ar, or, if present, it is were based on biotite-quartz-feldspar basement clasts; the beyond the limits of detection and unlikely to have affected resulting spectra from Stephan and Jessberger (1991) are what the final age beyond the limit of the 4% uncertainty. The would be expected for a mix of the quartz/feldspar and biotite intercept age is 41.7 ± 2.6 Ma (Fig. 7), but the line is slightly separate materials. biased by two points with low errors. Calculating the The infrared laser spot dating is able to circumvent the weighted mean of these data points using the square root of complexities of the whole-rock dating to some extent. It is MSWD times student “t” to increase the errors as used by possible to position the laser within melt-vein material, rather ISOPLOT (Ludwig 2003) yields an age of 39.1 ± 1.7 Ma. than in the host-rock minerals, and is able to avoid large Neither of these methods is entirely satisfactory, given that entrained refractory host-rock minerals. From the laser probe the resulting ages are actually a continuum of Eocene ages. data, it is still possible to see the influence of differing argon The youngest age of Jessberger (1988), which corresponds to reservoirs, which is a common problem in strongly the lowest-temperature step, is 41.9 ± 0.9 Ma and is also heterogeneous, rapidly cooled material, that was unable to Eocene. It is apparent from the complexity of the samples that equilibrate with regard to argon isotopes. The ages produced a precise age for the Haughton impact structure is beyond the by laser probe spot-dating are wide-ranging. The resolution of resolution of the current dating studies; it is clearly older than the infrared laser probe (∼75 µm) is such that it is impossible the original ∼23 Ma fission track and Ar/Ar ages and is likely to avoid all but the largest entrained target-rock clasts (e.g., to be ∼39 Ma, and is certainly Eocene. Sherlock and Hetzel 2001), and as such, laser probe dating of heterogeneous material almost universally results in widely Implications of an Eocene Age for the Haughton Impact scattered age and isotopic data. Conversely, a small number of Structure analyses appear to yield a younger age, which may be an indication of some glass devitrification. Notably, the laser An Eocene age for Haughton presents a number of probe spot ages fall within the entire range of individual paradoxes, and the validity of the 22.4 ± 1.4 Ma apatite heating steps reported in Jessberger (1988). fission-track age of Omar et al. (1987) must be questioned. 1786 S. C. Sherlock et al.

The new Ar/Ar laser probe data and a re-evaluation of the et al. 1989). However, an age of 39 ± 2 Ma is within error of data of Jessberger (1988) both indicate a mid-Eocene age for two other major North American craters: Wanapitei (37.2 ± the impact event, while the apatite fission-track age appears to 1.2 Ma, Ontario, Canada) (Winzler et al. 1976) and Mistastin indicate an early Miocene cooling event. It is possible that the (36.4 ± 4.0 Ma, Labrador, Canada) (Mak et al. 1976). In order fission tracks have been thermally reset, which can occur at to explore this further and to establish whether or not these temperatures around 100° or more. This could be explained, craters are a second Eocene cluster, it would be necessary to for example, if post-impact hydrothermal activity, albeit of co-irradiate and analyze suitable material from all three short duration at Haughton, was intense enough, at least craters. locally, to reset the fission track ages (Osinski et al. 2005c). Alternatively, where geothermal gradients are sufficient, CONCLUSIONS sedimentary burial can play an important role in resetting apatite fission tracks. However, there is no evidence to Laser probe Ar/Ar dating of impact melt material from suggest that Haughton was buried by more than a few tens of shocked basement clasts reveals a new Eocene age (∼39 Ma) meters of sedimentary crater-fill deposits following its for the Haughton impact event. This is at odds with the formation (Hickey et al. 1988), which would be insufficient to Miocene stratigraphic and apatite fission-track ages for both give rise to temperatures of even 70° C. In addition, apatite the impact event and lacustrine crater-fill of the Haughton fission track data from granite from outcrop on Devon Island, Formation. It is also at odds with the published age Ar/Ar of some 100 km to the east of the Haughton structure, suggest 23.4 ± 1.0 Ma, although the earlier data can also be that regional reheating has not exceeded 70 °C over the last interpreted as indicating an age of ∼40 Ma. Our new age data 150 Myr or more. Thus, the Miocene fission-track age must supports the conclusions of Robertson and Sweeney (1983) be questioned. and Osinski and Lee (2005) that the Haughton Formation was Given an Eocene age for the Haughton impact event, the not deposited immediately following impact but substantially imprecise stratigraphic age of 20 ± 5 Ma for the Haughton post-dates the formation of the crater. Formation now highlights a time gap of ∼15–20 Ma between the impact event and lacustrine deposition, raising the Acknowledgments–S. C. Sherlock gratefully acknowledges question of whether there was an immediate post-impact NERC Fellowship NER/I/S/2002/00692. S. P. Kelley crater lake at all. If such a crater lake existed, the sedimentary gratefully acknowledges Leverhulme grant F/00269/J. This record of its early history may have been eradicated. study was conducted in part under the auspices of the Alternatively, there was never an immediate post-impact lake, Haughton-Mars Project (HMP) under NASA cooperative only a lacustrine episode that post-dated the impact by some agreements NCC2-1185 and NCC2-1416 (P. Lee, P. I.) with 15–20 Ma. The immediacy of a post-impact crater lake is funding support from the U.S. National Aeronautics and Space inherent in the model for the hydrothermal system detailed by Administration (NASA) and the Canadian Space Agency Osinski et al. (2001), both for sustaining the system as a (CSA). The HMP is managed jointly by the SETI Institute and meteoric water contribution, but also in influencing the rate of the Mars Institute with logistical support provided in part by cooling from ∼650° to below 50° C within tens of thousands the Polar Continental Shelf Project of Natural Resources of years (Osinski et al. 2001). Importantly, Osinski and Lee Canada and the United States Marine Corps. Additional (2005) have confirmed the hypothesis of Robertson and support was provided by the Nunavut Research Institute and Sweeney (1983) that there was a significant temporal hiatus, the many sponsors of the HMP (www.marsonearth.org). during which substantial erosion of impact melt breccias Special thanks are owed to the Arctic communities of Grise occurred, between the formation of the crater and the Fiord and Resolute Bay. We also wish to acknowledge the deposition of the Haughton Formation. 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