Earth and Planetary Science Letters 406 (2014) 37–48

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Earth and Planetary Science Letters

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40 39 Ar/ Ar age of the Lake Saint Martin (Canada) – Unchaining the Late Triassic terrestrial impact craters ∗ Martin Schmieder a,b, , Fred Jourdan b, Eric Tohver a, Edward A. Cloutis c a School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia b Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia c Planetary Spectrophotometer Facility, Department of Geography, University of Winnipeg, Canada a r t i c l e i n f o a b s t r a c t

Article history: New 40Ar/39Ar dating of impact-melted K-feldspars and impact melt rock from the ∼40 km Lake Saint Received 4 December 2013 Martin impact structure in Manitoba, Canada, yielded three plateau ages and one mini-plateau age in Received in revised form 29 June 2014 agreement with inverse isochron ages for the K-feldspar melt aliquots and a minimum age for a whole- Accepted 30 August 2014 rock impact melt sample. A combination of two plateau ages and one isochron age, with a weighted Available online xxxx mean of 227.8 ± 0.9Ma[±1.1 Ma; including all sources of uncertainty] (2σ ; MSWD = 0.52; P = 0.59), Editor: C. Sotin is considered to represent the best-estimate age for the impact. The concordant 40Ar/39Ar ages for the Keywords: melted K-feldspars, derived from impact melt rocks in the eastern crater moat domain and the partially Lake Saint Martin melted Proterozoic central uplift granite, suggest that the new dates accurately reflect the Lake Saint 40Ar/39Ar dating Martin in the Carnian stage of the Late Triassic. With a relative error of ±0.4% on the impact melting 40Ar/39Ar age, the Lake Saint Martin impact structure counts among the most precisely dated impact K-feldspar structures on Earth. The new isotopic age for Lake Saint Martin significantly improves upon earlier Triassic Rb/Sr and (U–Th)/He results for this impact structure and contradicts the hypothesis that planet Earth multiple impact event experienced the formation of a giant ‘ chain’ during a major Late Triassic multiple impact event. © 2014 Elsevier B.V. All rights reserved.

1. Introduction and geologic background under the vernacular term ‘Saint Martin Series’ (McCabe and Ban- natyne, 1970; Simonds and McGee, 1979; Grieve, 2006). Shock 1.1. The Lake Saint Martin impact structure, Manitoba metamorphic effects in these rocks and minerals led McCabe and Bannatyne (1970) to conclude that Lake Saint Martin is an im- The ∼40 km-diameter Lake Saint Martin impact structure in the pact structure, while Currie (1970) still considered it a volcanic Interlake Region of southern Manitoba (McCabe and Bannatyne, structure. Largely concealed by post-impact Mid- 1970; Bannatyne and McCabe, 1984; Grieve, 2006; Fig. 1) is one dle Jurassic intra-crater red beds and evaporites of the Williston of the largest impact structures in North America. Target Basin, up to ∼30 m of Pleistocene glacial drift and the waters of rocks comprise Paleoproterozoic (∼2.4 Ga) granites of the Supe- Lake Saint Martin, the impact structure is considered one of the rior Province of the Canadian Shield overlain by ∼200–400 m of best preserved larger complex terrestrial impact craters formed Ordovician to Devonian sedimentary rocks that build up the north- in a layered, crystalline-sedimentary target. Post-impact evapor- eastern margin of the Paleozoic intracratonic Williston Basin (e.g., Grieve, 2006). The shocked and partially melted basement gran- itic sediments have preserved large parts of the impact crater ite is exposed in the central uplift of the impact structure. The and its impact lens from erosion (Bannatyne and McCabe, annular crater moat is filled with a sequence of in- 1984). However, due to the sedimentary cover surficial outcrop cluding carbonate-dominated and granitic lithic impact , of primary impactites is rather scarce. Numerous drillings since impact melt rocks, and suevitic breccias occasionally summarized the 1970s revealed the subsurface geology and specific rocks of the Lake Saint Martin impact structure. The composition of the Lake Saint Martin impactor is unknown, but very low concentra- * Corresponding author at: School of Earth and Environment, University of West- tions of siderophile elements in the melt rocks (Göbel et al., 1980; ern Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Tel.: +61 434 808 655. Palme, 1982; Reimold et al., 1990)suggested a differentiated pro- E-mail address: martin@.com (M. Schmieder). jectile, possibly an achondrite. http://dx.doi.org/10.1016/j.epsl.2014.08.037 0012-821X/© 2014 Elsevier B.V. All rights reserved. 38 M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48

North Dakota/USA, the ∼40 km Rochechouart impact structure in France, the ∼20 km Obolon impact structure in Ukraine, as well as possibly the ∼12 km Wells Creek impact structure in Ten- nessee/USA and the ∼3.2 km Newporte impact structure in North Dakota/USA, the Lake Saint Martin impact structure has been con- sidered the western member of a giant, ∼4500 km-long, ‘impact ◦ crater chain’ along the 22.8 paleolatitude on the Late Triassic Earth (Spray et al., 1998). All of these impact structures, plotted on a paleoglobe at ∼214 Ma, lie on reconstructed great circles ◦ of identical declination (37.2 ). Taking into account the overlap- ping isotopic and stratigraphic age data available in 1998 and tectonic plate reconstructions for the Late Triassic, Monte Carlo simulations suggested that a random array for the three largest of these impact structures – Manicouagan, Rochechouart, and Lake Saint Martin – was highly unlikely (Spray et al., 1998). The idea was that the proposed multiple impact scenario could have oc- curred similar to the ‘string-of-pearls’-style impact of the tidally fragmented comet Shoemaker–Levy 9 on Jupiter in the summer of 1994 (e.g., Crawford et al., 1994), within a short period of time. The Late Triassic multiple impact theory was soon vividly discussed Fig. 1. Landsat-5 satellite image of Lake Saint Martin in Manitoba, Canada (scene in the scientific literature (Kent, 1998; Melosh, 1998; Walkden path 032, row 024, acquired on 04 April 2010) and outline of the estimated outer et al., 2002; Tanner et al., 2004; Carporzen and Gilder, 2006; limits of the largely sediment- and water-covered ∼40 km-diameter impact struc- Grieve, 2006; Zivkovic and Gosnold, 2007; Schmieder and Buch- ture according to Bannatyne and McCabe (1984). Impact melt rock from the eastern ner, 2008; Schmieder et al., 2010, 2012; Jourdan et al., 2012) and crater moat and a partially melted Proterozoic granite from the central uplift (in- set images; both rock specimens ∼10 cm in maximum length) were sampled for scientific reviews (VanDecar, 1998; Monastersky, 1998; Sankaran, 40 39 40Ar/39Ar dating. Satellite image source: USGS. 1999). This study presents a set of new Ar/ Ar ages for aliquots of impact-melted K-feldspars and impact melt rocks from different 1.2. Previous age determinations for the Lake Saint Martin impact domains within the Lake Saint Martin impact structure. The new isotopic ages will demonstrate that the Lake Saint Martin impact Stratigraphically, the Lake Saint Martin impact can be placed occurred millions of years before the Manicouagan and Roche- between the Devonian and the Middle Jurassic. The youngest chouart impacts and that none of the impacts grouped together shock-deformed sedimentary rocks most likely belong to the De- into an alleged crater chain occurred synchronously. vonian Ashern and Winnipegosis Formations of the northeastern Williston Basin (e.g., Rosenthal, 1988); the evaporitic deposits of 2. Samples and analytical methods the Amaranth Formation that overlie the impact structure are of Middle Jurassic age (McCabe and Bannatyne, 1970). Previous dat- 2.1. Impact melt specimens and petrography ing of the Lake Saint Martin impact structure employed a wide range of techniques but has not yielded any satisfactory age so Whole-rock chips of impact melt rock from the crater moat far. Early results yielded rather imprecise K/Ar apparent ages of domain of the Lake Saint Martin impact structure, as well as 200 ± 25 and 250 ± 25 Ma (McCabe and Bannatyne, 1970) and chips of impact-melted K-feldspars separated from the melt rock a Rb/Sr apparent age of 223 ± 32 Ma (Reimold et al., 1990; re- and the impact-metamorphosed Precambrian basement granite ex- calculated from 219 ± 32 Ma according to the revised 87Rb decay posed in the central uplift (Fig. 1) were used for laser step-heating 40 39 constant of Nebel et al., 2011). Despite the large error on the age Ar/ Ar dating. This approach was chosen to gain isotopic ages (28%) and the notorious problems associated with this technique from different types of impact melt lithologies representative of in dating impact craters (Jourdan et al., 2009), the Rb/Sr age of different portions of the impact structure. We direct the reader Reimold et al. (1990) is currently accepted as the ‘official’ age to Simonds and McGee (1979), Reimold et al. (1990) and Grieve for the impact in most impact crater listings in the literature; a (2006) for summaries of the composition, petrography, and geo- slightly modified age value of 220 ± 32 Ma (as also given in Grieve, chemistry of the impact melt rocks and breccias at Lake Saint 2001) is listed in the (2014). Apatite and zir- Martin. The brownish to reddish, fine-grained crystalline impact con fission track dating gave an age of 208 ± 14 Ma (Kohn et al., melt rocks (Fig. 2A–B; typically containing ∼3.2–3.9 wt% K2O; 1995). Additional dating attempts using the low-temperature (U– Grieve, 2006) are moderately to intensely altered and have a ma- Th)/He technique on apatite and zircon separated from melt rocks trix of plagioclase, alkali feldspar, pyroxene (augite), quartz, and resulted in scattered ages with two apparent age peaks around Fe–Ti oxides. Groundmass crystals are typically ∼20–100 μm in ∼214 Ma and ∼234 Ma (Wartho et al., 2009; 2010). A strontium crystal length. Larger grains of plagioclase (oligoclase and ande- and sulfur isotopic study of the post-impact evaporites favored an sine) commonly exhibit a checkerboard texture, whereas shocked age around the Devonian/Carboniferous boundary (∼360 Ma) for quartz has been partially transformed into aggregates of ballen the impact (Leybourne et al., 2007), at odds with the previous iso- quartz with marginal melting textures and reaction rims (com- topic dating results. Clearly, despite several dating attempts using pare Grieve, 2006). The impact-metamorphosed basement granite different techniques, the age of the Lake Saint Martin impact still (Fig. 2C–D) contains fluidal-vesicular domains of feldspar melt of remains poorly constrained. variable composition with vugs locally lined by adularia. Quartz in the shocked granite mainly occurs in the form of ballen sil- 1.3. Lake Saint Martin as a member of a giant terrestrial impact crater ica, and biotite flakes have been decomposed into microcrystalline chain? opaques. Representative specimens were analyzed using an optical polarization microscope and a VEGA3 TESCAN scanning electron Together with the ∼100 km Manicouagan impact structure in microscope (SEM) equipped with an X-Max 50 silicon drift detector Québec/Canada, the ∼9 km Red Wing Creek impact structure in and an energy-dispersive X-ray spectroscopy system at the Centre M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48 39

Fig. 2. Optical microscopic and backscattered electron images (BEI) of the fine-grained, crystalline melt rock from the Lake Saint Martin impact structure and the impact metamorphosed granite exposed in the central uplift. A: General appearance of the melt rock with a larger grain of ballen quartz (plane polarized light). B: The melt rock is predominantly composed of plagioclase (Plg; a larger phenocryst of andesine is shown), K-feldspar (Kfs), Quartz (Qtz), pyroxene (Px), and Fe–Ti oxides. C: Optical image of ballen silica surrounded by fluidal-textured feldspar (cross polarized light). D: BEI close-up of the granite, locally with idiomorphic K-feldspar (adularia; Adu). The images are representative of the rock samples from which the dating aliquots were separated. for Microscopy, Characterisation and Analysis (CMCA) at the Uni- which had been irradiated and analyzed in an earlier batch of versity of Western Australia campus. Backscattered electron images aliquots. The melt rock samples were leached in 5N HF for 2 min- were acquired using an acceleration voltage of 15 kV. utes to remove alteration phases on the surface and along fissures (e.g., Baksi, 2007). 40 39 2.2. Ar/ Ar dating The samples were loaded in multi-pit aluminum discs along with the Fish Canyon sanidine (FCs, age: 28.294 ± 0.037 Ma; 40Ar/39Ar laser step-heating experiments on impact melt rock Jourdan and Renne, 2007; Renne et al., 2011) used as a neu- chips and separates of melted K-feldspar from the crushed rock tron fluence monitor, Cd-shielded in order to reduce isotopic in- (grain size ∼500–800 μm in diameter) were carried out at the terferences, and irradiated for 40 hours in the TRIGA (Denver) Western Australian Argon Isotope Facility, Curtin University. We nuclear reactor (central position, specific J-values given in Table- performed analyses on two single-grain aliquots of impact-melted K-feldspar extracted from a fluidal-vesicular impact melt rock re- Annex 1). After irradiation, the samples were step-heated using covered from the eastern sector of the impact crater moat (samples a 110 W Spectron Laser Systems instrument, with a continuous LSM-1A and LSM-1B); three particles of melted K-feldspar from Nd-YAG (IR; 1064 nm) laser rastered (1 minute; beam diameter the shocked and partially melted basement granite collected in the 350 μm) over the single-grain aliquots, moved on an x/y/z-mobile central uplift domain (samples LSM-2A, LSM-2B and LSM-2C); one sample stage. The gas was purified in a stainless steel extrac- whole-rock chip of a moderately altered, brownish impact melt tion line using one GP50 and two AP10 SAES getters. Argon iso- rock (sample LSM-3); and a whole-rock chip (∼200 μm in diam- topes were measured in static mode using a MAP 215-50 mass eter) of an intensely altered, reddish melt rock (sample LSM-4), spectrometer with a Balzers SEV 217 electron multiplier using 40 M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48

Fig. 3. 40Ar/39Ar age spectra and corresponding inverse isochron plots for two aliquots of impact-melted K-feldspars separated from the Lake Saint Martin impact melt rock. A: Age spectrum (plateau) for aliquot LSM-1A. B: Inverse isochron for LSM-1A. C: Age spectrum (mini-plateau) for aliquot LSM-1B. Note that this mini-plateau age is only used in support of the statistically more robust plateau ages. D: Inverse isochron for LSM-1B. Errors are given at the 2σ level. nine to ten cycles of peak hopping. The argon results were cor- 3. 40Ar/39Ar results and interpretation rected for mass discrimination, monitored using an automatic air pipette, and provided mean values of 1.004293 ±0.0025 per dalton 3.1. Melt rock-hosted K-feldspar (atomic mass unit). The correction factors for interfering isotopes 39 37 −4 36 37 −4 40 39 were ( Ar/ Ar)Ca = 7.06 ·10 (±7%), ( Ar/ Ar)Ca = 2.81 ·10 Ar/ Ar step-heating analysis of the impact-melted K-feldspar 40 39 −4 ± (±3%) determined on CaF2, as well as ( Ar/ Ar)K = 6.76 · 10 LSM-1A yielded a well-defined plateau age of 226.7 2.4Ma(2σ ; 38 39 −2 = = 39 (±10%) and ( Ar/ Ar)K = 1.24 · 10 (±32%) measured on K– MSWD 0.78; P 0.66; 84% of Ar released; Fig. 3A). This Fe glass. Blanks were monitored every 3 to 4 steps and typical aliquot did not develop a statistically representative isochron due − − 40Ar blanks range from 1 · 10 16 to 2 · 10 16 mol. All argon iso- to a low spreading (S; Jourdan et al., 2009) value. Except for a sin- topic data are given in the supplementary files (Table-Annex 1). gle step, all data points in the inverse isochron diagram cluster 40Ar/39Ar ages were calculated using the K decay constants of near the radiogenic axis (compare Schmieder and Jourdan, 2013) and resulted in an age of 227.2 ± 1.5Ma(2σ ; MSWD = 0.79; Renne et al. (2010; 2011) and the LabView-environment Argus pro- P = 0.64; 40Ar/36Ar intercept = 288 ± 13; S-value = 31%; Fig. 3B). gram by M.O. McWilliams, the ArArCALC software (Koppers, 2002), The second K-feldspar melt aliquot, LSM-1B, yielded a mini-plateau and Isoplot 4.1 written by K.R. Ludwig (Berkeley Geochronology apparent age of 227 ± 5Ma(2σ ; MSWD = 1.70; P = 0.15; 63% Center). All errors in this study are ±2σ , unless otherwise noted. of 39Ar released; Fig. 3C) for the first five steps within a tilde- A plateau is defined by 70–100%, a mini-plateau by 50–70% of the shaped, disturbed age spectrum. The mini-plateau and total fusion 39 total Ar released in at least 5 consecutive steps with ages that apparent ages of this sample are concordant within error with the overlap within 2σ error limits, and define a probability (P -value) plateau age obtained for LSM-1A which might suggest that the 2 of ≥0.05 using a χ (chi square) test (e.g., Baksi, 2007). Errors on perturbed spectrum is an effect of 39Ar recoil redistribution in- the age of the monitor and on the decay constants are combined duced by the irradiation process (Table 1). The inverse isochron for with the internal uncertainties and are provided in brackets (e.g., LSM-1B yielded an age of 228 ± 6Ma(MSWD = 1.73; P = 0.16; [1.0] Ma). 40Ar/36Ar intercept = 289 ± 20; S = 70%; Fig. 3D). The isochron M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48 41

40 36

Ar/ Ar intercept values for LSM-1A and LSM-1B are atmospheric (∼298.6; Lee et al., 2006; Mark et al., 2011) within error. calculation

Spreading factor (%) 3.2. K-feldspar from central uplift granite the

for

The aliquots of impact-melted K-feldspars separated from the used

±

P central uplift granite yielded a plateau age of 227.0 1.2Ma (2σ ; MSWD = 1.00; P = 0.46; 84% of 39Ar) for sample LSM-2A were (Fig. 4A). The inverse isochron for this aliquot, including degassing bold

step 1 (with a step apparent age concordant with the plateau in age), yielded a slightly sub-atmospheric 40Ar/36Ar intercept value MSWD of 288 ± 8 and an isochron age of 227.9 ± 1.4Ma(MSWD = noted 0.79; P = 0.71; S = 61%; Fig. 4B). The sub-atmospheric ratio might

Ages

be due to thermally activated isotopic fractionation upon melt 13 0.79 0.64 31 20 1.73 0.16 70 8 0.79 0.71 61 Ar ) 36 ± ± ± emplacement, or fluid circulation after the impact. Whatever the σ 2 Ar/

± cause for this effect, the isochron-derived age for sample LSM-2A, 40 intercept ( 288

structure. 40 36 ∼ directly accounting for the initial Ar/ Ar ratio below 298.6 (Lee et al., 2006), is here regarded as the more accurate age value * impact

for sample LSM-2A (compare, e.g., Jourdan et al., 2008). Sample 19 5 289 n LSM-2B gave a plateau age of 228.1 ± 1.4Ma(2σ ; MSWD = 1.40; = 39 Martin P 0.18; 73% of Ar; Fig. 4C). Data points for this aliquot cluster

near the radiogenic axis and fail to provide an isochron (Fig. 4D). Saint 5 12 288 )

. age A third feldspar particle picked from the granite, LSM-02C, yielded 1.4

1 σ 2 6 ± ± a disturbed and tilde-shaped age spectrum from which no plateau

Lake ±

2 ± . or mini-plateau age could be extracted (Fig. 4E). Individual steps Isochron (Ma, – no isochron – – no isochron – –noisochron– 227.9 Isochron characteristics rock, in the high-temperature extraction range gave older apparent ages ∗ of up to ∼260 Ma, suggesting partial inheritance of 40Ar from an melt incompletely degassed Precambrian precursor feldspar in this sam- P 0.46 ple (e.g., Jourdan et al., 2007). No isochron result was obtained for impact aliquot LSM-02C (Fig. 4F). and

3.3. Impact melt rock section.

particles 1.70 0.15 228

0.59) The moderately altered, brownish impact melt rock chip LSM-3 = melt

plateau did not yield a plateau in a disturbed, tilde-shaped age spec-

trum, with younger step ages (∼180–200 Ma) in the low- and age

0.52; P high-temperature extraction range and older apparent ages of feldspar = the

∼220–230 Ma in the mid-temperature steps (Fig. 5A). This age Mini- plateau Attribute MSWD spectrum is similar to some of the previous age spectra obtained with)

;MSWD on impact melt rocks from Lake Saint Martin (Schmieder et al., σ (2 18 Plateau 1.00 n 2012; an unpublished age spectrum for a drill core-derived melt recrystallized rock sample, acquired with W.H. Schwarz, M. Trieloff and E. Buch- for

concordant ner at the Ar laboratory at the University of Heidelberg, Germany)

age

and other spectra for terrestrial impact structures in which hump-

Ar statistics 39

and tilde-shapes have been attributed to alteration effects, such as

the Dhala impact structure in India (Pati et al., 2010)or the Söder- 3 5 apparent hump-shaped spectrum (minimum age) Total staircase-shaped spectrum (minimum age) 84 12 Plateau 0.78 0.66 227 released (%) 73 11 Plateau 1.38 0.18 – no isochron – 84 fjärden impact structure in Finland (Schmieder et al., 2014). When internal

step

alteration is indeed responsible for hump- and tilde-shaped spec- a

with

tra, the maxima of such age spectra must be strictly considered with ) age a minimum age for the formation of the melt rock (e.g., Buchner

2.4 1.4 (0.9) [1.1] Ma 1.2 σ along

2 56 ± ± ± ± et al., 2009; Jourdan, 2012). We note that the apparent minimum ± (but

± impact.

of age for melt rock sample LSM-3 is notably close to the statisti- ages, 220–230 209

Age spectrum characteristics Apparent (Ma, 226.7 227.0 ≥ ≥ 227.8 228.1 the

cally well-defined plateau and isochron ages for the melt rock- part

for and granite-derived K-feldspar melt aliquots. The K/Ca ratio for

not isochron

∼ ∼

age LSM-3 gave values between 1 and 3 suggesting no major crys- 1

and tallization of secondary K O-rich minerals. No isochron could be

2

step produced for this altered impact melt rock (Fig. 5B). granite,

rock, domain

altered

In contrast to the moderately altered melt rock specimen altered

melted

rock) rock) uplift melt

moat LSM-3, the intensely altered melt rock sample LSM-4 yielded plateau

age:

degassing

weighted-mean

mini-plateau 2

a distinct staircase-shaped age spectrum. Step apparent ages in of

Impact crater (single grain) Partially Precambrian central (single grain) Moderately (whole Intensely (whole the low-temperature extraction range of this spectrum lie around ∼

isochron 140 Ma and gradually increase until they reach a step-age max- includes plateau,

mean

1

∼ of ID imum at 209 Ma before the age values drop again (Fig. 5C). We

and

recommended note that the minimum age derived from the oldest apparent step

Isochron ages of this age spectrum is ∼15–20 Myr younger than the min- the LSM-2A LSM-2B LSM-1B 227 Sample K-feldspar melt aliquots LSM-1A Weighted ages LSM-4 LSM-2C – no plateau – Impact melt rock aliquots LSM-3

*

Table 1 Summary of imum age for the less altered melt rock aliquot LSM-3. The K/Ca 42 M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48

Fig. 4. 40Ar/39Ar age spectra and corresponding inverse isochron plots for three impact-melted K-feldspar specimens separated from the partially melted basement granite exposed in the central uplift of the Lake Saint Martin impact structure. A: Age spectrum (plateau) for aliquot LSM-2A. B: Inverse isochron for LSM-2A with inset for magnification of data point array. C: Age spectrum (plateau) for aliquot LSM-2B. D: Inverse isochron plot for LSM-2B with inset for zoom on data point cluster. E: Age spectrum (disturbed) for aliquot LSM-2C. F: Inverse isochron plot for LSM-2C. Errors are given at the 2σ level. M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48 43

Fig. 5. 40Ar/39Ar age spectra and corresponding K/Ca and inverse isochron plots for two whole-rock chips of the Lake Saint Martin impact melt rock. A: Age spectrum (disturbed) and K/Ca ratio for moderately altered aliquot LSM-3. B: Inverse isochron plot for LSM-3. C: Age spectrum (disturbed) and K/Ca ratio for intensely altered aliquot LSM-4. B: Inverse isochron plot for LSM-4. Errors are given at the 2σ level. ratio varies between ∼7 and ∼18 suggesting the presence of abun- 2012 about the suitability of the Rb/Sr method for the dating of dant secondary K2O-rich minerals relative to sample LSM-3. Such impact events). The more recent Rb/Sr apparent age of 219 [223] an age spectrum likely reflects a mixture between Ar loss and the ±32 Ma obtained for impact melt rocks by Reimold et al. (1990) recrystallization of alteration minerals. As for LSM-3, no isochron has been considered the most precise and accurate age for the was obtained for melt rock sample LSM-4 (Fig. 5D). Lake Saint Martin impact for more than two decades; however, the A summary of the 40Ar/39Ar ages and associated statistics for rather low precision of this result, along with the relevance of this all melt samples from the Lake Saint Martin impact structure is impact structure for a potential giant crater chain, warranted fur- presented in Table 1. ther investigations. Here we present a set of concordant plateau, mini-plateau, and inverse isochron ages obtained for the impact- 4. Discussion melted K-feldspars separated from impact melt rock and the par- tially melted central uplift granite. A combination of two plateau 4.1. Age of the Lake Saint Martin impact ages (aliquots LSM-1A and LSM-2B) and one inverse isochron age (LSM-2A; taking into account the sub-atmospheric 40Ar/36Ar inter- This study presents the first systematic set of 40Ar/39Ar ages for cept of 288 ± 8) yields a statistically compelling weighted mean the well-preserved Lake Saint Martin impact structure, obtained age of 227.8 ± 0.9Ma[±1.1 Ma] (2σ ; MSWD = 0.52; P = 0.59; from different mineral-melt and whole-rock impact melt speci- the number in brackets includes the full external error). This early mens. It thereby improves upon earlier dating attempts that re- Late Triassic mean age is considered as the most precise and accu- sulted in imprecise (and statistically unverifiable) whole-rock K/Ar rate age currently available for the Lake Saint Martin impact. As and Rb/Sr ages (cf. discussion in Jourdan et al., 2009 and Jourdan, mini-plateau ages are potentially inaccurate (e.g., Jourdan et al., 44 M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48

Fig. 6. Summary of measured 40Ar/39Ar plateau, mini-plateau, inverse isochron and minimum (alteration) apparent ages for the Lake Saint Martin impact melt aliquots along with statistical age combinations for plateau and isochron ages. Color code as in Figs. 3–5. All ages are in million years. The weighted mean of two plateau ages and one inverse isochron age is here considered the best-estimate age for the Lake Saint Martin impact. Error boxes are 2σ . (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

2009), we did not include the 40Ar/39Ar result for the impact- the Lake Saint Martin impact are within the range of the ‘min- melted K-feldspar LSM-1B in the final age calculation; however, imum age’ of ∼220–230 Ma for the impact melt rock sample this mini-plateau age is within error also equivalent with the LSM-3 and results from Schmieder et al. (2012) who reported a plateau and isochron ages obtained on the other K-feldspar melt similar minimum age of ∼225 Ma for an additional impact melt aliquots. The internal consistency of the 40Ar/39Ar dating results rock specimen. In this case, the minimum age for the moderately and the independent reproduction of coeval apparent ages from altered impact melt rock sample LSM-3 closely approaches the different types of impact melt lithologies gained from two different statistically more robust plateau and isochron ages. In contrast, sample localities across the impact structure provides good confi- the intensely altered impact melt rock LSM-4 yielded apparent dence that these matching ages accurately reflect the Lake Saint step ages no older than ∼209 Ma, which are still notably younger Martin impact. With a relative error on the age of only ±0.4% (all than the age of ∼228 Ma determined for the impact in this study. uncertainties included), Lake Saint Martin now represents one of Such alteration age spectra are generally difficult to interpret un- the most precisely dated impact structures on Earth (compare im- less more robust isotopic ages (e.g., a set of concordant 40Ar/39Ar pact crater ages in the reviews by Jourdan et al., 2009, 2012 and plateau ages from fresh or less altered impact melt specimens; or references therein). A summary of all measured 40Ar/39Ar plateau, statistically compelling U/Pb ages from melt-grown zircons) and mini-plateau and inverse isochron ages, along with the recom- stratigraphic age constraints are available. Minimum impact ages mended age for the impact, is presented in Fig. 6. Further dating inferred from step-age maxima in 40Ar/39Ar age spectra can be of impact melt samples from natural outcrop and drill cores into considerably – in cases up to several hundred Myr – younger than the impact structure will be desirable to create an even more ex- the real formation age of an impact melt rock. This effect is ex- tended isotopic data set for the Lake Saint Martin impact in the emplified by the ≥1.7 Ga Dhala impact structure in India that future (compare, e.g., Buchner et al., 2010). produced minimum apparent 40Ar/39Ar ages of ∼1.0 Ga, whereas According to the latest International Chronostratigraphic Chart field geologic relationships indicate a much older, Paleoproterozoic, published by the International Commission on Stratigraphy (Cohen minimum age for the impact (Pati et al., 2010). et al., 2013), the new 40Ar/39Ar age of 227.8 ±[1.1] Ma for the Late The concordant plateau, mini-plateau and inverse isochron Triassic Lake Saint Martin impact corresponds within error to the 40Ar/39Ar ages obtained in this study, moreover, critically question ∼227 Ma Carnian/Norian boundary. However, Lucas et al. (2012) the indiscriminate use of age data obtained from low-temperature pointed out that the base of the Norian is probably several million geochronometers, such as the (U–Th)/He dating method (e.g., years younger and lies around ∼220 Ma based on an extensive set Farley, 2002; Reiners, 2005) that records thermal events at clo- ◦ ◦ of radioisotopic data. This younger age would place the Lake Saint sure temperatures of ∼200 C for zircon and as low as ∼60 C for Martin impact well within the Carnian (lasting from ∼237 Ma to apatite. None of the previous (U–Th)/He age peaks at ∼180 Ma, ∼220 Ma). The strong reverse magnetization in the Lake Saint ∼214 Ma and ∼234 Ma for zircon and apatite grains recovered Martin melt rocks (Coles and Clark, 1982)is in agreement with from impact melt rocks from Lake Saint Martin (Wartho et al., a number of reversed magnetochrons (within a range of chrons 2009, 2010) have reliably identified the age of the impact within E1r to E13r) during the Carnian time stage (Kent et al., 1995; error. According to Kohn et al. (1995), rocks in the Williston Basin Lucas et al., 2012). have been overprinted by an Eocene thermal event that caused the partial resetting of low-temperature geochronometers, such 4.2. Implications for rock alteration effects and low-temperature as apatite fission track and (U–Th)/He. The cooling of larger im- geochronometers pact craters and long-lived hydrothermal activity may have an additional effect on isotopic systems with low closure tempera- The systematic study of a set of different impact melt samples tures. Recent 40Ar/39Ar results for the ∼23 km Lappajärvi impact from Lake Saint Martin helps with the interpretation and under- structure in predominantly crystalline rocks of Finland suggested standing of ‘alteration age’ spectra and minimum ages derived comparatively slow crater cooling over ∼0.6–1.6 Myr (Schmieder from 40Ar/39Ar step-heating analysis, in particular bell-shaped and Jourdan, 2013). However, because of the higher amounts of and/or tilde-shaped age spectra. The plateau and isochron ages for subsurface water, porosity, and permeability in sedimentary tar- M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48 45

Fig. 7. Compilation of selected terrestrial meteorite impacts during the Triassic and the postulated Late Triassic multiple impact theory, modified after Spray et al. (1998). ∗ Lucas et al. (2012) suggested an age of ∼220 Ma for the Carnian/Norian boundary, which has an age of ∼227 Ma in the current International Stratigraphic Chart (Cohen et al., 2013). Impact age data from Koeberl et al. (1996), Ramezani et al. (2005), Schmieder and Buchner (2008), Schmieder et al. (2010) and this study. gets and water-promoted convective cooling (e.g., Kirsimäe et al., al., 2010). The age of the ∼100 km Manicouagan impact struc- 2002), the Lake Saint Martin crater may have cooled considerably ture in Québec has been precisely determined at 215.56 ± 0.05 Ma faster. This ‘crater cooling lag’ would not explain, for example, (Ramezani et al., 2005) and 214 ± 1Ma(Hodych and Dun- the apparent zircon and apatite (U–Th)/He age clusters around ning, 1992)using U/Pb dating of impact melt-grown zircons. The ∼214 Ma and ∼180 Ma, respectively. So far, the (U–Th)/He tech- ∼40 km Rochechouart impact structure in France was previously nique has been applied to twelve impact structures (e.g., Wartho dated at an 40Ar/39Ar age of 214 ± 8Ma(Kelley and Spray, 1997), et al., 2012a), from which only one, the ∼7km Wetumpka im- obtained from UV laser ablation spots in the matrix of melt- pact structure in Alabama/USA (Wartho et al., 2012b), yielded a bearing pseudotachylitic breccia veins. However, these melt veins subset of apparent ages that are consistent within uncertainty are known to contain considerable amounts of undigested Pre- with biostratigraphic ages. Our understanding of the applicability cambrian to Paleozoic feldspar clasts and hydrothermal fillings of the (U–Th)/He technique to impact crater dating is currently (Reimold et al., 1987); moreover, spot fusion apparent ages within at an early stage. Inaccurate results prompt us to advise cau- a range between ∼230 Ma and ∼1.7 Ga and an isochron 40Ar/36Ar tion in using this technique to date impact events without any intercept value of >500 (Kelley and Spray, 1997) indicated con- robust age constraints. Extensive testing and calibration of the tamination by extraneous argon. The age for the Rochechouart method against well-dated (impact melt-hosted) mineral speci- impact was recently revised to [202.8 ± 2.2Ma] (Schmieder et mens is required until we fully understand what constitutes a al., 2010; recalculated) obtained from two 40Ar/39Ar plateau ages ‘valid’ (U–Th)/He impact crater age, especially with respect to the for K-feldspar from an impact-metamorphosed gneiss. The other potential effects of on the retention and candidate craters for the Triassic ‘crater chain’ have only been diffusion of He in zircon (e.g., van Soest et al., 2010). Some of imprecisely dated. The ∼20 km marine Obolon impact structure the previous results were compromised and of doubtful geologi- in Ukraine (Gurov et al., 2009) had earlier been estimated at a cal significance, either reflecting impact-induced crustal uplift and stratigraphic age of 215 ± 25 Ma (Masaitis et al., 1980; Spray et slow crater cooling (Biren et al., 2012), post-impact thermal heat- al., 1998), but K/Ar dating of the Obolon impact glass and melt ing and resetting not related to the impact (Wartho et al., 2010; rock gave a Jurassic apparent age of 169 ± 7Ma(Masaitis, 1999; van Soest et al., 2011), or the proposed impact age was based on Valter et al., 1999; Gurov et al., 2009). Asimple paleogeographic statistically invalid dating results (e.g., Young et al., 2013). test further supports a Jurassic maximum age for the Obolon im- pact structure; only from the Early Jurassic onward did the pale- 4.3. A geochronologic appraisal of the Late Triassic multiple impact oenvironmental conditions in the Donets Graben of the Ukrainian hypothesis Shield change from continental sedimentation into a marine en- vironment and provided the setting for the submarine Obolon To reliably assess the Late Triassic multiple impact theory impact (Schmieder and Buchner, 2008). The age of the ∼9km proposed by Spray et al. (1998), according to which several im- Red Wing Creek impact structure in the Williston Basin of North pact events occurred virtually synchronously, narrow error ranges Dakota is stratigraphically constrained to roughly ∼200–220 Ma associated with the individual impact ages are crucial (see im- (Koeberl et al., 1996). Other loose stratigraphic age estimates pact timing scheme in Fig. 7). At least two of the impacts in- for Red Wing Creek are 200 ± 25 Ma (Gerhard et al., 1982) and volved in the postulated scenario, Rochechouart in France and 200 ± 5Ma(Grieve, 1991), however, no fresh impact melt litholo- Obolon in Ukraine, were previously dismissed based on their non- gies are known from this impact site that could be used for more synchronous ages (Schmieder and Buchner, 2008; Schmieder et precise and accurate isotopic dating. Within the extended list of 46 M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48 potential craters in the suspected multiple impact system (Spray 2010), for example due to the incomplete homogenization of the et al., 1998), the ∼3km Newporte impact structure in North impactor-derived elements and the local impact melt, fractiona- Dakota has a Cambrian stratigraphic age of ∼500 Ma (Koeberl and tion of the meteoritic signature elements (e.g., Palme et al., 1979; Reimold, 1995), which is in conflict with the Late Triassic multi- Palme, 1982), differentiation of larger impact melt bodies (e.g., ple impact scenario. The age of the ∼12 km Wells Creek impact O’Connell-Cooper and Spray, 2011), as well as post-impact alter- structure in Tennessee is poorly known and given as 200 ± 100 Ma ation. For the given uncertainties on the exact composition of (Grieve, 2001). the that generated the Lake Saint Martin, Manicoua- From a geochronologic perspective, the refined 40Ar/39Ar age gan, Rochechouart, Obolon and Red Wing Creek impact structures of ∼228 Ma for Lake Saint Martin demonstrates that the Obolon, we stress that the geochemical constraints cannot serve as reliable Rochechouart, Manicouagan and Lake Saint Martin impact events evidence for or against a multiple impact scenario. were far from synchronous. At least ∼10 Myr, and up to >40 Myr, lie between each of the impact events (Fig. 7). This also rules out 4.5. Statistical considerations any potential connection between the postulated large-scale multi- impact scenario and extinction events during the Triassic (e.g., Finally, the statistical probability for a multiple impact event Benton, 1988; Hallam and Wignall, 1997; Sephton et al., 2002; on Earth in the Late Triassic should be briefly evaluated based on Tanner et al., 2004). the timing and paleopositions of the impact events in plate re- constructions for North America (Laurentia) and Eurasia (see Spray 4.4. Geochemical constraints on impacting bodies et al., 1998; Walkden et al., 2002). Using the estimated paleolati- tudes and -longitudes of the impact craters as input parameters in In addition to the time constraints gained from isotopic dat- Monte Carlo simulations, Spray et al. (1998) calculated a low prob- ing, the compositions of the respective impactors can be used as ability of P < 0.08 (<0.008 if the longitudinal range is included) an additional argument for or against a cosmic combination strike. for the event that the linear arrangement of the three ‘main’ E–W- We refer to Goderis et al. (2012), Koeberl et al. (2012) and Koeberl aligned craters, Rochechouart, Manicouagan, and Lake Saint Martin, (2014) for recent reviews on the identification of meteorite sig- is a random array. The probability for an aligned random strike natures in impactites. The proposed Late Triassic multiple impacts decreases even further for five impact structures (i.e., including should, thus, also be appraised with regards to their potential im- Obolon and Red Wing on great circles; however, no statistical pactor signatures. No conclusive impactor traces, with siderophile probability was given for such an event). As the crater alignment elements below detection limit, could be gained from the impact was very unlikely a result of random chance and because the im- melt rocks at Manicouagan (Palme et al., 1978) and Lake Saint precise age constraints for the craters agreed well within this cal- Martin (Göbel et al., 1980; Palme, 1982; Reimold et al., 1990), culation, Spray et al. (1998) concluded that the five craters may which might suggest a differentiated asteroid as the parent body represent a chain of impact structures probably formed ‘within for achondritic projectiles. In contrast, the Rochechouart impact hours’. structure in France (Koeberl et al., 2007; Tagle et al., 2009) and the The general expectation that linear arrays of apparently coeval Obolon impact structure in Ukraine (Valter and Ryabenko, 1977; impacts constitute a combination strike, as supported by statistical Masaitis, 1999)carry siderophile element patterns that favor the modeling, may be in analogy to some of the binary impact sys- impact of iron meteorites. The meteorite type for the Red Wing tems on Earth, including the proposed doublet craters of the West Creek impact structure is uncertain, but Koeberl et al. (1996) found and East Clearwater Lake impact structures in Canada and the Su- a possible chondritic contamination pattern in impact breccias re- vasvesi North and South impact structures in Finland. Despite very covered from drill cores. low calculated likelihoods for their random origin in close proxim- Despite this apparent difference in impacting bodies for the ity to the respective fellow crater (Miljkovic´ et al., 2013), differing Manicouagan, Lake Saint Martin, Rochechouart, Obolon, and Red 40Ar/39Ar dating results for impact melt rocks from each of these Wing Creek impact structures, one could argue that double and impact structures rather suggest that the impacts occurred millions multiple meteorite impacts could in fact involve a mixture of ma- of years apart (see Schmieder et al., 2014 for a discussion and ref- terials. One such example, yet on a much smaller scale, is the erences therein for earlier dating results). fall of the Almahata Sitta meteorite in northern Sudan on Octo- Based on the size-frequency distribution of Earth-crossing aster- ber 7, 2008, which comprised a variety of chondritic and achon- oids and their encounter frequency with the Earth–Moon system, dritic (ureilitic) lithologies (Bischoff et al., 2010). We acknowledge Bottke et al. (1997) estimated that the production rate for impact that asteroids commonly represent ‘rubble piles’ (Richardson et al., crater chains on Earth after S-type (Shoemaker–Levy 9) tidal dis- 1998, 2002), such as asteroid 25 143 Itokawa (e.g. Fujiwara et al., ruption is less than 0.1 crater chains in 3.8 Gyr. While we acknowl- 2006), and can hence be composed of different types of materi- als. Moreover, a larger asteroid can capture a smaller asteroid in edge that these statistics are derived from numerical modeling, space and thereby generate a binary asteroid system (such as, e.g., the results by Bottke et al. (1997) suggest a very low chance of asteroid 243 Ida and its satellite Dactyl; Chapman et al., 1995) producing a terrestrial impact crater chain in younger geologic his- of two bodies of different composition, fragments of which may tory. We recommend that proposed terrestrial impact crater chains eventually impact Earth. It seems somewhat unlikely that the in- and doublets should be systematically accompanied by isotope dat- coming fragments of a tidally disrupted larger rubble pile asteroid ing. Even more delicate is the case of extraterrestrial impact crater or comet, such as Shoemaker–Levy 9 (e.g., Shoemaker et al., 1993; chains and doublets, for which no absolute ages can be obtained Crawford et al., 1994), would be heterogeneous to the extent that without sample return missions. Although crater counting (e.g., they generate a chain of impact craters in which each one leaves Neukum et al., 2001) helps to some extent, this study shows that a different geochemical fingerprint; however, unless this type of fortuitously aligned, yet unrelated impact craters may also occur scenario (i.e., impact of variable-type cosmic rubble) is proven im- on asteroids and other planetary bodies in the Solar System. possible in terms of celestial mechanics, one should consider the theoretical possibility that a geochemically heterogeneous impact 5. Conclusions system can occur over geologic time. On the other hand, im- pact melt lithologies at terrestrial impact structures can lack a The ∼40 km Lake Saint Martin impact structure in Manitoba, clear and representative meteoritic signature (e.g., Goderis et al., Canada, has been 40Ar/39Ar-dated at a Carnian age of 227.8 ± M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48 47

0.9Ma[±1.1 Ma] (2σ ; MSWD = 0.52; P = 0.59) by combin- Cohen, K.M., Finney, S.M., Gibbard, P.L., Fan, J.-X., 2013. The ICS international ing two plateau ages and one isochron age obtained on differ- chronostratigraphic chart. Episodes 363, 199–204. ent aliquots of impact-melted K-feldspar. With a relative error of Coles, R.L., Clark, J.F., 1982. Lake St. Martin impact structure, Manitoba, Canada: ± magnetic anomalies and magnetizations. J. Geophys. Res., Solid Earth 87 (B8), 0.4% on the isotopic age, the Lake Saint Martin impact struc- 7087–7095. ture has advanced to one of the most precisely dated larger impact Crawford, D.A., Boslough, M.B., Trucano, T.G., Robinon, A.C., 1994. The impact of structures on Earth. The new dating results demonstrate that none comet Shoemaker–Levy 9 on Jupiter. Shock Waves 4, 47–50. of the impacts that constituted the postulated Late Triassic mul- Currie, K.L., 1970. New Canadian cryptoexplosion crater at Lake St Martin, Manitoba. 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