*Manuscript
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Accepted for publication in
Earth and Planetary Science Letters
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Ar/39Ar age of the Lake Saint Martin impact structure (Canada) –
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Unchaining the Late Triassic terrestrial impact craters
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Martin Schmieder1,2*, Fred Jourdan2, Eric Tohver1 and Edward A. Cloutis3
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School of Earth and Environment, University of Western Australia, 35 Stirling Highway,
Crawley, WA 6009, Australia.
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Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University,
GPO Box U1987, Perth, WA 6845, Australia.
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Planetary Spectrophotometer Facility, Department of Geography, University of Winnipeg, Canada.
*corresponding author. Tel.: 0061 434 808 655 – Email: martin@suevite.com.
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Abstract – New 40Ar/39Ar dating of impact-melted K-feldspars and impact melt rock
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from the ~40 km Lake Saint Martin impact structure in Manitoba, Canada, yielded
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three plateau ages and one mini-plateau age in agreement with inverse isochron ages
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for the K-feldspar melt aliquots and a minimum age for a whole-rock impact melt
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sample. A combination of two plateau ages and one isochron age, with a weighted
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mean of 227.8 ± 0.9 Ma [± 1.1 Ma; including all sources of uncertainty] (2σ; MSWD
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= 0.52; P = 0.59), is considered to represent the best-estimate age for the impact. The
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concordant
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rocks in the eastern crater moat domain and the partially melted Proterozoic central
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uplift granite, suggest that the new dates accurately reflect the Lake Saint Martin
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impact event in the Carnian stage of the Late Triassic. With a relative error of ± 0.4%
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on the 40Ar/39Ar age, the Lake Saint Martin impact structure counts among the most
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precisely dated impact structures on Earth. The new isotopic age for Lake Saint
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Martin significantly improves upon earlier Rb/Sr and (U–Th)/He results for this
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impact structure and contradicts the hypothesis that planet Earth experienced the
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formation of a giant ‗impact crater chain‘ during a major Late Triassic multiple
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impact event.
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Ar/39Ar ages for the melted K-feldspars, derived from impact melt
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Keywords: Lake Saint Martin; 40Ar/39Ar dating; impact melting; K-feldspar; Triassic; Carnian;
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multiple impact event. 5,896 words, 1 Table, 7 Figs., 1 supplementary Table.
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1.
INTRODUCTION AND GEOLOGIC BACKGROUND
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1.1.
The Lake Saint Martin impact structure, Manitoba
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The ~40 km-diameter Lake Saint Martin impact structure in the Interlake Region of southern
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Manitoba (McCabe and Bannatyne, 1970; Bannatyne and McCabe, 1984; Grieve, 2006; Fig. 1)
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is one of the largest meteorite impact structures in North America. Target rocks comprise
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Paleoproterozoic (~2.4 Ga) granites of the Superior Province of the Canadian Shield overlain by
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~200-400 m of Ordovician to Devonian sedimentary rocks that build up the northeastern margin
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of the Paleozoic intracratonic Williston Basin (e.g., Grieve, 2006). The shocked and partially
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melted basement granite is exposed in the central uplift of the impact structure. The annular
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crater moat is filled with a sequence of impactites including carbonate-dominated and granitic
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lithic impact breccias, impact melt rocks, and suevitic breccias occasionally summarized under
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the vernacular term ‗Saint Martin Series‘ (McCabe and Bannatyne, 1970; Simonds and McGee,
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1979; Grieve, 2006). Shock metamorphic effects in these rocks and minerals led McCabe and
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Bannatyne (1970) to conclude that Lake Saint Martin is an impact structure, while Currie (1970)
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still considered it a volcanic cryptoexplosion structure. Largely concealed by post-impact Middle
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Jurassic intra-crater red beds and evaporites of the Williston Basin, up to ~30 m of Pleistocene
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glacial drift and the waters of Lake Saint Martin, the impact structure is considered one of the
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best preserved larger complex terrestrial impact craters formed in a layered, crystalline-
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sedimentary target. Post-impact evaporitic sediments have preserved large parts of the impact
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crater and its impact breccia lens from erosion (McCabe and Bannatyne, 1984). However, due to
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the sedimentary cover surficial outcrop of primary impactites is rather scarce. Numerous
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drillings since the 1970s revealed the subsurface geology and specific rocks of the Lake Saint
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Martin impact structure. The composition of the Lake Saint Martin impactor is unknown, but
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very low concentrations of siderophile elements in the melt rocks (Göbel et al., 1980; Palme,
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1982; Reimold et al., 1990) suggested a differentiated projectile, possibly an achondrite.
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1.2.
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Stratigraphically, the Lake Saint Martin impact can be placed between the Devonian and the
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Middle Jurassic. The youngest shock-deformed sedimentary rocks most likely belong to the
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Devonian Ashern and Winnipegosis Formations of the northeastern Williston Basin (e.g.,
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Rosenthal, 1988); the evaporitic deposits of the Amaranth Formation that overlie the impact
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structure are of Middle Jurassic age (McCabe and Bannatyne, 1984). Previous dating of the Lake
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Saint Martin impact structure employed a wide range of techniques but has not yielded any
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satisfactory age so far. Early results yielded rather imprecise K/Ar apparent ages of 200 ± 25 and
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250 ± 25 Ma (McCabe and Bannatyne, 1970) and a Rb/Sr apparent age of 223 ± 32 Ma (Reimold
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et al., 1990; recalculated from 219 ± 32 Ma according to the revised
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Nebel et al., 2011). Despite the large error on the age (28%) and the notorious problems
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associated with this technique in dating impact craters (Jourdan et al., 2009), the Rb/Sr age of
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Reimold et al. (1990) is currently accepted as the ‗official‘ age for the impact in most impact
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crater listings in the literature; a slightly modified age value of 220 ± 32 Ma (as also given in
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Grieve, 2001) is listed in the Earth Impact Database (2013). Apatite and zircon fission track
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dating gave an age of 208 ± 14 Ma (Kohn et al., 1995). Additional dating attempts using the low-
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temperature (U–Th)/He technique on apatite and zircon separated from melt rocks resulted in
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scattered ages with two apparent age peaks around ~214 Ma and ~234 Ma (Wartho et al., 2009;
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2010). A strontium and sulfur isotopic study of the post-impact evaporites favored an age around
Previous age determinations for the Lake Saint Martin impact
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Rb decay constant of
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the Devonian/Carboniferous boundary (~360 Ma) for the impact (Leybourne et al., 2007), at
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odds with the previous isotopic dating results. Clearly, despite several dating attempts using
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different techniques, the age of the Lake Saint Martin impact still remains poorly constrained.
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1.3.
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Together with the ~100 km Manicouagan impact structure in Québec/Canada, the ~9 km Red
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Wing Creek impact structure in North Dakota/USA, the ~40 km Rochechouart impact structure
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in France, the ~20 km Obolon impact structure in Ukraine, as well as possibly the ~12 km Wells
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Creek impact structure in Tennessee/USA and the ~3.2 km Newporte impact structure in North
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Dakota/USA, the Lake Saint Martin impact structure has been considered the western member of
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a giant, ~4,500 km-long, ‗impact crater chain‘ along the 22.8° paleolatitude on the Late Triassic
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Earth (Spray et al., 1998). All of these impact structures, plotted on a paleoglobe at ~214 Ma, lie
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on reconstructed great circles of identical declination (37.2°). Taking into account the
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overlapping isotopic and stratigraphic age data available in 1998 and tectonic plate
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reconstructions for the Late Triassic, Monte Carlo simulations suggested that a random array for
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the three largest of these impact structures – Manicouagan, Rochechouart, and Lake Saint Martin
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– was highly unlikely (Spray et al., 1998). The idea was that the proposed multiple impact
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scenario could have occurred similar to the ‗string-of-pearls‘-style impact of the tidally
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fragmented comet Shoemaker-Levy 9 on Jupiter in the summer of 1994 (e.g., Crawford et al.,
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1994), within a short period of time. The Late Triassic multiple impact theory was soon vividly
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discussed in the scientific literature (Kent, 1998; Melosh, 1998; Walkden et al., 2002; Tanner et
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al., 2004; Carporzen and Gilder, 2006; Grieve, 2006; Zivkovic and Gosnold, 2007; Schmieder
Lake Saint Martin as a member of a giant terrestrial impact crater chain?
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and Buchner, 2008; Schmieder et al., 2010; 2012; Jourdan et al., 2012) and scientific reviews
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(VanDecar, 1998; Monastersky, 1998; Sankaran, 1999). This study presents a set of new
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Ar/39Ar ages for aliquots of impact-melted K-feldspars and impact melt rocks from different
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domains within the Lake Saint Martin impact structure. The new isotopic ages will demonstrate
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that the Lake Saint Martin impact occurred millions of years before the Manicouagan and
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Rochechouart impacts and that none of the impacts grouped together into an alleged crater chain
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occurred synchronously.
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2.
SAMPLES AND ANALYTICAL METHODS
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2.1.
Impact melt specimens and petrography
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Whole-rock chips of impact melt rock from the crater moat domain of the Lake Saint Martin
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impact structure, as well as chips of impact-melted K-feldspars separated from the melt rock and
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the impact-metamorphosed Precambrian basement granite exposed in the central uplift (Fig. 1)
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were used for laser step-heating 40Ar/39Ar dating. This approach was chosen to gain isotopic ages
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from different types of impact melt lithologies representative of different portions of the impact
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structure. We direct the reader to Simonds and McGee (1979), Reimold et al. (1990) and Grieve
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(2006) for summaries of the composition, petrography, and geochemistry of the impact melt
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rocks and breccias at Lake Saint Martin. The brownish to reddish, fine-grained crystalline impact
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melt rocks (Fig. 2A-B; typically containing ~3.2-3.9 wt% K2O; Grieve, 2006) are moderately to
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intensely altered and have a matrix of plagioclase, alkali feldspar, pyroxene (augite), quartz, and
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Fe-Ti oxides. Groundmass crystals are typically ~20-100 µm in crystal length. Larger grains of
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plagioclase (oligoclase and andesine) commonly exhibit a checkerboard texture, whereas
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shocked quartz has been partially transformed into aggregates of ballen quartz with marginal
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melting textures and reaction rims (compare Grieve, 2006; Jourdan et al., 2010). The impact-
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metamorphosed basement granite (Fig. 2C-D) contains fluidal-vesicular domains of feldspar
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melt of variable composition with vugs locally lined by adularia. Quartz in the shocked granite
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mainly occurs in the form of ballen silica, and biotite flakes have been decomposed into
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microcrystalline opaques. Representative specimens were analyzed using an optical polarization
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microscope and a VEGA3 TESCAN scanning electron microscope (SEM) equipped with an X-
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Max 50 silicon drift detector and an energy-dispersive X-ray spectroscopy system at the Centre
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for Microscopy, Characterisation and Analysis (CMCA) at the University of Western Australia
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campus. Backscattered electron images were acquired using an acceleration voltage of 15 kV.
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Ar/39Ar dating
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2.2.
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feldspar from the crushed rock (grain size ~500-800 µm in diameter) were carried out at the
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Western Australian Argon Isotope Facility, Curtin University. We performed analyses on two
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single-grain aliquots of impact-melted K-feldspar extracted from a fluidal-vesicular impact melt
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rock recovered from the eastern sector of the impact crater moat (samples LSM-1A and LSM-
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1B); three particles of melted K-feldspar from the shocked and partially melted basement granite
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collected in the central uplift domain (samples LSM-2A, LSM-2B and LSM-2C); one whole-
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rock chip of a moderately altered, brownish impact melt rock (sample LSM-3); and a whole-rock
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chip (~200 µm in diameter) of an intensely altered, reddish melt rock (sample LSM-4; see
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Jourdan et al., 2010), which had been irradiated and analyzed in an earlier batch of aliquots. The
Ar/39Ar laser step-heating experiments on impact melt rock chips and separates of melted K-
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melt rock samples were leached in 5N HF for 2 minutes to remove alteration phases on the
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surface and along fissures (e.g., Baksi, 2007).
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The samples were loaded in multi-pit aluminum discs along with the Fish Canyon sanidine (FCs,
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age: 28.294 ± 0.037 Ma; Jourdan and Renne, 2007; Renne et al., 2011) used as a neutron fluence
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monitor, Cd-shielded in order to reduce isotopic interferences, and irradiated for 40 hours in the
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TRIGA (Denver) nuclear reactor (central position, specific J-values given in Table-Annex 1).
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After irradiation, the samples were step-heated using a 110 W Spectron Laser Systems
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instrument, with a continuous Nd-YAG (IR; 1064 nm) laser rastered (1 minute; beam diameter
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350 µm) over the single-grain aliquots, moved on a x/y/z-mobile sample stage. The gas was
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purified in a stainless steel extraction line using one GP50 and two AP10 SAES getters. Argon
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isotopes were measured in static mode using a MAP 215-50 mass spectrometer with a Balzers
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SEV 217 electron multiplier using nine to ten cycles of peak hopping. The argon results were
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corrected for mass discrimination, monitored using an automatic air pipette, and provided mean
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values of 1.004293 ± 0.0025 per dalton (atomic mass unit). The correction factors for interfering
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isotopes were (39Ar/37Ar)Ca = 7.06 ∙ 10-4 (± 7%), (36Ar/37Ar)Ca = 2.81 ∙ 10-4 (± 3%) determined on
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CaF2, as well as (40Ar/39Ar)K = 6.76 ∙ 10-4 (± 10%) and (38Ar/39Ar)K = 1.24 ∙ 10-2 (± 32%)
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measured on K-Fe glass. Blanks were monitored every 3 to 4 steps and typical 40Ar blanks range
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from 1 ∙ 10-16 to 2 ∙ 10-16 mol. All argon isotopic data are given in the supplementary files (Table-
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Annex 1).
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2011) and the LabView-environment Argus program by M.O. McWilliams, the ArArCALC
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software (Koppers, 2002), and Isoplot 4.1 written by K. R. Ludwig (Berkeley Geochronology
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Center). All errors in this study are ± 2σ, unless otherwise noted. A plateau is defined by 70-
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100%, a mini-plateau by 50-70% of the total
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Ar/39Ar ages were calculated using the K decay constants of Renne et al., (2010;
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Ar released in at least 5 consecutive steps with
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ages that overlap within 2σ error limits, and define a probability (P-value) of ≥0.05 using a χ²
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(chi square) test (e.g., Baksi, 2007). Errors on the age of the monitor and on the decay constants
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are combined with the internal uncertainties and are provided in brackets (e.g., [1.0] Ma).
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3.
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3.1.
Melt rock-hosted K-feldspar
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40
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plateau age of 226.7 ± 2.4 Ma (2σ; MSWD = 0.78; P = 0.66; 84% of
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This aliquot did not develop a statistically representative isochron due to a low spreading (S;
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Jourdan et al., 2009) value. Except for a single step, all data points in the inverse isochron
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diagram cluster near the radiogenic axis (compare Schmieder and Jourdan, 2013) and resulted in
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an age of 227.2 ± 1.5 Ma (2σ; MSWD = 0.79; P = 0.64; 40Ar/36Ar intercept = 288 ± 13; S-value
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= 31%; Fig. 3B). The second K-feldspar melt aliquot, LSM-1B, yielded a mini-plateau apparent
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age of 227 ± 5 Ma (2σ; MSWD = 1.70; P = 0.15; 63% of 39Ar released; Fig. 3C) for the first five
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steps within a tilde-shaped, disturbed age spectrum. The mini-plateau and total fusion apparent
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ages of this sample are concordant within error with the plateau age obtained for LSM-1A which
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might suggest that the perturbed spectrum is an effect of 39Ar recoil redistribution induced by the
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irradiation process (Table 1). The inverse isochron for LSM-1B yielded an age of 228 ± 6 Ma
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(MSWD = 1.73; P = 0.16;
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Mark et al., 2011) within error.
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Ar/39Ar RESULTS AND INTERPRETATION
Ar/39Ar step-heating analysis of the impact-melted K-feldspar LSM-1A yielded a well-defined
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39
Ar released; Fig. 3A).
Ar/36Ar intercept = 289 ± 20; S = 70%; Fig. 3D). The isochron
Ar/36Ar intercept values for LSM-1A and LSM-1B are atmospheric (~298.6; Lee et al., 2006;
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3.2.
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The aliquots of impact-melted K-feldspars separated from the central uplift granite yielded a
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plateau age of 227.0 ± 1.2 Ma (2σ; MSWD = 1.00; P = 0.46; 84% of 39Ar) for sample LSM-2A
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(Fig. 4A). The inverse isochron for this aliquot, including degassing step 1 (with a step apparent
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age concordant with the plateau age), yielded a slightly sub-atmospheric
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value of 288 ± 8 and an isochron age of 227.9 ± 1.4 Ma (MSWD = 0.79; P = 0.71; S = 61%;
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Fig. 4B). The sub-atmospheric ratio might be due to thermally activated isotopic fractionation
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upon melt emplacement, or fluid circulation after the impact. Whatever the cause for this effect,
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the isochron-derived age for sample LSM-2A, directly accounting for the initial
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below ~298.6 (Lee et al., 2006), is here regarded as the more accurate age value for sample
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LSM-2A (compare, e.g., Jourdan et al., 2008). Sample LSM-2B gave a plateau age of 228.1 ±
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1.4 Ma (2σ; MSWD = 1.40; P = 0.18; 73% of 39Ar; Fig. 4C). Data points for this aliquot cluster
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near the radiogenic axis and fail to provide an isochron (Fig. 4D). A third feldspar particle picked
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from the granite, LSM-02C, yielded a disturbed and tilde-shaped age spectrum from which no
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plateau or mini-plateau age could be extracted (Fig. 4E). Individual steps in the high-temperature
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extraction range gave older apparent ages of up to ~260 Ma, suggesting partial inheritance of
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Jourdan et al., 2007). No isochron result was obtained for aliquot LSM-02C (Fig. 4F).
K-feldspar from central uplift granite
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Ar/36Ar intercept
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Ar/36Ar ratio
Ar* from an incompletely degassed Precambrian precursor feldspar in this sample (e.g.,
205
206
3.3.
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The moderately altered, brownish impact melt rock chip LSM-3 did not yield a plateau in a
208
disturbed, tilde-shaped age spectrum, with younger step ages (~180-200 Ma) in the low- and
Impact melt rock
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high-temperature extraction range and older apparent ages of ~220-230 Ma in the mid-
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temperature steps (Fig. 5A). This age spectrum is similar to some of the previous age spectra
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obtained on impact melt rocks from Lake Saint Martin (Schmieder et al., 2012; an unpublished
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age spectrum for a drill core-derived melt rock sample, acquired with W.H. Schwarz, M. Trieloff
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and E. Buchner at the Ar laboratory at the University of Heidelberg, Germany) and other spectra
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for terrestrial impact structures in which hump- and tilde-shapes have been attributed to
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alteration effects, such as the Dhala impact structure in India (Pati et al., 2010) or the
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Söderfjärden impact structure in Finland (Schmieder et al., 2014a). When alteration is indeed
217
responsible for hump- and tilde-shaped spectra, the maxima of such age spectra must be strictly
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considered a minimum age for the formation of the melt rock (e.g., Buchner et al., 2009;
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Jourdan, 2012). We note that the apparent minimum age for melt rock sample LSM-3 is notably
220
close to the statistically well-defined plateau and isochron ages for the melt rock- and granite-
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derived K-feldspar melt aliquots. The K/Ca ratio for LSM-3 gave values between ~1 and ~3
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suggesting no major crystallization of secondary K2O-rich minerals. No isochron could be
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produced for this altered impact melt rock (Fig. 5B).
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In contrast to the moderately altered melt rock specimen LSM-3, the intensely altered melt rock
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sample LSM-4 yielded a distinct staircase-shaped apparent age spectrum. Step ages in the low-
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temperature extraction range of this spectrum lie around ~140 Ma and gradually increase until
227
they reach a step-age maximum at ~209 Ma before the age values drop again (Fig. 5C). We note
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that the minimum age derived from the oldest apparent step ages of this age spectrum is ~15-
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20 Myr younger than the minimum age for the less altered melt rock aliquot LSM-3. The K/Ca
230
ratio varies between ~7 and ~18 suggesting the presence of abundant secondary K2O-rich
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minerals relative to sample LSM-3. Such an age spectrum likely reflects a mixture between Ar
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loss and the recrystallization of alteration minerals. As for LSM-3, no isochron was obtained for
233
melt rock sample LSM-4 (Fig. 5D).
234
A summary of the
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Saint Martin impact structure is presented in Table 1.
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Ar/39Ar ages and associated statistics for all melt samples from the Lake
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4.
DISCUSSION
238
4.1.
Age of the Lake Saint Martin impact
239
This study presents the first systematic set of
240
Martin impact structure, obtained from different mineral-melt and whole-rock impact melt
241
specimens. It thereby improves upon earlier dating attempts that resulted in imprecise (and
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statistically unverifiable) whole-rock K/Ar and Rb/Sr ages (cf. discussion in Jourdan et al., 2009
243
and Jourdan, 2012 about the suitability of the Rb/Sr method for the dating of impact events). The
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more recent Rb/Sr apparent age of 219 [223] ± 32 Ma obtained for impact melt rocks by
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Reimold et al. (1990) has been considered the most precise and accurate age for the Lake Saint
246
Martin impact for more than two decades; however, the rather low precision of this result, along
247
with the relevance of this impact structure for a potential giant crater chain, warranted further
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investigations. Here we present a set of concordant plateau, mini-plateau, and inverse isochron
249
ages obtained for the impact-melted K-feldspars separated from impact melt rock and the
250
partially melted central uplift granite. A combination of two plateau ages (aliquots LSM-1A and
251
LSM-2B) and one inverse isochron age (LSM-2A; taking into account the sub-atmospheric
252
40
253
227.8 ± 0.9 Ma [± 1.1 Ma] (2σ; MSWD = 0.52; P = 0.59; the number in brackets includes the
40
Ar/39Ar ages for the well-preserved Lake Saint
Ar/36Ar intercept of 288 ± 8) yields a statistically compelling weighted mean age of
254
full external error). This early Late Triassic mean age is considered as the most precise and
255
accurate age currently available for the Lake Saint Martin impact. As mini-plateau ages are
256
potentially inaccurate (e.g., Jourdan et al., 2009), we did not include the 40Ar/39Ar result for the
257
impact-melted K-feldspar LSM-1B in the final age calculation; however, this mini-plateau age is
258
within error also equivalent with the plateau and isochron ages obtained on the other K-feldspar
259
melt aliquots. The internal consistency of the
260
reproduction of coeval apparent ages from different types of impact melt lithologies gained from
261
two different sample localities across the impact structure provides good confidence that these
262
matching ages accurately reflect the Lake Saint Martin impact. With a relative error on the age of
263
only ± 0.4% (all uncertainties included), Lake Saint Martin now represents one of the most
264
precisely dated impact structures on Earth (compare impact crater ages in the reviews by Jourdan
265
et al., 2009; 2012 and references therein). A summary of all measured
266
plateau and inverse isochron ages, along with the recommended age for the impact, is presented
267
in Fig. 6. Further dating of impact melt samples from natural outcrop and drill cores into the
268
impact structure will be desirable to create an even more extended isotopic data set for the Lake
269
Saint Martin impact in the future (compare, e.g., Buchner et al., 2010).
270
According to the latest International Chronostratigraphic Chart published by the International
271
Commission on Stratigraphy (Cohen et al., 2013), the new 40Ar/39Ar age of 227.8 ± [1.1] Ma for
272
the Late Triassic Lake Saint Martin impact corresponds within error to the ~227 Ma
273
Carnian/Norian boundary. However, Lucas et al. (2012) pointed out that the base of the Norian is
274
probably several million years younger and lies around ~220 Ma based on an extensive set of
275
radioisotopic data. This younger age would place the Lake Saint Martin impact well within the
276
Carnian (lasting from ~237 Ma to ~220 Ma). The strong reverse magnetization in the Lake Saint
40
Ar/39Ar dating results and the independent
40
Ar/39Ar plateau, mini-
277
Martin melt rocks (Coles and Clark, 1982) is in agreement with a number of reversed
278
magnetochrons (within a range of chrons E1r to E13r) during the Carnian time stage (Kent et al.,
279
1995; Lucas et al., 2012).
280
281
4.2.
282
The systematic study of a set of different impact melt samples from Lake Saint Martin helps with
283
the interpretation and understanding of ‗alteration age‘ spectra and minimum ages derived from
284
40
285
plateau and isochron ages for the Lake Saint Martin impact are within the range of the ‗minimum
286
age‘ of ~220-230 Ma for the impact melt rock sample LSM-3 and results from Schmieder et al.
287
(2012) who reported a similar minimum age of ~225 Ma for an additional impact melt rock
288
specimen. In this case, the minimum age for the moderately altered impact melt rock sample
289
LSM-3 closely approaches the statistically more robust plateau and isochron ages. In contrast,
290
the intensely altered impact melt rock LSM-4 yielded apparent step ages no older than ~209 Ma,
291
which are still notably younger than the age of ~228 Ma determined for the impact in this study.
292
Such alteration age spectra are generally difficult to interpret unless more robust isotopic ages
293
(e.g., a set of concordant 40Ar/39Ar plateau ages from fresh or less altered impact melt specimens;
294
or statistically compelling U/Pb ages from melt-grown zircons) and stratigraphic age constraints
295
are available. Minimum impact ages inferred from step-age maxima in 40Ar/39Ar age spectra can
296
be considerably – in cases up to several hundred Myr – younger than the real formation age of an
297
impact melt rock. This effect is exemplified by the ≥1.7 Ga Dhala impact structure in India that
Implications for rock alteration effects and low-temperature geochronometers
Ar/39Ar step-heating analysis, in particular bell-shaped and/or tilde-shaped age spectra. The
40
Ar/39Ar ages of ~1.0 Ga, whereas field geologic relationships
298
produced minimum apparent
299
indicate a much older, Paleoproterozoic, minimum age for the impact (Pati et al., 2010).
300
The concordant plateau, mini-plateau and inverse isochron 40Ar/39Ar ages obtained in this study,
301
moreover, critically question the indiscriminate use of age data obtained from low-temperature
302
geochronometers, such as the (U–Th)/He dating method (e.g., Farley, 2002; Reiners, 2005) that
303
records thermal events at closure temperatures of ~200°C for zircon and as low as ~60°C for
304
apatite. None of the previous (U–Th)/He age peaks at ~180 Ma, ~214 Ma and ~234 Ma for
305
zircon and apatite grains recovered from impact melt rocks from Lake Saint Martin (Wartho et
306
al., 2009; 2010) have reliably identified the age of the impact within error. According to Kohn et
307
al. (1995), rocks in the Williston Basin have been overprinted by an Eocene thermal event that
308
caused the partial resetting of low-temperature geochronometers, such as apatite fission track and
309
(U–Th)/He. The cooling of larger impact craters and long-lived hydrothermal activity may have
310
an additional effect on isotopic systems with low closure temperatures. Recent 40Ar/39Ar results
311
for the ~23 km Lappajärvi impact structure in predominantly crystalline rocks of Finland
312
suggested comparatively slow crater cooling over ~0.6-1.6 Myr (Schmieder and Jourdan, 2013).
313
However, because of the higher amounts of subsurface water, porosity, and permeability in
314
sedimentary targets and water-promoted convective cooling (e.g., Kirsimäe et al., 2002), the
315
Lake Saint Martin crater may have cooled considerably faster. This ‗crater cooling lag‘ would
316
not explain, for example, the apparent zircon and apatite (U–Th)/He age clusters around ~214
317
Ma and ~180 Ma, respectively. So far, the (U–Th)/He technique has been applied to twelve
318
impact structures (e.g., Wartho et al., 2012a), from which only one, the ~7 km Wetumpka impact
319
structure in Alabama/USA (Wartho et al., 2012b), yielded a subset of apparent ages that are
320
consistent within uncertainty with biostratigraphic ages. Our understanding of the applicability of
321
the (U–Th)/He to impact crater dating is currently at an early stage. Inaccurate results prompt us
322
to advise caution in using this technique to date impact events without any robust age constraints.
323
Extensive testing and calibration of the method against well-dated (impact melt-hosted) mineral
324
specimens is required until we fully understand what constitutes a ‗valid‘ (U–Th)/He impact
325
crater age, especially with respect to the potential effects of shock metamorphism on the
326
retention and diffusion of He in zircon (e.g., van Soest et al., 2010). Some of the previous results
327
were compromised and of doubtful geological significance, either reflecting impact-induced
328
crustal uplift and slow crater cooling (Biren et al., 2012), post-impact thermal heating and
329
resetting not related to the impact (Wartho et al., 2010; van Soest et al., 2011), or the proposed
330
impact age was based on statistically invalid dating results (e.g., Young et al., 2013).
331
332
4.3.
333
To reliably assess the Late Triassic multiple impact theory proposed by Spray et al. (1998),
334
according to which several impact events occurred virtually synchronously, narrow error ranges
335
associated with the individual impact ages are crucial (see impact timing scheme in Fig. 7). At
336
least two of the impacts involved in the postulated scenario, Rochechouart in France and Obolon
337
in Ukraine, were previously dismissed based on their non-synchronous ages (Schmieder and
338
Buchner, 2008; Schmieder et al., 2010). The age of the ~100 km Manicouagan impact structure
339
in Québec has been precisely determined at 215.56 ± 0.05 Ma (Ramezani et al., 2005) and 214 ±
340
1 Ma (Hodych and Dunning, 1992) using U/Pb dating of impact melt-grown zircons.
341
The ~40 km Rochechouart impact structure in France was previously dated at an 40Ar/39Ar age of
342
214 ± 8 Ma (Kelley and Spray, 1997), obtained from UV laser ablation spots in the matrix of
A geochronologic appraisal of the Late Triassic multiple impact hypothesis
343
melt-bearing pseudotachylitic breccia veins. However, these melt veins are known to contain
344
considerable amounts of undigested Precambrian to Paleozoic feldspar clasts and hydrothermal
345
fillings (Reimold et al., 1987); moreover, spot fusion apparent ages within a range between
346
~230 Ma and ~1.7 Ga and an isochron
347
1997) indicated contamination by extraneous argon. The age for the Rochechouart impact was
348
recently revised to 202.8 ± 2.2 Ma (Schmieder et al., 2010) obtained from two 40Ar/39Ar plateau
349
ages for K-feldspar from an impact-metamorphosed gneiss. The other candidate craters for the
350
Triassic ‗crater chain‘ have only been imprecisely dated. The ~20 km marine Obolon impact
351
structure in Ukraine (Gurov et al., 2009) had earlier been estimated at a stratigraphic age of 215
352
± 25 Ma (Masaitis et al., 1980; Spray et al., 1998), but K/Ar dating of the Obolon impact glass
353
and melt rock gave a Jurassic apparent age of 169 ± 7 Ma (Masaitis, 1999; Valter et al., 1999;
354
Gurov et al., 2009). A simple paleogeographic test further supports a Jurassic maximum age for
355
the Obolon impact structure; only from the Early Jurassic onward did the paleoenvironmental
356
conditions in the Donets Graben of the Ukrainian Shield change from continental sedimentation
357
into a marine environment and provided the setting for the submarine Obolon impact (Schmieder
358
and Buchner, 2008). The age of the ~9 km Red Wing Creek impact structure in the Williston
359
Basin of North Dakota is stratigraphically constrained to roughly ~200-220 Ma (Koeberl et al.,
360
1996). Other loose stratigraphic age estimates for Red Wing Creek are 200 ± 25 Ma (Gerhard et
361
al., 1982) and 200 ± 5 Ma (Grieve, 1991), however, no fresh impact melt lithologies are known
362
from this impact site that could be used for more precise and accurate isotopic dating. Within the
363
extended list of potential craters within the suspected multiple impact system (Spray et al.,
364
1998), the ~3 km Newporte impact structure in North Dakota has a Cambrian stratigraphic age of
365
~500 Ma (Koeberl and Reimold, 1995), which is in conflict with the Late Triassic multiple
40
Ar/36Ar intercept value of >500 (Kelley and Spray,
366
impact scenario. The age of the ~12 km Wells Creek impact structure in Tennessee is poorly
367
known and given as 200 ± 100 Ma (Grieve, 2001).
368
From a geochronologic perspective, the refined 40Ar/39Ar age of ~228 Ma for Lake Saint Martin
369
demonstrates that the Obolon, Rochechouart, Manicouagan and Lake Saint Martin impact events
370
were far from synchronous. At least ~10 Myr, and up to >40 Myr, lie between each of the impact
371
events (Fig. 7). This also rules out any potential connection between the postulated large-scale
372
multi-impact scenario and extinction events during the Triassic (e.g., Benton, 1988; Hallam and
373
Wignall, 1997; Sephton et al., 2002; Tanner et al., 2004).
374
375
4.4.
376
In addition to the time constraints gained from isotopic dating, the compositions of the respective
377
impactors can be used as an additional argument for or against a cosmic combination strike. We
378
refer to Goderis et al. (2012), Koeberl et al. (2012) and Koeberl (2014) for recent reviews on the
379
identification of impactor signatures in impactites. The proposed Late Triassic multiple impacts
380
should, thus, also be appraised with regards to their potential impactor signatures. No conclusive
381
impactor traces, with siderophile elements below detection limit, could be gained from the
382
impact melt rocks at Manicouagan (Palme et al., 1978) and Lake Saint Martin (Göbel et al.,
383
1980; Palme, 1982; Reimold et al., 1990), which might suggest a differentiated asteroid as the
384
parent body for achondritic projectiles. In contrast, the Rochechouart impact structure in France
385
(Koeberl et al., 2007; Tagle et al., 2009) and the Obolon impact structure in Ukraine (Valter and
386
Ryabenko, 1977; Masaitis, 1999) carry siderophile element patterns that favour the impact of
387
iron meteorites. The meteorite type for the Red Wing Creek impact structure is uncertain, but
Geochemical constraints on impacting bodies
388
Koeberl et al. (1996) found a possible chondritic contamination pattern in impact breccias
389
recovered from drill cores.
390
Despite this apparent difference in impacting bodies for the Manicouagan, Lake Saint Martin,
391
Rochechouart, Obolon, and Red Wing Creek impact structures, one could argue that double and
392
multiple meteorite impacts could in fact involve a mixture of materials (Schmieder et al., 2013).
393
One such example, yet on a much smaller scale, is the fall of the Almahata Sitta meteorite in
394
northern Sudan on October 7, 2008, which comprised a variety of chondritic and achondritic
395
(ureilitic) lithologies (Bischoff et al., 2010). We acknowledge that asteroids commonly represent
396
‗rubble piles‘ (Richardson et al., 1998; 2002), such as asteroid 25143 Itokawa (e.g. Fujiwara et
397
al., 2006), and can hence be composed of different types of materials. Moreover, a larger asteroid
398
can capture a smaller asteroid in space and thereby generate a binary asteroid system (such as,
399
e.g., asteroid 243 Ida and its satellite Dactyl; Chapman et al., 1995) of two bodies of different
400
composition, fragments of which may eventually impact Earth. It seems somewhat unlikely that
401
the incoming fragments of a tidally disrupted larger rubble pile asteroid or comet, such as
402
Shoemaker Levy-9 (e.g., Shoemaker et al., 1993; Crawford et al., 1994), would be heterogeneous
403
to the extent that they generate a chain of impact craters in which each one leaves a different
404
geochemical fingerprint; however, unless this type of scenario (i.e., impact of variable-type
405
cosmic rubble) is proven impossible in terms of celestial mechanics, one should consider the
406
theoretical possibility that a geochemically heterogeneous impact system can occur over
407
geologic time. On the other hand, impact melt lithologies at terrestrial impact structures can lack
408
a clear and representative meteoritic signature (e.g., Goderis et al., 2010), for example due to the
409
incomplete homogenization of the impactor-derived elements and the local impact melt,
410
fractionation of the meteoritic signature elements (e.g., Palme et al., 1979; Palme 1982),
411
differentiation of larger impact melt bodies (e.g., O‘Connell-Cooper and Spray, 2011), as well as
412
post-impact alteration. For the given uncertainties on the exact composition of the meteorites that
413
generated the Lake Saint Martin, Manicouagan, Rochechouart, Obolon and Red Wing Creek
414
impact structures we stress that the geochemical constraints cannot serve as reliable evidence for
415
or against a multiple impact scenario.
416
417
4.5.
418
Finally, the statistical probability for a multiple impact event on Earth in the Late Triassic should
419
be briefly evaluated based on the timing and paleopositions of the impact events in plate
420
reconstructions for North America (Laurentia) and Eurasia (see Spray et al., 1998; Walkden et
421
al., 2002). Using the estimated paleolatitudes and -longitudes of the impact craters as input
422
parameters in Monte Carlo simulations, Spray et al. (1998) calculated a low probability of P
423
<0.08 (<0.008 if the longitudinal range is included) for the event that the linear arrangement of
424
the three ‗main‘ E-W-aligned craters, Rochechouart, Manicouagan, and Lake Saint Martin, is a
425
random array. The probability for an aligned random strike decreases even further for five
426
impact structures (i.e., including Obolon and Red Wing on great circles; however, no statistical
427
probability was given for such an event). As the crater alignment was very unlikely a result of
428
random chance and because the imprecise age constraints for the craters agreed well within this
429
calculation, Spray et al. (1998) concluded that the five craters may represent a chain of impact
430
structures probably formed ‗within hours‘.
431
The general expectation that linear arrays of apparently coeval impacts constitute a combination
432
strike, as supported by statistical modeling, may be in analogy to some of the binary impact
Statistical considerations
433
systems on Earth, including the proposed doublet craters of the West and East Clearwater Lake
434
impact structures in Canada and the Suvasvesi North and South impact structures in Finland.
435
Despite very low calculated likelihoods for their random origin in close proximity to the
436
respective fellow crater (Miljković et al., 2013), differing 40Ar/39Ar dating results for impact melt
437
rocks from each of these impact structures rather suggest that the impacts occurred millions of
438
years apart (see Schmieder et al., 2014b for a discussion and references therein for earlier dating
439
results).
440
Based on the size-frequency distribution of Earth-crossing asteroids and their encounter
441
frequency with the Earth-Moon system, Bottke et al. (1997) estimated that the production rate for
442
impact crater chains on Earth after S-type (Shoemaker-Levy 9) tidal disruption is less than 0.1
443
crater chains in 3.8 Gyr. While we acknowledge that these statistics are derived from numerical
444
modeling, the results by Bottke et al. (1997) suggest a very low chance of producing a terrestrial
445
impact crater chain in younger geologic history. We recommend that proposed terrestrial impact
446
crater chains and doublets should be systematically accompanied by isotope dating. Even more
447
delicate is the case of extraterrestrial impact crater chains and doublets, for which no absolute
448
ages can be obtained without sample return missions. Although crater counting (e.g., Neukum et
449
al., 2001) helps to some extent, this study shows that fortuitously aligned, yet unrelated impact
450
craters may also occur on asteroids and other planets in the Solar System.
451
452
5.
453
The ~40 km Lake Saint Martin impact structure in Manitoba, Canada, has been
454
at a Carnian age of 227.8 ± 0.9 Ma [± 1.1 Ma] (2σ; MSWD = 0.52; P = 0.59) by combining two
CONCLUSIONS
40
Ar/39Ar-dated
455
plateau ages and one isochron age obtained on different aliquots of impact-melted K-feldspar.
456
With a relative error of ± 0.4% on the isotopic age, the Lake Saint Martin impact structure has
457
advanced to one of the most precisely dated larger impact structures on Earth. The new dating
458
results demonstrate that none of the impacts that constituted the postulated Late Triassic multiple
459
impact event were synchronous. Moreover, our data suggest that high-temperature isotopic
460
chronometers are currently the preferred means of dating impact events. Where possible,
461
statistical model results arguing that linear arrays of multiple impact craters, as well as closely
462
spaced apparent crater pairs, are most likely synchronous should be assessed in the light of
463
precise and accurate isotopic dating results.
464
465
Acknowledgements: We thank Celia Mayers and Adam Frew, Curtin University, for their helpful
466
support during sample preparation, Ruth Bezys for the earlier supply of some of the sample
467
materials used, and Rebecca Wilks for review of an earlier draft of this manuscript. The authors
468
acknowledge the facilities, and the scientific and technical assistance of the Australian
469
Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterisation &
470
Analysis, The University of Western Australia, a facility funded by the University, State and
471
Commonwealth Governments. We thank the reviewers Vera A. Fernandes and Wolf Uwe
472
Reimold for their constructive comments that helped improve our manuscript, as well as the
473
Associate Editor Christophe Sotin for careful handling. Sample collection was supported by a
474
CARN grant from the Canadian Space Agency to EC. This project is supported by an ARC
475
Discovery grant to FJ and ET.
476
477
REFERENCES
478
Baksi, A.K., 2007. A quantitative tool for detecting alteration in undisturbed rocks and minerals
479
– II: Application to argon ages related to hotspots. In: Foulger, G. R., Jurdy, D. M. (Eds.),
480
Plates, Plumes and Planetary Processes. GSA Spec. Pap. 430, Boulder, CO, USA, pp. 305–
481
333.
482
Bannatyne, B.B., McCabe, H.R., 1984. Manitoba crater revealed. GEOS 1984/3, 10–13.
483
Benton, M.J., 1988. Late Triassic tetrapod extinction events. In: Padian, K. (Ed.), The beginning
484
of the age of dinosaurs: faunal change across the Triassic-Jurassic boundary. Cambridge
485
Univ. Press, UK, p. 303–320.
486
Biren, M.B., van Soest, M.C., Wartho, J.A., Spray, J.G., 2012. (U–Th)/He dating of uplift-
487
induced cooling in a complex terrestrial impact structure: The Manicouagan example.
488
Meteoritics Planet. Sci. suppl. 75, 5344.
489
Bischoff, A., Horstmann, M., Pack, A., Laubenstein, M., Haberer, S., 2010. Asteroid 2008
490
TC3—Almahata Sitta: A spectacular breccia containing many different ureilitic and
491
chondritic lithologies. Meteoritics Planet. Sci. 45, 1638–1656.
492
493
494
Bottke Jr., W.F., Richardson, D.C., Love, S.G., 1997. Can tidal disruption of asteroids make
crater chains on the Earth and Moon? Icarus 126, 470–474.
Buchner, E., Schmieder, M., Schwarz, W.H., Trieloff, M., Moilanen, J., Öhman, T., Stehlik, H.,
40
Ar/39Ar Age for the Suvasvesi South Structure Finland. Meteoritics
495
2009. A Proterozoic
496
Planet. Sci. suppl. 72, 5076.
497
Buchner E., Schwarz W.H., Schmieder M., Trieloff M., 2010. Establishing a 14.6 ± 0.2 Ma age
498
for the Nördlinger Ries impact (Germany) – a prime example for concordant isotopic ages
499
from various dating materials. Meteorit. Planet. Sci. 45, 662–674.
500
Carporzen, L., Gilder, S.A., 2006. Evidence for coeval Late Triassic terrestrial impacts from the
501
Rochechouart
502
doi:10.1029/2006GL027356.
France
meteorite
crater.
Geophys.
Res.
Lett.
33,
L19308.
503
Chapman, C.R., Veverka, J., Thomas, P.C., Klaasen, K., Belton, M.J.S., Harch, A., McEwen, A.,
504
Johnson, T.V., Helfenstein, P., Davies, M.E., Merline, W.J., Denk, T., 1995. Discovery and
505
physical properties of Dactyl, a satellite of asteroid 243 Ida. Nature, 374, 783–784.
506
507
508
509
510
511
512
513
Cohen, K.M., Finney, S.M., Gibbard, P.L., Fan, J.-X., 2013. The ICS International
Chronostratigraphic Chart. Episodes 363, 199–204.
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, 7087–7095.
Crawford, D.A., Boslough, M.B., Trucano, T.G., Robinon, A.C., 1994. The impact of comet
Shoemaker-Levy 9 on Jupiter. Shock Waves 4, 47–50.
Currie, K.L., 1970. New Canadian cryptoexplosion crater at Lake St Martin, Manitoba. Nature
226, 839–841.
514
Earth Impact Database, 2013. Online database maintained by the University of New Brunswick,
515
Canada. Available online at http://www.passc.net/EarthImpactDatabase/index.html (last
516
accessed 03 December, 2013).
517
518
Farley, K.A., 2002. (U–Th)/He dating: Techniques, calibrations, and applications. Rev. Mineral.
Geochem. 471, 819–844.
519
Fujiwara, A., Kawaguchi, J., Yeomans, D.K., Abe, M., Mukai, T., Okada, T., Saito, J., Yano, H.,
520
Yoshikawa, M., Scheeres, D.J., Barnouin-Jha, O., Cheng, A.F., Demura, H., Gaskell, R.W.,
521
Hirata, N., Ikeda, H., Kominato, T., Miyamoto, H., Nakamura, A.M., Nakamura, R., Sasaki,
522
S., Uesugi, K., 2006. The rubble-pile asteroid Itokawa as observed by Hayabusa. Science 312,
523
1330–1334.
524
Gerhard, L.C., Anderson, S.B., Lefever, J.A., Carlson, C.G., 1982. Geological development,
525
origin, and energy mineral resources of Williston Basin, North Dakota. AAPG Bull. 668,
526
989–1020.
527
528
Göbel, E., Reimold, W.U., Baddenhausen, H., Palme, H., 1980. The projectile of the Lappajärvi
crater. Zeitschr. Naturforsch. 35A, 197–203.
529
Goderis, S., Hertogen, J., Vanhaecke, F., Claeys, Ph., 2010. Siderophile elements from the
530
Eyreville drill cores of the Chesapeake Bay impact structure do not constrain the nature of the
531
projectile. In: Gibson, R.L., Reimold, W.U. (Eds.), Large Meteorite Impacts and Planetary
532
Evolution IV. GSA Spec. Pap. 465, Boulder, CO, USA, pp. 395–409.
533
Goderis, S., Paquay, F., Claeys, Ph., 2012. Projectile identification in terrestrial impact structures
534
and ejecta material. In: Osinski, G.R. and Pierazzo, E. (Eds.), Impact Cratering: Processes and
535
Products, Wiley-Blackwell, pp. 223–239.
536
Grieve, R.A.F., 1991. Terrestrial impact: The record in the rocks. Meteoritics 26, 175–194.
537
Grieve, R.A.F., 2001. The terrestrial cratering record. In: Peucker-Ehrenbrink, B., Schmitz, B.
538
(Eds.), Accretion of extraterrestrial matter throughout Earth‘s history. Kluwer, NY, USA, pp.
539
379–402.
540
541
542
543
544
545
546
547
548
549
550
551
552
Grieve, R.A.F., 2006. Impact structures in Canada. GeoTEXT 5, Geol. Assoc. Canada, St.
John‘s, 210 p.
Gurov, E.P., Gurova, E.P., Chernenko, Y., Yamnichenko, A., 2009. The Obolon impact
structure, Ukraine, and its ejecta deposits. Meteoritics Planet. Sci. 44, 389–404.
Hallam, A., Wignall, P.B., 1997. Mass extinctions and their aftermath. Oxford Univ. Press, New
York, NY, USA, 320 p.
Hodych, J.P., Dunning, G.R., 1992. Did the Manicouagan impact trigger end-of-Triassic mass
extinction? Geology 20, 51–54.
Jourdan, F., 2012. The
40
Ar/39Ar dating technique applied to planetary sciences and terrestrial
impacts. Austral. J. Earth Sci. 59, 199–224.
Jourdan, F., Renne, P.R., 2007. Age calibration of the Fish Canyon sanidine
40
Ar/39Ar dating
standard using primary K–Ar standards. Geochim. Cosmochim. Acta 71, 387–402.
Jourdan, F., Renne, P.R., Reimold, W.U., 2007. The problem of inherited 40Ar* in dating impact
40
Ar/39Ar method: Evidence from the Tswaing impact crater South Africa.
553
glass by the
554
Geochim. Cosmochim. Acta 71, 1214–1231.
555
556
557
558
559
560
561
562
Jourdan, F., Renne, P.R., Reimold, W.U., 2008. High-precision
40
Ar/39Ar age of the Jänisjärvi
impact structure (Russia). Earth Planet. Sci. Lett. 265, 438–449.
Jourdan, F., Renne, P.R., Reimold, W.U., 2009. An appraisal of the ages of terrestrial impact
structures. Earth Planet. Sci. Lett. 286, 1–13.
Jourdan, F., Reimold, W.U., Deutsch, A., 2012. Dating terrestrial impact structures. In: Jourdan,
F., Reimold, W.U. (Eds.), Impact! Elements 8, 49–53.
Kelley, S.P., Spray, J.G., 1997. A late Triassic age for the Rochechouart impact structure,
France. Meteoritics Planet. Sci. 32, 629–636.
563
Kent, D.V., Olsen, P.E., Witte, W.K., 1995. Late Triassic-earliest Jurassic geomagnetic polarity
564
sequence and paleolatitudes from drill cores in the Newark rift basin, eastern North America.
565
J. Geophys. Res. 100, B8, 14,965–14,998.
566
Kent, D.V., 1998. Impacts on Earth in the Late Triassic. Nature 395, 126–126.
567
Kirsimäe, K., Suuroja, S., Kirs, J., Kärki, A., Polikarpus, M., Puura, V., Suuroja, K. 2002.
568
Hornblende alteration and fluid inclusions in Kärdla impact crater, Estonia: evidence for
569
impact-induced hydrothermal activity. Meteoritics Planet. Sci. 37, 449–457.
570
Koeberl, C. 2014. Chapter 2.5 – The Geochemistry and Cosmochemistry of Impacts. In:
571
Turekian, K. K. and Holland, H. (Eds.), Treatise on Geochemistry (2nd Edition), Elsevier, pp.
572
73–118.
573
574
Koeberl, C., Reimold, W.U., 1995. The Newporte impact structure, North Dakota, USA.
Geochim. Cosmochim. Acta 59, 4747–4767.
575
Koeberl, C., Reimold, W.U., Brandt, D., 1996. Red Wing Creek structure, North Dakota:
576
Petrographical and geochemical studies, and confirmation of impact origin. Meteoritics
577
Planet. Sci. 31, 335–342.
578
Koeberl, C., Shukolyukov, A., Lugmair, G.W., 2007. Chromium isotopic studies of terrestrial
579
impact craters: Identification of meteoritic components at Bosumtwi, Clearwater East,
580
Lappajärvi, and Rochechouart. Earth Planet. Sci. Lett. 256, 534–546.
581
582
583
584
585
586
Koeberl, C., Claeys, Ph., Hecht, L., McDonald, I., 2012. Geochemistry of impactites. In:
Jourdan, F., Reimold, W.U. (Eds.), Impact! Elements 8, 37-42.
Kohn, B.P., Osadetz, K.G., Bezys, R.K., 1995. Apatite fission-track dating of two crater
structures in the Canadian Williston basin. Bull. Canadian Petrol. Geol. 431, 54–64.
Koppers, A.A., 2002. ArArCALC—software for
40
Ar/39Ar age calculations. Comp. Geosci. 28,
605–619.
587
Lee J. Y., Marti K., Severinghaus K., Kawamura K., Yoo H. S., Lee J. B., Kim J. S., 2006. A
588
redetermination of the isotopic abundances of atmospheric Ar. Geochim. Cosmochim. Acta
589
70, 4507–4512.
590
Leybourne, M.I., Denison, R.E., Cousens, B.L., Bezys, R.K., Gregoire, D.C., Boyle, D.R.,
591
Dobrzanski, E., 2007. Geochemistry, geology, and isotopic (Sr, S, and B) composition of
592
evaporites in the Lake St. Martin impact structure: New constraints on the age of melt rock
593
formation. Geochem. Geophys. Geosys. 8, 22 p . DOI: 10.1029/2006GC001481.
594
Lucas, S.G., Tanner, L.H., Kozur, H.W., Weems, R.E., Heckert, A.B., 2012. The Late Triassic
595
timescale: age and correlation of the Carnian–Norian boundary. Earth-Sci. Rev. 114, 1–18.
596
Mark D. F., Stuart F. M., de Podesta M., 2011. New high-precision measurements of the isotopic
597
598
599
600
601
602
603
composition of atmospheric argon. Geochim. Cosmochim. Acta 75, 7494–7501.
Masaitis, V.L., Danilin, A.N., Mashchak, M.S., Raikhlin, A.I., Selivanovskaia, T.V.,
Shadenkov, E.M., 1980. The geology of astroblemes. Nedra, Leningrad, 232 p.
Masaitis, V.L., 1999. Impact structures of northeastern Eurasia: The territories of Russia and
adjacent countries. Meteoritics Planet. Sci. 34, 691–711.
McCabe, H.R., Bannatyne, B.B., 1970. Lake St. Martin crypto-explosion crater and geology of
the surrounding area. Geol. Survey Manitoba Geol. Pap. 3/70, 1–69.
604
Melosh, H.J., 1998. Craters unchained. Nature 394, 221–223.
605
Miljković, K., Collins, G.S., Mannick, S., Bland, P.A., 2013. Morphology and population of
606
binary asteroid impact craters. Earth Planet. Sci. Lett. 363, 121–132.
607
Monastersky, R., 1998. Target Earth. Science News 153, 312–314.
608
Nebel, O., Scherer, E.E., Mezger, K., 2011. Evaluation of the
609
610
611
87
Rb decay constant by age
comparison against the U–Pb system. Earth Planet. Sci. Lett. 301, 1–8.
Neukum, G., Ivanov, B. A., Hartmann, W. K., 2001. Cratering records in the inner Solar System
in relation to the lunar reference system. Space Sci. Rev. 96, 55–86.
612
O‘Connell-Cooper, C.D., Spray, J.G., 2011. Geochemistry of the impact‐generated melt sheet at
613
Manicouagan: Evidence for fractional crystallization. J. Geophys. Res. 116, B06204,
614
doi:10.1029/2010JB008084, 22 p.
615
Palme, H., 1982. Identification of projectiles of large terrestrial impact craters and some
616
implications for the interpretation of Ir-rich Cretaceous/Tertiary boundary layers. In: Silver,
617
L.T., Schultz, P.H. (Eds.), Geological implications of impacts of large asteroids and comets
618
on the Earth, GSA Spec. Pap. 190, Boulder, CO, USA, pp. 223–233.
619
620
Palme, H., Janssens, M.J., Takahashi, H., Anders, E., Hertogen, J., 1978. Meteoritic material at
five large impact craters. Geochim. Cosmochim. Acta 42, 313–323.
621
Palme, H., Göbel, E., Grieve, R.A.F., 1979. The distribution of volatile and siderophile elements
622
in the impact melt of East Clearwater (Quebec). Proc. Lunar Planet. Sci. Conf. 10, 2465–
623
2492.
624
Pati, J.K., Jourdan, F., Armstrong, R.A., Reimold, W.U., Prakash, K., 2010. First SHRIMP U-Pb
625
and 40Ar/39Ar chronological results from impact melt breccia from the Paleoproterozoic Dhala
626
impact structure, India. In: Gibson, R.L., Reimold, W.U. (Eds.), Large Meteorite Impacts and
627
Planetary Evolution IV. GSA Spec. Pap. 465, Boulder, CO, USA, pp. 571–591.
628
Ramezani, J., Bowring, S.A., Pringle, M.S., Winslow III, F.D., Rasbury, E.T., 2005. The
629
Manicouagan impact melt rock: A proposed standard for the intercalibration of U‐Pb and
630
40
631
632
Ar/39Ar isotopic systems. Geochim. Cosmochim. Acta, 69, A321.
Reimold, W. U., Oskierski, W., Huth, J., 1987. The Pseudotachylite from Champagnac in the
Rochechouart Meteorite Crater, France. J. Geophys. Res. 92, B4, E737–E748.
633
Reimold, W.U., Barr, J.M., Grieve, R.A.F., Durrheim, R.J., 1990. Geochemistry of the melt and
634
country rocks of the Lake St. Martin impact structure, Manitoba, Canada. Geochim.
635
Cosmochim. Acta 54, 2093–2111.
636
637
638
Reiners, P.W., 2005. Zircon (U–Th)/He thermochronometry. Rev. Mineral. Geochem. 58, 151–
179.
Renne, P.R., Mundil, R., Balco, G., Min, K., Ludwig, K.R., 2010. Joint determination of
40
K
639
decay constants and 40Ar∗/40K for the Fish Canyon sanidine standard, and improved accuracy
640
for 40Ar/39Ar geochronology. Geochim. Cosmochim. Acta 74, 5349–5367.
641
Renne, P.R., Balco, G., Ludwig, K.R., Mundil, R., Min, K., 2011. Response to the comment by
642
WH Schwarz et al. on ―Joint determination of 40K decay constants and 40Ar∗/40K for the Fish
643
Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology‖ by PR Renne
644
et al. (2010). Geochim. Cosmochim. Acta 75, 5097–5100.
645
646
Richardson, D.C., Bottke, W.F., Love, S.G., 1998. Tidal distortion and disruption of Earthcrossing asteroids. Icarus 134, 47–76.
647
Richardson, D.C., Leinhardt, Z.M., Melosh, H.J., Bottke Jr., W.F., Asphaug, E., 2002.
648
Gravitational aggregates: evidence and evolution. In: Bottke Jr., W.F., Cellino, A., Paolicchi,
649
P., Binzel, R.P. (Eds.), Asteroids III. Univ. Arizona Press, Tucson, AZ, USA, pp. 501–515.
650
Rosenthal, L.R., 1988. The Winnipegosis Formation Middle Devonian of the northeastern
651
margin of the Williston Basin, Canada. In: McMillan, N.J., Embry, A.F., Glass, D.J. (Eds.),
652
Devonian of the World – Volume II, CSPG Mem. 14, Calgary, AB, Canada, pp. 463–475.
653
654
655
656
657
658
Sankaran, A.V., 1999. Geological findings on some postulated synchronous impact-craters on
Earth. Current Science Bangalore 76, 15–17.
Schmieder, M., Buchner, E., 2008. Dating impact craters: palaeogeographic versus isotopic and
stratigraphic methods – A brief case study. Geol. Mag 145, 586–590.
Schmieder, M., Jourdan, F., 2013. The Lappajärvi impact structure Finland: Age, duration of
crater cooling, and implications for early life. Geochim. Cosmochim. Acta 112, 321–339.
659
Schmieder, M., Buchner, E., Schwarz, W.H., Trieloff, M., Lambert, P., 2010. A Rhaetian
660
40
661
Triassic sedimentary record. Meteoritics Planet. Sci. 45, 1225–1242.
662
663
664
Ar/39Ar age for the Rochechouart impact structure France and implications for the latest
Schmieder, M., Schwarz, W.H., Buchner, E., Pesonen, L.J., 2012. Double and multiple impact
events on Earth – Hypotheses, tests, and problems. Meteoritics Planet. Sci. 47, S1, A341.
Schmieder, M., Jourdan, F., Öhman, T., Tohver, E., Mayers, C., Frew, A., 2014a. A Proterozoic
665
40
Ar/39Ar age for the Söderfjärden impact structure, Finland. Lunar Planet. Sci. Conf. 45,
666
abstr. no. 1301.
667
Schmieder, M., Trieloff, M., Schwarz, W.H., Buchner, E., Jourdan, F., 2014b. Supportive
668
comment on: ―Morphology and population of binary asteroid impact craters‖, by K.
669
Miljković, G. S. Collins, S. Mannick and P. A. Bland [Earth Planet. Sci. Lett. 363 2013 121–
670
132] – An updated assessment. Earth Planet. Sci. Lett. (in press).
671
Sephton, M.A., Amor, K., Franchi, I.A., Wignall, P.B., Newton, R., Zonneveld, J.P., 2002.
672
Carbon and nitrogen isotope disturbances and an end-Norian Late Triassic extinction event.
673
Geology 30, 1119–1122.
674
675
676
677
678
679
Shoemaker, C.S., Shoemaker, E.M., Levy, D.H., Scotti, J.V., Bendjoya, P., Mueller, J., 1993.
Comet Shoemaker-Levy (1993e). In: Marsden, B.G. (Ed.), IAU Circ., 5725, 1.
Simonds C.H., McGee, P.E., 1979. Petrology of impactites from Lake St. Martin structure,
Manitoba. Proc. Lunar Planet. Sci. Conf. 10, 2493–2518.
Spray, J.G., Kelley, S.P., Rowley, D.B., 1998. Evidence for a late Triassic multiple impact event
on Earth. Nature 392, 171–173.
680
Tagle, R., Schmitt, R.T., Erzinger, J., 2009. Identification of the projectile component in the
681
impact structures Rochechouart, France and Sääksjärvi, Finland: Implications for the impactor
682
population for the earth. Geochim. Cosmochim. Acta 73, 4891–4906.
683
684
Tanner, L.H., Lucas, S.G., Chapman, M.G., 2004. Assessing the record and causes of Late
Triassic extinctions. Earth-Sci. Rev. 65, 103–139.
685
686
Valter, A.A., Ryabenko, V.A., 1977. Explosion craters on the Ukrainian Shield. Naukova
Dumka, Kiev, Ukraine, 155 p. (in Russian).
687
Valter, A.A., Asimov, A.T., Voizizkji, E.J., Sirchenko, W.W., 1999. The probably unique
688
geological structure of the Obolon astrobleme: Complicated crater form within the central
689
hole. In: Churyumova, K.I. (Ed.), Current problems with Comets, Asteroids, Meteors,
690
Meteorites, Astroblemes, and Craters – Proc. CAMMAC ‗99 Meeting, Vinnitsa, Ukraine, pp.
691
343–366.
692
VanDecar, J., 1998. Planetary science: Crater row. Nature, 392, 131–131.
693
van Soest, M.C., Cooper, F.J., Wartho, J.-A., Hodges, K., Buchner, E., Schmieder, M., Koeberl,
694
C., 2010. Can single crystal (U–Th)/He zircon ages from Nördlinger Ries suevite be linked to
695
impact-related shock effects? AGU, Fall Meeting 2010, abstract #P44B-08.
696
van Soest, M.C., Hodges, K.V., Wartho, J.A., Biren, M.B., Monteleone, B.D., Ramezani, J.,
697
Spray, J.G., Thompson, L.M., 2011. (U‐Th)/He dating of terrestrial impact structures: The
698
Manicouagan example. Geochem. Geophys. Geosys. 12, 8 p. DOI: 10.1029/2010GC003465.
699
Young, K.E., van Soest, M.C., Hodges, K.V., Watson, E.B., Adams, B.A., Lee, P., 2013. Impact
700
thermochronology and the age of Haughton impact structure, Canada. Geophys. Res. Lett. 40,
701
3836–3840.
702
703
704
Walkden, G.M., Parker, J., Kelley, S.P., 2002. A Late Triassic impact ejecta layer in
southwestern Britain. Science 298, 2185–2188.
Wartho, J.-A., Schmieder, M., van Soest, M.C., Buchner, E., Hodges, K.V., Bezys, R.K.,
705
Reimold, W.U., 2009. New (U–Th)/He zircon and apatite ages for the Lake Saint Martin
706
impact structure (Manitoba, Canada) and implications for the Late Triassic multiple impact
707
theory. Lunar Planet. Sci. Conf. 40, abstr. no. 2004.
708
Wartho, J.-A., van Soest, M.C., Cooper, F.J., Hodges, K.V., Spray, J.G., Schmieder, M.,
709
Buchner, E., Bezys, R.K., Reimold, W.U., 2010. Updated (U–Th)/He Zircon Ages for the
710
Lake Saint Martin impact structure (Manitoba, Canada) and implications for the Late Triassic
711
multiple impact theory. Lunar Planet. Sci. Conf. 41, abstr. no. 1930.
712
Wartho, J.-A., van Soest, M.C., Cooper, F.J., Spray, J.G., Schmieder, M., Buchner, E., King,
713
D.T., Ukstins Peate, I., Koeberl, C., Reimold, W.U., Biren, M.B., Petruny, L.W., Hodges, K.
714
V., 2012a. (U–Th)/He dating of impact structures – The big, the small, and the potential
715
limitations. Meteoritics Planet. Sci. 75, S1, A401.
716
Wartho, J.-A., van Soest, M.C., King, D.T., Petruny, L.W., 2012b. An (U‐Th)/He age for the
717
shallow‐marine Wetumpka impact structure, Alabama, USA. Meteoritics Planet. Sci. 47,
718
1243–1255.
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
Zivkovic, V.B., Gosnold, W., 2007. The Lake Saint Martin impact structure and its relation to a
multiple impact event. AGU Fall Meeting 2007, abstr. no. U23A-0857.
735
FIGURE CAPTIONS
736
Fig. 1: Landsat-5 satellite image of Lake Saint Martin in Manitoba, Canada (scene path 032, row
737
024, acquired on 04 April 2010) and outline of the estimated outer limits of the largely sediment-
738
and water-covered ~40 km-diameter impact structure according to Bannatyne and McCabe
739
(1984). Impact melt rock from the eastern crater moat and a partially melted Proterozoic granite
740
from the central uplift (inset images; both rock specimens ~10 cm in maximum length) were
741
sampled for 40Ar/39Ar dating. Satellite image source: USGS.
742
Fig. 2: Optical microscopic and backscattered electron images (BEI) of the fine-grained,
743
crystalline melt rock from the Lake Saint Martin impact structure and the impact metamorphosed
744
granite exposed in the central uplift. A: General appearance of the melt rock with a larger grain
745
of ballen quartz (plane polarized light). B: The melt rock is predominantly composed of
746
plagioclase (Plg; a larger phenocryst of andesine is shown), K-feldspar (Kfs), Quartz (Qtz),
747
pyroxene (Px), and Fe-Ti oxides. C: Optical image of ballen silica surrounded by fluidal-textured
748
feldspar (cross polarized light). D: BEI close-up of the granite, locally with idiomorphic K-
749
feldspar (adularia; Adu). The images are representative of the rock samples from which the
750
dating aliquots were separated.
751
Fig. 3:
752
impact-melted K-feldspars separated from the Lake Saint Martin impact melt rock. A: Age
753
spectrum (plateau) for aliquot LSM-1A. B: Inverse isochron for LSM-1A. C: Age spectrum
754
(mini-plateau) for aliquot LSM-1B. D: Inverse isochron for LSM-1B. Errors are given at the 2σ
755
level.
756
Fig. 4:
757
K-feldspar specimens separated from the partially melted basement granite exposed in the central
758
uplift of the Lake Saint Martin impact structure. A: Age spectrum (plateau) for aliquot LSM-2A.
759
B: Inverse isochron for LSM-2A with inset for magnification of data point array. C: Age
760
spectrum (plateau) for aliquot LSM-2B. D: Inverse isochron plot for LSM-2B with inset for
761
zoom on data point cluster. E: Age spectrum (disturbed) for aliquot LSM-2C. F: Inverse
762
isochron plot for LSM-2C. Errors are given at the 2σ level.
40
40
Ar/39Ar age spectra and corresponding inverse isochron plots for two aliquots of
Ar/39Ar age spectra and corresponding inverse isochron plots for three impact-melted
763
Fig. 5: 40Ar/39Ar age spectra and corresponding K/Ca and inverse isochron plots for two whole-
764
rock chips of the Lake Saint Martin impact melt rock. A: Age spectrum (disturbed) and K/Ca
765
ratio for moderately altered aliquot LSM-3. B: Inverse isochron plot for LSM-3. C: Age
766
spectrum (disturbed) and K/Ca ratio for intensely altered aliquot LSM-4. B: Inverse isochron plot
767
for LSM-4. Errors are given at the 2σ level.
768
Fig. 6: Summary of measured
769
(alteration) apparent ages for the Lake Saint Martin impact melt aliquots along with statistical
770
age combinations for plateau and isochron ages. All ages are in million years. The weighted
771
mean of two plateau ages and one inverse isochron age is here considered the best-estimate age
772
for the Lake Saint Martin impact. Error boxes are 2σ.
773
Fig. 7: Compilation of selected terrestrial meteorite impacts during the Triassic and the
774
postulated Late Triassic multiple impact theory, modified after Spray et al. (1998). *Lucas et al.
775
(2012) suggested an age of ~220 Ma for the Carnian/Norian boundary, which has an age of
776
~227 Ma in the current International Stratigraphic Chart (Cohen et al., 2013). Impact age data
777
from Koeberl et al. (1996), Ramezani et al. (2005), Buchner and Schmieder (2008), Schmieder et
778
al., (2010) and this study.
40
Ar/39Ar plateau, mini-plateau, inverse isochron and minimum
Fig. 1
Click here to download high resolution image
Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 6
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Fig. 7
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Table 1
Click here to download Table: Table1.pdf
Table 1: Summary of plateau, mini-plateau and isochron ages, along with internal statistics for recrystallized feldspar melt particles and impact melt rock, Lake Saint Martin impact structure. Ages noted in bold were used for the
calculation of the recommended weighted-mean age for the impact.
Age spectrum characteristics
Isochron characteristics
40
n
Ar/36Ar
intercept
(±2σ)
MSWD
P
Spreading
factor (%)
227.2 ± 1.5
12
288 ± 13
0.79
0.64
31
0.15
228 ± 6
5
289 ± 20
1.73
0.16
70
1.00
0.46
227.9 ± 1.4
19*
288 ± 8
0.79
0.71
61
1.38
0.18
39
Sample ID
Apparent age
(Ma, ±2σ)
Total Ar
released
(%)
n
Attribute
MSWD
226.7 ± 2.4
84
12
Plateau
227 ± 5
63
5
227.0 ± 1.2
84
228.1 ± 1.4
73
P
Isochron age
(Ma, ±2σ)
0.78
0.66
Mini-plateau
1.70
18
Plateau
11
Plateau
K-feldspar melt aliquots
LSM-1A
LSM-1B
Impact melt rock,
crater moat domain
(single grain)
LSM-2A
LSM-2B
Partially melted Precambrian
granite, central uplift
(single grain)
LSM-2C
― no isochron ―
― no plateau ―
― no isochron ―
Impact melt rock
aliquots
LSM-3
moderately altered
(whole rock)
≥220-230
hump-shaped spectrum (minimum age)
― no isochron ―
LSM-4
intensely altered
(whole rock)
≥209
staircase-shaped spectrum (minimum age)
― no isochron ―
Weighted mean of 2 plateau ages and 1
isochron age:
*
227.8 ± (0.9) [1.1] Ma
(2σ; MSWD = 0.52; P = 0.59)
isochron includes degassing step 1 not part of (but with a step apparent age concordant with) the age plateau section.
Supplementary Table-Annex 1
Click here to download Supplementary material for on-line publication only: Table-Annex1.xlsx