Earth and Planetary Science Letters 406 (2014) 37–48
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Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
40
Ar/39 Ar age of the Lake Saint Martin impact structure (Canada) –
Unchaining the Late Triassic terrestrial impact craters
Martin Schmieder a,b,∗ , Fred Jourdan b , Eric Tohver a , Edward A. Cloutis c
a
b
c
School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
Western Australian Argon Isotope Facility, Department of Applied Geology and JdL Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
Planetary Spectrophotometer Facility, Department of Geography, University of Winnipeg, Canada
a r t i c l e
i n f o
Article history:
Received 4 December 2013
Received in revised form 29 June 2014
Accepted 30 August 2014
Available online xxxx
Editor: C. Sotin
Keywords:
Lake Saint Martin
40
Ar/39 Ar dating
impact melting
K-feldspar
Triassic
multiple impact event
a b s t r a c t
New 40 Ar/39 Ar dating of impact-melted K-feldspars and impact melt rock from the ∼40 km Lake Saint
Martin impact structure in Manitoba, Canada, yielded three plateau ages and one mini-plateau age in
agreement with inverse isochron ages for the K-feldspar melt aliquots and a minimum age for a wholerock impact melt sample. A combination of two plateau ages and one isochron age, with a weighted
mean of 227.8 ± 0.9 Ma [±1.1 Ma; including all sources of uncertainty] (2σ ; MSWD = 0.52; P = 0.59),
is considered to represent the best-estimate age for the impact. The concordant 40 Ar/39 Ar ages for the
melted K-feldspars, derived from impact melt rocks in the eastern crater moat domain and the partially
melted Proterozoic central uplift granite, suggest that the new dates accurately reflect the Lake Saint
Martin impact event in the Carnian stage of the Late Triassic. With a relative error of ±0.4% on the
40
Ar/39 Ar age, the Lake Saint Martin impact structure counts among the most precisely dated impact
structures on Earth. The new isotopic age for Lake Saint Martin significantly improves upon earlier
Rb/Sr and (U–Th)/He results for this impact structure and contradicts the hypothesis that planet Earth
experienced the formation of a giant ‘impact crater chain’ during a major Late Triassic multiple impact
event.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction and geologic background
1.1. The Lake Saint Martin impact structure, Manitoba
The ∼40 km-diameter Lake Saint Martin impact structure in the
Interlake Region of southern Manitoba (McCabe and Bannatyne,
1970; Bannatyne and McCabe, 1984; Grieve, 2006; Fig. 1) is one
of the largest meteorite impact structures in North America. Target
rocks comprise Paleoproterozoic (∼2.4 Ga) granites of the Superior Province of the Canadian Shield overlain by ∼200–400 m of
Ordovician to Devonian sedimentary rocks that build up the northeastern margin of the Paleozoic intracratonic Williston Basin (e.g.,
Grieve, 2006). The shocked and partially melted basement granite is exposed in the central uplift of the impact structure. The
annular crater moat is filled with a sequence of impactites including carbonate-dominated and granitic lithic impact breccias,
impact melt rocks, and suevitic breccias occasionally summarized
*
Corresponding author at: School of Earth and Environment, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Tel.: +61 434 808
655.
E-mail address: martin@suevite.com (M. Schmieder).
http://dx.doi.org/10.1016/j.epsl.2014.08.037
0012-821X/© 2014 Elsevier B.V. All rights reserved.
under the vernacular term ‘Saint Martin Series’ (McCabe and Bannatyne, 1970; Simonds and McGee, 1979; Grieve, 2006). Shock
metamorphic effects in these rocks and minerals led McCabe and
Bannatyne (1970) to conclude that Lake Saint Martin is an impact structure, while Currie (1970) still considered it a volcanic
cryptoexplosion structure. Largely concealed by post-impact Middle Jurassic intra-crater red beds and evaporites of the Williston
Basin, up to ∼30 m of Pleistocene glacial drift and the waters of
Lake Saint Martin, the impact structure is considered one of the
best preserved larger complex terrestrial impact craters formed
in a layered, crystalline-sedimentary target. Post-impact evaporitic sediments have preserved large parts of the impact crater
and its impact breccia lens from erosion (Bannatyne and McCabe,
1984). However, due to the sedimentary cover surficial outcrop
of primary impactites is rather scarce. Numerous drillings since
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 concentrations of siderophile elements in the melt rocks (Göbel et al., 1980;
Palme, 1982; Reimold et al., 1990) suggested a differentiated projectile, possibly an achondrite.
38
M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48
Fig. 1. Landsat-5 satellite image of Lake Saint Martin in Manitoba, Canada (scene
path 032, row 024, acquired on 04 April 2010) and outline of the estimated outer
limits of the largely sediment- and water-covered ∼40 km-diameter impact structure according to Bannatyne and McCabe (1984). Impact melt rock from the eastern
crater moat and a partially melted Proterozoic granite from the central uplift (inset images; both rock specimens ∼10 cm in maximum length) were sampled for
40
Ar/39 Ar dating. Satellite image source: USGS.
1.2. Previous age determinations for the Lake Saint Martin impact
Stratigraphically, the Lake Saint Martin impact can be placed
between the Devonian and the Middle Jurassic. The youngest
shock-deformed sedimentary rocks most likely belong to the Devonian Ashern and Winnipegosis Formations of the northeastern
Williston Basin (e.g., Rosenthal, 1988); the evaporitic deposits of
the Amaranth Formation that overlie the impact structure are of
Middle Jurassic age (McCabe and Bannatyne, 1970). Previous dating of the Lake Saint Martin impact structure employed a wide
range of techniques but has not yielded any satisfactory age so
far. Early results yielded rather imprecise K/Ar apparent ages of
200 ± 25 and 250 ± 25 Ma (McCabe and Bannatyne, 1970) and
a Rb/Sr apparent age of 223 ± 32 Ma (Reimold et al., 1990; recalculated from 219 ± 32 Ma according to the revised 87 Rb decay
constant of Nebel et al., 2011). Despite the large error on the age
(28%) and the notorious problems associated with this technique
in dating impact craters (Jourdan et al., 2009), the Rb/Sr age of
Reimold et al. (1990) is currently accepted as the ‘official’ age
for the impact in most impact crater listings in the literature; a
slightly modified age value of 220 ± 32 Ma (as also given in Grieve,
2001) is listed in the Earth Impact Database (2014). Apatite and zircon fission track dating gave an age of 208 ± 14 Ma (Kohn et al.,
1995). Additional dating attempts using the low-temperature (U–
Th)/He technique on apatite and zircon separated from melt rocks
resulted in scattered ages with two apparent age peaks around
∼214 Ma and ∼234 Ma (Wartho et al., 2009; 2010). A strontium
and sulfur isotopic study of the post-impact evaporites favored an
age around the Devonian/Carboniferous boundary (∼360 Ma) for
the impact (Leybourne et al., 2007), at odds with the previous isotopic dating results. Clearly, despite several dating attempts using
different techniques, the age of the Lake Saint Martin impact still
remains poorly constrained.
1.3. Lake Saint Martin as a member of a giant terrestrial impact crater
chain?
Together with the ∼100 km Manicouagan impact structure in
Québec/Canada, the ∼9 km Red Wing Creek impact structure in
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 Tennessee/USA and the ∼3.2 km Newporte impact structure in North
Dakota/USA, the Lake Saint Martin impact structure has been considered 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 overlapping 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 occurred 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
in the scientific literature (Kent, 1998; Melosh, 1998; Walkden
et al., 2002; Tanner et al., 2004; Carporzen and Gilder, 2006;
Grieve, 2006; Zivkovic and Gosnold, 2007; Schmieder and Buchner, 2008; Schmieder et al., 2010, 2012; Jourdan et al., 2012) and
scientific reviews (VanDecar, 1998; Monastersky, 1998; Sankaran,
1999). This study presents a set of new 40 Ar/39 Ar ages for aliquots
of impact-melted K-feldspars and impact melt rocks from different
domains within the Lake Saint Martin impact structure. The new
isotopic ages will demonstrate that the Lake Saint Martin impact
occurred millions of years before the Manicouagan and Rochechouart impacts and that none of the impacts grouped together
into an alleged crater chain occurred synchronously.
2. Samples and analytical methods
2.1. Impact melt specimens and petrography
Whole-rock chips of impact melt rock from the crater moat
domain of the Lake Saint Martin impact structure, as well as
chips of impact-melted K-feldspars separated from the melt rock
and the impact-metamorphosed Precambrian basement granite exposed in the central uplift (Fig. 1) were used for laser step-heating
40
Ar/39 Ar dating. This approach was chosen to gain isotopic ages
from different types of impact melt lithologies representative of
different portions of the impact structure. We direct the reader
to Simonds and McGee (1979), Reimold et al. (1990) and Grieve
(2006) for summaries of the composition, petrography, and geochemistry of the impact melt rocks and breccias at Lake Saint
Martin. The brownish to reddish, fine-grained crystalline impact
melt rocks (Fig. 2A–B; typically containing ∼3.2–3.9 wt% K2 O;
Grieve, 2006) are moderately to intensely altered and have a matrix of plagioclase, alkali feldspar, pyroxene (augite), quartz, and
Fe–Ti oxides. Groundmass crystals are typically ∼20–100 μm in
crystal length. Larger grains of plagioclase (oligoclase and andesine) commonly exhibit a checkerboard texture, whereas shocked
quartz has been partially transformed into aggregates of ballen
quartz with marginal melting textures and reaction rims (compare Grieve, 2006). The impact-metamorphosed basement granite
(Fig. 2C–D) contains fluidal-vesicular domains of feldspar melt of
variable composition with vugs locally lined by adularia. Quartz
in the shocked granite mainly occurs in the form of ballen silica, and biotite flakes have been decomposed into microcrystalline
opaques. Representative specimens were analyzed using an optical
polarization microscope and a VEGA3 TESCAN scanning electron
microscope (SEM) equipped with an X-Max 50 silicon drift detector
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 University of Western Australia campus. Backscattered electron images
were acquired using an acceleration voltage of 15 kV.
2.2.
40
40
Ar/39 Ar dating
Ar/39 Ar laser step-heating experiments on impact melt rock
chips and separates of melted K-feldspar from the crushed rock
(grain size ∼500–800 μm in diameter) were carried out at the
Western Australian Argon Isotope Facility, Curtin University. We
performed analyses on two single-grain aliquots of impact-melted
K-feldspar extracted from a fluidal-vesicular impact melt rock recovered from the eastern sector of the impact crater moat (samples
LSM-1A and LSM-1B); three particles of melted K-feldspar from
the shocked and partially melted basement granite collected in the
central uplift domain (samples LSM-2A, LSM-2B and LSM-2C); one
whole-rock chip of a moderately altered, brownish impact melt
rock (sample LSM-3); and a whole-rock chip (∼200 μm in diameter) of an intensely altered, reddish melt rock (sample LSM-4),
which had been irradiated and analyzed in an earlier batch of
aliquots. The melt rock samples were leached in 5N HF for 2 minutes to remove alteration phases on the surface and along fissures
(e.g., Baksi, 2007).
The samples were loaded in multi-pit aluminum discs along
with the Fish Canyon sanidine (FCs, age: 28.294 ± 0.037 Ma;
Jourdan and Renne, 2007; Renne et al., 2011) used as a neutron fluence monitor, Cd-shielded in order to reduce isotopic interferences, and irradiated for 40 hours in the TRIGA (Denver)
nuclear reactor (central position, specific J-values given in TableAnnex 1). After irradiation, the samples were step-heated using
a 110 W Spectron Laser Systems instrument, with a continuous
Nd-YAG (IR; 1064 nm) laser rastered (1 minute; beam diameter
350 μm) over the single-grain aliquots, moved on an x/ y / z-mobile
sample stage. The gas was purified in a stainless steel extraction line using one GP50 and two AP10 SAES getters. Argon isotopes were measured in static mode using a MAP 215-50 mass
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. 40 Ar/39 Ar 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 corrected for mass discrimination, monitored using an automatic air
pipette, and provided mean values of 1.004293 ± 0.0025 per dalton
(atomic mass unit). The correction factors for interfering isotopes
were (39 Ar/37 Ar)Ca = 7.06 · 10−4 (±7%), (36 Ar/37 Ar)Ca = 2.81 · 10−4
(±3%) determined on CaF2 , as well as (40 Ar/39 Ar)K = 6.76 · 10−4
(±10%) and (38 Ar/39 Ar)K = 1.24 · 10−2 (±32%) measured on K–
Fe glass. Blanks were monitored every 3 to 4 steps and typical
40
Ar blanks range from 1 · 10−16 to 2 · 10−16 mol. All argon isotopic data are given in the supplementary files (Table-Annex 1).
40
Ar/39 Ar ages were calculated using the K decay constants of
Renne et al. (2010; 2011) and the LabView-environment Argus program by M.O. McWilliams, the ArArCALC software (Koppers, 2002),
and Isoplot 4.1 written by K.R. Ludwig (Berkeley Geochronology
Center). All errors in this study are ±2σ , unless otherwise noted.
A plateau is defined by 70–100%, a mini-plateau by 50–70% of the
total 39 Ar released in at least 5 consecutive steps with ages that
overlap within 2σ error limits, and define a probability ( P -value)
of ≥0.05 using a χ 2 (chi square) test (e.g., Baksi, 2007). Errors on
the age of the monitor and on the decay constants are combined
with the internal uncertainties and are provided in brackets (e.g.,
[1.0] Ma).
3.
40
Ar/39 Ar results and interpretation
3.1. Melt rock-hosted K-feldspar
40
Ar/39 Ar step-heating analysis of the impact-melted K-feldspar
LSM-1A yielded a well-defined plateau age of 226.7 ± 2.4 Ma (2σ ;
MSWD = 0.78; P = 0.66; 84% of 39 Ar released; Fig. 3A). This
aliquot did not develop a statistically representative isochron due
to a low spreading (S; Jourdan et al., 2009) value. Except for a single step, all data points in the inverse isochron diagram cluster
near the radiogenic axis (compare Schmieder and Jourdan, 2013)
and resulted in an age of 227.2 ± 1.5 Ma (2σ ; MSWD = 0.79;
P = 0.64; 40 Ar/36 Ar intercept = 288 ± 13; S-value = 31%; Fig. 3B).
The second K-feldspar melt aliquot, LSM-1B, yielded a mini-plateau
apparent age of 227 ± 5 Ma (2σ ; MSWD = 1.70; P = 0.15; 63%
of 39 Ar released; Fig. 3C) for the first five steps within a tildeshaped, disturbed age spectrum. The mini-plateau and total fusion
apparent ages of this sample are concordant within error with the
plateau age obtained for LSM-1A which might suggest that the
perturbed spectrum is an effect of 39 Ar recoil redistribution induced by the irradiation process (Table 1). The inverse isochron for
LSM-1B yielded an age of 228 ± 6 Ma (MSWD = 1.73; P = 0.16;
40
Ar/36 Ar intercept = 289 ± 20; S = 70%; Fig. 3D). The isochron
41
61
3.2. K-feldspar from central uplift granite
0.79
0.71
The aliquots of impact-melted K-feldspars separated from the
central uplift granite yielded a plateau age of 227.0 ± 1.2 Ma
(2σ ; MSWD = 1.00; P = 0.46; 84% of 39 Ar) for sample LSM-2A
(Fig. 4A). The inverse isochron for this aliquot, including degassing
step 1 (with a step apparent age concordant with the plateau
age), yielded a slightly sub-atmospheric 40 Ar/36 Ar intercept value
of 288 ± 8 and an isochron age of 227.9 ± 1.4 Ma (MSWD =
0.79; P = 0.71; S = 61%; Fig. 4B). The sub-atmospheric ratio might
be due to thermally activated isotopic fractionation upon melt
emplacement, or fluid circulation after the impact. Whatever the
cause for this effect, the isochron-derived age for sample LSM-2A,
directly accounting for the initial 40 Ar/36 Ar ratio below ∼298.6
(Lee et al., 2006), is here regarded as the more accurate age value
for sample LSM-2A (compare, e.g., Jourdan et al., 2008). Sample
LSM-2B gave a plateau age of 228.1 ± 1.4 Ma (2σ ; MSWD = 1.40;
P = 0.18; 73% of 39 Ar; Fig. 4C). Data points for this aliquot cluster
near the radiogenic axis and fail to provide an isochron (Fig. 4D).
A third feldspar particle picked from the granite, LSM-02C, yielded
a disturbed and tilde-shaped age spectrum from which no plateau
or mini-plateau age could be extracted (Fig. 4E). Individual steps
in the high-temperature extraction range gave older apparent ages
of up to ∼260 Ma, suggesting partial inheritance of 40 Ar∗ from an
incompletely degassed Precambrian precursor feldspar in this sample (e.g., Jourdan et al., 2007). No isochron result was obtained for
aliquot LSM-02C (Fig. 4F).
Isochron includes degassing step 1 not part of (but with a step apparent age concordant with) the age plateau section.
*
227.8 ± (0.9) [1.1] Ma
Weighted mean of 2 plateau
ages and 1 isochron age:
staircase-shaped spectrum (minimum age)
≥209
LSM-4
Intensely altered
(whole rock)
hump-shaped spectrum (minimum age)
≥220–230
Impact melt rock aliquots
LSM-3
Moderately altered
(whole rock)
(2σ ; MSWD = 0.52; P = 0.59)
– no isochron –
– no isochron –
19*
227.9 ± 1.4
– no isochron –
– no isochron –
84
73
227.0 ± 1.2
228.1 ± 1.4
– no plateau –
Partially melted
Precambrian granite,
central uplift
(single grain)
LSM-2A
LSM-2B
LSM-2C
18
11
Plateau
Plateau
1.00
1.38
0.46
0.18
288 ± 8
31
70
0.64
0.16
0.79
1.73
288 ± 13
289 ± 20
12
5
227.2 ± 1.5
228 ± 6
0.66
0.15
84
63
226.7 ± 2.4
227 ± 5
K-feldspar melt aliquots
LSM-1A
Impact melt rock,
LSM-1B
crater moat domain
(single grain)
12
5
Plateau
Miniplateau
0.78
1.70
n
Isochron age
(Ma, ±2σ )
Isochron characteristics
MSWD
Attribute
n
Total 39 Ar
released
(%)
Apparent age
(Ma, ±2σ )
Age spectrum characteristics
Ar/36 Ar intercept values for LSM-1A and LSM-1B are atmospheric
(∼298.6; Lee et al., 2006; Mark et al., 2011) within error.
P
40
Ar/36 Ar
intercept
(±2σ )
MSWD
P
Spreading
factor
(%)
40
Sample ID
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.
M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48
3.3. Impact melt rock
The moderately altered, brownish impact melt rock chip LSM-3
did not yield a plateau in a disturbed, tilde-shaped age spectrum, with younger step ages (∼180–200 Ma) in the low- and
high-temperature extraction range and older apparent ages of
∼220–230 Ma in the mid-temperature steps (Fig. 5A). This age
spectrum is similar to some of the previous age spectra obtained
on impact melt rocks from Lake Saint Martin (Schmieder et al.,
2012; an unpublished age spectrum for a drill core-derived melt
rock sample, acquired with W.H. Schwarz, M. Trieloff and E. Buchner at the Ar laboratory at the University of Heidelberg, Germany)
and other spectra for terrestrial impact structures in which humpand tilde-shapes have been attributed to alteration effects, such as
the Dhala impact structure in India (Pati et al., 2010) or the Söderfjärden impact structure in Finland (Schmieder et al., 2014). When
alteration is indeed responsible for hump- and tilde-shaped spectra, the maxima of such age spectra must be strictly considered
a minimum age for the formation of the melt rock (e.g., Buchner
et al., 2009; Jourdan, 2012). We note that the apparent minimum
age for melt rock sample LSM-3 is notably close to the statistically well-defined plateau and isochron ages for the melt rockand granite-derived K-feldspar melt aliquots. The K/Ca ratio for
LSM-3 gave values between ∼1 and ∼3 suggesting no major crystallization of secondary K2 O-rich minerals. No isochron could be
produced for this altered impact melt rock (Fig. 5B).
In contrast to the moderately altered melt rock specimen
LSM-3, the intensely altered melt rock sample LSM-4 yielded
a distinct staircase-shaped age spectrum. Step apparent ages in
the low-temperature extraction range of this spectrum lie around
∼140 Ma and gradually increase until they reach a step-age maximum at ∼209 Ma before the age values drop again (Fig. 5C). We
note that the minimum age derived from the oldest apparent step
ages of this age spectrum is ∼15–20 Myr younger than the minimum 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. 40 Ar/39 Ar 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. 40 Ar/39 Ar 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 abundant secondary K2 O-rich minerals relative to sample LSM-3. Such
an age spectrum likely reflects a mixture between Ar loss and the
recrystallization of alteration minerals. As for LSM-3, no isochron
was obtained for melt rock sample LSM-4 (Fig. 5D).
A summary of the 40 Ar/39 Ar ages and associated statistics for
all melt samples from the Lake Saint Martin impact structure is
presented in Table 1.
4. Discussion
4.1. Age of the Lake Saint Martin impact
This study presents the first systematic set of 40 Ar/39 Ar ages for
the well-preserved Lake Saint Martin impact structure, obtained
from different mineral-melt and whole-rock impact melt specimens. It thereby improves upon earlier dating attempts that resulted in imprecise (and statistically unverifiable) whole-rock K/Ar
and Rb/Sr ages (cf. discussion in Jourdan et al., 2009 and Jourdan,
2012 about the suitability of the Rb/Sr method for the dating of
impact events). The more recent Rb/Sr apparent age of 219 [223]
±32 Ma obtained for impact melt rocks by Reimold et al. (1990)
has been considered the most precise and accurate age for the
Lake Saint Martin impact for more than two decades; however, the
rather low precision of this result, along with the relevance of this
impact structure for a potential giant crater chain, warranted further investigations. Here we present a set of concordant plateau,
mini-plateau, and inverse isochron ages obtained for the impactmelted K-feldspars separated from impact melt rock and the partially melted central uplift granite. A combination of two plateau
ages (aliquots LSM-1A and LSM-2B) and one inverse isochron age
(LSM-2A; taking into account the sub-atmospheric 40 Ar/36 Ar intercept of 288 ± 8) yields a statistically compelling weighted mean
age of 227.8 ± 0.9 Ma [±1.1 Ma] (2σ ; MSWD = 0.52; P = 0.59;
the number in brackets includes the full external error). This early
Late Triassic mean age is considered as the most precise and accurate age currently available for the Lake Saint Martin impact. As
mini-plateau ages are potentially inaccurate (e.g., Jourdan et al.,
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M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48
Fig. 6. Summary of measured 40 Ar/39 Ar 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 40 Ar/39 Ar result for the impactmelted K-feldspar LSM-1B in the final age calculation; however,
this mini-plateau age is within error also equivalent with the
plateau and isochron ages obtained on the other K-feldspar melt
aliquots. The internal consistency of the 40 Ar/39 Ar dating results
and the independent reproduction of coeval apparent ages from
different types of impact melt lithologies gained from two different
sample localities across the impact structure provides good confidence that these matching ages accurately reflect the Lake Saint
Martin impact. With a relative error on the age of only ±0.4% (all
uncertainties included), Lake Saint Martin now represents one of
the most precisely dated impact structures on Earth (compare impact crater ages in the reviews by Jourdan et al., 2009, 2012 and
references therein). A summary of all measured 40 Ar/39 Ar plateau,
mini-plateau and inverse isochron ages, along with the recommended age for the impact, is presented in Fig. 6. Further dating
of impact melt samples from natural outcrop and drill cores into
the impact structure will be desirable to create an even more extended isotopic data set for the Lake Saint Martin impact in the
future (compare, e.g., Buchner et al., 2010).
According to the latest International Chronostratigraphic Chart
published by the International Commission on Stratigraphy (Cohen
et al., 2013), the new 40 Ar/39 Ar age of 227.8 ± [1.1] Ma for the Late
Triassic Lake Saint Martin impact corresponds within error to the
∼227 Ma Carnian/Norian boundary. However, Lucas et al. (2012)
pointed out that the base of the Norian is probably several million
years younger and lies around ∼220 Ma based on an extensive set
of radioisotopic data. This younger age would place the Lake Saint
Martin impact well within the Carnian (lasting from ∼237 Ma to
∼220 Ma). The strong reverse magnetization in the Lake Saint
Martin melt rocks (Coles and Clark, 1982) is in agreement with
a number of reversed magnetochrons (within a range of chrons
E1r to E13r) during the Carnian time stage (Kent et al., 1995;
Lucas et al., 2012).
4.2. Implications for rock alteration effects and low-temperature
geochronometers
The systematic study of a set of different impact melt samples
from Lake Saint Martin helps with the interpretation and understanding of ‘alteration age’ spectra and minimum ages derived
from 40 Ar/39 Ar step-heating analysis, in particular bell-shaped
and/or tilde-shaped age spectra. The plateau and isochron ages for
the Lake Saint Martin impact are within the range of the ‘minimum age’ of ∼220–230 Ma for the impact melt rock sample
LSM-3 and results from Schmieder et al. (2012) who reported a
similar minimum age of ∼225 Ma for an additional impact melt
rock specimen. In this case, the minimum age for the moderately
altered impact melt rock sample LSM-3 closely approaches the
statistically more robust plateau and isochron ages. In contrast,
the intensely altered impact melt rock LSM-4 yielded apparent
step ages no older than ∼209 Ma, which are still notably younger
than the age of ∼228 Ma determined for the impact in this study.
Such alteration age spectra are generally difficult to interpret unless more robust isotopic ages (e.g., a set of concordant 40 Ar/39 Ar
plateau ages from fresh or less altered impact melt specimens; or
statistically compelling U/Pb ages from melt-grown zircons) and
stratigraphic age constraints are available. Minimum impact ages
inferred from step-age maxima in 40 Ar/39 Ar age spectra can be
considerably – in cases up to several hundred Myr – younger than
the real formation age of an impact melt rock. This effect is exemplified by the ≥1.7 Ga Dhala impact structure in India that
produced minimum apparent 40 Ar/39 Ar ages of ∼1.0 Ga, whereas
field geologic relationships indicate a much older, Paleoproterozoic,
minimum age for the impact (Pati et al., 2010).
The concordant plateau, mini-plateau and inverse isochron
40
Ar/39 Ar ages obtained in this study, moreover, critically question
the indiscriminate use of age data obtained from low-temperature
geochronometers, such as the (U–Th)/He dating method (e.g.,
Farley, 2002; Reiners, 2005) that records thermal events at closure temperatures of ∼200 ◦ C for zircon and as low as ∼60 ◦ C for
apatite. None of the previous (U–Th)/He age peaks at ∼180 Ma,
∼214 Ma and ∼234 Ma for zircon and apatite grains recovered
from impact melt rocks from Lake Saint Martin (Wartho et al.,
2009, 2010) have reliably identified the age of the impact within
error. According to Kohn et al. (1995), rocks in the Williston Basin
have been overprinted by an Eocene thermal event that caused
the partial resetting of low-temperature geochronometers, such
as apatite fission track and (U–Th)/He. The cooling of larger impact craters and long-lived hydrothermal activity may have an
additional effect on isotopic systems with low closure temperatures. Recent 40 Ar/39 Ar results for the ∼23 km Lappajärvi impact
structure in predominantly crystalline rocks of Finland suggested
comparatively slow crater cooling over ∼0.6–1.6 Myr (Schmieder
and Jourdan, 2013). However, because of the higher amounts of
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.,
2002), the Lake Saint Martin crater may have cooled considerably
faster. This ‘crater cooling lag’ would not explain, for example,
the apparent zircon and apatite (U–Th)/He age clusters around
∼214 Ma and ∼180 Ma, respectively. So far, the (U–Th)/He technique has been applied to twelve impact structures (e.g., Wartho
et al., 2012a), from which only one, the ∼7 km Wetumpka impact structure in Alabama/USA (Wartho et al., 2012b), yielded a
subset of apparent ages that are consistent within uncertainty
with biostratigraphic ages. Our understanding of the applicability
of the (U–Th)/He technique to impact crater dating is currently
at an early stage. Inaccurate results prompt us to advise caution in using this technique to date impact events without any
robust age constraints. Extensive testing and calibration of the
method against well-dated (impact melt-hosted) mineral specimens is required until we fully understand what constitutes a
‘valid’ (U–Th)/He impact crater age, especially with respect to the
potential effects of shock metamorphism on the retention and
diffusion of He in zircon (e.g., van Soest et al., 2010). Some of
the previous results were compromised and of doubtful geological significance, either reflecting impact-induced crustal uplift and
slow crater cooling (Biren et al., 2012), post-impact thermal heating and resetting not related to the impact (Wartho et al., 2010;
van Soest et al., 2011), or the proposed impact age was based on
statistically invalid dating results (e.g., Young et al., 2013).
4.3. A geochronologic appraisal of the Late Triassic multiple impact
hypothesis
To reliably assess the Late Triassic multiple impact theory
proposed by Spray et al. (1998), according to which several impact events occurred virtually synchronously, narrow error ranges
associated with the individual impact ages are crucial (see impact timing scheme in Fig. 7). At least two of the impacts involved in the postulated scenario, Rochechouart in France and
Obolon in Ukraine, were previously dismissed based on their nonsynchronous ages (Schmieder and Buchner, 2008; Schmieder et
al., 2010). The age of the ∼100 km Manicouagan impact structure in Québec has been precisely determined at 215.56 ± 0.05 Ma
(Ramezani et al., 2005) and 214 ± 1 Ma (Hodych and Dunning, 1992) using U/Pb dating of impact melt-grown zircons. The
∼40 km Rochechouart impact structure in France was previously
dated at an 40 Ar/39 Ar age of 214 ± 8 Ma (Kelley and Spray, 1997),
obtained from UV laser ablation spots in the matrix of meltbearing pseudotachylitic breccia veins. However, these melt veins
are known to contain considerable amounts of undigested Precambrian to Paleozoic feldspar clasts and hydrothermal fillings
(Reimold et al., 1987); moreover, spot fusion apparent ages within
a range between ∼230 Ma and ∼1.7 Ga and an isochron 40 Ar/36 Ar
intercept value of >500 (Kelley and Spray, 1997) indicated contamination by extraneous argon. The age for the Rochechouart
impact was recently revised to [202.8 ± 2.2 Ma] (Schmieder et
al., 2010; recalculated) obtained from two 40 Ar/39 Ar plateau ages
for K-feldspar from an impact-metamorphosed gneiss. The other
candidate craters for the Triassic ‘crater chain’ have only been
imprecisely dated. The ∼20 km marine Obolon impact structure
in Ukraine (Gurov et al., 2009) had earlier been estimated at a
stratigraphic age of 215 ± 25 Ma (Masaitis et al., 1980; Spray et
al., 1998), but K/Ar dating of the Obolon impact glass and melt
rock gave a Jurassic apparent age of 169 ± 7 Ma (Masaitis, 1999;
Valter et al., 1999; Gurov et al., 2009). A simple paleogeographic
test further supports a Jurassic maximum age for the Obolon impact structure; only from the Early Jurassic onward did the paleoenvironmental conditions in the Donets Graben of the Ukrainian
Shield change from continental sedimentation into a marine environment and provided the setting for the submarine Obolon
impact (Schmieder and Buchner, 2008). The age of the ∼9 km
Red Wing Creek impact structure in the Williston Basin of North
Dakota is stratigraphically constrained to roughly ∼200–220 Ma
(Koeberl et al., 1996). Other loose stratigraphic age estimates
for Red Wing Creek are 200 ± 25 Ma (Gerhard et al., 1982) and
200 ± 5 Ma (Grieve, 1991), however, no fresh impact melt lithologies are known from this impact site that could be used for more
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
et al., 1998), the ∼3 km Newporte impact structure in North
Dakota has a Cambrian stratigraphic age of ∼500 Ma (Koeberl and
Reimold, 1995), which is in conflict with the Late Triassic multiple impact scenario. The age of the ∼12 km Wells Creek impact
structure in Tennessee is poorly known and given as 200 ± 100 Ma
(Grieve, 2001).
From a geochronologic perspective, the refined 40 Ar/39 Ar age
of ∼228 Ma for Lake Saint Martin demonstrates that the Obolon,
Rochechouart, Manicouagan and Lake Saint Martin impact events
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
any potential connection between the postulated large-scale multiimpact scenario and extinction events during the Triassic (e.g.,
Benton, 1988; Hallam and Wignall, 1997; Sephton et al., 2002;
Tanner et al., 2004).
4.4. Geochemical constraints on impacting bodies
In addition to the time constraints gained from isotopic dating, the compositions of the respective impactors can be used as
an additional argument for or against a cosmic combination strike.
We refer to Goderis et al. (2012), Koeberl et al. (2012) and Koeberl
(2014) for recent reviews on the identification of meteorite signatures in impactites. The proposed Late Triassic multiple impacts
should, thus, also be appraised with regards to their potential impactor signatures. No conclusive impactor traces, with siderophile
elements below detection limit, could be gained from the impact
melt rocks at Manicouagan (Palme et al., 1978) and Lake Saint
Martin (Göbel et al., 1980; Palme, 1982; Reimold et al., 1990),
which might suggest a differentiated asteroid as the parent body
for achondritic projectiles. In contrast, the Rochechouart impact
structure in France (Koeberl et al., 2007; Tagle et al., 2009) and the
Obolon impact structure in Ukraine (Valter and Ryabenko, 1977;
Masaitis, 1999) carry siderophile element patterns that favor the
impact of iron meteorites. The meteorite type for the Red Wing
Creek impact structure is uncertain, but Koeberl et al. (1996) found
a possible chondritic contamination pattern in impact breccias recovered from drill cores.
Despite this apparent difference in impacting bodies for the
Manicouagan, Lake Saint Martin, Rochechouart, Obolon, and Red
Wing Creek impact structures, one could argue that double and
multiple meteorite impacts could in fact involve a mixture of materials. One such example, yet on a much smaller scale, is the
fall of the Almahata Sitta meteorite in northern Sudan on October 7, 2008, which comprised a variety of chondritic and achondritic (ureilitic) lithologies (Bischoff et al., 2010). We acknowledge
that asteroids commonly represent ‘rubble piles’ (Richardson et al.,
1998, 2002), such as asteroid 25 143 Itokawa (e.g. Fujiwara et al.,
2006), and can hence be composed of different types of materials. Moreover, a larger asteroid can capture a smaller asteroid in
space and thereby generate a binary asteroid system (such as, e.g.,
asteroid 243 Ida and its satellite Dactyl; Chapman et al., 1995)
of two bodies of different composition, fragments of which may
eventually impact Earth. It seems somewhat unlikely that the incoming fragments of a tidally disrupted larger rubble pile asteroid
or comet, such as Shoemaker–Levy 9 (e.g., Shoemaker et al., 1993;
Crawford et al., 1994), would be heterogeneous to the extent that
they generate a chain of impact craters in which each one leaves
a different geochemical fingerprint; however, unless this type of
scenario (i.e., impact of variable-type cosmic rubble) is proven impossible in terms of celestial mechanics, one should consider the
theoretical possibility that a geochemically heterogeneous impact
system can occur over geologic time. On the other hand, impact melt lithologies at terrestrial impact structures can lack a
clear and representative meteoritic signature (e.g., Goderis et al.,
2010), for example due to the incomplete homogenization of the
impactor-derived elements and the local impact melt, fractionation of the meteoritic signature elements (e.g., Palme et al., 1979;
Palme, 1982), differentiation of larger impact melt bodies (e.g.,
O’Connell-Cooper and Spray, 2011), as well as post-impact alteration. For the given uncertainties on the exact composition of
the meteorites that generated the Lake Saint Martin, Manicouagan, Rochechouart, Obolon and Red Wing Creek impact structures
we stress that the geochemical constraints cannot serve as reliable
evidence for or against a multiple impact scenario.
4.5. Statistical considerations
Finally, the statistical probability for a multiple impact event
on Earth in the Late Triassic should be briefly evaluated based on
the timing and paleopositions of the impact events in plate reconstructions for North America (Laurentia) and Eurasia (see Spray
et al., 1998; Walkden et al., 2002). Using the estimated paleolatitudes and -longitudes of the impact craters as input parameters in
Monte Carlo simulations, Spray et al. (1998) calculated a low probability of P < 0.08 (<0.008 if the longitudinal range is included)
for the event that the linear arrangement of the three ‘main’ E–Waligned craters, Rochechouart, Manicouagan, and Lake Saint Martin,
is a random array. The probability for an aligned random strike
decreases even further for five impact structures (i.e., including
Obolon and Red Wing on great circles; however, no statistical
probability was given for such an event). As the crater alignment
was very unlikely a result of random chance and because the imprecise age constraints for the craters agreed well within this calculation, Spray et al. (1998) concluded that the five craters may
represent a chain of impact structures probably formed ‘within
hours’.
The general expectation that linear arrays of apparently coeval
impacts constitute a combination strike, as supported by statistical
modeling, may be in analogy to some of the binary impact systems on Earth, including the proposed doublet craters of the West
and East Clearwater Lake impact structures in Canada and the Suvasvesi North and South impact structures in Finland. Despite very
low calculated likelihoods for their random origin in close proximity to the respective fellow crater (Miljković et al., 2013), differing
40
Ar/39 Ar dating results for impact melt rocks from each of these
impact structures rather suggest that the impacts occurred millions
of years apart (see Schmieder et al., 2014 for a discussion and references therein for earlier dating results).
Based on the size-frequency distribution of Earth-crossing asteroids and their encounter frequency with the Earth–Moon system,
Bottke et al. (1997) estimated that the production rate for impact
crater chains on Earth after S-type (Shoemaker–Levy 9) tidal disruption is less than 0.1 crater chains in 3.8 Gyr. While we acknowledge that these statistics are derived from numerical modeling,
the results by Bottke et al. (1997) suggest a very low chance of
producing a terrestrial impact crater chain in younger geologic history. We recommend that proposed terrestrial impact crater chains
and doublets should be systematically accompanied by isotope dating. Even more delicate is the case of extraterrestrial impact crater
chains and doublets, for which no absolute ages can be obtained
without sample return missions. Although crater counting (e.g.,
Neukum et al., 2001) helps to some extent, this study shows that
fortuitously aligned, yet unrelated impact craters may also occur
on asteroids and other planetary bodies in the Solar System.
5. Conclusions
The ∼40 km Lake Saint Martin impact structure in Manitoba,
Canada, has been 40 Ar/39 Ar-dated at a Carnian age of 227.8 ±
M. Schmieder et al. / Earth and Planetary Science Letters 406 (2014) 37–48
0.9 Ma [±1.1 Ma] (2σ ; MSWD = 0.52; P = 0.59) by combining two plateau ages and one isochron age obtained on different aliquots of impact-melted K-feldspar. With a relative error of
±0.4% on the isotopic age, the Lake Saint Martin impact structure has advanced to one of the most precisely dated larger impact
structures on Earth. The new dating results demonstrate that none
of the impacts that constituted the postulated Late Triassic multiple impact event were synchronous. Moreover, our data suggest
that high-temperature isotopic chronometers are currently the preferred means of dating impact events. Where possible, statistical
model results arguing that linear arrays of multiple impact craters,
as well as apparent crater pairs, were most likely generated by
synchronous impacts should be assessed in the light of precise and
accurate isotopic dating results.
Acknowledgements
We thank Celia Mayers and Adam Frew, Curtin University, for
their helpful support during sample preparation, Ruth Bezys for
the earlier supply of some of the sample materials used, and Rebecca Wilks for review of an earlier draft of this manuscript. The
authors acknowledge the facilities, and the scientific and technical
assistance of the Australian Microscopy & Microanalysis Research
Facility at the Centre for Microscopy, Characterisation & Analysis, The University of Western Australia, a facility funded by the
University, State and Commonwealth Governments. We thank the
reviewers Vera A. Fernandes and Wolf Uwe Reimold for their constructive comments that helped improve our manuscript, as well
as the Associate Editor Christophe Sotin for careful handling. Sample collection was supported by a CARN grant from the Canadian
Space Agency to EC. This project is supported by the ARC Discovery
grant DP110104818 – “Consequences of extraterrestrial impacts on
the biosphere and geosphere” to FJ and ET.
Appendix A. Supplementary material
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2014.08.037.
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