New 40Ar/39Ar and K–Ar ages of the Viluy traps (Eastern Siberia

Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
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Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo
New 40Ar/39Ar and K–Ar ages of the Viluy traps (Eastern Siberia):
Further evidence for a relationship with the Frasnian–Famennian
mass extinction
J. Ricci a,b,⁎, X. Quidelleur a,b, V. Pavlov c, S. Orlov c, A. Shatsillo c, V. Courtillot d,e
a
Univ Paris-Sud, Laboratoire IDES, UMR8148, Orsay F-91405, France
CNRS, Orsay F-91405, France
c
Inst. Phys. Earth, Moscow 123995, Russia
d
Equipe de Paléomagnétisme, Institut de Physique du Globe, UMR 7154, Sorbonne Paris Cité, F-75005 Paris, France
e
Sciences de la Terre, de l'Environnement et des Planètes, Université Paris Diderot, Sorbonne Paris Cité, F-75013 Paris, France
b
a r t i c l e
i n f o
Article history:
Received 20 March 2013
Received in revised form 11 June 2013
Accepted 13 June 2013
Available online 25 June 2013
Keywords:
Viluy traps
40
Ar/39Ar dating
K–Ar dating
Frasnian–Famennian
Mass extinction
End-Devonian
a b s t r a c t
We present here new ages, combining both K–Ar and 40Ar/39Ar techniques, reinforcing the possible link between
the Viluy traps (eastern Siberia) with the Late-Devonian mass extinctions. The Viluy traps (also referred to as the
Yakutsk large igneous province) are associated with the breakup of the eastern margin of the Siberian platform as
a triple-junction rift system, with volcanic outcropping along the Lena, Markha and Viluy rivers and an initial volume of volcanic products of about one million cubic kilometers. K–Ar ages obtained on seven samples from
the Viluy rift display values ranging from 366 ± 5 to 381 ± 5 Ma, with a mean age of 374 ± 5 Ma. Our new
40
Ar/39Ar results display well-defined step-heating plateau ages ranging from 362.3 ± 3.4 to 379.7 ± 3.5 Ma.
Although most K–Ar and 40Ar/39Ar ages agree within about 1% uncertainty, which reinforces the validity of the
K–Ar ages obtained with the Cassignol and Gillot unspiked technique (even in the Paleozoic), a slightly clearer
scenario appears when only the 40Ar/39Ar ages are considered. Effectively, they cluster in two different groups
with weighted mean ages of 364.4 ± 1.7 and 376.7 ± 1.7 Ma. These ages support a multi-phases emplacement
of the Viluy traps, as is the case for many continental flood basalts. Because the age of the first volcanic phase is
undistinguishable from recent determinations of the age of the Frasnian–Famennian boundary, our results further strengthen the hypothesis of a causal relationship between the two events. The second phase of emplacement of the Viluy traps occurred at the end of the Devonian, apparently slightly before the Devonian–
Carboniferous boundary. Finally, except for the Ordovician–Silurian extinction which remains to be associated
with a volcanic province, all major Phanerozoic mass extinctions have been correlated with the emplacement
of a CFB, thereby reinforcing the possible causal link between these events.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Among the several causes proposed to explain the main mass
extinctions observed throughout the Phanerozoic, the emplacement of
large igneous provinces (LIP) is one of the best candidates. Continental
flood basalts (CFB), which are on-land volcanic constructions with surface extent from ~1 up to ~10 million square kilometers and original
volumes between 1 and 10 million cubic kilometers, have been correlated with main oceanic anoxia events and mass extinctions back to
the end-Guadalupian (Courtillot and Renne, 2003). A detailed causal
relationship model between volcanism and environmental effects has
been proposed for the Siberian traps (Wignall, 2001) and can be
⁎ Corresponding author at: UMR 8148 CNRS IDES, Bât. 504, Dpt Sciences de la Terre,
Université Paris Sud, 91405 Orsay Cedex, France. Tel.: +33 1 69 15 67 71.
E-mail address: [email protected] (J. Ricci).
0031-0182/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.palaeo.2013.06.020
extended to most CFB, providing their volume is sufficient (over a million cubic kilometer), and their emplacement is of short duration and
marked by high intensity phases.
Three out of the five most important events of the Phanerozoic,
the Cretaceous–Tertiary boundary (KTB), the Triassic–Jurassic and
the Permo-Triassic boundaries, are now strongly associated with the
emplacement of the Deccan, Central Atlantic Magmatic Province
(CAMP) and Siberian traps, respectively (Courtillot and Renne, 2003).
Based on paleomagnetic data and a few K–Ar and 40Ar/39Ar ages, it
has recently been suggested that this scenario can be extended back
to the late Devonian, another one of the five main mass extinctions,
with the emplacement of the Viluy traps (eastern Siberia) at the time
of the Frasnian–Famennian (FF) boundary (Kravchinsky et al., 2002;
Courtillot et al., 2010). In addition, Kravchinsky (2012) suggested that
the correlation between LIPs of Northern Eurasia and mass extinction
could be valid throughout the Paleozoic, although high precision radiometric ages are still needed to better support any causal relationship.
532
J. Ricci et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
Documented by House (1971, 1975), the Frasnian–Famennian mass
extinction has mainly affected the sea ecosystem causing the disappearance of more than 75% of marine species (McGhee, 1982, 1996; Elewa,
2008; Elewa and Rhawn, 2009). Many contradictory scenarios, involving either major marine regression or transgression episodes, a large
erosional event, or the massive development of terrestrial plants have
been proposed to explain this crisis (e.g., Buggisch, 1991; Joachimski
and Buggisch, 1993; Becker and House, 1994; Algeo et al., 1995). Notably, these models must be compatible with the occurrence of a large
marine anoxia deposit identified in many sections just below the FF
boundary and referred to as the Kellwasser anoxic event (Buggisch,
1991). The hypothesis of a large meteorite impact is not supported in
the absence of any iridium anomaly or any other marker at the FF
boundary, despite a 40Ar/39Ar age close to the FF obtained for the, rather
small, Siljan impact crater (Sweden; Henkel and Aaro, 2005). A volcanic
event occurring around the FF and having strong links to it remains the
most plausible trigger mechanism to date (Racki, 1999). The age of the
FF remains debated and recent values between 372.0 ± 1.6 and
376.1 ± 3.6 Ma have been proposed in the past few years (Kaufmann,
2006; Gradstein et al., 2012).
Because in the exploratory study of Courtillot et al. (2010), only one
40
Ar/39Ar plateau age was considered as reliable by the authors, the objective of the present study is to better constrain the timing of the Viluy
traps in order to better assess its possible relationship with the FF mass
extinction event. For that purpose, K–Ar and 40Ar/39Ar dating have been
performed on samples distributed in the southern part of the Viluy rift.
a)
96°
The Viluy (or Vilyui) traps, also referred as the Yakutsk LIP, are
located in the northeastern part of the Siberian platform (Fig. 1). The
Viluy LIP has been associated with late-Devonian breakup initiation
on the eastern margin of the Siberian Craton (Ernst and Buchan,
1997) that changed to a compressional setting in early Carboniferous.
The Viluy rift is the south-western branch of a triple-junction rift
system. The two others rift branches form the modern margin of the
Siberian platform (Fig. 1; Zonenshain et al., 1991; Kiselev et al., 2006).
Presently, the Viluy rift is 800 km long in the northeast direction,
with a width of 450 km (Gaiduk, 1987; Kiselev et al., 2006), and dips
below the Verkhoyansk folded belt (Kuzmin et al., 2010). Viluy volcanics can be found within and outside of the rift. They outcrop along
the Lena, Markha and Viluy rivers, along the rift margin faults, and on
the slopes of the Aldan and Anabar shields (Kravchinsky et al., 2002).
The intrusive rocks include dykes, sills and layered basalt breccias. Outcrops of basalt are often intercalated with ash and tuff. Generally, the
lavas are composed of olivine- and plagioclase-bearing basalts. A significant thickness of sediment, up to 8 km, covers the volcanic products
and sediments of late Proterozoic and early Paleozoic to Silurian age.
The rift is bounded by two large NE–SW trending dike swarms that extend along 700 km. The total volume of volcanic material of the Viluy
rift is presently estimated to be more than 300 × 103 km3. Extrapolating this volume to the other branches of the rift, the initial volume is estimated to have been one million cubic kilometers (Kiselev et al., 2006).
b)
102° 108° 114° 120° 126° 132° 138° 144°
Taymir fold belt
2. Geological setting
108°
114°
120°
Laptev sea
64°
72°
u
Vil
ive
yr
Vil-6a
r
a
Verkhoy
Anabar shield
68°
y
lu
Mirniy
Vi
Vil-13
Kolyma
blocks
t
rif
leo
pa
nsk
belt
rif
t
TL-18a
leo
pa
r
60°
a
Len
er
a riv
belt
ded
Pat
fol
om
l-
a
aik
Bolsh
Len
rive
B
TO-29 TO-35
Aldan
a
Vi
Mirniy
TL-20a
TO-41a
TL-19a
60°
luy
ar
64°
Ch
ed
r
tom
fold
rive
oy Pa
Vilu
y
TO-9a
shield
Upper Permian – Lower Triassic basaltic sills
Precambrian rocks
Upper Permian – Lower Triassic basaltic lavas
and tuffs
Upper Devonian – Lower Carboniferous traps
Viluy paleorift zone under sedimentary cover
Edge of Siberian platform
Mesozoic sedimentary cover
Studied site
Paleozoic sedimentary cover
Sampling site of Courtillot et al. (2010)
Rejected site (see text)
Fig. 1. a) General structure of the Siberian and Viluy traps in Siberia. The inside frame shows the location of panel b. b) Location of the samples studied here is shown with closed red
circles, while sites of rejected samples are located with red open circles. Drill site from Courtillot et al. (2010) is shown with the black triangle.
J. Ricci et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
Much of this material has most likely been eroded away or is still covered by the Siberian Traps to the West and by a thick cover of Mesozoic
sediments to the East. Lavas dominate in the lower part of the Devonian
rift sequence, and occur also in higher horizons, demonstrating the
multi-phase character of volcanism.
The timing of eruptions of these traps was estimated to be late
Devonian–early Carboniferous based on position of some effusive layers
bracketed by middle Paleozoic sedimentary strata (Masaitis et al.,
1975). Although many K–Ar whole-rock analyses were performed
in the framework of geologic mapping and studies of the eastern Siberia
craton (e.g., Nenashev, 1970; Masaitis et al., 1975; Shpount and
Oleinikov, 1987), they cannot be considered here as they lack description of analytical procedures, precise sampling location, and ages often
show stratigraphic incoherence. Although based on only two samples,
a recent study provided the first high-resolution ages and suggested
the occurrence of two phases of activity around 370 and 360 Ma
(Courtillot et al., 2010).
3. Sampling and mineralogical preparation
About 20 samples were taken throughout the southern part of the
Viluy rift (see Supp. Mat. 1 for geographic location of all samples), but
about half of them were discarded because evidence for weathering
was revealed by the petrologic examination of thin sections (Supp.
Mat. 2). The nine samples selected for analyses have been collected
along the Lena and Viluy rivers (Fig. 1). Seven come from sills located
to the north of the “Baikal-Patom fold arc”. Sample TO-9a has been
taken along the Chara river, samples TO-29, TO-35 and TO-41a on the
edge of Bolshoy Patom river, and samples TL-18a, TL-19 and TL-20
along the Lena river. The other two samples, Vil-6a and Vil-13a, were collected further north, from basaltic lava flows in the western part of the
Viluy depression (Fig. 1). Each selected sample has been dated using
the K–Ar technique, and eight were also analyzed by the 40Ar/39Ar
technique.
Since the groundmass glass fraction is likely to have been affected
by K and Ar loss and hence cannot be used for such old rocks, we have
selected plagioclase phenocrysts, i.e. the remaining phase in which K
is concentrated. Samples were crushed in the 125–250 μm size range
and ultrasonically cleaned in a 10% nitric acid solution. In order to remove lighter minerals, possibly affected by weathering, and to obtain
a very homogeneous fraction, we have used heavy liquids in a narrow
density interval in order to remove heavy and light aggregates, which
include mafic minerals and/or microcrysts. Finally, a magnetic separator was used to remove plagioclase phenocrysts with titanomagnetite
inclusions or with remaining glass.
533
Because both plagioclase phenocrysts and microcrysts from sample
Vil-13a were suitable for dating, a double step preparation was applied.
It consisted of, first, crushing, 0.5–1 mm sieving and heavy liquid
separation to isolate plagioclase phenocrysts, second, crushing the
remaining heavy fraction, sieving in a small size fraction (80–120 μm),
and heavy liquids separation again to isolate plagioclase microcrysts.
4. Analytical techniques
4.1. K–Ar procedure
The K–Ar analyses were performed in the IDES geochronology laboratory (Université Paris-Sud). Our aim was to check the coherency of
both K–Ar and 40Ar/39Ar techniques, which could reinforce the suitability of our (and earlier) K–Ar data, as well as demonstrate that the 40Ar/
39
Ar ages are not biased by any irradiation-induced effect. Potassium
content was measured by flame absorption spectrometry and was compared with reference values for the MDO-G (Gillot et al., 1992) and the
BCR-2 (USGS Standard) standards. The relative uncertainty attached to
this measure is 1%. Then, argon was measured with a mass spectrometer following the K–Ar method, as described by Gillot and Cornette
(1986). This procedure is based on the comparison of argon isotopes
36 and 40 from atmospheric and sample aliquots. Because the mass
spectrometer has been designed and its electronic parameters are set
for that purpose, the signals remain very stable throughout data acquisition, with 40Ar and 36Ar varying with a standard deviation of about
only 0.01% and 0.05% respectively, during a typical run. Such signals stability allows us to use the 40Ar signal of a 0.1 cm3 air aliquot, without the
need for any 38Ar spike (Gillot and Cornette, 1986): this air pipette is
calibrated by routine measurements of the GL-O (Odin et al., 1982)
and HD-B1 (Fuhrmann et al., 1987) standards. The former is characterized by a value of 6.679 × 1014 atoms of radiogenic 40Ar/g, while the age
of 24.2 Ma (with K = 7.995%) is used for the latter (Hess and Lippolt,
1994; Schwarz and Trieloff, 2007). Note that for each determination,
the total relative age uncertainty is 1.4% and is calculated as the
quadratic sum of the 1% uncertainties attached to both the K content
determination and the 40Ar calibration, the radiogenic argon content
uncertainty being here negligible.
4.2. 40Ar/39Ar procedure
The 40Ar/39Ar analyses were also performed in the IDES geochronology laboratory using the same mineralogical preparation as for the K–Ar
technique. The major advantage of this technique, when compared to the
K–Ar technique, is to allow the identification of thermal disturbances,
Table 1
New K–Ar ages. Column headings are: locality, sample name, coordinates using the World Geodetic System 84 (WGS84) projection, plagioclase size (P: phenocrysts and
M: microcrysts), potassium content (K) in percent, radiogenic 40Ar (40Ar*) content in atoms per gram (×1014 at g−1), radiogenic 40Ar content in percent (%), ages ± 1 σ uncertainty
(in Ma), and, weighted (by the radiogenic content) mean ages ± 1 σ uncertainty (in Ma).
Locality
Sample
Position
Plagioclase size
K
(%)
40
Ar*
(×1014 at g−1)
40
Ar*
(%)
Age ± un.
(Ma)
Mean ± un.
(Ma)
Chara river
TO-9a
P
0.216
TO-29
P
0.464
Bolshoy Patom river
TO-35
P
0.531
Lena river
TL-18a
P
0.351
Lena river
TL-19a
P
0.257
Lena river
TL-20a
P
0.361
Viluy river
Vil-6a
P
0.150
Viluy river
Vil-13a
M
P
0.148
0.228
0.93514
0.93470
2.2728
2.2744
2.3289
2.2733
1.5747
1.5337
1.1117
1.1453
1.5709
1.5916
0.63888
0.63438
0.63951
0.96401
90.6
89.8
88.4
87.0
91.9
91.2
92.1
94.3
90.6
93.3
90.9
89.5
85.6
87.8
90.2
90.6
373.1
373.0
416.9
417.1
377.5
369.4
385.3
376.2
372.8
383.0
374.9
379.3
367.6
365.3
372.5
365.2
373.1 ± 5.3
Bolshoy Patom river
58°54′45.04″N
118°29′17.10″E
60°06′32.16″N
115°26′22.62″E
60°07′24.72″N
115°53′43.02″E
59°50′14.70″N
113°26′14.70″E
60°37′54.20″N
114°45′17.50″E
60°38′23.34″N
114°46′15.42″E
63°00′22.32″N
115°09′03.84″E
62°38′46.20″N
115°20′40.00″E
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
±
5.3
5.3
5.9
5.9
5.4
5.2
5.5
5.3
5.3
5.4
5.3
5.4
5.2
5.2
5.3
5.2
417.0 ± 5.9
373.5 ± 5.3
380.7 ± 5.4
378.0 ± 5.4
377.1 ± 5.3
366.5 ± 5.2
368.8 ± 5.2
534
J. Ricci et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
Table 2
40
Ar/39Ar ages obtained in this study. Column headings are: locality, sample name, plagioclase size (P: phenocrysts and M: microcrysts), plateau ages ± 1σ analytical uncertainty (and with total uncertainty in parentheses), and integrated ages ± 1σ total
uncertainty.
Locality
Sample
Plagioclase
size
Plateau age ± an.
un. (±tot. un.)
(Ma)
Integrated age
(±tot. un.)
(Ma)
Chara river
Bolshoy Patom
river
Bolshoy Patom
river
Bolshoy Patom
river
Lena river
Lena river
Viluy river
Viluy river
TO-9a
TO-29
P
P
385.7 ± 0.61 (±3.5)
None
382.3 ± 5.8
410.5 ± 4.3
TO-35
P
368.0 ± 0.26 (±3.3)
369.0 ± 3.4
TO-41a
P
378.9 ± 0.33 (±3.4)
378.8 ± 3.7
TL-18a
TL-19a
Vil-6a
Vil-13a
P
P
P
P
379.7
376.3
362.3
363.8
379.6
374.9
371.6
368.4
±
±
±
±
0.22
0.39
0.88
0.62
(±3.4)
(±3.4)
(±3.4)
(±3.3)
±
±
±
±
3.7
3.6
5.3
4.2
and to check that the initial 40Ar/36Ar ratio is atmospheric. Moreover, the
40
Ar/39Ar technique can yield a lower analytical uncertainty, not including the systematic error induced by the age of the flux monitor, to better
constrain the duration of geological processes. In order to avoid 39Ar recoil occurring during irradiation, we have only used feldspar phenocrysts
from a size fraction of at least 125 μm (McDougall and Harrison, 1999).
Samples and flux monitor were irradiated in the CLICIT position of the
Triga reactor (Radiation Center, Oregon State University, USA) during
60 h and measured using the multi-collector spectrometer described
in Coulié et al. (2004). Gas extraction from the samples was performed
by step-heating using a high frequency furnace from about 500 to
1250 °C, whereas standards were directly fused using a 120 W Nd-YAG
laser operating in the infrared at 1064 nm. In order to determine the J
factors, positioned between each sample, we have used 20 mg aliquots
of the Fish Canyon Tuff sanidine (FCs), with the recommended age of
28.201 Ma (Kuiper et al., 2008). CBC neutron reactions of Renne et al.
(1998), which were determined on pure compounds of K2SO4 and CaF2
irradiated in the same reactor, have been used. Finally, the mass discrimination of the mass spectrometer was corrected using a linear law, from
analyses of air aliquots measured under pressure condition similar to
those of typical steps for each sample. 40K decay constants and the K isotopic ratio of Steiger and Jäger (1977) have been used. Unless specified,
all uncertainties throughout this manuscript are quoted at the 1σ level.
Accurate and precise measurements are essential to better constrain
the age, duration, and to correlate geological events. Precision of the
40
Ar/39Ar can be lower than 0.1% under favorable conditions, but it is
now acknowledged that standard age and decay constant uncertainties
limit in fact the accuracy of this technique to at least 1% (Kuiper et al.,
2008). This is largely due to changes of the recommended age for flux
monitors, such as the FCs. For instance, Jourdan and Renne (2007) have
calibrated this standard based on four primary K–Ar standards and
obtained a mean age of 28.03 ± 0.08 Ma, supporting the age of
28.02 ± 0.20 Ma in uses for about a decade, which was derived from
the astronomical calibration of quaternary paleomagnetic reversals and
inter-standard comparisons (Renne et al., 1998). Then, based on the
comparison between astronomical and 40Ar/39Ar ages of tephra from
marine deposits in Morocco, Kuiper et al. (2008) proposed the value of
28.201 ± 0.023 Ma for FCs. Even more recently, Renne et al. (2010)
obtained an age of 28.305 ± 0.018 Ma from the comparison between
U/Pb and 40Ar/39Ar ages. It is interesting that all these recent FCs age determinations have uncertainties much lower than 1% (from 28.03 to
28.305 Ma), making them significantly different. Because these ages
range within about 1%, we conservatively impose a 1% uncertainty to
the systematic errors involved in 40Ar/39Ar age calculations, namely uncertainties on the flux monitor and on the K decay constants.
5. Results
5.1. K–Ar dating
Eight new K–Ar ages shown in Table 1 have been obtained here.
Both K and Ar analyses were duplicated and all are consistent at the
1 σ level, except sample TO-41a for which K measurement could
not be duplicated, probably due to inter-grain heterogeneity, and
hence was discarded for K–Ar determination. The K contents range
between 0.150 and 0.530% (Table 1).
As expected since K is an incompatible element and because they
crystallized last, K content of Vil-13 microcrysts is higher than the phenocryst (0.228 and 0.148% respectively). Nevertheless, they display
consistent ages of 372.5 ± 5.3 and 365.2 ± 5.2 Ma, for the microcrysts
and phenocrysts, respectively (Table 1).
Mean K–Ar ages shown in Table 1 range from 366.5 ± 5.2 (Vil-6a)
to 380.7 ± 5.4 Ma (TL-18a), except TO-29 which displays a significantly older K–Ar age. Since this sample is associated with a strongly
disturbed 40Ar/39Ar age spectrum (see below), this sample did not
remain closed for the K–Ar chronometer and hence will not be further
considered. Nonetheless, the remaining 7 out of 8 K–Ar ages obtained
here are statistically identical at the 2σ level (95% confidence) with
most compatible at 1σ level (66% confidence). With such coherence,
we can calculate their weighted mean (by the inverse of the variance;
e.g., Taylor, 1982) K–Ar age of 374.0 ± 5.3 Ma.
5.2. 40Ar/39Ar dating
The complete 40Ar/39Ar data can be found in Supp. Mat. 3, and plateau (when obtained) and integrated ages are given in Table 2. Fig. 2
shows well-defined plateau ages, with more than 50% of 39Ar gas
released and with a minimum of 3 steps, all steps included in the plateau being consistent at the 2σ level (Fleck et al., 1977), for 6 out of 8
samples analyzed.
TO-35 displays relatively scattered steps within the plateau, with a
slightly undulating pattern also present in the Ca/K spectra. Such behavior was observed previously for Viluy trap samples (Courtillot et al.,
2010). It could be explained by an irradiation effect such as an isotopic
reorganization due to 37Ar and 39Ar recoil within the minerals under the
effect of the neutron flux, or as evidence of slight sericitization. Apparent age spectra of both TL-19a and TO-9a samples display a much
lower age for the initial step, which could be interpreted in terms of
slight superficial argon loss. On the other hand, samples Vil-6a and
Vil-13a display relatively old ages for the initial steps (Fig. 2). A loss of
39
Ar by recoil cannot be invoked given the phenocryst size used here
(N 125 μm; McDougall and Harrison, 1999). Alternatively, such age disturbance could highlight a slight loss of potassium. But, since the Ca/K
diagrams exhibit increasing values for the initial steps for these samples, both hypotheses can be ruled out. The age spectrum obtained for
TO-29 appears very disturbed (Fig. 2), with apparent ages ranging between 343 and 745 Ma, preventing the determination of a plateau
age. Following a first step with an age of 745 Ma, apparent ages are decreasing from 370 to 343 Ma during the first half of the 39Ar release.
Then, they increase up to 490 Ma before decreasing again to 390 Ma
at the highest temperature step, during the second half. As mentioned
above for TO-35, such undulating pattern could illustrate a more intense
sericitization process occurring here, preventing any reliable age determination for this sample. Nevertheless, we note that the integrated age
of 410.5 ± 4.3 Ma is consistent with the K–Ar age (Table 1).
Fig. 2. 40Ar/39Ar apparent ages spectra and Ca/K spectrum of Viluy samples. Arrows shows the steps included in the plateau age calculation. Plateau ages are calculated as weighted
means (by the inverse of the variance) of individual age of each step included in the plateau. Analytical uncertainties on plateau ages are given at the 1σ level (total uncertainties,
including a 1% uncertainty on the FCs standard, are given in parentheses). Integrated ages (or total-gas ages) are given with the total uncertainty.
60
60
40
40
Ca/K
Ca/K
J. Ricci et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
20
0
400
Apparent age (Ma)
Apparent age (Ma)
20
0
800
TO-9a
380
385.7 ± 0.6 (3.9) Ma
360
340
320
300
0
Integrated age: 382.3 ± 5.8 Ma
20
40
60
80
TO-29
700
600
500
400
300
200
100
0
20
60
60
40
40
20
20
0
400
0
400
TO-41a
Apparent age (Ma)
Apparent age (Ma)
TO-35
380
360
368.0 ± 0.3 (3.7) Ma
340
320
Integrated age: 369.0 ± 3.4 Ma
0
20
40
60
80
380
340
320
300
100
378.9 ± 0.3 (3.8) Ma
360
Integrated age: 378.8 ± 3.7 Ma
0
20
60
60
40
40
20
0
400
100
TL-19a
Apparent age (Ma)
Apparent age (Ma)
80
20
TL-18a
379.7 ± 0.2 (3.4) Ma
360
340
320
Integrated age: 379.6 ± 3.7 Ma
0
20
40
60
80
380
360
340
320
300
100
376.3 ± 0.4 (3.8) Ma
Integrated age: 374.9 ± 3.6 Ma
0
20
Cumulative 39Ar (%)
80
80
60
60
40
20
60
80
100
40
20
0
400
0
400
Vil-6a
Vil-13a
Apparent age (Ma)
380
360
362.3 ± 0.9 (3.6) Ma
340
320
300
0
40
Cumulative 39Ar (%)
Ca/K
Ca/K
60
0
400
380
300
40
Cumulative 39Ar (%)
Ca/K
Ca/K
Cumulative 39Ar (%)
Apparent age (Ma)
Integrated age: 410.5 ± 4.3 Ma
100
40
60
80
Cumulative 39Ar (%)
Ca/K
Ca/K
Cumulative 39Ar (%)
300
535
Integrated age: 371.6 ± 5.3 Ma
20
40
60
80
Cumulative 39Ar (%)
100
380
360
363.8 ± 0.6 (3.6) Ma
340
320
300
Integrated age: 368.4 ± 4.2
0
20
40
60
80
Cumulative 39Ar (%)
100
J. Ricci et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
Inverse isochron 40Ar/36Ar intercepts are rather poorly defined
because of the high radiogenic content of those samples, which is
greater than 95% for most steps incorporated in the plateaus. As
shown in Fig. 3a, plateau and integrated ages are undistinguishable
at the 1 σ level, except for Vil-6a which has consistent ages only at
the 2 σ level. Such coherence argues for the lack of significant excess
argon, recoil or thermal effects having affected these spectra. Nevertheless, because plateau ages are better defined, they will be considered as the age of emplacement of these volcanic lavas and dikes.
Plateau ages range from 362.3 ± 0.9 to 385.7 ± 0.6 Ma (Vil-6a and
TO-9a, respectively; Table 2). Note that only the analytical uncertainty, as calculated from the reproducibility of successive steps, is given
here. When the total uncertainty is considered (see previous section),
ages range from 362.3 ± 3.6 to 385.7 ± 3.9 Ma (Table 2).
a)
380
375
370
365
Vil-6a
360
355
6. Discussion
40
Ar/39Ar techniques
6.2. Age and duration of the Viluy traps
If only the seven K–Ar ages, which range from 366.5 ± 5.2 and
380.7 ± 5.4 Ma (Table 1), are considered, two contrasting hypotheses can be proposed. Either, Viluy volcanism lasted between about 5
up to 25 myr, or either, it happened in one main volcanic phase centered on 373.8 ± 2.0 Ma. The latter being the weighted mean of all
K–Ar ages, which is statistically supported since they all overlap at
the 2σ level (Fig. 4a).
However, when we consider only the 40Ar/39Ar plateau ages, a more
interesting pattern is revealed. Effectively, it is noticeable in Table 2 that
the six remaining ages are distributed in two distinct groups. The older
group includes samples TL-19a, TL-18a and TO-41a, with plateau ages of
376.3 ± 3.4, 379.7 ± 3.4 and 378.9 ± 3.4 Ma, respectively (Table 2).
The younger group is made of samples Vil-6a, Vil-13a and TO-35, with
plateau ages of 363.3 ± 3.4, 363.8 ± 3.3 and 368.0 ± 3.3 Ma, respectively (Table 2).
355
360
365
370
375
380
385
390
380
385
390
380
385
390
Integrated ages (Ma)
b) 390
TO-9a
385
380
Integrated Age (Ma)
In order to compare results from different dating techniques, it is
necessary to take into account the total age uncertainty. As stated
above, a 1% uncertainty has been assigned here for the FC-s standard.
Fig. 3b shows the correlation graph of the six samples dated here
using both K–Ar and 40Ar/39Ar techniques. In principle, if interfering
Ar isotopes produced during irradiation are correctly accounted for
both ages should be identical. Fig. 3b shows that it is indeed the
case at the 1σ level for 5 out of 6 samples. Only TO-9a displays a significantly older 40Ar/39Ar integrated than K–Ar age.
The 40Ar/39Ar stepwise technique is more efficient to remove an
undesirable inherited argon component and best suited to precisely
determine the cooling age of the samples. The correlation between
40
Ar/39Ar integrated and plateau ages (Fig. 3a) shows that most samples are consistent at the 1σ level, except Vil-6a which has a slightly
older integrated age due to the high ages displayed by the first four
steps. Nevertheless, such correlation highlights the good behavior of
these samples and demonstrates that the system has remained closed
since cooling time. Fig. 3c shows that K–Ar and 40Ar/39Ar plateau ages
for 5 out of 6 samples agree at the 1σ level with slight systematically
older K–Ar ages. Sample TO-9a appears clearly as an outlier with
much older 40Ar/39Ar than K–Ar ages. Although the plateau seems
well-defined using the highest temperature interval, there is a puzzling sharp age increase at 825 °C, between steps 6 and 7, while the
Ca/K ratio evolves rather smoothly (Fig. 2). Because no other sample
from this study display such behavior, and because K–Ar and 40Ar/39Ar
are not consistent, for sake of homogeneity, we prefer to only consider
samples with matching ages using both techniques, and hence, we prefer not to consider further this sample.
350
350
375
370
365
360
355
350
350
355
360
365
370
375
K-Ar ages (Ma)
c) 390
TO-9a
385
380
Plateau Age (Ma)
6.1. Comparison of the K–Ar and
390
385
Plateau Age (Ma)
536
375
370
365
360
355
350
350
355
360
365
370
375
K-Ar ages (Ma)
Fig. 3. Comparison between a) 40Ar/39Ar integrated and plateau ages; b) K–Ar and
40
Ar/39Ar integrated (or total-gas) ages; and c) K–Ar and 40Ar/39Ar plateau ages.
J. Ricci et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
a)
Kellwasser horizons at Steinbruch Schmidt (Germany), Kaufmann
(2006) proposed a revised age of 376.1 ± 3.6 Ma for the FF boundary.
Recently, based on a cubic spline fit between 17 selected Devonian ages
ranging from 419.2 ± 3.2 and 358.9 ± 0.4 Ma, including the Steinbruch
Schmidt given above (Kaufmann, 2006), an age of 372.2 ± 1.6 Ma has
been obtained for the FF boundary in GTS2012 (Gradstein et al., 2012).
Nevertheless, the cubic fit used shows relatively strong curvatures,
which appears not strongly constrained due to the lack of ages between
364 and 390 Ma. Thus, we prefer to rely on the approach of Kaufmann
(2006) where the Devonian time scale is constructed simply from a linear
interpolation between the available radiometric ages. Furthermore, the
U/Pb age of 377.2 ± 3.7 Ma obtained from a bentonite intercalated between the two Kellwasser layers, and hence close to the boundary,
strongly supports the age of 376.1 ± 3.6 Ma derived for the FF boundary
(Kaufmann, 2006).
The age of the Devonian–Carboniferous boundary appears rather well
defined, with a recent determination of 358.9 ± 0.4 Ma (Gradstein et al.,
2012), which is based on two unpublished ages bracketing the boundary
and the Hangenberg event (Davidov et al., 2011). While more precise, it is
undistinguishable from the age of 360.7 ± 2.7 Ma (Kaufmann, 2006)
previously proposed.
350
355
K-Ar ages (Ma)
360
365
370
375
380
385
390
b) 350
6.4. The Viluy traps and the late Devonian mass extinctions
355
Plateau Age (Ma)
360
nd
2 phase:
364.4±1.7 Ma
365
370
st
375
537
1 phase:
376.7±1.7 Ma
380
385
390
Fig. 4. a) K–Ar ages obtained here (Table 1); b) 40Ar/39Ar plateau ages from this study
(closed symbols, Table 2) and from Courtillot et al. (2010; open symbols).
In their previous study, Courtillot et al. (2010) also identified two
phases of volcanic activity at about the same ages. Recalculating their
two high-quality 40Ar/39Ar plateau ages with the value of 28.201 for
the FCs, we can combine the two datasets, which includes all the
40
Ar/39Ar presently available for the Viluy volcanism. Fig. 4b shows
that the two volcanic phase hypotheses are now well supported and
the weighted mean ages of each volcanic phase are 376.7 ± 1.7 and
364.4 ± 1.7 Ma. Note that such a volcanic pattern with multiple
phases of activity is frequently observed for large igneous provinces
(e.g. Courtillot et al., 1999; Chenet et al., 2007; Jourdan et al., 2007;
Chenet et al., 2009).
6.3. Age of the Frasnian–Famennian and Devonian–Carboniferous
boundaries
The age of the FF boundary has experienced strong changes within
the last 40 years, with ages ranging from 360 to 376 Ma. Based on a
spline–line interpolation between U/Pb ages obtained from zircon
extracted from volcanic ashes throughout the Devonian, the age of the
FF boundary was placed at 374.5 ± 2.6 Ma in GTS2004 (Gradstein
et al., 2004). Note that the only two ages that bracket the FF boundary
are 363.6 ± 1.6 and 381.1 ± 1.6 Ma (Tucker et al., 1998). Then, with
the incorporation of a new U/Pb age of 377.2 ± 3.7 Ma, obtained
from a volcanic bentonite intercalated between the Upper Frasnian
It is presently often proposed that eruption of CFBs causes severe
harm to biodiversity, leading, in extreme cases, to mass extinction
(i.e. Courtillot and Renne, 2003). The primary factor involved is gaseous
emissions of volcanism (SO2 and CO2), which can be aggravated if sedimentary rocks (sulfates and carbonates) are intruded by the magmatic
complex (Retallack and Jahren, 2008; Ganino and Arndt, 2009; Svensen
et al., 2009). Sulfuric aerosols, if released into the stratosphere through
heat-driven ascending convection (Kaminski et al., 2011), can have a
strong, albeit short-term, cooling effect, and be deadly for ecosystems
when they precipitate (McCormick et al., 1995; Thordarson and Self,
2003; Chenet et al., 2005; Grattan, 2005). Because CFB volcanism occurs
by successive large volume pulses, as shown for the Deccan and Karoo
traps (Chenet et al., 2009; Moulin et al., 2011), such effects can be
enhanced dramatically. In addition, another dramatic effects of CFB emplacement is recorded by the strong perturbations of the carbon cycle,
probably by dissociation of methane hydrates, as evidenced by a rapid
shift of the δ13C isotopic records or the presence of anoxic oceanic event
(AOE) deposits often observed in stratigraphic successions (Wignall,
2001).
Among several stratigraphic markers, which indicate contradicting,
though possibly rapidly changing, climatic conditions, a sharp decrease
of sea-level is reported at the FF boundary (Sandberg et al., 2002), as
well as the occurrence of the Kellwasser oceanic anoxic event. Because
a volcanic origin for these Earth surface perturbations appears to be the
most probable cause (i.e., Racki, 1999), our new 40Ar/39Ar ages further
support the hypothesis previously proposed by Courtillot et al. (2010)
that the emplacement of the large volume Viluy traps might have a
causal link with the Frasnian–Famennian mass extinction. More precisely, the first volcanic phase, dated here at 376.7 ± 1.7 Ma (Fig. 4b),
relative to the FCs standard with an age of 28.201 Ma (Kuiper et al.,
2008), is in excellent agreement with the age of 376.1 ± 3.6 Ma proposed for the FF boundary (Kaufmann, 2006) from U/Pb zircon ages
(Fig. 5). Note that the use of the 28.201 Ma age for the FCs allows reconciliation of U/Pb and 40Ar/39Ar ages, which were previously offset by
about 1% (i.e., Min et al., 2000).
The second volcanic phase identified here by the 40Ar/39Ar technique at 364.4 ± 1.7 Ma appears statistically much younger than the
FF boundary. It is coeval with the Devonian–Carboniferous boundary if
the age of 360.7 ± 2.7 Ma (Kaufmann, 2006) is used, while it appears
somewhat older if the most recent determination of 358.9 ± 0.4 Ma
(Gradstein et al., 2012) is considered (Fig. 5). Notwithstanding, since
the latter age for the Devonian–Carboniferous boundary appears rather
J. Ricci et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
355
Kaufmann
2006
GTS
2012
Finally, it can be noted that in addition to the Viluy traps, two
other volcanic provinces are thought to have been emplaced at the
end-Famennian: the Pripyat–Dnieper–Donets and the Kola provinces
situated in the East European Platform (Courtillot et al., 1999; Courtillot
and Renne, 2003; Kravchinsky, 2012). Although the stratigraphic age of
the former is relatively well constrained close to the Frasnian–Famennian
boundary and throughout the Famennian (Kusznir et al., 1996), no accurate radiometric ages are presently available. Furthermore, their original
volume remains currently poorly determined, which prevents any strong
correlation with patterns of mass extinction (Wilson and Lyashkevitch,
1996).
Tournaisian
Carbo.
350
Tournaisian
GTS
2004
Tournaisian
538
Giv.
385
Giv.
380
Frasnian
Frasnian
1st phase
Giv.
375
7. Conclusions
Frasnian
Devonian
370
Famennian
2nd phase
Famennian
365
Famennian
360
390
Fig. 5. Comparison of the 40Ar/39Ar weighted-mean plateau ages obtained for the first
and second phase of the Viluy traps, with recently published Devonian geological time
scales (Gradstein et al., 2004: GTS2004; Kaufmann, 2006; Gradstein et al., 2012:
GTS2012).
well constrained, the relationship between the second volcanic phase
and a global impact on biodiversity appears unsupported with the
presently available dataset.
Seven new K–Ar and seven 40Ar/39Ar ages from the Viluy traps have
been obtained in the present study, with six of them analyzed using
both techniques. Five out of six K–Ar and 40Ar/39Ar ages agree within
about 1% uncertainty, which reinforces the coherency between the two
techniques, and extends the suitability of the K–Ar Cassignol and Gillot
unspiked technique, applied to fresh and undisturbed lavas, to the
Paleozoic. The remaining age displays a slightly disturbed 40Ar/39Ar age
spectra and was not used. Including two previously published highquality ages (Courtillot et al., 2010), the eight 40Ar/39Ar plateau ages, calculated relative to an age of 28.201 Ma for the FCs flux monitor (Kuiper
et al., 2008), cluster in two different groups with weighted mean ages of
364.4 ± 1.7 and 376.7 ± 1.7 Ma. These ages support a multi-phase emplacement of the Viluy traps, as recognized for many CFBs (Courtillot and
Renne, 2003).
Because it is based on the interpolation between two bentonites
(dated using U/Pb) that are stratigraphically distant from the limit,
the age of the Frasnian–Famennian boundary remains debated. When
the age of 376.1 ± 3.6 Ma (Kaufmann, 2006) is considered, the first volcanic phase of the Viluy traps is undistinguishable from the Frasnian–
Famennian limit, while the second phase of Viluy traps volcanism
occurred close to the end of the Devonian, but possibly slightly before
the Devonian–Carboniferous boundary (Gradstein et al., 2012).
Finally, since we have shown here that the Frasnian–Famennian
boundary and the Viluy traps are coeval, the correlation between
mass extinctions and emplacement of CFBs can be further extended
to yet another one of the five major Phanerozoic mass extinctions
Viluy traps
600
5
Deccan
traps
3 4
CAMP
2
Siberian traps
Number of families
1
300
Mesozoic - Cenozoic fauna
Paleozoic fauna
Cambrian fauna
Cambrian
Ph.
Ordovician Silur. Devonian Carbon.
Paleozoic
Permian
Tria.
Jurassic
Cretaceous
Mesozoic
Pal. Neo.
Cenozoic
Fig. 6. The Phanerozoic biodiversity changes shown by the evolution of the marine families (Sepkoski, 1984). The Cambrian, Paleozoic and Mesozoic–Cenozoic (Pal.: Paleogene
Period, Neo.: Neogene Period.) faunas are shown with dark blue, green and blue colors, respectively. The numbers indicate the 5 major extinctions events of the Phanerozoic.
J. Ricci et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 386 (2013) 531–540
(Fig. 6), leaving only the Ordovician–Silurian mass extinction to be
associated with another CFB province.
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.palaeo.2013.06.020.
Acknowledgments
We Thank V.I. Powerman, I.V. Fedyukin and V.O. Ponomarev for the
help during field work, and Claire Boukari for the laboratory assistance
during measurements. We are additionally indebted to Finn Surlyk and
an anonymous reviewer for the comments to the manuscript. Financial
support was provided by RFBR project numbers 11-05-00705,
12-05-00403 and 12-05-91051. This is LGMT contribution number 109.
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