Earth and Planetary Science Letters 434 (2016) 349–352
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Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Comment to paper: Evaluating the temporal link between the Karoo
LIP and climatic–biologic events of the Toarcian Stage with
high-precision U–Pb geochronology by Bryan Sell, Maria Ovtcharova,
Jean Guex, Annachiara Bartolini, Fred Jourdan, Jorge E. Spangenberg,
Jean-Claude Vicente, Urs Schaltegger in Earth and Planetary Science
Letters 408 (2014) 48–56
F. Corfu a,b,∗ , H. Svensen b , A. Mazzini b
a
b
University of Oslo, Department of Geosciences, Postbox 1047, Blindern, N-0316 Oslo, Norway
University of Oslo, Centre for Earth Evolution and Dynamics (CEED), Postbox 1028, Blindern, N-0315 Oslo, Norway
a r t i c l e
i n f o
Article history:
Received 17 February 2015
Received in revised form 12 May 2015
Accepted 5 July 2015
Editor: A. Yin
1. Introduction
The paper by Sell et al. (2014) reports new, high quality and
high precision data that constrain the Lower Toarcian time scale,
and provides further dates from the K-LIP,1 emplaced during the
same time interval. The Early Toarcian records an episode of
mass extinction of marine fauna correlated with a global negative CIE. The extensive magmatism of the K-LIP has been proposed as the leading explanation for the environmental changes
(Svensen et al., 2007, 2012; McElwain et al., 2005). In past studies we and coauthors have investigated this potential association
by dating sills of the K-LIP event across South Africa (Svensen
et al., 2012) and verifying the stratigraphic correlation using a
fossiliferous bentonite-bearing shale section in western Argentina
(Mazzini et al., 2010). We have also conducted a number of detailed investigations of the mode of emplacement of the Karoo sills
and their effects on the intruded sedimentary rocks, concluding
that the heat from the intrusions contributed to devolatilization
DOI of original article: http://dx.doi.org/10.1016/j.epsl.2014.10.008.
DOI of reply to comment: http://dx.doi.org/10.1016/j.epsl.2015.07.016.
Corresponding author at: University of Oslo, Department of Geosciences, Postbox 1047, Blindern, N-0316 Oslo, Norway.
E-mail addresses: fernando.corfu@geo.uio.no (F. Corfu),
henrik.svensen@mn.uio.no (H. Svensen), adriano.mazzini@geo.uio.no (A. Mazzini).
1
Abbreviations: K-LIP = Karoo Large Igneous Province; CIE = Carbon Isotope Excursion; TOAE = Toarcian oceanic anoxic event; NIGL= NERC Isotope Geosciences
Laboratory.
*
http://dx.doi.org/10.1016/j.epsl.2015.07.010
0012-821X/© 2015 Elsevier B.V. All rights reserved.
and venting of 12 C-enriched CO2 and CH4 (Svensen et al., 2007;
Aarnes et al., 2011).
In their new paper Sell et al. (2014) provide U–Pb results on
a different fossiliferous Toarcian stratigraphic section in the Andes,
reporting high precision U–Pb zircon ages of 183.22 ± 0.26 Ma and
181.99 ± 0.13 Ma for bentonites that bracket the critical strata containing the TOAE. They also provide dates for one mafic and one
granophyric sill from the Karoo basin as well as one rhyolitic and
one granophyric sill from the Lebombo rift in the eastern parts of
South Africa. Based on the latter dates, Sell et al. (2014) suggest
that the K-LIP may have extended for a longer period of time and
may thus not have been solely responsible for the TOAE. That contrasts with the results of Svensen et al. (2012), who documented a
short duration of the K-LIP sill emplacement and the likelihood of
a causal connection to the TOAE. We disagree with the conclusion
of Sell et al. (2014) and will argue against their interpretation in
this reply.
We appreciate nevertheless the contribution by Sell et al.
(2014). Their paper tightly constrains the timing of the stratigraphy of the studied section in Peru, adding high precision U–Pb data
and new perspectives on the TOAE problem and shedding more
light on some of the links between the TOAE and the K-LIP. The
paper also raises some questions which we would like to address
in this comment. We discuss first a technical factor which skews
the comparison between our dates and those from other laboratories; in this comments we report an adaptation of our results to
the same calibration solution widely used in the geochronological
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F. Corfu et al. / Earth and Planetary Science Letters 434 (2016) 349–352
community. We also use the opportunity to discuss some of the
natural factors that can complicate the interpretation of high precision U–Pb data, with implications for the conclusions by Sell et
al. (2014). Finally, we argue why the samples collected by Sell et
al. (2014) cannot be used to draw conclusions about the duration
of the sill emplacement in the Karoo Basin.
2. Bias in the published U–Pb results from Oslo
The U–Pb data reported in Mazzini et al. (2010) and Svensen
et al. (2012) were obtained with our own mixed 202 Pb–205 Pb–235 U
spike, which has been calibrated against various reference solutions and having a U/Pb ratio considered to be known to better than 0.2%. More recent repeated measurements, using the
same spike, of an artificial U–Pb solution (ET100) provided by the
EARTHTIME initiative (Condon et al., 2007) reveal a small but reproducible difference, yielding an average value of 206 Pb/238 U =
0.0156513 versus the value of 0.015623 obtained with the ET2535
spike at NIGL (Condon, personal comm., 2014). The data published by Sell et al. (2014) were obtained using the same spike
as NIGL. Such a bias has negligible effects on the data of most
projects handled in the Oslo laboratory, but it becomes a more
significant factor in projects involving time-scale dating or the resolution of short-lived magmatic and volcanic processes requiring
the comparison of data sets from different sources. It is not evident whether the bias can be linked to the original calibration
or whether the spike composition might slowly change with time,
due for example to absorption in walls of the storage FEP bottle. The data in Mazzini et al. (2010) and Svensen et al. (2012)
were obtained recently enough that this consideration is less relevant. They can therefore be adapted to match those obtained
with the ET spike, for this Jurassic age involving a correction of
+0.35 Ma.
3. Comparison
The corrected ages are listed in Table 1 and plotted in Fig. 1.
The two estimates of the timing of the CIE, from Sell et al. (2014)
based on the Peru section and Mazzini et al. (2010) from the Argentinian section, overlap within uncertainty, although the latter
extends to a younger time period. Conversely, Mazzini et al. (2010)
extrapolate an age of 182.5 ± 0.6 Ma for the boundary between
Pliensbachian and Toarcian, whereas Sell et al. (2014) suggest that
Fig. 1. Compilation and comparison of results from Encarnacion et al. (1996), Sell
et al. (2014) and Burgess et al. (2015) and the corrected ages of Svensen et al.
(2007, 2012) and Mazzini et al. (2010). The extrapolated age of 182.5 ± 0.6 Ma for
the Pliensbachian–Toarcian boundary proposed by Mazzini et al. (2010) may be too
young depending on the stratigraphic interpretation. Further work on the details of
the biostratigraphy is in progress. CIE = Carbon Isotopic Excursion; badd = baddeleyite.
the boundary must be older than 183.5 Ma. Neither of the two
studies could find datable volcanic horizons near the top of the
Pliensbachian and the extrapolation relies on a proper estimate
of the sedimentation rates in the strata below the lowest dated
volcanic tuff. The bottom of the section investigated in Mazzini et al. (2010) has limited biostratigraphic constrains and thus
the extrapolated age of the boundary is subjected to some uncertainty.
More pronounced differences are evident with respect to the
timing of the rocks associated with the K-LIP. Svensen et al. (2007,
2012) reported data for 15 mafic sills and dykes collected on a traverse across South Africa (Figs. 1–2). The ages obtained range from
182.7 to 183.4 Ma (Table 1), but essentially all overlap within error
and were modelled to represent emplacement over a period of less
than 0.47 myr. By contrast, Sell et al. (2014) dated two sills from
the Karoo Basin (one giving a reliable age of 183.014 ± 0.072 Ma)
Table 1
Ages of Karoo sills and dykes, from Svensen et al. (2007, 2012), and Argentinian tuffs from Mazzini et al. (2010), adapted to ET spike; 2σ − int = analytical uncertainty;
2σ + tr = analytical + tracer uncertainty; 2σ + tr + λ = analytical + tracer + decay constant uncertainty.
No.
Sample
Karoo (Fig. 2)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
UZ1
SP-07-05
SP-06-05
K08-13
K08-16
K08-34
K08-1
K08-6
SP08-05
K08-47
K08-48
K08-41
K08-31
QU1-2
K08-9
G39974
AS29
AS16.
Age
(Ma)
2σ − int
2σ + tr
2σ + tr + λ
Dyke
Dyke
Sill
Sill
Sill
Sill
Sill
Sill
Sill
Sill
Sill
Sill
Sill
Sill
Sill
183.0
182.9
183.2
182.7
183.3
183.4
182.9
183.4
182.8
183.3
182.9
183.1
183.1
183.2
182.9
0.3
0.3
0.4
0.6
0.5
0.5
0.3
0.3
0.7
0.3
0.5
0.9
0.3
0.4
0.4
0.4
0.4
0.5
0.6
0.5
0.5
0.3
0.4
0.8
0.4
0.5
0.9
0.4
0.5
0.4
0.5
0.5
0.6
0.7
0.6
0.6
0.4
0.5
0.8
0.5
0.6
1.0
0.5
0.6
0.5
tuff
tuff
180.9
181.8
0.4
0.2
0.5
0.3
0.6
0.4
Neuquen
F. Corfu et al. / Earth and Planetary Science Letters 434 (2016) 349–352
351
Fig. 2. Geological map of the Karoo Basin showing sampling sites, numbered as in Fig. 1 and Table 1 (adapted from Svensen et al., 2012).
together with two sills from the Lebombo rift giving ages of
181.31 ± 0.19 and 179.32 ± 0.18 Ma (Fig. 1). Thus, in summary,
Sell et al. (2014) only present one new reliable age from the sill
complex in the Karoo Basin.
The two Karoo sills represent the main K-LIP sill complex
crossed by the traverse sampled in our study (Fig. 2; Svensen
et al., 2012). The granophyre studied by Sell et al. (2014) yields
their oldest age of 183.014 Ma, which is well within the range
of our dates. The other sample from the Karoo Basin only yielded
baddeleyite, which show a range of ages for different grains. The
results are deemed inconclusive by the authors, who suggest that
the age range may represent protracted crystallization of baddeleyite in the magma chamber, although they also discuss a potential effect of Pb loss. A protracted crystallization of zircon in
large silicic or bimodal magma systems is well documented, but
whether the same applies for baddeleyite is more speculative. Baddeleyite is only stable in silica undersaturated systems, and it is a
question whether one can maintain basaltic magma in the crust
for extended periods of time without inducing extensive crustal
melting and contamination, the consequent silicification of the
magma destroying the stability field of baddeleyite and producing zircon. In general the mafic sill complexes have the character
of relatively fast crystallized systems with little crustal contamination, evidenced also by the rarity of zircon xenocrysts in such
rocks.
The potential of having partially reset U–Pb ages of zircon and
baddeleyite because of loss of Pb is also an important consideration. The authors have treated the zircon grains with chemical
abrasion (Mattinson, 2005), which we agree is a very efficient way
to eliminate zircon domains affected by Pb loss. However, the zircon populations in differentiated mafic sills can be extremely rich
in U and zoned, and in some cases it is virtually impossible to
eliminate all secondary Pb loss, unless one dissolves away all the
grain. Based on our experience (e.g. Corfu et al., 2013) we would
argue that it is next to impossible to avoid partially reset baddeleyite ages, unless the grains are large enough to minimize the
effect of Pb loss from near-surface layers, surficial crystallization
to zircon, and ideally are large enough so they can be air-abraded.
Nucleous recoil during alpha decay leads automatically to some
loss of daughter nuclides in the outer region of grains, and this
effect can cause pronounced effects in very thin and small baddeleyite blades (Davis and Davis, 2010). The fact that the oldest
age of 182.8 Ma in sample SA91 of Sell et al. (2014) was obtained
from what seems to have been the largest baddeleyite grain analyzed (judging from the amount of Pb recovered) would support
the suggestion that this rock is at least that old, again in agreement
with our data. The second sample of that area, SA97, contains both
zircon and baddeleyite, which show remarkably matched ages of
183.014 Ma for zircon and 183.07 Ma for baddeleyite. Some baddeleyite grains, however, yield older ages (up to 183.3 Ma). We
accept Sell et al. (2014)’s interpretation, but nevertheless cannot
avoid speculating whether Pb loss rather than protracted crystallization was the mechanism that caused the spread in baddeleyite
ages, and hence a somewhat older age is likely.
In any case, the data in these two samples from the southern
domain of the Karoo Basin sill complex are perfectly in agreement with our (adapted) results indicating a relatively short-lived
but areally extensive period of mafic sill emplacement across
the Karoo Basin. The data are also consistent with the previous age of Encarnacion et al. (1996) from a nearby intrusion
(Figs. 1–2).
4. The Lebombo volcanics and concluding remarks
The two samples with the deviating younger ages of 181.31 ±
0.19 and 179.32 ± 0.18 Ma are a granophyre and a rhyolite from
the Lebombo rift much further north. Although not mentioned
by Sell et al. (2014), it is well established that the Lebombo
volcanism is geologically and geochemically different from the
main Karoo volcanism (e.g., Riley et al., 2004; Klausen, 2009). The
Lebombo province represents a complex rift-setting with an overall succession of nephelinites at the bottom, overlain by picrites
and picritic basalts, then by low-Mg basalts with local rhyolites,
followed at the top by the main rhyolitic sequence with local
basalts. The chronostratigraphy within the sequence, and its comparison with the ages of sills in the Karoo Basin, are not fully resolved by the existing dates. Three SHRIMP U–Pb zircon ages range
from about 182 to 179 Ma but have uncertainties of ±2–3 Ma
(Riley et al., 2004). The Ar–Ar results for whole rocks and minerals (plagioclase, biotite, hornblende) of the Lebombo felsic volcanic
rocks yield dates of about 180–178 Ma, but the latter can be partially affected by disturbances related to alteration and locally by
Ar-excess (Duncan et al., 1997; Jourdan et al., 2007). Nevertheless,
the published data suggest that the main rhyolite volcanism of the
Lebombo rift post-dates the Karoo Basin sill emplacement. This age
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F. Corfu et al. / Earth and Planetary Science Letters 434 (2016) 349–352
difference is nicely demonstrated by the new data from Sell et al.
(2014).
The comparison of samples from the Lebombo rift with samples from the main Karoo Basin is an interesting exercise in itself,
but does not help understand the environmental effects of the
main Karoo volcanism and sills – or the duration of sill emplacement. Sell et al. (2014) conclude that “our data are not sufficient
to address eruptive volumes per unit time”, and judging from their
one good new data point from the Karoo Basin, we couldn’t agree
more. We stress that the new results by Sell et al. (2014) reinforce
our earlier interpretation that the K-LIP sill emplacement was a
rapid event that likely triggered the TOAE.
References
Aarnes, I., Svensen, H., Polteau, S., Planke, S., 2011. Contact metamorphic devolatilization of shales in the Karoo Basin, South Africa, and the effects of
multiple sill intrusions. Chem. Geol. 281, 181–194.
Burgess, S.D., Bowring, S.A., Fleming, T.H., Elliot, D.H., 2015. High-precision
geochronology links the Ferrar large igneous province with early-Jurassic ocean
anoxia and biotic crisis. Earth Planet. Sci. Lett. 415, 90–99. http://dx.doi.org/
10.1016/j.epsl.2015.01.037.
Condon, D., Schoene, B., Bowring, S., Parrish, R., Mclean, N., Noble, S., Crowley, Q.,
2007. EARTHTIME: isotopic tracers and optimized solutions for high-precision
U–Pb ID-TIMS geochronology. Eos Trans. AGU Fall Meeting Abstracts 88 (52).
Fall Meeting Supplement, Abstract V41E–06.
Corfu, F., Polteau, S., Planke, S., Faleide, J.I., Svensen, H., Zayoncheck, A., Stolbov, N.,
2013. U–Pb geochronology of Cretaceous magmatism on Svalbard and Franz
Josef Land, Barents Sea Large Igneous Province. Geol. Mag. 150, 1127–1135.
http://dx.doi.org/10.1017/S0016756813000162.
Davis, W.J., Davis, D.W., 2010. Evaluation of alpha recoil loss from baddeleyite crystals by depth profiling and numerical modelling: implications for U–Pb age
interpretations. Int. Assoc. Gondwana Res. Conf. Ser. 10; International workshop
on zircon geochronology, Beijing, China, Abstract volume, 18–19.
Duncan, R.A., Hooper, P.R., Rehacek, J., Marsh, J.S., Duncan, A.R., 1997. The timing and
duration of the Karoo igneous event, southern Gondwana. J. Geophys. Res. 102,
18127–18138.
Encarnacion, J., Fleming, T., Elliot, D., Eales, H., 1996. Synchronous emplacement of
Ferrar and Karoo dolerites and the early breakup of Gondwana. Geology 24,
535–538.
Jourdan, F., Féraud, G., Bertrand, H., Watkeys, M.K., 2007. From flood basalts to
the inception of oceanization: example from the 40Ar/39Ar high-resolution
picture of the Karoo large igneous province. Geochem. Geophys. Geosyst. 8.
http://dx.doi.org/10.1029/2006GC001392.
Klausen, M.B., 2009. The Lebombo monocline and associated feeder dyke swarm: diagnostic of a successful and highly volcanic rifted margin? Tectonophysics 468,
42–62.
Mattinson, J.M., 2005. Zircon U–Pb chemical abrasion (“CA-TIMS”) method: combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chem. Geol. 220, 47–66. http://dx.doi.org/
10.1016/j.chemgeo.2005.03.011.
Mazzini, A., Svensen, H., Leanza, H., Corfu, F., Planke, S., 2010. Early Jurassic shale
chemostratigraphy and U–Pb ages from the Neuquén Basin (Argentina): implications for the Toarcian Oceanic Anoxic Event. Earth Planet. Sci. Lett. 297,
631–643. http://dx.doi.org/10.1016/j.epsl.2010.07.017.
McElwain, J.C., Wade-Murphy, J., Hesselbo, S.P., 2005. Changes in carbon dioxide
during an oceanic anoxic event linked to intrusion into Gondwana coals. Nature 435, 479–482.
Riley, T.R., Millar, I.L., Watkeys, M.K., Curtis, M.L., Leat, P.T., Klausen, M.B., Fanning,
C.M., 2004. U–Pb zircon (SHRIMP) ages for the Lebombo rhyolites, South Africa:
refining the duration of Karoo volcanism. Q. J. Geol. Soc. Lond. 161, 547–550.
Sell, B., Ovtcharova, M., Guex, J., Bartolini, A., Jourdan, F., Spangenberg, J.E., Vicente,
J.-C., Schaltegger, U., 2014. Evaluating the temporal link between the Karoo LIP
and climatic–biologic events of the Toarcian Stage with high-precision U–Pb
geochronology. Earth Planet. Sci. Lett. 408, 48–56.
Svensen, H., Corfu, F., Polteau, S., Hammer, Ø., Planke, S., 2012. Rapid magma emplacement in the Karoo Large Igneous Province. Earth Planet. Sci. Lett. 325–326,
1–9. http://dx.doi.org/10.1016/j.epsl.2012.01.015.
Svensen, H., Planke, S., Chevallier, L., Malthe-Sørenssen, A., Corfu, F., Jamtveit, B.,
2007. Hydrothermal venting of greenhouse gases triggering Early Jurassic global
warming. Earth Planet. Sci. Lett. 256, 554–566.