EGQSJE&G Quaternary Science JournalEGQSJE&G Quaternary Sci. J.2199-9090Copernicus PublicationsGöttingen, Germany10.5194/egqsj-67-7-2018Capability of U–Pb dating of zircons from Quaternary tephra: Jemez
Mountains, NM, and La Sal Mountains, UT, USACapability of U–Pb dating of zircons from Quaternary tephraKrautzJanajana.krautz@tu-dresden.deHofmannMandyGärtnerAndreashttps://orcid.org/0000-0002-1670-7305LinnemannUlfKleberArnoInstitute of Geography, Technische Universität Dresden, Helmholtzstr. 10, 01690 Dresden, GermanySenckenberg Naturhistorische Sammlungen Dresden, Museum für Mineralogie und Geologie, Sektion Geochronologie, GeoPlasma Lab, Königsbrücker Landstraße 159, 01109 Dresden, GermanyJana Krautz (jana.krautz@tu-dresden.de)31January2018671716This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://egqsj.copernicus.org/articles/67/7/2018/egqsj-67-7-2018.htmlThe full text article is available as a PDF file from https://egqsj.copernicus.org/articles/67/7/2018/egqsj-67-7-2018.pdf
Two Quaternary tephras derived from the Jemez Mountains, New Mexico – the
Guaje and Tsankawi tephras – are difficult to distinguish due to their
similar glass-shard chemical composition. Differences in bulk chemical
composition are small as well. Here we examine the feasibility to assign an
age to a distal tephra layer in the La Sal Mountains, Utah, by U–Pb dating
of zircons and to correlate it with one of the two Jemez eruptions. We also
dated original Jemez tephras for comparison. Even though the tephras are very
young, we obtained reasonable age determinations using the youngest cluster
of zircon grains overlapping in age at 2σ. Thereafter, the Guaje
tephra is 1.513 ± 0.021 Myr old. The La Sal Mountains tephra is
correlated with the Tsankawi tephra. Three samples yielded a common age range
of 1.31–1.40 Myr. All ages are in slight disagreement with published age
determinations obtained by 40Ar /39Ar dating. These findings
indicate that distal Jemez tephras can be distinguished by U–Pb dating.
Furthermore, we encourage giving this method a try for age assignments even
of Quaternary volcanic material.
Zwei quartäre Tephren aus den Jemez Mountains, New
Mexico, – Guaje- und Tsankawi-Tephra – sind durch die ähnliche
chemische Zusammensetzung ihrer Gläser nur schwer zu unterscheiden. Dies
gilt auch, bis auf geringfügige Unterschiede, für die Totalanalyse.
Wir haben die Möglichkeit untersucht, das Alter einer distalen Tephralage
in den La Sal Mountains, Utah, zu bestimmen und einer der Tephren aus den
Jemez Mountains zuzuordnen. Zur Vergleichbarkeit haben wir auch die Zirkone
der Tephren aus den Jemez Mountains U–Pb datiert. Obwohl die Tephren alle
sehr jung sind, haben wir reliable Alter durch das Cluster der jüngsten,
im 2σ Fehler überlappenden Zirkone erhalten. Demzufolge ist die
Guaje-Tephra 1.513 ± 0.021 Myr alt. Die Tephra aus den La Sal
Mountains wurde mit der Tsankawi-Tephra korreliert: Drei Proben aus den Jemez
Mountains (1×) und den La Sal Mountains (2×) ergaben eine
Altersspanne von 1.31–1.40 Myr. Alle Alter weichen etwas von bereits
publizierten 40Ar /39Ar ab. Die Ergebnisse deuten darauf hin,
dass die distalen Jemez-Tephren durch U–Pb Datierung unterschieden werden
können. Wir wollen dazu ermutigen, diese Methode der Altersbestimmung
auch für quartäres vulkanisches Material in Erwägung zu ziehen.
citationstatementKrautz, J., Hofmann, M., Gärtner, A., Linnemann, U., and Kleber, A.: Capability of U–Pb dating of zircons from Quaternary tephra: Jemez
Mountains, NM, and La Sal Mountains, UT, USA, E&G Quaternary Sci. J., 67, 7–16, https://doi.org/10.5194/egqsj-67-7-2018, 2018.Introduction
Tephra is eruptive rock material deposited as airborne fallout often quite
distant from its source volcano. Because of its chemical composition –
usually obtained from glass shards – it often may be related to a
particular volcanic eruption (Westgate et al., 1994). Therefore,
tephrochronology has become an established method, using tephra intercalated
between other deposits as a stratigraphic marker bed, provided the original
eruption is well dated (Lowe, 2011).
Areas under study. Source of maps: Google Maps 2016
(http://maps.google.com).
There are a lot of reliable methods for dating Quaternary tephra
(Dickinson and Gehrels, 2009). Most commonly the
40Ar /39Ar method is applied using K-rich minerals (Lowe,
2011). This utilizes the fact that embedded argon completely leaves the mineral
lattice by disturbances such as a volcanic eruption. After this the
enrichment by radioactive decay of K re-starts, and thenceforward the accrued
isotopes may be measured. So ages can be calculated via the half-life of the
isotopes (Worsley, 1998).
A distal tephra layer discovered in the La Sal Mountains, Utah, was linked
to the volcanic province of the Jemez Mountains, New Mexico, based on
glass-shard chemistry. However, correlation with a particular eruption
remained ambiguous (Kleber, 2013), because two tephras derived from
there have closely similar chemical compositions (Slate et al., 2007) – one of the major threads of tephrochronology (Lowe, 2011).
Zimmerer et al. (2016) state that both tephras are
difficult to date by Ar–Ar dating, asking for elaborate sample preparation and
calculation of the results. Though still not done very often on such young
zircons (Lee, 2012), there have been a few successful applications
of U–Pb dating of zircons to young material in recent years (e.g.,
Ito et al., 2016; Sakata et al., 2017). Zircons have the
advantage of being outstandingly chemically and physically robust. They are
unsusceptible to alteration and weathering even under extreme conditions
(Wilson et al., 2008). Thus, we tried dating the tephra layer using
zircon dating.
Here we demonstrate reasonable age determinations of zircons from the La Sal
Mountains tephra layer and of the two suspect tephras in the Jemez
Mountains. Through this, the Jemez tephra layers may be discriminated with
high certainty. Furthermore, we encourage giving the U–Pb method – which
is available in a variety of labs worldwide – a try for dating volcanic
material of undisclosed age even if the assumed age is as young as 1 Myr,
after having tested the total uranium contents.
Geological settingJemez Mountains, New Mexico
The Jemez Mountains (Fig. 1) are calderas of various volcanic eruptions,
among which the Valles, Antonio, and Toledo calderas are still recognizable
as concentric mountain ranges. Their eruptive products, mainly
basalt–andesite–dacite–rhyolite associations, range from about 15 Myr
(mid-Miocene) to < 2 Myr (Pleistocene) (Kues et al., 2007). The
Neogene and Quaternary formations are divided into three groups, named after
Indian nations, from oldest to youngest: the Keres, the Polvadera, and the
Tewa group (Bailey et al., 1969). We took our samples from the Tewa group.
This comprises the Bandelier Tuff, which is mainly the result of two large
ignimbrite- and caldera-forming eruptions. The lower Otowi (including the
Guaje tephra) and the upper Tshirege (including the Tsankawi tephra)
sequences were deposited approximately 1.6 and 1.2 Ma, respectively (Self et al., 1996;
Slate et al., 2007). Today large parts of these ignimbrite and tephra
sequences belong to the Bandelier National Monument.
The Jemez Mountains are known to be the source area of the La Sal Mountains
tephra layer. We took samples approximately 8 km southeast of Los Alamos, New
Mexico, from a slope along New Mexico State Road 502 (Guaje and Tsankawi tephras,
located at 35∘52′05′′ N, 106∘11′59′′ W and at
35∘52′05′′ N, 106∘12′00′′ W, respectively). The site is
depicted in Goff (2009) and in Fig. 2a.
La Sal Mountains, Utah
The chain of the La Sal Mountains lies at the eastern border of Utah (Fig. 1). Like the Jemez Mountains, it is part of the Colorado Plateau Province and
together with Mount Peale (3877 m a.s.l.) is the highest peak of the plateau
(Henning, 1975; Grahame and Sisk, 2002). The La Sal Mountains are
remnants of laccoliths and mainly consist of granitoid rocks
(Henning, 1975; Ross, 2006). The Precambrian basement is
unconformably overlain by Paleozoic and Mesozoic sedimentary rocks, which
were intruded by monzonite and diorite porphyry during the Paleogene (K–Ar
ages are 25–28 Myr; Ross, 2006). The laccolithic structures preserve
Mesozoic rocks at the mountain flanks, mainly clays and sandstones
(Richmond, 1962; Henning, 1975). Within the adjacent Paradox Basin,
the Mesozoic rock sequence is underlain by marine sediments, which include
limestone, dolomite, slate, and a several-hundreds-of-meters-thick diapiric layer of salt
and gypsum (Henning, 1975).
(a) Sampling sites of tephras in the Jemez Mountains. All
visible rocks are volcanic in origin. Photo: Jana Krautz (22 August 2014).
(b) Sampling site in the La Sal Mountains. The whitish tephra
intercalates between periglacial cover beds and is to the left of the picture
cut by a gully fill. Photo: Arno Kleber (27 July 2009). The sampling spot
visible in the La Sal Mountains tephra was for radiofluorescence dating, not
for the present dating.
A distal tephra layer was found in the northwestern La Sal Mountains, Utah,
USA (located 38∘34′33′′ N, 109∘17′32′′ W),
approximately 20 km linear distance from Moab, Utah, at 2130 m a.s.l., on
a 22∘ steep slope, exposed by a road cut of the Manti-La Sal
Circuit (Kleber, 2013 and
Fig. 2b). The tephra was identified by the US Geological Survey,
Tephrochronology Laboratory, Menlo Park, CA, via the chemical composition of
its glass shards. It was correlated with either the approximately 1.25 Myr old (Phillips et al., 2007) Tsankawi
tephra or – because of the Fe contents somewhat more likely – the
approximately 1.65 Myr old (Spell and Harrison, 1993) Guaje tephra, both
derived from the Jemez Mountains, New Mexico (Kleber, 2013).
Methods
We took two samples from the deposition area in the La Sal Mountains, UT,
USA, and one from each original tephra layer, derived from the Toledo Caldera
(Guaje tephra) and from the Valles Caldera (Tsankawi tephra). The latter two
– taken from well-known tephra locations – were mainly measured to disclose
whether the results of the U–Pb determinations are consistent with the
aforementioned earlier 40Ar /39Ar
datings and may, thus, yield reliable
ages of distal tephra layers.
We performed sample preparation for cathodoluminescence (CL) images,
LA-ICP-MS (laser ablation with inductively coupled plasma mass spectrometry)
U–Pb analyses, and age calculations at the Geochronology Department of
Senckenberg Naturhistorische Sammlungen Dresden, Germany. Circa 1 kg of
material was collected for each sample. After crushing in a jaw crusher, the
samples were sieved for the fraction 36 to
400 µm. Density separation of this fraction was accomplished with
LST (solution of lithium heteropolytungstates in water). We used a Frantz
isodynamic separator for the magnetic separation of the extracted heavy
minerals. Single zircon grains of all grain sizes, colors, and morphological
types were randomly picked under a binocular microscope and subsequently
analyzed regarding their morphology based on backscatter electron (BSE)
images of the unmounted zircon grain surfaces using a Zeiss EVO50SEM at
20 kV and a spot size of 300 nm. Then the grains were mounted in resin
blocks and polished to approximately half their thickness, in order to expose
their internal structure. We obtained CL images using a Zeiss EVO50SEM
coupled to a CL detector system at 20 kV and a spot size of 500 nm. Zircons
were analyzed for U, Th, and Pb isotopes by LA-ICP-MS, utilizing a Thermo
Scientific ELEMENT 2 XR sector
field ICP-MS coupled to a New Wave UP-193 excimer laser system with laser
spot sizes of 20 to 35 µm. Fifteen seconds of background acquisition was
followed by 25 s of data acquisition during each analysis. The signal was
tuned for a maximum sensitivity for Pb and U, whereas oxide production
(235UO vs. 238U) was kept well below 1 %. Raw data were
corrected for background signal, common Pb, laser-induced elemental
fractionation, instrumental mass discrimination, and time- and
depth-dependent elemental fractionation of Pb / Th and Pb / U using an
Excel® macro developed by Axel Gerdes
(Geosciences Inst., Goethe University Frankfurt, Germany). Reported uncertainties
were propagated by quadratic addition of the external reproducibility
obtained from the standard zircon GJ-1 (∼ 0.6 and 0.5–1 % for
207Pb /206Pb and 206Pb /238U, respectively)
during individual analytical sessions and the within-run precision of each
analysis. Concordia diagrams (2σ error ellipses) and concordia ages
(95 % confidence level) were created using Isoplot/Ex 2.49 (Ludwig,
2001). 207Pb /206Pb ages were used for concordant analyses of
zircons above 1.0 Ga, and 206Pb /238U ages for younger ones.
For ages younger than 10 Myr, we corrected for 230Th disequilibrium
using the formula of Simon et al. (2008).
Electron microprobe analyses of glass shards from tephra layers.
±: standard deviation. Values are weight-percent oxide, re-calculated to
be 100 % fluid-free. Normalized data (raw data are available in
Supplement).
Ages of tephra layers as derived from the youngest cluster of grain
ages overlapping at the 2σ level. (a) Guaje tephra,
(b) Tsankawi tephra, (c) La Sal Mountains tephra sampled in
2013, (d) same but sampled in 2014.
Geochemical analyses of bulk samples were performed at Activation
Laboratories Ltd. (Ancaster, Ontario, Canada) using their standard protocols
RX4 for sample preparation and 4LITHO-Quant Major Elements Fusion ICP (WRA)/Trace Elements Fusion ICP-MS (WRA4B2) for the analyses as described on
their website (ActLabs, 2014). The samples from the La Sal
Mountains were contaminated with pedogenic carbonates, whereas the samples
from the Jemez Mountains were not, or at least not to the same degree.
Therefore, the major elements (and the total percentages) were re-calculated
on a carbonate-free basis, i.e., without considering MgO, CaO, and loss on
ignition (LOI), though the original values of these three measurements are
given so that one could re-assemble all original quantities. In addition to
the aforementioned samples, we analyzed a confirmed Guaje tephra sample
provided by David B. Dethier (Slate et al., 2007).
Microprobe analyses were conducted aided by a CAMECA SX51 electron
microprobe with five wavelength-dispersive spectrometers at the Earth
Sciences Institute at Heidelberg University. The standard operating
conditions were 15 kV accelerating voltage, 20 nA beam current, and a beam
diameter of ca. 20 µm. Counting times during analyses were 10 s for
Na and K; 20 s for Fe; 30 s for Mn and P; and 50 s for Si, Ti, Al, Mg, and Ca.
Detection limits were 0.02 wt % for Si, Al, and Ca, 0.001 wt % for Ti
and Mn, 0.08 wt % for Fe, and 0.09 wt % for K and Na. Calibration was
performed using natural and synthetic oxide and silicate standards. Values
given are weight-percent oxide, re-calculated to be 100 % fluid-free.
Results and discussion
The microprobe analyses of glass shards corroborate the great similarity of
the Guaje and the La Sal Mountains tephras (Table 1; cf. Supplement for raw
data). Even the differences in Fe contents, typically acknowledged as the
only clue to distinguish Guaje from Tsankawi tephras
(Andrei M. Sarna-Wojcicki, personal communication, 1990), are within the
standard deviations of the analyses.
Major and trace element concentrations of tephra samples. The table
displays analyses from bulk samples. Major element percentages are calculated
carbonate-free, i.e., without considering the columns in italic font. Trace
elements which show remarkable differences between Guaje and Tsankawi tephras
are identified in bold font. Source: Actlab report number A14-07544; report
date: 24 October 2014.
Weighted average ages of tephra layers to compare with the age
displays. (a) Guaje tephra, (b) Tsankawi tephra,
(c) La Sal Mountains tephra sampled in 2013, (d) same but
sampled in 2014.
Table 2 shows that the major and especially the trace element concentrations
from bulk samples of the La Sal Mountains tephra are very close to the
Tsankawi tephra from the Jemez Mountains but somewhat dissimilar to the
Guaje tephra sample as well as to the Guaje sample DN-97-117 submitted by
David B. Dethier. This holds especially true for the elements shaded in
yellow in Table 2, with the most remarkable being Cr, Rb, Nb, and Th. The
differences in the Sr and Ba contents between the La Sal Mountains and Jemez
Mountains samples may be explained by eolian contamination, as both elements
are frequent components of eolian deposits (Jones, 1986). Similar
differences in Tl contents may be due to different durations of sample
materials being exposed to oxidation. These findings render the La Sal
Mountains tephra correlative to the Tsankawi rather than the Guaje tephra.
In all samples, primary uranium contents in zircons were sufficiently high
to allow reliable age determinations. Given the apparently young ages of the
tephras, 207Pb could not be accumulated in quantities remarkably above
the detection limit of the instrument due to the extremely long half-life of
235U and/or insufficiently high U contents to produce enough Pb in
such short intervals of time (compare young grains in the Supplement). Thus, we
could use only the 206Pb /238U for age estimations (cf.
Gehrels, 2014). Therefore, 207Pb /235U and
207Pb /206Pb ratios for cross-validation are not available; the
degree of concordance cannot be calculated for these young zircon grains, and
those data are left blank (Supplement). Accordingly, the ages we
report are regarded as model ages.
To establish the age of each tephra sample, we used the youngest cluster
of zircon-derived U–Pb ages overlapping at 2σ. The mean age of the
youngest cluster of grain ages that overlap in age at 2σ is
regarded as the most conservative measure of age (Dickinson and
Gehrels, 2009). These clusters may be seen as groups of analyses resulting
in ages close together, thereby validating each other even without a
reliable Pb–Pb age. Grains with younger 238U /206Pb ages than the
ones used for the calculation of the concordia ages (cf. Supplement) are
not part of such a cluster in the concordia plot and, thus, cannot be
cross-validated. Accordingly, they were not considered sufficiently
reliable.
CL images of selected zircons which have been included in the age
displays (including laser ablation mark). (a) La Sal Mountains tephra
sampled in 2013: c13; (b) same but sampled in 2014:
a22; (c) Tsankawi tephra: a38; (d) Guaje tephra: a36.
The grains used for age determination are accentuated in tables in the
Supplement. The clusters are sufficiently large for the ages to be
constrained to small confidence intervals (2σ); see also Figs. 3 and
4: we assigned an age of 1.513 ± 0.021 Myr to the Guaje tephra from the
Jemez Mountains, which is somewhat younger than the published Ar–Ar-derived
ages of 1.651 ± 0.011 Myr (Zimmerer et al., 2016) or 1.613 ± 0.011 Myr (Izett and Obradovich, 1994). The
other three samples yielded ages incompatible with the Guaje tephra: the
Tsankawi tephra from the Jemez Mountains was determined to be as old as
1.316 ± 0.012 Myr. The two samples from the La Sal Mountains yielded
ages of 1.327 ± 0.017 Myr (sample from the year 2013) and 1.341 ± 0.059 Myr (2014 sample, which had the smallest number of zircon ages within
the overlapping cluster). The confidence intervals of the latter three
samples do all overlap within errors. Therefore, we correlate these
tephra-layer samples with the same, the Tsankawi eruption. The common age
range within 2σ of both samples is 1.31–1.40 Myr. We assume this
is the most likely age array. Zoning of zircons indicates steady growth. If
there is a core depicted in the CL images, the measuring spot may not be
located at a core's edge (Fig. 5).
The ages derived via Ar–Ar dating are 1.264 ± 0.010 Myr
(Phillips et al., 2007; recalculated by Zimmerer et al., 2016) and 1.223 ± 0.018 Myr (Izett and
Obradovich, 1994); i.e., they are slightly younger than ours. Though being
very close to each other, the U–Pb ages are slightly older. The common
notion is that Ar–Ar ages approximate the eruption ages and U–Pb ages
indicate the (earlier) time of crystal closure (Simon et al., 2008).
However, this does not work for the Guaje tephra. Zimmerer
et al. (2016) observed similar differences between
40Ar /39Ar and uranium-series (U / Th) ages for other tephras of the
Jemez Mountains. They explain their findings with a complicated
crystallization history of the magma, leading to disequilibrium between the
uranium isotopes in the melt. Another explanation could be that the zircon
crystal lattices of the Guaje tephra were not completely closed during
eruption, as our sample was taken close to an underlying mafic lava bed
which still could have been hot enough to achieve this effect. Or there
still are problems with the Ar–Ar dating of some Jemez tephras not yet
understood.
Older zircons (cf. Supplement for raw data) are assumed to be inherited from rocks
melted during magma rise, with those zircons being their most
temperature-resistant components.
Conclusions
Our findings demonstrate that U–Pb dating of zircons from Quaternary
volcanic material may result in valuable age determination. U–Pb dating of
zircons seems to allow – at least combined with bulk geochemical analyses –
confident distinction between the two tephras derived from the Jemez
Mountains, which are too similar to be clearly kept apart by glass-shard
chemistry alone. This approach avoids the complications accompanying the
Ar–Ar dating of Bandelier tephras (Phillips et al., 2007; Zimmerer et al., 2016).
We recommend considering U–Pb dating as a possible approach to identifying
rather young tephras or to distinguish such tephras, as in our study.
However, before application, we recommend measuring total uranium contents in
zircon minerals, which might indicate whether this dating method will be
applicable.
In Quaternary research, dating of zircons as young as 1 Myr may well become a
tool for better defining age models of sedimentary archives – such as
loesses, cover beds, or paleosols – with interbedded or admixed tephra
layers.
All underlying data can be found in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/egqsj-67-7-2018-supplement.
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank David B. Dethier, Williamstown, MA, USA, for sending samples from Jemez
tephras and Hans-Peter Meyer, Heidelberg, Germany, for the electron-microprobe
analyses. We also thank Rita Krause (Senckenberg Naturhistorische Sammlungen
Dresden) for her invaluable support of our lab work. We are grateful to the
San Ildefonso Indian Nation for allowing access to the Tsankawi sampling
site. We thank two anonymous reviewers and Ludwig Zöller for their
critical comments and helpful advice. Our work was supported by the German
Research Foundation (DFG, KL 701/12-0).
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