Article
Volume 11
4 August 2010
Q0AA08, doi:10.1029/2010GC003073
ISSN: 1525‐2027
The 40Ar/39Ar and U/Pb dating of young rhyolites
in the Kos‐Nisyros volcanic complex, Eastern Aegean Arc,
Greece: Age discordance due to excess 40Ar in biotite
O. Bachmann
Earth and Space Sciences, University of Washington, Mailstop 351310, Seattle, Washington
98195‐1310, USA (bachmano@u.washington.edu)
B. Schoene
Section des Sciences de la Terre, Université de Genève, 13, rue des Maraîchers, CH‐1205 Geneva 4,
Switzerland
Now at Department of Geosciences, Princeton University, Guyot Hall, Princeton, New Jersey 08544,
USA
C. Schnyder
Section des Sciences de la Terre, Université de Genève, 13, rue des Maraîchers, CH‐1205 Geneva 4,
Switzerland
Now at Departement de Mineralogie et Petrographie, Museum d’Histoire Naturelle, 1, Route de
Malagnou, CH‐1211 Geneva 6, Switzerland
R. Spikings
Section des Sciences de la Terre, Université de Genève, 13, rue des Maraîchers, CH‐1205 Geneva 4,
Switzerland
[1] High‐precision dating of Quaternary silicic magmas in the active Kos‐Nisyros volcanic center (Aegean
Arc, Greece) by both 40Ar/39Ar on biotite and U/Pb on zircon reveals a complex geochronological story.
U/Pb ID‐TIMS multi and single‐grain zircon analyses from 3 different units (Agios Mammas and Zini
domes, Kefalos Serie pyroclasts) range in age from 0.3 to 0.5 to 10–20 Ma. The youngest dates provide
the maximum eruption age, while the oldest zircons indicate inheritance from local continental crust
(Miocene and older). Step‐heating 40Ar/39Ar experiments on 1–3 crystals of fresh biotite yielded highly
disturbed Ar‐release patterns with plateau ages typically older than most U/Pb ages. These old plateau ages
are probably not a consequence of inheritance from xenocrystic biotites because Ar diffuses extremely fast at
magmatic temperatures and ratios are reset within a few days. On the basis of (1) elevated and/or imprecise
40
Ar/36Ar ratios, (2) shapes of the Ar release spectra, and (3) a high mantle 3He flux in the Kos‐Nisyros area,
we suggest that biotite crystals retained some mantle 40Ar that led to the observed, anomalously old ages.
In contrast, sanidine crystals from the only sanidine‐bearing unit in the Kos‐Nisyros volcanic center (the
caldera‐forming Kos Plateau Tuff) do not appear to store any excess 40Ar relative to atmospheric composition. The eastern edge of the Aegean Arc is tectonically complex, undergoing rapid extension and located
close to a major structural boundary. In such regions, which are characterized by high fluxes of mantle
volatiles, 40Ar/39Ar geochronology on biotite can lead to erroneous results due to the presence of excess
40
Ar and should be checked either against 40Ar/39Ar sanidine or U/Pb zircon ages.
Copyright 2010 by the American Geophysical Union
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Components: ~7900 words, 4 figures, 4 tables.
Keywords: geochronology; excess argon; Aegean Arc; rhyolite.
Index Terms: 1115 Geochronology: Radioisotope geochronology.
Received 4 February 2010; Revised 28 May 2010; Accepted 9 June 2010; Published 4 August 2010.
Bachmann, O., B. Schoene, C. Schnyder, and R. Spikings (2010), The 40Ar/39Ar and U/Pb dating of young rhyolites in the
Kos‐Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40Ar in biotite, Geochem.
Geophys. Geosyst., 11, Q0AA08, doi:10.1029/2010GC003073.
Theme: EarthTime: Advances in Geochronological Technique
Guest Editors: D. Condon, G. Gehrels, M. Heizler, and F. Hilgen
1. Introduction
[2] Understanding the generation, transport, storage
and eruption of magma in subduction zones is
important for building models for the generation of
continental crust. Geochronology has become an
important tool for such research because of its ability
to determine the rates of magma crystallization and
the tempo of volcanism. For silicic volcanic rocks,
the commonly used 40Ar/39Ar and U/Pb dating techniques have the potential to provide different views
on the evolution of the unit of interest. When results
from both techniques are compared, U/Pb often
yields older ages. The reason for this are twofold:
(1) uncertainties in the decay constant for 40K and/or
mineral reference standards lead to calculated dates
that are ∼0.5–1% too young compared to U/Pb ages
[e.g., Renne et al., 1998; Min et al., 2000; Kuiper
et al., 2008; Simon et al., 2008], and/or (2) retention of daughter products varies due to different
closure temperatures for Pb in zircon and Ar in a host
of minerals. Because of fast diffusion and therefore
low closure temperatures (<550°C) within common
high‐K minerals, Ar is commonly assumed to record
the timing of eruption [McDougall and Harrison,
1999], assuming insignificant post‐eruption burial.
In contrast, U and Pb are retained in zircon at magmatic temperatures [Lee et al., 1997] and therefore
U‐Pb dates record mineral crystallization, which
occurs on a range of timescales prior to eruption
[Charlier et al., 2005; Simon and Reid, 2005;
Bachmann et al., 2007a; Crowley et al., 2007; Costa,
2008]. The result is that zircons can retain information about older processes within xenocrystic cores
or antecrystic material (co‐genetic, but up to >100 ky
older than eruption ages [e.g., Reid et al., 1997;
Brown and Fletcher, 1999; Vazquez and Reid,
2004; Charlier et al., 2005; Bachmann et al.,
2007b; Bindeman et al., 2008]). In contrast, several
recent studies have documented that 40Ar/39Ar dates
from sanidine and biotite can also predate eruption
due to problems with 39Ar recoil, inheritance, or
excess 40Ar [Smith et al., 2006; Hora et al., 2007;
Lipman and McIntosh, 2008; Smith et al., 2008].
[3] In a geochronological effort to constrain the
magmatic evolution that predates one of the two
largest Quaternary caldera‐forming eruptions in the
Mediterranean region (the 161 ky Kos Plateau Tuff
(hereafter KPT) [Smith et al., 1996; this paper], we
discovered significant discrepancies between zircon
U/Pb and biotite 40Ar/39Ar ages that are contrary to
“standard” behavior: 40Ar/39Ar ages are older than
most U/Pb ages, in a few cases by several million
years. Furthermore, the biotite 40Ar/39Ar ages proved
to be extremely complex and disturbed. This paper
provides a basis to reconcile this age discrepancy in
terms of Ar systematics in biotite and also to present
new and improved ages on volcanic units that pre‐
date the KPT on the Kefalos Peninsula (Kos Island,
Greece). The results from this study also highlight
the potential to extend ID‐TIMS U/Pb dating to sub‐
million year old rocks with high precision in order
to elucidate complex magma evolution.
2. Geological Setting and Sample
Description
[4] The South Aegean Arc is the result of subduction of the African plate under the Aegean
microplate [Le Pichon and Angelier, 1979; Jolivet,
2001], which was probably initiated in the early
Tertiary (with Oligo‐Miocene magmatic rocks in
northern Greece [e.g., Pe‐Piper and Piper, 2002]).
The convergent velocities are some of the slowest
in the world (about 5 to 10 mm/yr [DeMets et al.,
1990; Jackson, 1993]). The volcanic centers of the
South Aegean are (from W to E): Crommyonia,
Aegina, Methana, and Poros, all situated in the
Saronic Golf, and Milos, Santorini, and Kos‐
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Table 1.
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Summary of Published Geochronological Data for Kefalos, Kos, and Nisyros Units
Stratigraphy
Age (ka)
Method
Kefalos Dacite
Zini Dome
Agios Mammas dome
Latra dome
Kos Plateau Tuff
Kos Plateau Tuff
Nisyros
Yali obsidian
Yali pumice fall
Upper Pumice
Lower Pumice
Avlaki Rhyolite
2542–2990; 2600–3100
1000 ± 200, 540–560 ± 30
2700 ± 150
2500 ± 500
161 ± 1
160–480
K‐Ar
K‐Ar
K‐Ar
K‐Ar
Ar‐Ar on sanidine
U‐Th‐Pb on zircon
Matsuda et al. [1999]
Bellon and Jarrige [1979]; Pasteels et al. [1986]
Bellon and Jarrige [1979]
Bellon and Jarrige [1979]
Smith et al. [1996, 2000]
Bachmann et al. [2007a]
24
31
>44
24 ± 0.56
200 ± 50; 66.6 ± 2
FT on volcanic glass
Oxygen isotope
14
C
14
C in paleosoil
K‐Ar (WR) K‐Ar (Pl)
Wagner et al. [1976]
Federman and Carey [1980]
Limburg and Varekamp [1991]
Rehren [1988]
Di Paola [1974]; Keller et al. [1990]
Nisyros, which form the three voluminous complexes in the Aegean Sea [Fytikas et al., 1976].
[5] Previously published K/Ar ages on different
volcanic units of Kos Island [Bellon and Jarrige,
1979; Pasteels et al., 1986; Matsuda et al., 1999]
indicate that volcanism in the area started around
3 Ma ago and was episodic until recent activity (see
Table 1). However, much of the data has large errors
and could be significantly improved by applying
the Ar‐Ar technique to K‐rich minerals. The best‐
constrained age for any Kos Island unit was obtained
using the 40Ar/39Ar method on the only sanidine‐
bearing unit in the area, the KPT (161 ± 1 ky). As
this unit is an important time marker (both locally
and within the Eastern Mediterranean region), much
effort has been invested in acquiring a precise and
accurate age [Smith et al., 1996, 2000].
[6] Obtaining accurate ages for the Nisyros units
has proved much more challenging than for the
Kefalos‐Kos systems. None of the erupted material
on Nisyros Island contains K‐bearing phases, and
hence the K/Ar (and 40Ar/39Ar) method has not
been applied successfully. Other methods have been
applied, with some success (Table 1).
[7] All the samples in this study were collected on
the Kefalos Peninsula, in the southwest of Kos
Island. Dome rocks include, from north to south:
Mt. Zini, Mt. Cumianas, Mt. Latra and Mt. Agios
Mammas. We also collected several samples of the
Kefalos Series pyroclastic rocks [Dabalakis and
Vougioukalakis, 1993] in areas around Kefalos
village (Figure 1 and Table S1).1
[8] The dome rocks of Mt. Zini (KD02, Zini),
Mt. Latra (KD03), Mt. Cumianas (KD04, CS11–05)
and Mt. Agios Mammas (KD07, CS12–05) are crystal‐
poor rhyolites with ∼5 wt% phenocrysts, comprising
2–3% plagioclase, 1–2% biotite, 1–2% quartz and
1
Auxiliary material data sets are available at ftp://ftp.agu.org/
apend/gc/2010gc003073. Other auxiliary materials are in the HTML.
Reference
less than 1% of Fe‐Ti oxides and accessory phases
(monazite, zircon). These crystals reside in a generally glassy matrix, but spherulites (devitrification)
can be abundant in the matrix glasses in some domes.
The Kefalos dacite (KD01) is porphyritic, with 30
to 40 vol% crystals (plagioclase, hornblende, biotite and Fe‐Ti oxides). This rock hosts numerous
mafic microgranular enclaves of basaltic‐andesite
to andesitic composition. These enclaves exhibit
ovoid shapes with sharp contacts with the host rock,
and a mineral assemblage similar to the dacite.
[9] Whole‐rock chemical analyses indicate that
rhyolites have ∼76 wt% SiO2 (anhydrous basis), with
relatively low Al2O3 content (12–13 wt%), and fairly
high K2O (4.3 wt%). The silica content of the dacite is
∼64 wt%, with 16 wt% Al2O3, 3.8 wt% Na2O and
2.7 wt% K2O. The most noticeable feature is the
“adakitic” signature of the dacitic composition, with
high Sr (>400 ppm) and low Y (≤18 ppm) leading to a
Sr/Y ratio of 50–60 [Pe‐Piper and Moulton, 2008].
[10] The biotite crystals are euhedral in the rhyolitic
domes (Zini, Latra, Cumianas, Agios Mammas),
and slightly subhedral in the dacite, but without
any evidence of disequilibrium (no resorption rims
or any alteration were noticed during optical
examination; Figure 2). Microprobe data on biotite
crystals from all dated units yield good totals and
typical biotite compositions (Table 2), which can be
classified as Mg‐biotites (according to Tischendorf
et al. [2001]). Individual crystals appear homogeneous, as no significant chemical zonation was
found during core‐to‐rim traverses.
3. The
40
Ar/39Ar Method and Results
3.1. Method
[11] All samples were gently crushed with a
hydraulic press to obtain coarsely crushed material,
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Figure 1. Map of area with sample locations.
which was then sieved. The fraction <500 mm was
washed in water and dried for 24 h in an oven at
70°C. The minerals were separated using a Frantz
magnetic separator and hand‐picked. Crystals were
subsequently cleaned for 2 min in an ultrasonic
bath using de‐ionized water, then dried under an
infrared lamp for several minutes.
[12] Packets of 20–40 mg of whole grains of biotite
from Kefalos units were wrapped in >99.99% pure
copper foil and were sent for irradiation at Oregon
State University (CLICIT facility) for 1 h, along
with Fish Canyon Tuff sanidine grains (fluence
monitors) that were dispersed in 5mm intervals
along the length of the irradiation package. The
samples were analyzed at the University of Geneva
using an Argus (GV Instruments) multicollector
mass spectrometer, equipped with four high‐gain
(1012 ohms) Faraday collectors for the analysis of
39
Ar, 38Ar, 37Ar and 36Ar, as well as a single Faraday
collector (1011 ohms) for the analysis of 40Ar.
Collector gain was calibrated performing a classic
current‐collector gain (CC gain) procedure, normalizing CC gains for the two high mass and two
low mass collectors to the CC gains value obtained
for the axial collector.
[13] Cup efficiency was measured in two ways:
(1) focused CO2 beams were put onto each collector
in sequence (single collector, peak hopping) in
dynamic mode, and then compared. This assumes
that the dynamic CO2 abundance is constant, which
is considered to be true over the time scale of the
experiment (∼2 h) because it is buffered to an
equilibrium state after 10 days of operation since
the previous bake‐out of the mass spectrometer
(cup efficiency is performed at least ten days after
the previous MS bake‐out). (2) A single pipette of
air was extracted and focused on the highest mass
collector, then individual air shots were focused on
the subsequent Faraday cups. Cup efficiency is
ensured by a volume calibrated air‐pipette system,
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Figure 2. Photomicrographs of biotite crystals in the different units that were dated by Ar‐Ar and U/Pb (all but the
Kefalos Dacite).
Table 2.
Average Composition Analyses of Kefalos Biotite Crystals by Electron Microprobea
Kefalos Dacite
Zini Dome
Latra Dome
Cumianas Dome
Kefalos Series
KPT Pumice
N
wt%
SiO2
TiO2
Al2O3
FeO
MnO
MgO
BaO
CaO
Na2O
K2O
F
Cl
Sum
6
20
5
73
6
22
38.03
4.17
14.31
14.88
0.15
14.70
0.66
0.00
0.78
8.71
0.55
0.14
96.95
38.22
4.40
13.54
15.76
0.48
13.91
0.57
0.05
0.57
8.75
0.41
0.14
96.66
38.10
4.25
13.68
15.24
0.45
14.40
0.62
0.02
0.65
8.83
0.33
0.12
96.59
37.87
4.40
13.51
16.04
0.48
13.84
0.57
0.05
0.59
8.59
0.45
0.14
96.37
37.82
4.37
13.61
15.58
0.52
13.61
0.57
0.12
0.45
8.48
0.33
0.14
95.46
36.95
4.38
13.53
17.74
0.51
12.54
0.50
0.03
0.46
8.77
0.31
0.12
96.40
Mg#
0.64
0.61
0.63
0.61
0.61
0.56
a
N, number of analyses. KPT pumice is shown for comparison. The microprobe analyses were carried with a Cameca SX50 of the University of
Lausanne, with the following parameters: 15 kV accelerating voltage, 10 nA current, with 20 to 30 s of counting on the detected signals and 5 to
15 s on the background. All Fe as Fe+2. Mineral formulas calculated as by Dymek [1983] (11 oxygen, OH+F + Cl = 2, cations‐(Ca + Na+K)+Ti = 7;
octahedral Al is always low).
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which controls the number of moles delivered to
each Faraday in turn (see also Bendezú et al.
[2008] for analytical details).
[14] The automated, UHV stainless steel gas extrac-
tion line incorporates one SAES AP10 getter, and
one SAES GP50‐ST707 getter, and the extracted
gas from biotites and sanidine grains was cooled
to ∼−150°C by a Polycold P100 cryogenic refrigeration unit mounted over a cold finger. Single to
2–3 grains of biotite were step‐heated using a
defocused 30W, MIR10 IR (CO2) laser that was
rastered over the samples to provide even‐heating of
the grains. Samples were measured on the Faraday
collectors and time‐zero regressions were fitted to
data collected from twelve cycles. Peak heights and
blanks were corrected for mass discrimination,
isotopic decay of 39Ar and 37Ar and interfering
nucleogenic Ca‐, K‐ and Cl‐derived isotopes. The
high stability of the Faraday baseline measurements renders it unnecessary to record baselines
during each analysis. Error calculations include
the errors on mass discrimination measurement
(done on 20 individual measurements prior to this
project and repeated once a day during the analyses;
40
Ar/36Ar measured at 298.25 and normalized to
295.5 [Nier, 1950]), and the J value. 40Ar, 39Ar,
38
Ar, 37Ar and 36Ar blanks were calculated before
every new sample and after every three heating
steps. 40Ar blanks were between 6.5*10−16 and
10−15 moles (signal to blank ratios are typically in
the order of 10,000–1,000,000). Blank values for
m/e 39 to 36 were all <6.5−17 moles. Age plateaus
were determined using the criteria of Dalrymple
and Lanphere [1971]. The automated analytical
process uses the software ArArCalc [Koppers,
2002] (Data Set S1).
[15] Samples from the Kos Plateau Tuff were also
analyzed at the University of Geneva following the
procedure given by Singer et al. [1999], several
years prior to the Kefalos units. KPT samples were
irradiated for 20 min. at the OSU Triga reactor, and
were analyzed using a CO2 laser and MAP 216 Ar
spectrometer. All ages were calculated relative to
1.19 Ma Alder Creek sanidine [Turrin et al., 1994].
Full analytical details are given in Data Set S2.
3.2. Results
3.2.1. Kefalos Units
[16] Out of the six Kefalos units analyzed by biotite
40
Ar/39Ar, only the Kefalos Dacite gave a plausible
age of 2.958 ± 0.024 Ma, which overlaps with
previous K/Ar ages of 2.6 ± 0.2 and 3.1 ± 0.3 Ma
40
Ar IN AEGEAN RHYOLITES
10.1029/2010GC003073
[Matsuda et al., 1999]. The concordance of these
two sets of ages provides firm evidence for Pliocene
calc‐alkaline magmatism on the Island of Kos.
All the other analyses yielded extremely disturbed
Ar release patterns, with large errors and conflicting
plateau and isochron ages (see Figures 3a–3f,
Table 3, and Data Set S1). 40Ar/36Ar intercepts for
the inverse isochrons were either higher than the
atmospheric value of 295.5 (in two cases), while the
two other cases overlapped with 295.5, but within
very large errors. The biotite 40Ar/39Ar ages appear
older than expected from stratigraphic relations and
previous K/Ar dates [Bellon and Jarrige, 1979;
Pasteels et al., 1986]. Therefore, three of these
units (Zini dome, Agios Mammas Dome and the
Kefalos Series) have been re‐dated by zircon U/Pb
to better constrain the possible range of eruption
ages for these units.
3.2.2. Kos Plateau Tuff
[17] Sanidine, plagioclase and quartz crystals from
the KPT have already been dated by 40Ar/39Ar
[Smith et al., 1996, 2000]. Sanidine crystals yielded
a very precise age of 161 ± 1 ky on multiple samples,
but plagioclase and quartz produced a more complex pattern. Plagioclase and quartz crystals (the
latter having most of its Ar released from melt
inclusions) show the presence of multiple populations up to 1730 ky old, indicating complex recycling
of crystals in the magma chambers.
[18] In this study, we report additional sanidine
40
Ar/39Ar ages on both pumices and granitoid
enclaves present in the KPT (Table 4). The ages
obtained are very similar to those of Smith et al.
[1996, 2000], and cluster around 165 to 178 ky.
Our data set shows that granitoid enclaves are
slightly older than the pumiceous material, which
is in agreement with the interpretation that they
are co‐magmatic rinds of the magma chambers
[Bachmann et al., 2007a].
4. U‐Pb Method and Results
4.1. Method
[19] U/Pb geochronology was done using isotope
dilution thermal ionization mass spectrometry
(ID‐TIMS). Zircons were separated from bulk
samples using standard mineral separation techniques. Individual zircons from each sample were
picked for chemical abrasion [Mattinson, 2005]
and combined in a quartz beaker for annealing at
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Figure 3. Ar release spectra and isochron plot for Kefalos samples.
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Table 3.
Summary of
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40
Ar/39Ar Results for Kefalos Units
Unit
Weighted
“Plateau”
Age (ka)
MSWD
on
“Plateau”
Total Fusion
Age (ka)
Inverse
Isochron
Age (ka)
MSWD
on
Isochron
Kefalos Dacite (KD01)
Zini dome (KD02)
Latra dome (KD03)
Cumianas Dome (KD04)
Agios Mammas Dome (KD07)
Kefalos Series Pyroclasts (KS03)
2958 ± 24
687 ± 16
1370 ± 110
5310 ± 320
3080 ± 580
5490 ± 170
1.3
0.76
23.10
2.28
0.69
1.68
2844.1 ± 18.5
999.0 ± 13.9
14420 ± 021
5710 ± 160
18640 ± 1120
6600 ± 210
2991.1 ± 44.5
438.6 ± 264.5
790 ± 360
1580 ± 1530
1650 ± 3100
10 ± 20
0.49
0.17
7.22
0.22
1.36
0.13
900°C for ∼60 h. All grains from a single sample
were leached together in 3 ml Savillex beakers in
HF(aq) + trace HNO3(aq) for ∼12 h at 180°C, rinsed
with water and acetone and then placed in 6N HCl
(aq) on a hotplate at ∼110°C overnight. These were
then washed several times with water, HCl, and
HNO3. Single or multiple (up to three) grains were
then handpicked for dissolution; zircons exhibited
a range of morphologies from obviously resorbed
to short and stubby prisms to euhedral needles.
Multigrain fractions consisted of zircons of similar morphology. Each fraction was spiked with
∼0.004 g of the EARTHTIME 205Pb‐233U‐235U
(ET535) tracer solution. 235U/205Pb = 100.20 was
used for the ET535 tracer, to which a total uncertainty of 0.1 was assigned. Zircons were dissolved
in ∼70 ml 40% HF and trace HNO3 in 200 ml Savillex
capsules at 210°C for 48+ hours, dried down and
re‐dissolved in 6N HCl overnight. Samples were
then dried down and re‐dissolved in 3N HCl and
put through a modified single 50 ml anion exchange
column [Krogh, 1973]. U and Pb were collected
in the same beaker and dried down with a drop of
0.05 M H3PO4 and analyzed on a single outgassed
Re filament in a Si‐gel emitter, modified from
Gerstenberger and Haase [1997]. Measurements
were performed on a Thermo‐Finnigan Triton thermal ionization mass spectrometer at the University
of Geneva.
[20] Pb was measured in dynamic mode on a
modified Masscom secondary electron multiplier
(SEM). Deadtime for the SEM was determined
by periodic measurement of NBS‐982 for up to
1.3 Mcps and observed to be constant at 23.5 ns.
Table 4.
Summary of Total Fusion
Ar/36Ar
Intercept
290.5 ± 5.8
330.6 ± 39.3
319.6 ± 16.4
315.4 ± 10.2
315.1 ± 256.8
323.8 ± 16.1
Multiplier linearity was monitored every few days
between 1.3 × 106 and <100 cps by a combination
of measurements of NBS‐981, −982 and −983, and
observed to be constant if the Faraday to SEM yield
was kept between ∼93–94% by adjusting SEM
voltage. Baseline measurements were made at
masses 203.5 and 204.5 and the average was subtracted from each peak after beam decay correction.
Interferences on 205Pb were monitored by measuring
masses 201, 202 and 203 and also by monitoring
mass 205 in unspiked samples. As a result, no corrections were applied. Pb fractionation was determined both by measuring aliquots of NBS‐982 and
also by using fractionation values determined in
other studies at Geneva that employed a 202Pb‐205Pb
double spike, and the value 0.13 ± 0.04%/a.m.u. was
used (2‐sigma standard deviation).
[21] U was measured in static mode on Faraday cups
and 1012 ohm resistors as UO+2 . Oxygen isotopic
composition was monitored by measurement of
mass 272 on large U500 loads [Wasserburg et al.,
1981]. Though the 18O/16O typically grew from
0.00200 to 0.00208 over the course of an analysis,
the most drastic increase occurred at the beginning
toward an average value of ∼0.00205. As a result,
early blocks of data were deleted and the average
value was used for all data, and corrected during
mass spectrometry. Baselines were measurement at
±0.5 mass units for 30 s every 50 ratios. Correction
for mass‐fractionation for U was done with the
double spike assuming a sample 238U/235U ratio of
137.88.
[22] All common Pb was assigned to blank, whose
composition was measured at UNIGE over the
40
Ar/39Ar Results on Samples From the KPT
Sample
K2O (wt%)
Material
Na
Granitoids
Pumices
All
11.5
11.5
11.5
K‐spar
Sanidine
K‐spar and sanidine
11 of 13
10 of 11
21 of 24
a
40
39
Ar (%)
1–57
14–81
1–81
Total Fusion Age
Age (ky) ± 2s
MSWD
177.6 ± 6.6
165.1 ± 2.0
166.1 ± 2.0
0.8
3.1
2.4
N, number of total fusion analyses. See auxiliary material for full analytical details.
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Ar IN AEGEAN RHYOLITES
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Figure 4. Comparison of 40Ar/39Ar and U/Pb results on three samples of the Kefalos Peninsula (precursors to the
KPT). Note that the 40Ar/39Ar age is always older than the youngest zircon dated.
course of this study as total procedural blanks.
After 2‐sigma outlier rejection, the composition of
fifteen blanks was: 206Pb/204Pb = 18.08 ± 0.66,
207
Pb/204Pb = 15.79 ± 0.45, 208Pb/204Pb = 37.55 ±
0.93 (2‐sigma standard deviations). Measured ratios
were reduced using the algorithms of Schmitz
and Schoene [2007] and Crowley et al. [2007].
230
Th disequilibrium was corrected using measured
Th/Umagma values from pumices in the volcanic
units sampled: KS06–7 = 3.8 ± 1.4 (2‐sigma standard deviation); Zini and CS12–05 = 4.3 ± 1.4;
KS06–3 = 3.5 ± 1.4; CS03 = 5.0 ± 1.4, and assuming
secular equilibrium in the magma.
4.2. Results
[23] Twenty‐nine single and multigrain (up to
3 grains) zircon analyses were undertaken from five
samples for which we have comparable 40Ar/39Ar
dates. Relatively large uncertainties (3–12%) are
due to (1) the very low Pb contents of these zircons
and (2) the uncertainty in the 230Th‐correction. The
latter accounts for, on average 57% of the total
uncertainty for analyses <1 Ma. This is relatively
small, considering that for the ca. 300 ka Agios
Mammas dome, the 230Th correction raises the
206
Pb/238U date by ∼25%.
[24] When these data are compared to the biotite
40
Ar/39Ar ages of the same unit (Figure 4, Table 3,
and Data Set S3), two main observations can be
drawn from the Kefalos units (Zini and Agios
Mammas rhyolitic domes and 3 samples of the
Kefalos Series pyroclasts). First, the youngest U/Pb
age is always younger than the average 40Ar/39Ar
ages. Second, several zircons were much older than
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any possible eruption age, and imply that zircon
xenocrysts are present in the erupted magma. It is
interesting to note that the much larger KPT did not
show any evidence of xenocrystic zircons, despite
having been erupted in the same general area
[Bachmann et al., 2007a]. The KPT, however, does
show a significant amount of antecrystic material,
with zircon ages 200–300 ky older than the eruption
age. Excluding xenocrystic zircons, 4 of 5 samples
from this study also show evidence for growth of
antecrystic zircon over at least 50 ka. Each sample
except CS03 has 4 or more analyses that cluster
within 100 ka, but yield high MSWDs of weighted
mean 206Pb/238U dates. The Zini dome sample,
however, yields a cluster with a weighted mean
MSWD < 1, suggesting pre‐eruptive crystallization
of zircon was shorter in this sample. For each
sample, we use the youngest zircon date as the
maximum age of eruption.
5. Discussion
[25] Comparing the 40Ar/39Ar and U‐Pb age results
on Kefalos rhyolites leads to two major findings:
(1) the biotite 40Ar/39Ar age spectra are highly
disturbed, but when a plateau is obtained, the
40
Ar/39Ar age is older than most of the zircons in
the same unit. The 40Ar/39Ar plateau age is therefore too old to represent the eruption age. (2) The
presence of xenocrystic zircons is obvious in the
three units that we have U‐Pb data for (ages up to
21 Ma). This is a common observation in silicic
units [e.g., Lanphere and Baadsgaard, 2001; Schmitt
et al., 2003; Simon and Reid, 2005; Bindeman et
al., 2006, 2008], and is expected this area, where
the pre‐existing crust hosts components that are
at least 370 Ma old [Smith et al., 2000].
[26] The cause of the disturbed and anomalous
40
Ar/39Ar spectra for the Kefalos rhyolites could be
due to either (1) inherited Ar (Ar from older
crystals), (2) 39Ar recoil effects during irradiation
[e.g., Onstott et al., 1995; Smith et al., 2008] or
(3) excess 40Ar (40Ar‐rich reservoir not equilibrated
with atmosphere) in the mineral structure or within
fluid inclusions. On the basis of the U‐Pb zircon
dates showing xenocrystic material, it is possible that some of the anomalously old ages could be
due to older biotite xenocrysts. However, Ar
isotope systematics in biotite crystals re‐equilibrate
extremely fast at magmatic temperatures (>650–
750°C); a few days would be enough for complete
resetting of Ar [McDougall and Harrison, 1999;
Gardner et al., 2002]. In addition, inherited Ar
generally resides in the most retentive parts of the
40
Ar IN AEGEAN RHYOLITES
10.1029/2010GC003073
crystals [Bachmann et al., 2007b] and leads to Ar
release spectra with ages getting older during later
heating steps. This staircase pattern is not consistent to what is observed in the Kefalos case (see
Figures 3a–3f).
[27] The importance of recoil effect in redistributing
39
Ar in minerals has long been recognized [Turner
and Cadogan, 1974], and is a known problem for
40
Ar/39Ar dating [Onstott et al., 1995]. However,
internal redistribution of 39Ar by recoil in biotite
generally occurs when crystals are small [Paine
et al., 2006] and/or between K‐rich and K‐poor
zones of the mineral when those are intergrown at
the micron to submicron scale due to weathering
[Roberts et al., 2001] or presence of alteration
phases [Smith et al., 2006, 2008]. In the sample
suite we analyzed, large (>100 microns) biotite
crystals were extracted from young, unaltered rock
fragments, and they do not show any evidence of
alteration (as indicated by observation under the
optical microscope and by electron microprobe
analyses with no conspicuous zoning in major
elements and totals summing between 96 ± 1%).
We therefore suggest that 39Ar recoil effects are
unlikely to be the dominant process by which the
reported Ar spectra are disturbed.
[28] Biotite crystals readily incorporate excess
40
Ar
due to its relatively high Ar mineral/fluid partition coefficient [Dahl, 1996; Kelley, 2002b], and
previous studies have noted disturbed patterns,
anomalously old ages and high 40Ar/36Ar intercepts
(∼320 to >500) in biotite, particularly when compared to sanidine [e.g., Renne, 1995; Villeneuve
et al., 2000; Hora et al., 2007; Lipman and
McIntosh, 2008]. In addition, biotite is likely to
retain more excess 40Ar than sanidine due to a
slightly higher partition coefficient for Ar and a
lower stochiometric K‐content [Hora et al., 2010].
In the Kefalos case, the presence of excess 40Ar is
supported by (1) the inverse isochron intercepts
of all analyses (except the dacite) showing large
errors and/or ratios higher than the atmospheric ratio
(295.5 [Nier, 1950]), (2) previously published K/Ar
ages with high 40Ar/36 Ar ratios [Matsuda et al.,
1999], and (3) 40Ar/39Ar age spectra with older
ages in low temperatures steps, indicative of partial
diffusive re‐equilibration with a magma that had
40
Ar/36Ar higher than atmosphere [Lanphere and
Dalrymple, 1976; Harrison and McDougall, 1981;
Harrison et al., 1985; Hess et al., 1987].
[29] In the Kefalos area, two factors may be
enhancing the entrapment of excess 40Ar in the
biotite structure. First, the magmas are volatile‐rich
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[Massare et al., 1978; Bachmann et al., 2009;
Hora et al., 2010] and minerals should have ample
opportunity to interact with an Ar‐rich gas phase.
Second, the very high 3He/4He measured in the
region suggests a high mantle flux [Shimizu et al.,
2005]. As Ar and He are generally coupled, it is
possible that some of 40Ar is derived from the
mantle. As the 40Ar/36Ar of the mantle is extremely
high (lower limit in the presence of potential air
contamination is 30’000 to 40’000 [Farley and
Neroda, 1998]), only a small amount of such a
reservoir would significantly disturb the Ar spectra.
Melt or fluid inclusions have not been noticed in
biotite crystals from Kefalos, but the presence of
brine inclusions has been documented in biotites
from the KPT [Bachmann, 2010]. Such inclusions
could provide a reservoir for mantle Ar [Kelley,
2002a].
[30] We suspect that excess
40
Ar might be more
problematic in areas of rapid crustal extension, as is
presently occurring in the eastern Aegean region
[Pe‐Piper et al., 2005]. The mantle reservoir is
close to the surface (crust only about 25–30 km
thick underneath Kos Island [Wortel and Spakman,
2000]), and numerous extensional faults facilitate
efficient transfer of magmas and volatiles, carrying
high amounts of mantle‐derived 40Ar and 3He
toward the surface. Other areas undergoing rapid
crustal extensions and/or high mantle flux (e.g.,
Taupo Volcanic Zone, New Zealand [e.g., Hulston
and Lupton, 1996], and hot spot related activity
[e.g., Renne, 1995]) might be more susceptible to
storing excess 40Ar than magmatic provinces
located in areas undergoing compression.
Ar/39Ar ages on biotite crystals from
Kefalos units do not appear to carry any useful age
information, the youngest zircon U/Pb date best
approximates the eruption age. However, we note
that the presence of antecrystic zircon in these
samples spanning tens of thousands of years implies
that we may not have captured the youngest
grains (we report maximum eruption ages). Agios
Mammas dome (Southern edge of the Kefalos
Peninsula) is the youngest (eruption age younger
than 309 ± 17 ka), while the Zini dome (528 ±
32 Ma) and the Kefalos Series pyroclasts (542 ±
27 ka) appear to be part of the same eruptive episode
at around 500 ka, in agreement with field observations and previous dating (K/Ar dating of glass
dating by Pasteels et al. [1986], yielding 550 ±
20 ka for Mt Zini on three samples and 560 ±
30 ka on one sample of the Kefalos Series pyroclasts).
These are the youngest ID‐TIMS U/Pb dates
reported for any geologic environment and empha[31] As the
40
40
Ar IN AEGEAN RHYOLITES
10.1029/2010GC003073
size the potential for extending this method of
dating zircon growth well into the Pleistocene.
Reducing the analytical blank as well the uncertainty in its composition are important ways of
reducing age uncertainty in such low‐Pb zircons.
Another significant source of uncertainty is the
230
Th‐disequilibrium correction. In higher‐U zircons
of similar age (e.g., the Bishop Tuff [Crowley
et al., 2007]), this is the largest source of uncertainty, and can be reduced through a better understanding of the Th/U of the magma from which
zircons crystallized. Even with these present limitations, ID‐TIMS U‐Pb dating can yield precision
and accuracy of better than 2–3% (2‐sigma) for
single Pleistocene grains.
Acknowledgments
[32] The project was supported by Swiss NSF grant 200021‐
111709/1 to Bachmann, who was also supported by U.S.
NSF‐EAR grant 0809828 during the writing of this paper.
Caroline Bouvet de Maisonneuve, Alexandra Skopelitis and
Daniel Selles are thanked for the help during field work, mineral
separation and isotopic analyses respectively. The microprobe
analyses were carried under the supervision of Catherine
Ginibre. We thank Georges Vougioukalakis and the Institute
of Geology and Mineral Exploration Greece (IGME) for kindly
providing the fieldwork permits. We thank Brad Singer and an
anonymous reviewer for constructive reviews.
References
Bachmann, O. (2010), The petrologic evolution and pre‐
eruptive conditions of the rhyolitic Kos Plateau Tuff (Aegean
Arc), Cent. Eur. J. Geosci., doi:10.2478/v10085-010-0009-4,
in press.
Bachmann, O., B. L. A. Charlier, and J. B. Lowenstern
(2007a), Zircon crystallization and recycling in the magma
chamber of the rhyolitic Kos Plateau Tuff (Aegean Arc),
Geology, 35, 73–76, doi:10.1130/G23151A.1.
Bachmann, O., F. Oberli, M. A. Dungan, M. Meier, R. Mundil,
and H. Fischer (2007b), 40Ar/39Ar and U‐Pb dating of the
Fish Canyon magmatic system, San Juan Volcanic field,
Colorado: Evidence for an extended crystallization history,
Chem. Geol., 236, 134–166, doi:10.1016/j.chemgeo.
2006.09.005.
Bachmann, O., P. J. Wallace, and J. Bourquin (2009), The
melt inclusion record from the rhyolitic Kos Plateau Tuff
(Aegean Arc), Contrib. Mineral. Petrol., doi:10.1007/
s00410-009-0423-4.
Bellon, H., and J. J. Jarrige (1979), L’activité magmatique
néogene et quaternaire dans l’ile de Kos, Greece: Données
radiochronologiques, C. R. Acad. Sci. Paris, 288, 1359–1362.
Bendezú, R., L. Page, R. Spikings, Z. Pecskay, and L. Fontboté
(2008), New 40Ar/39Ar alunite ages from the Colquijirca
district, Peru: Evidence of a long period of magmatic
SO2 degassing during formation of epithermal Au‐Ag and
Cordilleran polymetallic ores, Miner. Deposita, 43,
777–789, doi:10.1007/s00126-008-0195-1.
11 of 14
Geochemistry
Geophysics
Geosystems
3
G
BACHMANN ET AL.: EXCESS
Bindeman, I., A. Schmitt, and J. Valley (2006), U‐Pb zircon
geochronology of silicic tuffs from the Timber Mountain/
Oasis Valley caldera complex, Nevada: Rapid generation
of large volume magmas by shallow‐level remelting, Contrib.
Mineral. Petrol., 152, 649–665, doi:10.1007/s00410-0060124-1.
Bindeman, I. N., B. Fu, N. T. Kita, and J. W. Valley (2008),
Origin and evolution of silicic magmatism at Yellowstone
based on ion microprobe analysis of isotopically zoned zircons,
J. Petrol., 49, 163–193, doi:10.1093/petrology/egm075.
Brown, S. J. A., and I. R. Fletcher (1999), SHRIMP U‐Pb
dating of the pre‐eruption growth history of zircons from the
340 ka Whakamaru Ignimbrite, New Zealand: Evidence for
>250 k.y. magma residence times, Geology, 27, 1035–1038,
doi:10.1130/0091-7613(1999)027<1035:SUPDOT>2.3.
CO;2.
Charlier, B. L. A., C. J. N. Wilson, J. B. Lowenstern, S. Blake,
P. W. Van Calsteren, and J. P. Davidson (2005), Magma
generation at a large, hyperactive silicic volcano (Taupo,
New Zealand) revealed by U‐Th and U‐Pb systematics in
zircons, J. Petrol., 46, 3–32, doi:10.1093/petrology/egh060.
Costa, F. (2008), Residence times of silicic magmas associated
with calderas, in Development in Volcanology, vol. 10,
edited by J. Marti and J. Gottsmann, pp. 1–55, Elsevier,
New York.
Crowley, J. L., B. Schoene, and S. A. Bowring (2007), U‐Pb
dating of zircon in the Bishop Tuff at the millennial scale,
Geology, 35, 1123–1126, doi:10.1130/G24017A.1.
Dabalakis, P., and G. Vougioukalakis (1993), The Kefalos
Tuff ring (W. Kos): Depositional mechanisms, vent position,
and model of the evolution of the eruptive activity, Bull.
Geol. Soc. Greece, 28, 259–273.
Dahl, P. S. (1996), The crystal‐chemical basis for Ar retention
in micas: Inferences from interlayer partitioning and implications for geochronology, Contrib. Mineral. Petrol., 123,
22–39, doi:10.1007/s004100050141.
Dalrymple, G. B., and M. A. Lanphere (1971), 40Ar/39Ar technique of K‐Ar dating: A comparison with the conventional
technique, Earth Planet. Sci. Lett., 12, 300–3 08,
doi:10.1016/0012-821X(71)90214-7.
DeMets, C., R. G. Gordon, D. F. Argus, and S. Stein (1990),
Current plate motions, Geophys. J. Int., 101, 425–478,
doi:10.1111/j.1365-246X.1990.tb06579.x.
Di Paola, G. M. (1974), Volcanology and petrology of Nisyros
Island (Dodecanese, Greece), Bull. Volcanol., 38, 944–987,
doi:10.1007/BF02597100.
Dymek, R. F. (1983), Titanium, aluminium and interlayer
cation substitutions in biotite from high‐grade gneisses,
West Greenland, Am. Mineral., 68, 880–899.
Farley, K. A., and E. Neroda (1998), Noble gases in the Earth’s
mantle, Annu. Rev. Earth Planet. Sci., 26(1), 189–218,
doi:10.1146/annurev.earth.26.1.189.
Federman, A. N., and S. N. Carey (1980), Electron microprobe
correlation of tephras from eastern Mediterranean abyssal
sediments and the island of Santorini, Quat. Res., 13,
160–171, doi:10.1016/0033-5894(80)90026-5.
Fytikas, M., O. Giuliani, F. Innocenti, G. Marinelli, and
R. Mazzuoli (1976), Geochronological data on recent magmatism of the Aegean Sea, Tectonophysics, 31, T29–T34,
doi:10.1016/0040-1951(76)90161-X.
Gardner, J. E., P. W. Layer, and M. J. Rutherford (2002),
Phenocrysts versus xenocrysts in the youngest Toba Tuff:
Implications for the petrogenesis of 2800 km3 of magma,
Geology, 30, 347–350, doi:10.1130/0091-7613(2002)
030<0347:PVXITY>2.0.CO;2.
40
Ar IN AEGEAN RHYOLITES
10.1029/2010GC003073
Gerstenberger, H., and G. Haase (1997), A highly effective
emitter substance for mass spectrometric Pb isotope ratio
determinations, Chem. Geol., 136, 309–312, doi:10.1016/
S0009-2541(96)00033-2.
Harrison, T. M., and I. McDougall (1981), Excess 40Ar in
metamorphic rocks from Broken Hill, New South Wales:
Implications for 40Ar/39Ar age spectra and the thermal history
of the region, Earth Planet. Sci. Lett., 55, 123–149,
doi:10.1016/0012-821X(81)90092-3.
Harrison, T. M., I. Duncan, and I. McDougall (1985), Diffusion
of 40Ar in biotite: Temperature, pressure and compositional
effects, Geochim. Cosmochim. Acta, 49, 2461–2468,
doi:10.1016/0016-7037(85)90246-7.
Hess, J. C., H. J. Lippolt, and R. Wirth (1987), Interpretation
of 40Ar/39Ar biotites: Evidence from hydrothermal degassing
experiments and TEM studies, Chem. Geol. Isotope Geosci.
Sect., 66, 137–149, doi:10.1016/0168-9622(87)90036-4.
Hora, J. M., B. S. Singer, and G. Worner (2007), Volcano
evolution and eruptive flux on the thick crust of the Andean
Central Volcanic Zone: 40Ar/39Ar constraints from Volcan
Parinacota, Chile, Geol. Soc. Am. Bull., 119, 343–362,
doi:10.1130/B25954.1.
Hora, J. M., B. S. Singer, B. R. Jicha, B. L. Beard, C. M.
Johnson, S. de Silva, and M. Salisbury (2010), Biotite‐
sanidine 40Ar/39Ar age discordances reflect complex Ar
partitioning and pre‐eruption closure in biotite, Geology,
in press.
Hulston, J. R., and J. E. Lupton (1996), Helium isotope studies
of geothermal fields in the Taupo Volcanic Zone, New
Zealand, J. Volcanol. Geotherm. Res., 74, 297–321,
doi:10.1016/S0377-0273(96)00031-5.
Jackson, J. (1993), Rates of active deformation in the
Eastern Mediterranean, in Recent Evolution and Seismicity
of the Mediterranean Region, edited by E. Boschi et al.,
pp. 53–64, Kluwer, Dordrecht, Netherlands.
Jolivet, L. (2001), A comparison of geodetic and finite strain
pattern in the Aegean, geodynamic implications, Earth
Planet. Sci. Lett., 187(1–2), 95–104, doi:10.1016/S0012821X(01)00277-1.
Keller, J., T. H. Rehren, and E. Stadlbauer (1990), Explosive
volcanism in the Hellenic Arc: A summary and review,
in Proceedings of the Third International Congress on the
Volcano of Thera, edited by D. A. Hardy et al., pp 13–26,
Thera Found., London.
Kelley, S. (2002a), Excess argon in K‐Ar and Ar‐Ar geochronology, Chem. Geol., 188, 1–22, doi:10.1016/S0009-2541
(02)00064-5.
Kelley, S. (2002b), K‐Ar and Ar‐Ar dating, Rev. Mineral.
Geochem., 47, 785–818, doi:10.2138/rmg.2002.47.17.
Koppers, A. A. P. (2002), ArArCALC—Software for
40
Ar/39Ar age calculations, Comput. Geosci., 28, 605–619,
doi:10.1016/S0098-3004(01)00095-4.
Krogh, T. E. (1973), A low contamination method for hydrothermal decomposition of zircon and extraction of U and
Pb for isotopic age determination, Geochim. Cosmochim.
Acta, 37, 485–494, doi:10.1016/0016-7037(73)90213-5.
Kuiper, K. F., A. Deino, F. J. Hilgen, W. Krijgsman, P. R.
Renne, and J. R. Wijbrans (2008), Synchronizing rocks
clocks of Earth history, Science, 320, 500–504, doi:10.1126/
science.1154339.
Lanphere, M. A., and H. Baadsgaard (2001), Precise K‐Ar,
40
Ar/39Ar, Rb‐Sr, U/Pb mineral ages from the 27.5 Ma Fish
Canyon Tuff reference standard, Chem. Geol., 175, 653–671,
doi:10.1016/S0009-2541(00)00291-6.
12 of 14
Geochemistry
Geophysics
Geosystems
3
G
BACHMANN ET AL.: EXCESS
Lanphere, M. A., and B. G. Dalrymple (1976), Identification of
excess 40Ar by the 40Ar/39Ar age spectrum technique, Earth
Planet. Sci. Lett., 32, 141–148, doi:10.1016/0012-821X(76)
90052-2.
Lee, J. K. W., I. S. Williams, and D. J. Ellis (1997), Pb, U, and
Th diffusion in natural zircon, Nature, 390, 159–162,
doi:10.1038/36554.
Le Pichon, X., and J. Angelier (1979), The Hellenic arc and
trench system: A key to the Neotectonic evolution of the
eastern Mediterranean area, Tectonophysics, 60, 1–42,
doi:10.1016/0040-1951(79)90131-8.
Limburg, E. M., and J. C. Varekamp (1991), Young pumice
deposits on Nisyros, Greece, Bull. Volcanol., 54(1), 68–77,
doi:10.1007/BF00278207.
Lipman, P. W., and W. C. McIntosh (2008), Eruptive and
noneruptive calderas, northeastern San Juan Mountains,
Colorado: Where did the ignimbrites come from?, Geol.
Soc. Am. Bull., 120(7–8), 771–795, doi:10.1130/B26330.1.
Massare, D., M. Chaineau, and R. Clocchatti (1978), Composition chimique (éléments majeurs et volatils) des inclusions
vitreuses des phénocristaux contenus dans les ponces rhyolitiques de l’île de Kos (Arc égéen): Conséquences pétrogénétiques, C. R. Sommaires Soc. Geol. France, 5, 246–249.
Matsuda, J., K. Senoh, T. Maruoka, H. Sato, and P. Mitropoulos
(1999), K‐Ar ages of the Aegean volcanic rocks and their
implication for the arc‐trench system, Geochem. J., 33,
369–377.
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, doi:10.1016/
j.chemgeo.2005.03.011.
McDougall, I., and T. M. Harrison (1999), Geochronology and
Thermochronology by the 40 Ar/ 39 Ar Method, 269 pp.,
Oxford Univ. Press, Oxford, U. K.
Min, K., R. Mundil, P. R. Renne, and K. R. Ludwig (2000), A
test for systematic errors in 40Ar/39Ar geochronology through
comparison with U/Pb analysis of a 1.1‐Ga rhyolite, Geochim.
Cosmochim. Acta, 64, 73–98, doi:10.1016/S0016-7037(99)
00204-5.
Nier, A. O. (1950), A redetermination of the relative abundances
of the isotopes of carbon, nitrogen, oxygen, argon, and potassium, Phys. Rev., 77, 789–793, doi:10.1103/PhysRev.77.789.
Onstott, T. C., M. L. Miller, R. C. Ewing, G. W. Arnold, and
D. S. Walsh (1995), Recoil refinements: Implications for the
40
Ar/39Ar dating technique, Geochim. Cosmochim. Acta, 59,
1821–1834, doi:10.1016/0016-7037(95)00085-E.
Paine, J. H., S. Nomade, and P. R. Renne (2006), Quantification of 39Ar recoil ejection from GA1550 biotite during
neutron irradiation as a function of grain dimensions,
Geochim. Cosmochim. Acta, 70, 1507–1517, doi:10.1016/
j.gca.2005.11.012.
Pasteels, P., N. Kolios, A. Boven, and E. Saliba (1986), Applicability of the K/Ar method to whole‐rock samples of acid
lava and pumice: Case of the Upper Pleistocene domes
and pyroclasts on Kos Island, Aegean Sea, Greece, Chem.
Geol., 57, 145–154, doi:10.1016/0009-2541(86)90099-9.
Pe‐Piper, G., and B. Moulton (2008), Magma evolution in the
Pliocene‐Pleistocene succession of Kos, South Aegean arc
(Greece), Lithos, 106, 110–124, doi:10.1016/j.lithos.
2008.07.002.
Pe‐Piper, G., and D. J. W. Piper (2002), The Igneous Rocks of
Greece, 573 pp., Borntreager, Berlin.
Pe‐Piper, G., D. J. W. Piper, and C. Perissoratis (2005), Neotectonics and the Kos Plateau Tuff eruption of 161 ka, South
40
Ar IN AEGEAN RHYOLITES
10.1029/2010GC003073
Aegean arc, J. Volcanol. Geotherm. Res., 139, 315–338,
doi:10.1016/j.jvolgeores.2004.08.014.
Rehren, T. (1988), Geochemie und Petrologie von Nisyros
(Ofstliche Agais). Ph.D. thesis, Dep. of Geol., Univ. of
Freiburg, Freiburg, Germany.
Reid, M. R., C. D. Coath, T. M. Harrison, and K. D. McKeegan
(1997), Prolonged residence times for the youngest rhyolites
associated with Long Valley Caldera: 230Th‐238U microprobe
dating of young zircons, Earth Planet. Sci. Lett., 150, 27–39,
doi:10.1016/S0012-821X(97)00077-0.
Renne, P. R. (1995), Excess 40Ar in biotite and hornblende
from the Noril’sk 1 intrusion, Siberia: Implications for the
age of the Siberian Traps, Earth Planet. Sci. Lett., 131,
165–176, doi:10.1016/0012-821X(95)00015-5.
Renne, P. R., D. B. Karner, and K. R. Ludwig (1998), Absolute
ages aren’t exactly, Science, 282, 1840–1841, doi:10.1126/
science.282.5395.1840.
Roberts, H. J., S. P. Kelley, and P. S. Dahl (2001), Obtaining
geologically meaningful 40Ar/39Ar ages from altered biotite,
Chem. Geol., 172, 277–290, doi:10.1016/S0009-2541(00)
00255-2.
Schmitt, A. K., J. M. Lindsay, S. de Silva, and R. B. Trumbull
(2003), U‐Pb zircon chronostratigraphy of early Pliocene
ignimbrites from La Pacana, north Chile: Implications for
the formation of stratified magma chambers, J. Volcanol.
Geotherm. Res., 120, 43–53, doi:10.1016/S0377-0273(02)
00359-1.
Schmitz, M. D., and B. Schoene (2007), Derivation of isotope
ratios, errors and error correlations for U‐Pb geochronology
using 205Pb‐235U‐233U‐spike isotope dilution thermal ionization mass spectrometric data, Geochem. Geophys. Geosyst., 8,
Q08006, doi:10.1029/2006GC001492.
Shimizu, A., H. Sumino, K. Nagao, K. Notsu, and P. Mitropoulos
(2005), Variation in noble gas isotopic composition of gas
samples from the Aegean arc, Greece, J. Volcanol. Geotherm.
Res., 140(4), 321–339, doi:10.1016/j.jvolgeores.2004.08.016.
Simon, J. I., and M. R. Reid (2005), The pace of rhyolite differentiation and storage in an ‘archetypical’ silicic magma
system, Long Valley, California, Earth Planet. Sci. Lett.,
235, 123–140, doi:10.1016/j.epsl.2005.03.013.
Simon, J. I., P. R. Renne, and R. Mundil (2008), Implications
of pre‐eruptive magmatic histories of zircons for U‐Pb geochronology of silicic extrusions, Earth Planet. Sci. Lett., 266,
182–194, doi:10.1016/j.epsl.2007.11.014.
Singer, B. S., K. A. Hoffman, A. Chauvin, R. S. Coe, and
M. S. Pringle (1999), Dating transitionally magnetized lavas
of the late Matuyama Chron: Toward a new 40 Ar/ 39 Ar
timescale of reversals and events, J. Geophys. Res., 104,
679–693, doi:10.1029/1998JB900016.
Smith, M. E., B. S. Singer, A. R. Carroll, and J. H. Fournelle
(2006), High‐resolution calibration of Eocene strata:
40
Ar/39Ar geochronology of biotite in the Green River Formation, Geology, 34, 393–396, doi:10.1130/G22265.1.
Smith, M. E., B. S. Singer, A. R. Carroll, and J. H. Fournelle
(2008), Precise dating of biotite in distal volcanic ash: Isolating
subtle alteration using 40Ar/39Ar laser incremental heating and
electron microprobe techniques, Am. Mineral., 93, 784–795,
doi:10.2138/am.2008.2517.
Smith, P. E., D. York, Y. Chen, and N. M. Evensen (1996),
Single crystal 40Ar/39Ar dating of a late Quaternary paroxysm
on Kos, Greece: Concordance of terrestrial and marine ages,
Geophys. Res. Lett., 23, 3047–3050, doi:10.1029/
96GL02759.
Smith, P. E., N. M. Evensen, and D. York (2000), Under the
volcano: A new dimension in Ar‐Ar dating of volcanic
13 of 14
Geochemistry
Geophysics
Geosystems
3
G
BACHMANN ET AL.: EXCESS
ash, Geophys. Res. Lett., 27, 585–588, doi:10.1029/
1999GL008451.
Tischendorf, G., H.‐J. Förster, and B. Gottesmann (2001),
Minor and trace‐element composition of trioctahedral micas:
A review, Mineral. Mag., 65, 249–276, doi:10.1180/
002646101550244.
Turner, G., and P. H. Cadogan (1974), Possible effect of 39Ar
recoil in 40Ar/39Ar dating, in Proceedings of the Fifth Lunar
Conference, vol. 2, pp. 1601–1615, Lunar Planet. Inst.,
Houston, Tex.
Turrin, B. D., J. M. Donnelly Nolan, and B. C. Hearn (1994),
40
Ar/39Ar ages from the rhyolite of Alder Creek, California:
Age of the Cobb Mountain Normal‐Polarity Subchron
revisited, Geology, 22, 251–254, doi:10.1130/0091-7613
(1994)022<0251:AAAFTR>2.3.CO;2.
Vazquez, J. A., and M. R. Reid (2004), Probing the accumulation history of the voluminous Toba magma, Science, 305,
991–994, doi:10.1126/science.1096994.
40
Ar IN AEGEAN RHYOLITES
10.1029/2010GC003073
Villeneuve, M., H. A. Sandeman, and W. J. Davis (2000), A
method for intercalibrations of U‐Th‐Pb and 40Ar/39Ar
ages in the Phanerozoic, Geochim. Cosmochim. Acta, 64,
4017–4030, doi:10.1016/S0016-7037(00)00484-1.
Wagner, G. J., D. Storzer, and J. Keller (1976), Spaltspurendatierungen quartarer Gesteinsglaser aus dem Mittelmeerraum,
Neues Jahrb. Miner. Monatsh., 2, 84–94.
Wasserburg, G. J., S. B. Jacousen, D. J. DePaolo, M. T.
McCulloch, and T. Wen (1981), Precise determinations of
Sm/Nd ratios, Sm and Nd isotopic abundances in standard
solutions, Geochim. Cosmochim. Acta, 45(12), 2311–2323,
doi:10.1016/0016-7037(81)90085-5.
Wortel, M. J. R., and W. Spakman (2000), Subduction and slab
detachment in the Mediterranean‐Carpathian region, Science,
290, 1910–1917, doi:10.1126/science.290.5498.1910.
14 of 14
Table S1: Coordinates of samples location
Sample
CS03
KS03
KS06-3
KS06-7
KD01
KD02
KD04,
CS11-05
KD07,
CS12-05
KD03,
CS14-05
Location
Kefalos Series, Fountain outcrop
Kefalos Series, Fountain outcrop
Kefalos Series, Agios Stefanos Road
Kefalos Series, Agios Stefanos Road
Dacitic dyke
Mt. Zini summit's quarry
Latitude
36°43.798'
36°43.798'
36°45.481
36°45.551
36°43.416'
36°43.394'
Longitude
26°57.321'
26°57.321'
26°59.436
26°59.436
26°57.388'
26°58.382'
Alt. m
156
156
49
55
230
328
Mt. Cumianas
36°42.283'
26°56.768'
250
Mt. Agios Mammas
36°40.680'
26°57.914'
210-190
Mt. Latra
36°42.204'
26°57.064'
352
Data Set S1: 40Ar/39Ar isotopic data for Kefalos biotites.
40
Ar/39Ar
37
Ar/39Ara
36
Ar/39Ar
40
Ar* (moles) %40Ar* K/Ca
Total fusion analyses
KD01 (bio)
apparent age (ky ± 1σ)
In bold = used in plateau
J=2.2E-4
N=18
GE33-10
40.17894
0.00000
1.07235
5.714E-16
11.25
0.00
15876.8 ±4561.7
GE33-11
39.65347
0.19948
0.86283
4.577E-15
13.46
2.16
15670.1 ±882.7
GE33-17
2.71985
0.11172
0.12986
8.293E-15
6.62
3.85
1079.2 ±89.2
GE33-18
5.26668
0.09820
0.10561
8.327E-15
14.44
4.38
2089.1 ±109.05
GE33-19
8.41150
0.10555
0.05992
5.601E-15
32.20
4.07
3335.4 ±59.05
GE33-20
8.14440
0.10395
0.04209
3.804E-15
39.57
4.14
3229.6 ±64.6
GE33-21
8.40018
0.09192
0.03920
3.902E-15
42.03
4.68
3330.9 ±71.8
GE33-22
7.82566
0.06648
0.03637
5.206E-15
42.13
6.47
3103.3 ±42.2
GE33-23
8.71532
0.07594
0.03652
6.697E-15
44.68
5.66
3455.8 ±39.75
GE33-24
7.69219
0.05674
0.02817
7.132E-15
48.03
7.58
3050.4 ±28.85
GE33-25
7.85000
0.04795
0.03909
1.311E-14
40.46
8.97
3113.0 ±24.05
GE33-26
7.38222
0.05915
0.02200
8.479E-15
53.17
7.27
2927.6 ±22.9
GE33-27
7.44963
0.09333
0.01912
1.107E-14
56.87
4.61
2954.3 ±17.85
GE33-28
7.43004
0.05784
0.02630
1.759E-14
48.88
7.43
2946.6 ±16.8
GE33-29
7.49475
0.05685
0.01016
2.260E-14
71.38
7.56
2972.2 ±11.15
GE33-30
7.26063
0.03036
0.01130
9.484E-15
68.50
14.17
2879.4 ±13.55
GE33-31
6.99880
0.02602
0.00982
1.902E-14
70.68
16.52
2775.7 ±9.9
GE33-32
6.21958
0.03858
0.01390
2.627E-14
60.22
11.15
2466.8 ±5.6
KD02 (bio) J=2.17E-4 N=21
GE36-10
11.25519
0.05496
0.62814
2.631E-14
5.72
7.82
4400.9 ±241.05
GE36-11
20.47214
0.02897
0.44572
1.285E-14
13.45
14.84
7996.8 ±204.1
GE36-12
9.91683
0.01876
0.27910
1.389E-14
10.73
22.92
3878.1 ±108.25
GE36-13
6.81752
0.01342
0.15734
1.258E-14
12.79
32.04
2667.0 ±62.55
GE36-14
4.41982
0.03187
0.10053
9.027E-15
12.95
13.49
1729.5 ±37.25
GE36-15
3.42990
0.02526
0.06728
8.756E-15
14.71
17.02
1342.3 ±27.55
GE36-16
2.83740
0.02299
0.05187
8.143E-15
15.62
18.71
1110.5 ±24.65
GE36-17
2.46834
0.02574
0.04204
8.198E-15
16.57
16.70
966.1 ±22
GE36-18
2.15265
0.01928
0.03482
7.103E-15
17.30
22.30
842.5 ±23.70
GE36-19
2.09097
0.01974
0.02898
7.841E-15
19.62
21.78
818.4 ±12.43
GE36-20
2.00896
0.02906
0.02554
7.064E-15
21.02
14.79
786.3 ±15.55
GE36-21
1.84447
0.02954
0.02603
5.719E-15
19.34
14.56
721.9 ±20.25
GE36-22
1.81351
0.02797
0.02172
5.348E-15
22.03
15.38
709.8 ±18.25
GE36-23
1.79302
0.02564
0.01973
5.281E-15
23.52
16.77
701.8 ±18.35
GE36-24
1.75422
0.02146
0.01888
5.215E-15
23.91
20.04
686.6 ±16.10
GE36-25
1.82822
0.02522
0.01970
4.220E-15
23.89
17.05
715.6 ±24.55
GE36-26
1.78196
0.02477
0.01876
4.467E-15
24.32
17.36
697.5 ±19.95
GE36-27
1.74376
0.02471
0.01759
4.413E-15
25.11
17.40
682.5 ±19.55
GE36-28
1.80839
0.02539
0.01828
3.315E-15
25.08
16.94
707.8 ±29.50
GE36-30
1.82786
0.02763
0.01957
2.777E-15
24.02
15.56
715.4 ±41.20
GE36-31
1.70595
0.02222
0.01672
5.771E-15
25.66
19.35
667.7 ±13.95
40
Ar/39Ar
37
Ar/39Ara
36
Ar/39Ar
40
Ar* (moles) %40Ar* K/Ca
apparent age (ky ± 1σ)
KD03 (bio) J=2.21E-4 N=17
GE36-10
69.22878
0.05770
2.27462
6.168E-14
9.34
7.45
27390 ±635
GE36-11
47.33328
0.02796
2.25902
1.164E-13
6.62
15.38
18770 ±4790
GE36-12
572.43388
0.01227
1.18141
3.755E-13
62.12
35.03
214910 ±980
GE36-13
59.89645
0.00897
1.42218
1.975E-13
12.47
47.96
23720 ±740
GE36-14
141.58211
0.00828
0.60447
1.105E-13
44.22
51.92
55580 ±755
GE36-15
12.59420
0.00343
0.29795
9.704E-14
12.51 125.35
5010 ±245
GE36-16
11.76421
0.00611
0.16998
3.232E-14
18.98
70.37
4680 ±50
GE36-17
10.01172
0.00438
0.13875
3.614E-14
19.63
98.27
3990 ±55
GE36-18
4.51466
0.00397
0.08637
1.746E-14
15.03 108.26
1800 ±30
GE36-19
4.84719
0.00232
0.07799
6.075E-14
17.38 185.32
1930 ±60
GE36-21
3.65624
0.00048
0.07064
1.138E-14
14.91 888.17
1460 ±25
GE36-22
3.48123
0.00000
0.05626
9.991E-15
17.31
0.00
1390 ±25
GE36-23
3.35891
0.00000
0.05051
1.133E-14
18.37
0.00
1340 ±25
GE36-24
3.09593
0.00273
0.05399
1.178E-14
16.25 157.62
1230 ±20
GE36-25
3.90449
0.00185
0.07773
1.978E-14
14.53 231.94
1560 ±30
GE36-26
5.81549
0.00357
0.11442
2.129E-14
14.68 120.32
2320 ±40
GE36-28
4.72911
0.00173
0.11090
1.785E-14
12.61 249.05
1880 ±40
KD04 (bio) J=2.16E-4 N=24
GE36-13
110.70588
0.00000
4.18064
4.753E-15
8.22
0.00
42630 ±4100
GE36-14
64.28293
0.00000
1.68354
3.201E-15
11.44
0.00
24880 ±2775
GE36-15
32.85913
0.00000
1.20514
5.543E-15
8.45
0.00
12760 ±1060
GE36-16
17.14463
0.00308
0.65997
6.091E-15
8.08 139.61
6670 ±490
GE36-17
14.22037
0.00000
0.49520
6.809E-15
8.86
0.00
5530 ±315
GE36-18
13.29128
0.00000
0.49617
7.671E-15
8.31
0.00
5170 ±245
GE36-19
16.35882
0.01815
0.52480
3.840E-14
9.54
23.69
6360 ±325
GE36-20
10.52053
0.01959
0.37791
1.564E-14
8.61
21.95
4090 ±170
GE36-21
10.57235
0.02233
0.37840
9.643E-15
8.64
19.26
4120 ±120
GE36-22
11.80473
0.04329
0.43157
8.209E-15
8.47
9.93
4590 ±175
GE36-23
16.76730
0.03213
0.57670
1.434E-14
8.96
13.38
6520 ±265
GE36-24
18.58314
0.04269
0.66424
1.706E-14
8.65
10.07
7230 ±310
GE36-25
16.54090
0.03835
0.58280
1.137E-14
8.76
11.21
6430 ±245
GE36-26
11.13561
0.06768
0.37357
4.772E-15
9.16
6.35
4330 ±410
GE36-27
12.73458
0.05874
0.43901
6.831E-15
8.94
7.32
4960 ±315
GE36-28
13.19247
0.05358
0.48879
6.512E-15
8.37
8.03
5130 ±350
GE36-30
15.50001
0.03892
0.56525
1.272E-14
8.49
11.05
6030 ±220
GE36-31
13.63750
0.04192
0.49101
6.351E-15
8.59
10.26
5310 ±385
GE36-32
13.58968
0.05060
0.48663
5.718E-15
8.63
8.50
5290 ±435
GE36-33
14.21553
0.04512
0.45320
5.169E-15
9.60
9.53
5530 ±515
GE36-34
12.63233
0.04447
0.40022
3.927E-15
9.65
9.67
4920 ±600
GE36-35
13.35092
0.03971
0.47765
7.614E-15
8.64
10.83
5200 ±215
GE36-36
11.65098
0.02658
0.37605
5.124E-15
9.49
16.18
4530 ±340
GE36-37
12.73529
0.03792
0.40209
1.381E-14
9.68
11.34
4960 ±205
8.72167
8.403E-15
10.02
6.37
110400 ±5435
KD07 (bio) J=2.2E-4
GE36-10
286.85167
N=13
0.06755
40
Ar/39Ar
37
Ar/39Ara
36
Ar/39Ar
40
Ar* (moles) %40Ar* K/Ca
apparent age (ky ± 1σ)
GE36-11
489.79383
0.00000 14.70848
5.029E-14
10.13
0.00
184620 ±11165
GE36-12
130.44871
0.00000
5.15771
2.413E-14
7.88
0.00
51050 ±2005
GE36-13
60.76381
0.00327
1.98555
1.401E-14
9.38 131.45
GE36-14
28.03600
0.00000
0.83625
6.033E-15
10.19
0.00
11090 ±630
GE36-15
12.40857
0.00000
0.44844
6.409E-15
8.56
0.00
4920 ±285
GE36-16
7.04084
0.02679
0.18425
2.649E-15
11.45
16.05
2790 ±380
GE36-17
8.36144
0.02478
0.19829
1.622E-15
12.49
17.35
3320 ±665
GE36-18
9.00508
0.03627
0.17669
1.353E-15
14.71
11.85
3570 ±590
GE36-19
3.40830
0.07810
0.09523
7.149E-16
10.80
5.51
1350 ±360
GE36-20
1.46602
0.23844
0.10930
1.994E-16
4.34
1.80
580 ±1040
GE36-22
33.06937
0.00000
0.24328
2.631E-16
31.51
0.00
13080 ±2685
GE36-23
34.06564
0.00000
0.18262
2.666E-16
38.70
0.00
13470 ±2010
KS03 (bio) J=2.23E-4 N=23
GE36-11
20.13275 0.02657
0.62548
1.208E-13
9.82
16.18
8080 ±310
GE36-12
16.27802
0.00000
0.57406
4.309E-15
8.76
0.00
6540 ±610
GE36-13
16.89337
0.01825
0.53365
2.464E-14
9.68
23.57
6780 ±580
GE36-14
15.44520
0.00185
0.53888
1.529E-14
8.84 232.21
6200 ±480
GE36-15
15.66792
0.00000
0.51202
1.048E-14
9.38
0.00
6290 ±195
GE36-16
14.85265
0.03605
0.48194
1.093E-14
9.44
11.93
5970 ±165
GE36-17
14.02664
0.05566
0.49257
1.253E-14
8.79
7.73
5630 ±195
GE36-19
14.10437
0.04428
0.50218
1.181E-14
8.68
9.71
5670 ±190
GE36-20
14.72900
0.05430
0.51887
1.143E-14
8.76
7.92
5920 ±190
GE36-21
13.25613
0.05134
0.46167
1.978E-14
8.86
8.38
5330 ±255
GE36-22
14.17826
0.05099
0.48515
1.496E-14
9.00
8.43
5700 ±230
GE36-23
13.47452
0.06157
0.48356
9.193E-15
8.62
6.98
5410 ±185
GE36-24
12.86450
0.06387
0.45259
8.498E-15
8.77
6.73
5170 ±175
GE36-25
12.79047
0.07677
0.45402
8.602E-15
8.70
5.60
5140 ±190
GE36-26
14.05699
0.08942
0.48881
7.697E-15
8.87
4.81
5650 ±260
GE36-27
13.42298
0.13145
0.47833
6.886E-15
8.67
3.27
5390 ±290
GE36-28
15.10576
0.10061
0.50018
6.149E-15
9.27
4.27
6070 ±395
GE36-29
16.19229
0.09827
0.51484
5.914E-15
9.62
4.38
6500 ±450
GE36-30
16.73740
0.06984
0.49781
5.372E-15
10.22
6.16
6720 ±495
GE36-31
15.68982
0.15374
0.53744
5.659E-15
8.99
2.80
6300 ±430
GE36-32
17.49143
0.08794
0.57330
4.417E-15
9.36
4.89
7020 ±630
GE36-33
17.10382
0.11288
0.54050
4.658E-15
9.67
3.81
6870 ±600
GE36-34
16.58121
0.16262
0.56240
9.771E-15
9.07
2.64
6660 ±240
23960 ±605
Data Set S2: 40Ar/39Ar isotopic data for each individual analysis on KPT samples
40
Ar/39Ar
37
Ar/39Ara
36
Ar/39Ar
40
Ar* %40Ar* K/Ca Apparent ageb step used
in
(10-14 mol)
(ky) ± 1σ
regression
Total fusion analyses
KosPum2
(san)
17/D3K-2
17/D3K-6
17/D3K-8
KosPum2
(san)
17/D3K-12
17/D3K-12B
KosPum1
(san)
17/D2K-16
17/D2K-16B
17/D2K-2B
17/D2K-2C
17/D2K-2D
KosXeno1 (Kspar)
17/D4S-2
17/D4K-4
17/D4K-4B
KosXeno4 (Kspar)
17/D1K-16
17/D1K-20
17/D1K-2
17/D1K-6
17/D1K-6B
17/D1K-6C
17/D1K-6D
17/D1K-6E
a
J=9.16E-5
N=3 of 3
1.029
1.027
1.005
0.1442
0
0.009
J=9.05E-5
N=2 of 2
1.029
1.029
0
0.0107
J=8.775E-5
N=5 of 6
0.713
1.017
0.991
1.09
1.035
0
0.0023
0.0647
0
0
J=8.58E-5
N=3 of 5
1.158
0.96
0.98
0.0359
0.003
0.0978
J=9.05E-5
N=8 of 8
1.072
2.943
1.092
1.332
2.12
1.057
1.101
1.113
0.0275
2.217
0.0234
0.8725
0.2364
0.0757
0.0784
0.0514
0.01551
0.00083
0.00142
1.7
1.1
1.3
18.4
80.8
70.6
- 170.11 ± 10.43
- 169.68 ± 2.72
- 166.02 ± 3.16
*
0.0145
0.0011
0.5
1.7
14.3
76.1
- 116.56 ± 20.56
- 167.96 ± 2.16
*
*
0.00515
0.00182
0.00363
0.00138
0.00121
0.4
2.2
0.4
0.8
1.6
31.9
65.4
48.2
72.8
74.3
-
± 17.4
± 1.95
± 13.12
± 4.6
± 2.1
*
0.04017
0.07233
0.16351
0.5
0.3
1.3
26.9
13.9
55.8
- 179.14 ± 12.19
- 148.6 ± 20.27
- 151.61 ± 29.60
*
0.0921
0.05559
0.00281
0.01055
0.87964
0.03299
0.01616
0.05434
18.4
0.4
1.3
0
7
1.9
2.2
13.8
3.8
15.3
56.8
30.4
0.8
9.8
18.8
6.5
-
*
*
*
*
*
*
*
*
112.9
160.86
156.99
172.47
163.87
175.05
480.39
178.28
217.42
346.22
172.69
179.78
181.72
±
±
±
±
±
±
±
±
18.46
138.5
4.08
261.5
501.9
20.01
9.79
12.61
*
*
*
*
Corrected for 37Ar and 39Ar decay: half-lives of 35 days and 259 years respectively. bAll
ages are calculated relative to 1.19 Ma Alder Creek sanidine (Turrin et al. 1994). Decay
constants: λE = 0.581x10-10/yr; λB = 4.692x10-10/yr. Power of CO2 laser used: 25 W. 2
analyses of KosXeno1 (17/D4K-2, 17/D4s-1) and 1 of KosPum1 (17/D2K-2) were not
used in the calculations due to clearly aberrant values (negative age or ages in excess of
6 Ma with very large errors).
3.4
0.4
0.3
4.2
0.6
0.5
7.0
21.8
0.3
0.2
0.3
0.2
4.3
0.3
2.6
0.4
0.3
1.0
1.2
3.5
1.0
0.4
1.0
0.9
0.4
0.6
0.8
1.2
5.9
CS12-05 z1 (1 sb,e)
CS12-05 z2 (1 eu,st)
CS12-05 z3 (1 eu,e)
CS12-05 z4 (1 eu,e)
CS12-05 z5 (2 eu,e)
CS12-05 z6 (2 eu,e)
KS06-7 z1 (2 sb,st)
KS06-7 z2 (2 an,bl)
KS06-7 z3 (1 eu,e)
KS06-7 z4 (1 eu,e)
KS06-7 z5 (1 eu,e)
KS06-7 z6 (1 eu,e)
KS06-3 z1 (3 eu,e)
KS06-3 z2 (3 r,bl)
KS06-3 z3 (1 sb,e)
KS06-3 z4 (1 sb,st)
KS06-3 z5 (3 eu,e)
zini z1 (1 eu,bl)
zini z2 (3 eu,bl)
zini z3 (3 sb,e)
zini z4 (1 sb,bl)
zini z5 (1 sb,bl)
zini z6 (1 sb,bl)
CS03 z1 (2 sb,st)
CS03 z2 (2 sb,st)
CS03 z3 (2 r,st)
CS03 z4 (2 r,st)
CS03 z5 (2 r,st)
CS03 z6 (2 r,st)
1.6
0.7
1.1
1.7
0.8
0.6
1.7
0.8
2.0
1.3
0.8
1.5
0.9
1.2
2.4
1.4
1.5
1.7
1.5
0.3
1.8
2.2
1.9
6.3
10.7
6.4
15.4
16.1
2.0
(d)
(c)
0.8
1.3
1.1
0.5
0.4
0.6
1.5
0.9
1.7
0.5
0.4
2.1
1.3
1.9
0.4
0.7
0.7
1.6
1.2
3.8
0.7
2.2
0.5
0.5
2.9
0.6
0.5
0.5
0.5
Th
U
Pbc
(pg)
Pb
Pb
126
30
26
153
36
34
242
725
27
26
27
26
156
27
102
30
30
51
59
131
51
31
52
42
27
32
44
59
207
(e)
204
206
Pb
Pb
0.603
0.294
0.482
0.657
0.334
0.270
0.538
0.263
0.697
0.490
0.315
0.546
0.304
0.441
0.814
0.509
0.554
0.608
0.545
0.094
0.653
0.791
0.691
1.953
3.150
1.991
4.664
5.052
0.650
(f)
206
208
Pb
Pb
0.06050
0.06607
0.07221
0.06015
0.07382
0.06842
0.07200
0.04702
0.05405
0.05597
0.04179
0.06762
0.04816
0.07030
0.05087
0.05715
0.05454
0.04939
0.05253
0.06111
0.05126
0.05463
0.05049
0.35783
0.85911
0.59007
0.04691
0.04297
0.05910
(f)
206
207
3.20
37.65
50.78
2.53
78.61
73.59
1.14
0.59
47.59
493.05
403.91
43.98
2.85
34.49
5.79
95.25
107.49
13.13
9.35
2.59
21.42
28.91
26.45
21.90
9.87
21.88
37.57
15.71
1.78
(g)
% err
Pb
U
0.000567
0.000323
0.000356
0.000400
0.000423
0.000414
0.016596
0.009222
0.000618
0.000562
0.000419
0.000647
0.005068
0.000730
0.000537
0.000578
0.000560
0.000508
0.000521
0.027988
0.000543
0.000561
0.000521
0.006241
0.026247
0.012807
0.004107
0.010032
0.018235
(f)
235
207
3.44
38.91
52.58
2.71
78.93
74.02
1.24
0.63
49.79
493.57
404.47
46.54
3.02
36.68
6.09
95.88
108.14
13.87
9.91
2.79
21.84
30.51
26.85
22.02
10.26
22.00
37.97
16.20
1.91
(g)
% err
Pb
U
0.0000680
0.0000354
0.0000357
0.0000483
0.0000416
0.0000438
0.0016716
0.0014225
0.0000830
0.0000728
0.0000727
0.0000694
0.0007633
0.0000753
0.0000766
0.0000734
0.0000745
0.0000746
0.0000719
0.0033216
0.0000768
0.0000745
0.0000748
0.0001265
0.0002216
0.0001574
0.0006350
0.0016931
0.0022377
(f)
238
206
0.25
1.97
2.87
0.20
1.52
1.63
0.12
0.07
2.65
4.16
3.39
3.09
0.18
2.59
0.34
2.04
2.16
0.78
0.59
0.20
0.76
1.78
0.81
1.21
2.83
1.73
0.93
0.65
0.14
(g)
% err
0.95
0.65
0.64
0.90
0.22
0.27
0.90
0.60
0.84
0.13
0.17
0.84
0.96
0.86
0.89
0.32
0.31
0.95
0.96
0.98
0.56
0.91
0.50
0.13
0.27
0.11
0.44
0.77
0.91
(h)
corr.
coef.
Pb
U
2.67
2.34
3.85
3.69
2.22
1.96
0.20
0.11
4.38
4.65
3.44
4.69
0.29
3.61
5.58
3.76
4.26
2.61
2.33
0.20
2.76
3.84
3.00
5.06
5.94
4.36
2.62
1.14
0.17
(g)
% err
Th-corrected
0.0000785
0.0000497
0.0000482
0.0000585
0.0000554
0.0000582
0.0016811
0.0014358
0.0000911
0.0000838
0.0000859
0.0000791
0.0007757
0.0000864
0.0000821
0.0000836
0.0000850
0.0000850
0.0000830
0.0033374
0.0000866
0.0000827
0.0000842
0.0001221
0.0002022
0.0001527
0.0006000
0.0016557
0.0022477
(f)
238
206
230
0.47
0.56
0.50
0.73
0.16
0.23
0.58
0.41
0.53
0.12
0.17
0.58
0.62
0.64
0.51
0.19
0.17
0.37
0.35
0.98
0.21
0.47
0.19
0.14
0.35
0.13
0.19
0.46
0.80
(h)
corr.
coef.
Pb
U
0.58
0.33
0.36
0.41
0.43
0.42
16.71
9.32
0.63
0.57
0.43
0.66
5.13
0.74
0.55
0.59
0.57
0.52
0.53
28.03
0.55
0.57
0.53
6.32
26.31
12.92
4.16
10.14
18.35
(i)
235
207
0.02
0.13
0.19
0.01
0.34
0.31
0.21
0.06
0.31
2.82
1.72
0.31
0.15
0.27
0.03
0.56
0.61
0.07
0.05
0.77
0.12
0.17
0.14
1.39
2.66
2.83
1.58
1.63
0.35
(j)
±
Pb
U
0.506
0.321
0.311
0.377
0.357
0.375
10.828
9.249
0.587
0.540
0.554
0.510
4.998
0.557
0.529
0.539
0.548
0.548
0.535
21.478
0.558
0.533
0.543
0.787
1.303
0.984
3.867
10.664
14.474
(i)
238
206
±
0.013
0.008
0.012
0.014
0.008
0.007
0.022
0.010
0.026
0.025
0.019
0.024
0.014
0.020
0.030
0.020
0.023
0.014
0.012
0.044
0.015
0.020
0.016
0.040
0.077
0.043
0.101
0.121
0.024
(j)
Dates (230Th-corrected)
(a) z1, z2 etc. are labels for zircon fractions (# of grains, grain description). sb = subhedral, eu = euhedral, r = rounded/resorbed, e=elongate, st = stubby, bl = blocky, (b) Ratio of radiogenic Pb to
common Pb, excluding 208Pb from both., (c) Total weight of common Pb, including 208Pb., (d) Model Th/U ratio calculated from radiogenic 208Pb/206Pb ratio and 206Pb/238U age. (e) Measured ratio
corrected for spike and fractionation only. Mass fractionation corrections were based on analysis of NBS-981, and samples measured at UNIGE with the 202Pb-205Pb tracer. Correction was 0.13 ±
0.04%/amu (atomic mass unit). (f) Corrected for fractionation, spike, and blank. All common Pb was assumed to be procedural blank. (g) Errors are 2 sigma, propagated using the algorithms of) and
Crowley et al. (2007). (h) correlation coefficient for 206Pb/238U and 207Pb/235U ratios, (i) Calculations are based on the decay constants of Jaffey et al. (1971). 206Pb/238U date corrected for initial
disequilibrium in 230Th/238U using values and uncertainties listed in the text, assuming activity ratio of one. (j) Errors are 2 sigma.
Pbc
(b)
Sample
(a)
Pb*
Uncorrected for 230Th disequilibrium
Data Set S3: Summary of U/Pb isotopic data and ages.