The Ar/ Ar and U/Pb dating of young rhyolites Greece: Age ......in the Kos‐Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40Ar in biotite O

Article

Volume 114 August 2010

Q0AA08, doi:10.1029/2010GC003073

ISSN: 1525‐2027

The 40Ar/39Ar and U/Pb dating of young rhyolitesin the Kos‐Nisyros volcanic complex, Eastern Aegean Arc,Greece: Age discordance due to excess 40Ar in biotite

O. BachmannEarth and Space Sciences, University of Washington, Mailstop 351310, Seattle, Washington98195‐1310, USA ([email protected])

B. SchoeneSection 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. SchnyderSection 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 deMalagnou, CH‐1211 Geneva 6, Switzerland

R. SpikingsSection 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 (AegeanArc, 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 Zinidomes, Kefalos Serie pyroclasts) range in age from 0.3 to 0.5 to 10–20 Ma. The youngest dates providethe 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 highlydisturbed Ar‐release patterns with plateau ages typically older than most U/Pb ages. These old plateau agesare probably not a consequence of inheritance from xenocrystic biotites because Ar diffuses extremely fast atmagmatic temperatures and ratios are reset within a few days. On the basis of (1) elevated and/or imprecise40Ar/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 (thecaldera‐forming Kos Plateau Tuff) do not appear to store any excess 40Ar relative to atmospheric composi-tion. The eastern edge of the Aegean Arc is tectonically complex, undergoing rapid extension and locatedclose to a major structural boundary. In such regions, which are characterized by high fluxes of mantlevolatiles, 40Ar/39Ar geochronology on biotite can lead to erroneous results due to the presence of excess40Ar and should be checked either against 40Ar/39Ar sanidine or U/Pb zircon ages.

Copyright 2010 by the American Geophysical Union 1 of 14

http://dx.doi.org/10.1029/2010GC003073

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 theKos‐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 TechniqueGuest Editors: D. Condon, G. Gehrels, M. Heizler, and F. Hilgen

1. Introduction

[2] Understanding the generation, transport, storageand eruption of magma in subduction zones isimportant for building models for the generation ofcontinental crust. Geochronology has become animportant tool for such research because of its abilityto determine the rates of magma crystallization andthe tempo of volcanism. For silicic volcanic rocks,the commonly used 40Ar/39Ar and U/Pb dating tech-niques have the potential to provide different viewson the evolution of the unit of interest. When resultsfrom both techniques are compared, U/Pb oftenyields older ages. The reason for this are twofold:(1) uncertainties in the decay constant for 40K and/ormineral reference standards lead to calculated datesthat are ∼0.5–1% too young compared to U/Pb ages[e.g., Renne et al., 1998; Min et al., 2000; Kuiperet al., 2008; Simon et al., 2008], and/or (2) reten-tion of daughter products varies due to differentclosure temperatures for Pb in zircon and Ar in a hostof minerals. Because of fast diffusion and thereforelow closure temperatures (<550°C) within commonhigh‐Kminerals, Ar is commonly assumed to recordthe timing of eruption [McDougall and Harrison,1999], assuming insignificant post‐eruption burial.In contrast, U and Pb are retained in zircon at mag-matic temperatures [Lee et al., 1997] and thereforeU‐Pb dates record mineral crystallization, whichoccurs 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 informa-tion about older processes within xenocrystic coresor antecrysticmaterial (co‐genetic, but up to >100 kyolder 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, severalrecent studies have documented that 40Ar/39Ar dates

from sanidine and biotite can also predate eruptiondue to problems with 39Ar recoil, inheritance, orexcess 40Ar [Smith et al., 2006; Hora et al., 2007;Lipman and McIntosh, 2008; Smith et al., 2008].

[3] In a geochronological effort to constrain themagmatic evolution that predates one of the twolargest Quaternary caldera‐forming eruptions in theMediterranean region (the 161 ky Kos Plateau Tuff(hereafter KPT) [Smith et al., 1996; this paper], wediscovered significant discrepancies between zirconU/Pb and biotite 40Ar/39Ar ages that are contrary to“standard” behavior: 40Ar/39Ar ages are older thanmost U/Pb ages, in a few cases by several millionyears. Furthermore, the biotite 40Ar/39Ar ages provedto be extremely complex and disturbed. This paperprovides a basis to reconcile this age discrepancy interms of Ar systematics in biotite and also to presentnew and improved ages on volcanic units that pre‐date the KPT on the Kefalos Peninsula (Kos Island,Greece). The results from this study also highlightthe potential to extend ID‐TIMSU/Pb dating to sub‐million year old rocks with high precision in orderto elucidate complex magma evolution.

2. Geological Setting and SampleDescription

[4] The South Aegean Arc is the result of sub-duction of the African plate under the Aegeanmicroplate [Le Pichon and Angelier, 1979; Jolivet,2001], which was probably initiated in the earlyTertiary (with Oligo‐Miocene magmatic rocks innorthern Greece [e.g., Pe‐Piper and Piper, 2002]).The convergent velocities are some of the slowestin the world (about 5 to 10 mm/yr [DeMets et al.,1990; Jackson, 1993]). The volcanic centers of theSouth Aegean are (from W to E): Crommyonia,Aegina, Methana, and Poros, all situated in theSaronic Golf, and Milos, Santorini, and Kos‐

GeochemistryGeophysicsGeosystems G3G3 BACHMANN ET AL.: EXCESS 40Ar IN AEGEAN RHYOLITES 10.1029/2010GC003073

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Nisyros, which form the three voluminous com-plexes in the Aegean Sea [Fytikas et al., 1976].

[5] Previously published K/Ar ages on differentvolcanic units of Kos Island [Bellon and Jarrige,1979; Pasteels et al., 1986; Matsuda et al., 1999]indicate that volcanism in the area started around3 Ma ago and was episodic until recent activity (seeTable 1). However, much of the data has large errorsand could be significantly improved by applyingthe Ar‐Ar technique to K‐rich minerals. The best‐constrained age for anyKos Island unit was obtainedusing the 40Ar/39Ar method on the only sanidine‐bearing unit in the area, the KPT (161 ± 1 ky). Asthis unit is an important time marker (both locallyand within the Eastern Mediterranean region), mucheffort has been invested in acquiring a precise andaccurate age [Smith et al., 1996, 2000].

[6] Obtaining accurate ages for the Nisyros unitshas proved much more challenging than for theKefalos‐Kos systems. None of the erupted materialon Nisyros Island contains K‐bearing phases, andhence the K/Ar (and 40Ar/39Ar) method has notbeen applied successfully. Other methods have beenapplied, with some success (Table 1).

[7] All the samples in this study were collected onthe Kefalos Peninsula, in the southwest of KosIsland. Dome rocks include, from north to south:Mt. Zini, Mt. Cumianas, Mt. Latra and Mt. AgiosMammas. We also collected several samples of theKefalos Series pyroclastic rocks [Dabalakis andVougioukalakis, 1993] in areas around Kefalosvillage (Figure 1 and Table S1).1

[8] The dome rocks of Mt. Zini (KD02, Zini),Mt. Latra (KD03), Mt. Cumianas (KD04, CS11–05)andMt.AgiosMammas (KD07,CS12–05) are crystal‐poor rhyolites with ∼5 wt% phenocrysts, comprising2–3% plagioclase, 1–2% biotite, 1–2% quartz and

less than 1% of Fe‐Ti oxides and accessory phases(monazite, zircon). These crystals reside in a gen-erally glassy matrix, but spherulites (devitrification)can be abundant in the matrix glasses in some domes.The Kefalos dacite (KD01) is porphyritic, with 30to 40 vol% crystals (plagioclase, hornblende, bio-tite and Fe‐Ti oxides). This rock hosts numerousmafic microgranular enclaves of basaltic‐andesiteto andesitic composition. These enclaves exhibitovoid shapes with sharp contacts with the host rock,and a mineral assemblage similar to the dacite.

[9] Whole‐rock chemical analyses indicate thatrhyolites have ∼76 wt% SiO2 (anhydrous basis), withrelatively low Al2O3 content (12–13 wt%), and fairlyhighK2O (4.3wt%). The silica content of the dacite is∼64 wt%, with 16 wt% Al2O3, 3.8 wt% Na2O and2.7 wt% K2O. The most noticeable feature is the“adakitic” signature of the dacitic composition, withhigh Sr (>400 ppm) and lowY (≤18 ppm) leading to aSr/Y ratio of 50–60 [Pe‐Piper and Moulton, 2008].

[10] The biotite crystals are euhedral in the rhyoliticdomes (Zini, Latra, Cumianas, Agios Mammas),and slightly subhedral in the dacite, but withoutany evidence of disequilibrium (no resorption rimsor any alteration were noticed during opticalexamination; Figure 2). Microprobe data on biotitecrystals from all dated units yield good totals andtypical biotite compositions (Table 2), which can beclassified as Mg‐biotites (according to Tischendorfet al. [2001]). Individual crystals appear homoge-neous, as no significant chemical zonation wasfound during core‐to‐rim traverses.

3. The 40Ar/39Ar Method and Results

3.1. Method

[11] All samples were gently crushed with ahydraulic press to obtain coarsely crushed material,

Table 1. Summary of Published Geochronological Data for Kefalos, Kos, and Nisyros Units

Stratigraphy Age (ka) Method Reference

Kefalos Dacite 2542–2990; 2600–3100 K‐Ar Matsuda et al. [1999]Zini Dome 1000 ± 200, 540–560 ± 30 K‐Ar Bellon and Jarrige [1979]; Pasteels et al. [1986]Agios Mammas dome 2700 ± 150 K‐Ar Bellon and Jarrige [1979]Latra dome 2500 ± 500 K‐Ar Bellon and Jarrige [1979]Kos Plateau Tuff 161 ± 1 Ar‐Ar on sanidine Smith et al. [1996, 2000]Kos Plateau Tuff 160–480 U‐Th‐Pb on zircon Bachmann et al. [2007a]NisyrosYali obsidian 24 FT on volcanic glass Wagner et al. [1976]Yali pumice fall 31 Oxygen isotope Federman and Carey [1980]Upper Pumice >44 14C Limburg and Varekamp [1991]Lower Pumice 24 ± 0.56 14C in paleosoil Rehren [1988]Avlaki Rhyolite 200 ± 50; 66.6 ± 2 K‐Ar (WR) K‐Ar (Pl) Di Paola [1974]; Keller et al. [1990]

1Auxiliary material data sets are available at ftp://ftp.agu.org/apend/gc/2010gc003073. Other auxiliary materials are in the HTML.


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which was then sieved. The fraction <500 mm waswashed in water and dried for 24 h in an oven at70°C. The minerals were separated using a Frantzmagnetic separator and hand‐picked. Crystals weresubsequently cleaned for 2 min in an ultrasonicbath using de‐ionized water, then dried under aninfrared lamp for several minutes.

[12] Packets of 20–40 mg of whole grains of biotitefrom Kefalos units were wrapped in >99.99% purecopper foil and were sent for irradiation at OregonState University (CLICIT facility) for 1 h, alongwith Fish Canyon Tuff sanidine grains (fluencemonitors) that were dispersed in 5mm intervalsalong the length of the irradiation package. Thesamples were analyzed at the University of Genevausing an Argus (GV Instruments) multicollectormass spectrometer, equipped with four high‐gain(1012 ohms) Faraday collectors for the analysis of39Ar, 38Ar, 37Ar and 36Ar, as well as a single Faradaycollector (1011 ohms) for the analysis of 40Ar.

Collector gain was calibrated performing a classiccurrent‐collector gain (CC gain) procedure, nor-malizing CC gains for the two high mass and twolow mass collectors to the CC gains value obtainedfor the axial collector.

[13] Cup efficiency was measured in two ways:(1) focused CO2 beams were put onto each collectorin sequence (single collector, peak hopping) indynamic mode, and then compared. This assumesthat the dynamic CO2 abundance is constant, whichis considered to be true over the time scale of theexperiment (∼2 h) because it is buffered to anequilibrium state after 10 days of operation sincethe previous bake‐out of the mass spectrometer(cup efficiency is performed at least ten days afterthe previous MS bake‐out). (2) A single pipette ofair was extracted and focused on the highest masscollector, then individual air shots were focused onthe subsequent Faraday cups. Cup efficiency isensured by a volume calibrated air‐pipette system,

Figure 1. Map of area with sample locations.


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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 theKefalos 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 6 20 5 73 6 22wt%SiO2 38.03 38.22 38.10 37.87 37.82 36.95TiO2 4.17 4.40 4.25 4.40 4.37 4.38Al2O3 14.31 13.54 13.68 13.51 13.61 13.53FeO 14.88 15.76 15.24 16.04 15.58 17.74MnO 0.15 0.48 0.45 0.48 0.52 0.51MgO 14.70 13.91 14.40 13.84 13.61 12.54BaO 0.66 0.57 0.62 0.57 0.57 0.50CaO 0.00 0.05 0.02 0.05 0.12 0.03Na2O 0.78 0.57 0.65 0.59 0.45 0.46K2O 8.71 8.75 8.83 8.59 8.48 8.77F 0.55 0.41 0.33 0.45 0.33 0.31Cl 0.14 0.14 0.12 0.14 0.14 0.12Sum 96.95 96.66 96.59 96.37 95.46 96.40

Mg# 0.64 0.61 0.63 0.61 0.61 0.56aN, 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 to15 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 toeach 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, andone SAES GP50‐ST707 getter, and the extractedgas from biotites and sanidine grains was cooledto ∼−150°C by a Polycold P100 cryogenic refrig-eration unit mounted over a cold finger. Single to2–3 grains of biotite were step‐heated using adefocused 30W, MIR10 IR (CO2) laser that wasrastered over the samples to provide even‐heating ofthe grains. Samples were measured on the Faradaycollectors and time‐zero regressions were fitted todata collected from twelve cycles. Peak heights andblanks were corrected for mass discrimination,isotopic decay of 39Ar and 37Ar and interferingnucleogenic Ca‐, K‐ and Cl‐derived isotopes. Thehigh stability of the Faraday baseline measure-ments renders it unnecessary to record baselinesduring each analysis. Error calculations includethe errors on mass discrimination measurement(done on 20 individual measurements prior to thisproject and repeated once a day during the analyses;40Ar/36Ar measured at 298.25 and normalized to295.5 [Nier, 1950]), and the J value. 40Ar, 39Ar,38Ar, 37Ar and 36Ar blanks were calculated beforeevery new sample and after every three heatingsteps. 40Ar blanks were between 6.5*10−16 and10−15 moles (signal to blank ratios are typically inthe order of 10,000–1,000,000). Blank values form/e 39 to 36 were all <6.5−17 moles. Age plateauswere determined using the criteria of Dalrympleand Lanphere [1971]. The automated analyticalprocess uses the software ArArCalc [Koppers,2002] (Data Set S1).

[15] Samples from the Kos Plateau Tuff were alsoanalyzed at the University of Geneva following theprocedure given by Singer et al. [1999], severalyears prior to the Kefalos units. KPT samples wereirradiated for 20 min. at the OSU Triga reactor, andwere analyzed using a CO2 laser and MAP 216 Arspectrometer. All ages were calculated relative to1.19 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 biotite40Ar/39Ar, only the Kefalos Dacite gave a plausibleage of 2.958 ± 0.024 Ma, which overlaps withprevious K/Ar ages of 2.6 ± 0.2 and 3.1 ± 0.3 Ma

[Matsuda et al., 1999]. The concordance of thesetwo sets of ages provides firm evidence for Pliocenecalc‐alkaline magmatism on the Island of Kos.All the other analyses yielded extremely disturbedAr release patterns, with large errors and conflictingplateau and isochron ages (see Figures 3a–3f,Table 3, and Data Set S1). 40Ar/36Ar intercepts forthe inverse isochrons were either higher than theatmospheric value of 295.5 (in two cases), while thetwo other cases overlapped with 295.5, but withinvery large errors. The biotite 40Ar/39Ar ages appearolder than expected from stratigraphic relations andprevious K/Ar dates [Bellon and Jarrige, 1979;Pasteels et al., 1986]. Therefore, three of theseunits (Zini dome, Agios Mammas Dome and theKefalos Series) have been re‐dated by zircon U/Pbto better constrain the possible range of eruptionages for these units.

3.2.2. Kos Plateau Tuff

[17] Sanidine, plagioclase and quartz crystals fromthe KPT have already been dated by 40Ar/39Ar[Smith et al., 1996, 2000]. Sanidine crystals yieldeda very precise age of 161 ± 1 ky onmultiple samples,but plagioclase and quartz produced a more com-plex pattern. Plagioclase and quartz crystals (thelatter having most of its Ar released from meltinclusions) show the presence of multiple popula-tions up to 1730 ky old, indicating complex recyclingof crystals in the magma chambers.

[18] In this study, we report additional sanidine40Ar/39Ar ages on both pumices and granitoidenclaves present in the KPT (Table 4). The agesobtained 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 areslightly older than the pumiceous material, whichis in agreement with the interpretation that theyare 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 isotopedilution thermal ionization mass spectrometry(ID‐TIMS). Zircons were separated from bulksamples using standard mineral separation techni-ques. Individual zircons from each sample werepicked 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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900°C for ∼60 h. All grains from a single samplewere leached together in 3 ml Savillex beakers inHF(aq) + trace HNO3(aq) for ∼12 h at 180°C, rinsedwith water and acetone and then placed in 6N HCl(aq) on a hotplate at ∼110°C overnight. These werethen washed several times with water, HCl, andHNO3. Single or multiple (up to three) grains werethen handpicked for dissolution; zircons exhibiteda range of morphologies from obviously resorbedto short and stubby prisms to euhedral needles.Multigrain fractions consisted of zircons of sim-ilar morphology. Each fraction was spiked with∼0.004 g of the EARTHTIME 205Pb‐233U‐235U(ET535) tracer solution. 235U/205Pb = 100.20 wasused for the ET535 tracer, to which a total uncer-tainty of 0.1 was assigned. Zircons were dissolvedin ∼70 ml 40% HF and trace HNO3 in 200 ml Savillexcapsules at 210°C for 48+ hours, dried down andre‐dissolved in 6N HCl overnight. Samples werethen dried down and re‐dissolved in 3N HCl andput through a modified single 50 ml anion exchangecolumn [Krogh, 1973]. U and Pb were collectedin the same beaker and dried down with a drop of0.05 M H3PO4 and analyzed on a single outgassedRe filament in a Si‐gel emitter, modified fromGerstenberger and Haase [1997]. Measurementswere performed on a Thermo‐Finnigan Triton ther-mal ionization mass spectrometer at the Universityof Geneva.

[20] Pb was measured in dynamic mode on amodified Masscom secondary electron multiplier(SEM). Deadtime for the SEM was determinedby periodic measurement of NBS‐982 for up to1.3 Mcps and observed to be constant at 23.5 ns.

Multiplier linearity was monitored every few daysbetween 1.3 × 106 and <100 cps by a combinationof measurements of NBS‐981, −982 and −983, andobserved to be constant if the Faraday to SEM yieldwas kept between ∼93–94% by adjusting SEMvoltage. Baseline measurements were made atmasses 203.5 and 204.5 and the average was sub-tracted from each peak after beam decay correction.Interferences on 205Pbweremonitored bymeasuringmasses 201, 202 and 203 and also by monitoringmass 205 in unspiked samples. As a result, no cor-rections were applied. Pb fractionation was deter-mined both by measuring aliquots of NBS‐982 andalso by using fractionation values determined inother studies at Geneva that employed a 202Pb‐205Pbdouble spike, and the value 0.13 ± 0.04%/a.m.u. wasused (2‐sigma standard deviation).

[21] U was measured in static mode on Faraday cupsand 1012 ohm resistors as UO2

+. Oxygen isotopiccomposition was monitored by measurement ofmass 272 on large U500 loads [Wasserburg et al.,1981]. Though the 18O/16O typically grew from0.00200 to 0.00208 over the course of an analysis,the most drastic increase occurred at the beginningtoward an average value of ∼0.00205. As a result,early blocks of data were deleted and the averagevalue was used for all data, and corrected duringmass spectrometry. Baselines were measurement at±0.5 mass units for 30 s every 50 ratios. Correctionfor mass‐fractionation for U was done with thedouble spike assuming a sample 238U/235U ratio of137.88.

[22] All common Pb was assigned to blank, whosecomposition was measured at UNIGE over the

Table 3. Summary of 40Ar/39Ar Results for Kefalos Units

Unit

Weighted“Plateau”Age (ka)

MSWDon

“Plateau”Total FusionAge (ka)

InverseIsochronAge (ka)

MSWDon

Isochron

40Ar/36ArIntercept

Kefalos Dacite (KD01) 2958 ± 24 1.3 2844.1 ± 18.5 2991.1 ± 44.5 0.49 290.5 ± 5.8Zini dome (KD02) 687 ± 16 0.76 999.0 ± 13.9 438.6 ± 264.5 0.17 330.6 ± 39.3Latra dome (KD03) 1370 ± 110 23.10 14420 ± 021 790 ± 360 7.22 319.6 ± 16.4Cumianas Dome (KD04) 5310 ± 320 2.28 5710 ± 160 1580 ± 1530 0.22 315.4 ± 10.2Agios Mammas Dome (KD07) 3080 ± 580 0.69 18640 ± 1120 1650 ± 3100 1.36 315.1 ± 256.8Kefalos Series Pyroclasts (KS03) 5490 ± 170 1.68 6600 ± 210 10 ± 20 0.13 323.8 ± 16.1

Table 4. Summary of Total Fusion 40Ar/39Ar Results on Samples From the KPT

Sample K2O (wt%) Material Na

Total Fusion Age39Ar (%) Age (ky) ± 2s MSWD

Granitoids 11.5 K‐spar 11 of 13 1–57 177.6 ± 6.6 0.8Pumices 11.5 Sanidine 10 of 11 14–81 165.1 ± 2.0 3.1All 11.5 K‐spar and sanidine 21 of 24 1–81 166.1 ± 2.0 2.4

aN, number of total fusion analyses. See auxiliary material for full analytical details.


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course of this study as total procedural blanks.After 2‐sigma outlier rejection, the composition offifteen blanks was: 206Pb/204Pb = 18.08 ± 0.66,207Pb/204Pb = 15.79 ± 0.45, 208Pb/204Pb = 37.55 ±0.93 (2‐sigma standard deviations). Measured ratioswere reduced using the algorithms of Schmitzand Schoene [2007] and Crowley et al. [2007].230Th disequilibrium was corrected using measuredTh/Umagma values from pumices in the volcanicunits sampled: KS06–7 = 3.8 ± 1.4 (2‐sigma stan-dard deviation); Zini and CS12–05 = 4.3 ± 1.4;KS06–3 = 3.5 ± 1.4; CS03 = 5.0 ± 1.4, and assumingsecular equilibrium in the magma.

4.2. Results

[23] Twenty‐nine single and multigrain (up to3 grains) zircon analyses were undertaken from five

samples for which we have comparable 40Ar/39Ardates. Relatively large uncertainties (3–12%) aredue to (1) the very low Pb contents of these zirconsand (2) the uncertainty in the 230Th‐correction. Thelatter accounts for, on average 57% of the totaluncertainty for analyses <1 Ma. This is relativelysmall, considering that for the ca. 300 ka AgiosMammas dome, the 230Th correction raises the206Pb/238U date by ∼25%.

[24] When these data are compared to the biotite40Ar/39Ar ages of the same unit (Figure 4, Table 3,and Data Set S3), two main observations can bedrawn from the Kefalos units (Zini and AgiosMammas rhyolitic domes and 3 samples of theKefalos Series pyroclasts). First, the youngest U/Pbage is always younger than the average 40Ar/39Arages. Second, several zircons were much older than

Figure 4. Comparison of 40Ar/39Ar and U/Pb results on three samples of the Kefalos Peninsula (precursors to theKPT). Note that the 40Ar/39Ar age is always older than the youngest zircon dated.


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any possible eruption age, and imply that zirconxenocrysts are present in the erupted magma. It isinteresting to note that the much larger KPT did notshow any evidence of xenocrystic zircons, despitehaving been erupted in the same general area[Bachmann et al., 2007a]. The KPT, however, doesshow a significant amount of antecrystic material,with zircon ages 200–300 ky older than the eruptionage. Excluding xenocrystic zircons, 4 of 5 samplesfrom this study also show evidence for growth ofantecrystic zircon over at least 50 ka. Each sampleexcept CS03 has 4 or more analyses that clusterwithin 100 ka, but yield high MSWDs of weightedmean 206Pb/238U dates. The Zini dome sample,however, yields a cluster with a weighted meanMSWD < 1, suggesting pre‐eruptive crystallizationof zircon was shorter in this sample. For eachsample, we use the youngest zircon date as themaximum age of eruption.

5. Discussion

[25] Comparing the 40Ar/39Ar and U‐Pb age resultson Kefalos rhyolites leads to two major findings:(1) the biotite 40Ar/39Ar age spectra are highlydisturbed, but when a plateau is obtained, the40Ar/39Ar age is older than most of the zircons inthe same unit. The 40Ar/39Ar plateau age is there-fore too old to represent the eruption age. (2) Thepresence of xenocrystic zircons is obvious in thethree units that we have U‐Pb data for (ages up to21 Ma). This is a common observation in silicicunits [e.g., Lanphere and Baadsgaard, 2001; Schmittet al., 2003; Simon and Reid, 2005; Bindeman etal., 2006, 2008], and is expected this area, wherethe pre‐existing crust hosts components that areat least 370 Ma old [Smith et al., 2000].

[26] The cause of the disturbed and anomalous40Ar/39Ar spectra for the Kefalos rhyolites could bedue to either (1) inherited Ar (Ar from oldercrystals), (2) 39Ar recoil effects during irradiation[e.g., Onstott et al., 1995; Smith et al., 2008] or(3) excess 40Ar (40Ar‐rich reservoir not equilibratedwith atmosphere) in the mineral structure or withinfluid inclusions. On the basis of the U‐Pb zircondates showing xenocrystic material, it is possi-ble that some of the anomalously old ages could bedue to older biotite xenocrysts. However, Arisotope systematics in biotite crystals re‐equilibrateextremely fast at magmatic temperatures (>650–750°C); a few days would be enough for completeresetting of Ar [McDougall and Harrison, 1999;Gardner et al., 2002]. In addition, inherited Argenerally resides in the most retentive parts of the

crystals [Bachmann et al., 2007b] and leads to Arrelease spectra with ages getting older during laterheating steps. This staircase pattern is not consis-tent to what is observed in the Kefalos case (seeFigures 3a–3f).

[27] The importance of recoil effect in redistributing39Ar in minerals has long been recognized [Turnerand Cadogan, 1974], and is a known problem for40Ar/39Ar dating [Onstott et al., 1995]. However,internal redistribution of 39Ar by recoil in biotitegenerally occurs when crystals are small [Paineet al., 2006] and/or between K‐rich and K‐poorzones of the mineral when those are intergrown atthe micron to submicron scale due to weathering[Roberts et al., 2001] or presence of alterationphases [Smith et al., 2006, 2008]. In the samplesuite we analyzed, large (>100 microns) biotitecrystals were extracted from young, unaltered rockfragments, and they do not show any evidence ofalteration (as indicated by observation under theoptical microscope and by electron microprobeanalyses with no conspicuous zoning in majorelements and totals summing between 96 ± 1%).We therefore suggest that 39Ar recoil effects areunlikely to be the dominant process by which thereported Ar spectra are disturbed.

[28] Biotite crystals readily incorporate excess 40Ardue to its relatively high Ar mineral/fluid parti-tion coefficient [Dahl, 1996; Kelley, 2002b], andprevious studies have noted disturbed patterns,anomalously old ages and high 40Ar/36Ar intercepts(∼320 to >500) in biotite, particularly when com-pared to sanidine [e.g., Renne, 1995; Villeneuveet al., 2000; Hora et al., 2007; Lipman andMcIntosh, 2008]. In addition, biotite is likely toretain more excess 40Ar than sanidine due to aslightly higher partition coefficient for Ar and alower stochiometric K‐content [Hora et al., 2010].In the Kefalos case, the presence of excess 40Ar issupported by (1) the inverse isochron interceptsof all analyses (except the dacite) showing largeerrors and/or ratios higher than the atmospheric ratio(295.5 [Nier, 1950]), (2) previously published K/Arages with high 40Ar/36Ar ratios [Matsuda et al.,1999], and (3) 40Ar/39Ar age spectra with olderages in low temperatures steps, indicative of partialdiffusive re‐equilibration with a magma that had40Ar/36Ar higher than atmosphere [Lanphere andDalrymple, 1976; Harrison and McDougall, 1981;Harrison et al., 1985; Hess et al., 1987].

[29] In the Kefalos area, two factors may beenhancing the entrapment of excess 40Ar in thebiotite 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 ampleopportunity to interact with an Ar‐rich gas phase.Second, the very high 3He/4He measured in theregion suggests a high mantle flux [Shimizu et al.,2005]. As Ar and He are generally coupled, it ispossible that some of 40Ar is derived from themantle. As the 40Ar/36Ar of the mantle is extremelyhigh (lower limit in the presence of potential aircontamination is 30’000 to 40’000 [Farley andNeroda, 1998]), only a small amount of such areservoir would significantly disturb the Ar spectra.Melt or fluid inclusions have not been noticed inbiotite crystals from Kefalos, but the presence ofbrine inclusions has been documented in biotitesfrom the KPT [Bachmann, 2010]. Such inclusionscould provide a reservoir for mantle Ar [Kelley,2002a].

[30] We suspect that excess 40Ar might be moreproblematic in areas of rapid crustal extension, as ispresently occurring in the eastern Aegean region[Pe‐Piper et al., 2005]. The mantle reservoir isclose to the surface (crust only about 25–30 kmthick underneath Kos Island [Wortel and Spakman,2000]), and numerous extensional faults facilitateefficient transfer of magmas and volatiles, carryinghigh amounts of mantle‐derived 40Ar and 3Hetoward the surface. Other areas undergoing rapidcrustal extensions and/or high mantle flux (e.g.,Taupo Volcanic Zone, New Zealand [e.g., Hulstonand Lupton, 1996], and hot spot related activity[e.g., Renne, 1995]) might be more susceptible tostoring excess 40Ar than magmatic provinceslocated in areas undergoing compression.

[31] As the 40Ar/39Ar ages on biotite crystals fromKefalos units do not appear to carry any useful ageinformation, the youngest zircon U/Pb date bestapproximates the eruption age. However, we notethat the presence of antecrystic zircon in thesesamples spanning tens of thousands of years impliesthat we may not have captured the youngestgrains (we report maximum eruption ages). AgiosMammas dome (Southern edge of the KefalosPeninsula) is the youngest (eruption age youngerthan 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 episodeat around 500 ka, in agreement with field observa-tions and previous dating (K/Ar dating of glassdating by Pasteels et al. [1986], yielding 550 ±20 ka for Mt Zini on three samples and 560 ±30 ka on one sample of theKefalos Series pyroclasts).These are the youngest ID‐TIMS U/Pb datesreported for any geologic environment and empha-

size the potential for extending this method ofdating zircon growth well into the Pleistocene.Reducing the analytical blank as well the uncer-tainty in its composition are important ways ofreducing age uncertainty in such low‐Pb zircons.Another significant source of uncertainty is the230Th‐disequilibrium correction. In higher‐U zirconsof similar age (e.g., the Bishop Tuff [Crowleyet al., 2007]), this is the largest source of uncer-tainty, and can be reduced through a better under-standing of the Th/U of the magma from whichzircons crystallized. Even with these present lim-itations, ID‐TIMS U‐Pb dating can yield precisionand accuracy of better than 2–3% (2‐sigma) forsingle 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 andDaniel Selles are thanked for the help during field work, mineralseparation and isotopic analyses respectively. The microprobeanalyses were carried under the supervision of CatherineGinibre. We thank Georges Vougioukalakis and the Instituteof Geology and Mineral Exploration Greece (IGME) for kindlyproviding the fieldwork permits. We thank Brad Singer and ananonymous reviewer for constructive reviews.

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The Ar/ Ar and U/Pb dating of young rhyolites Greece: Age ......in the Kos‐Nisyros volcanic complex, Eastern Aegean Arc, Greece: Age discordance due to excess 40Ar in biotite O