39Ar dating methods in the 14C age range

Quaternary Geochronology 6 (2011) 530e538

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Research Paper

Effectiveness of combined unspiked KeAr and 40Ar/39Ar dating methods in the 14C age range Hervé Guillou a, *, Sébastien Nomade a, Juan Carlos Carracedo b, Catherine Kissel a, Carlo Laj a, Francisco José Perez Torrado c, Camille Wandres a a

Laboratoire des Sciences du Climat et de L’Environnement (IPSL-CEA-CNRS-UVSQ), Domaine du CNRS Bât. 12, Avenue de la Terrasse, 91198 Gif sur Yvette, France Estación Volcanológica de Canarias, Consejo Superior de Investigaciones Cientificas, La Laguna, Tenerife, Spain Departamento de Física-Geología, Facultad de Ciencias del Mar, Campus de Tafira, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Canary Islands, Spain b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 October 2010 Received in revised form 28 March 2011 Accepted 31 March 2011 Available online 8 April 2011

In this study, the effectiveness of combined unspiked KeAr and 40Ar/39Ar dating methods as currently applied now at LSCE in the 14C age range was evaluated by studying two 30,000 years old phonolites from Tenerife (Canary Islands). New 40Ar/39Ar ages obtained in this work are comparable with the KeAr ages, and of similar precision. This remarkable agreement obtained between the two dating methods in this young age range, validates the new LSCE 40Ar/39Ar facility and provides a very promising approach to calibrate the Quaternary timescale and to date key events such as geomagnetic field instabilities and climatic changes. The approach combining the 40Ar/39Ar and the unspiked KeAr methods has the advantage that all the basic assumptions, which rule the KeAr clock, are checked via the isochron approach. In the event of positive check, the 40Ar/39Ar and unspiked KeAr ages can be pooled to produce very precise ages. Ó 2011 Elsevier B.V. All rights reserved.

Keywords: Geochronology Canary islands Volcanism

1. Introduction During the last decade, several studies evidenced the strong endeavor to push toward younger and younger ages the 40Ar/39Ar chronology (Renne et al., 1997; De Vivo et al., 2001; Sharp et al., 1996; Sharp and Renne, 2005; Ton-That et al., 2001; Guillou et al., 2004; Singer et al., 2009). These investigations focused both on methodological and geological objectives. Several of the methodological studies concentrated on the refinement of the ages of the neutron fluence monitors by improving the precision of 40Ar/39Ar analysis of historically dated samples (Renne et al., 1997; Lanphere et al., 2000) or astronomically tuned sedimentary sections (Kuiper et al., 2004, 2008) or comparing U/Pb and 40Ar/39Ar ages obtained on the same material (Renne et al., 2010) to provide better calibrated dates. 40Ar/39Ar chronology is also considered as a possible approach to calibrate cosmic-ray dependent dating methods such as 14C and 3He/4He (Hu et al., 1994; Ackert et al., 2003). In parallel, all these advancements have extended the scope of application of 40 Ar/39Ar chronology especially in paleoclimatic studies (Smith et al., 1996; Ton-That et al., 2001; Singer et al., 2000, 2004) to

* Corresponding author. Tel.: þ33 1 69 82 35 56; fax: þ33 1 69 82 35 68. E-mail address: [email protected] (H. Guillou). 1871-1014/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.quageo.2011.03.011

refine in an astronomically independent way the O-isotope stratigraphy (Guillou et al., 2004; Singer et al., 2009). Most of these promising results are based on 40Ar/39Ar dating of K-rich feldspars, from tephra layers, which allows a direct comparison between oceanic and continental records. However, as outlined by Lanphere (2000), improvements to date effusive volcanic rocks, which cover a much wider area than K-rich feldspars-bearing tephra, have also to be addressed to enlarge the field of chronological studies in Quaternary times. In an initial approach, McDougall (1985), Singer et al. (1997) and Lanphere (2000) provided the first direct comparisons between the conventional KeAr and the 40Ar/39Ar incremental heating methods. To participate in this challenging task, a new 40Ar/39Ar laboratory has been established at the LSCE to complete the already existing unspiked KeAr facility. Both the unspiked KeAr and the 40Ar/39Ar methods have their own distinct advantages and drawbacks, and the combination of these two methods is an efficient means to obtain accurate and reliable age determinations as demonstrated by the recent dating of the Laschamp excursion (Guillou et al., 2004). To extend this approach toward younger ages, we have studied two phonolitic lava flows from the Teide e Pico Viejo volcanic complex in the Canary Islands which were emplaced around 30 ka. The remarkable agreement obtained between the two dating methods in this young age range validates the new LSCE 40Ar/39Ar

H. Guillou et al. / Quaternary Geochronology 6 (2011) 530e538

facility and appears to constitute a very promising approach to calibrate the Quaternary timescale. 2. Geological Setting The two studied samples belong to the Teide Volcano complex (Tenerife, Canary Islands, Spain). A complete and detailed description of this volcanic complex can be found in Carracedo et al. (2007). This stratovolcano formed on the floor of the Las Cañadas Caldera. The earliest stage of Teide has been dated at 198  5 ka (Carracedo et al., 2007). At the end of this first eruptive phase which lasted about 50 ka, a central volcano probably formed. Subsequently, over the following 70e120 ka, eruptions of intermediate lava occurred. The development of the stratovolcano was completed at ca. 30 ka. At this time, differentiation of magmas within a shallow phonolitic magmatic chamber fed the Teide phonolitic eruptions. The two studied flows are from this last stage of activity. The first one (sample CITF-98), is from a phonolitic flow inside the Orotava valley. The second one (sample CITF-301), is from the spectacular phonolitic flow from Playa San Marcos (Table 1, Fig. 1). 3. Analytical technique 3.1. The unspiked KeAr technique The KeAr ages of these two samples have already been published in Carracedo et al. (2007) but we take the opportunity in this study to provide further details of our technical approach. The isotopic composition and abundance of Ar were determined using an unspiked technique described in Charbit et al. (1998). This technique differs from the conventional isotope dilution method in that argon extracted from the sample is measured in sequence with purified aliquots of atmospheric argon at the same working gas pressure in the mass spectrometer. This allows mass discrimination effects to be suppressed between the atmospheric reference and the unknown, and allows percentages of radiogenic 40Ar (40Ar*) as small as 0.14% to be detected on a single-run basis (Scaillet and Guillou, 2004). The mass spectrometer sensitivity is 5.7  103 mol/A @ m/ e ¼ 40 with amplifier backgrounds of 75  1012 A @ m/e ¼ 40 (109 U resistor), and 5.75  1014 A @ m/e ¼ 36 (1011 U resistor). Combined with high within-run signal stability, this permits individual samples of 40e50 ka basalt containing 2 wt. % K2O to be dated with an analytical precision of 2 ka (2, Δ on a single-run basis (Guillou et al., 2004). Groundmass splits from fresh samples were prepared following the methods described in Guillou et al. (1998). Aliquots from the same separates were used for both unspiked KeAr and 40Ar/39Ar age determinations. Phenocrysts and xenocrysts, which are potential carriers of extraneous 40Ar (including excess and inherited components), were eliminated using magnetic, gravimetric, and visual hand-picking separation. Replicate unspiked KeAr age determinations were made on the microcrystalline groundmass of the two samples. The determination of K was carried out by atomic absorption (flame photometry). Argon was extracted by radio frequency heating of 1.5e2.5 g of sample, then transferred to an ultra-high-vacuum glass line and purified with titanium sponge and ZreAr getters. Isotopic analyses were performed on total 40Ar Table 1 Latitude and Longitude of the dated samples. Sample id

Latitude

Longitude

CITF-98 CITF-301

28 200 13.000 N 28 220 39.800 N

16 350 0.100 W 16 430 20.300 W

531

contents ranging between 1.5 and 2.2  1011 mol using a 180 , 6 cm radius mass spectrometer with an accelerating potential of 620 V. The spectrometer was operated in static mode, but its volume was varied to give equal 40Ar signals for the air aliquots and the samples. Beam sizes were measured simultaneously on a double Faraday collector in sets of 100 online acquisitions with a 1 s integration time. The atmospheric correction is monitored via two separate measurements of atmospheric argon for each sample. A first atmospheric argon aliquot (Air-1: reference dose) is measured at the same gas pressure as the sample, and serves as an isotopic reference for the determination of its radiogenic content under identical mass-discrimination conditions. The second (Air-2: calibration dose) consists in a manometrically-calibrated dose of atmospheric argon (from a separate reservoir of known 40Ar content). This is used to convert beam intensities into atomic abundances. As both reference aliquots (isotopic and manometric) are atmospheric in composition, they provide a cross check on the radiogenic composition of the sample. All the calculated isotopic ratios (unknown, Air-1 and Air-2) are reported in Appendix A and summarized in Table 2. Periodic cross-calibration of zero-age standards precisely constrains the mass discrimination to within 0.5& on the 40Ar/36Ar ratios (Scaillet and Guillou, 2004). The manometric calibration of the Air-2 reference is based on periodic, replicate determinations of international dating standards of known K-Ar age using the same procedure for the unknown samples to be measured as described in Charbit et al. (1998). This allows the total 40Ar content of the sample to be determined with a precision of about  0.2% (2s). Used standards include LP-6 127.8  0.7 Ma, (Odin, 1982) and HD-B1 (24.21  0.32 Ma, (Fuhrmann et al., 1987; Hess and Lippolt, 1994; Hautmann and Lippolt, 2000). At the 95% confidence level, the values adopted here are consistent with those obtained for several 40Ar/39Ar standards through the intercalibration against biotite GA-1550 by Renne et al. (1998) and Spell and McDougall (2003). 3.2.

40

Ar/39Ar method

Equivalent groundmass samples to the KeAr experiments were analyzed at the new 40Ar/39Ar facility developed in our laboratory. The 120 mg samples were wrapped into 99.5% copper foil packets, loaded in aluminum disks and then irradiated either 60 min (Irradiation # 13) or 45 min (Irradiation # 21) in the b1 tube of the OSIRIS reactor (CEA Saclay, France). The total neutron flux received by the samples ranged from 1.2 10þ14 n/cm2 to 2.7 10þ14 n/cm2. Neutron fluence (J) was monitored by co-irradiation of Alder Creek Sanidine (Nomade et al., 2005) placed in three positions around the aluminum disk. The J value for each sample was determined from at least 8 single grain laser fusion analyses of ACR-2 crystals. Corresponding J values were calculated using an age of 1.193 Ma (Nomade et al., 2005) and the total decay constants of Steiger and Jäger (1977). The J values vary by about 1% across each disk pit. Correction factors for interfering neutron reactions were determined on pure compounds (K2O, CaF2) irradiated in the same position and were: (39Ar/37Ar)Ca ¼ 2.71 104, (36Ar/37Ar)Ca ¼ 4.06 104, (40Ar/39Ar)K ¼ 3.52 103; (38Ar/39Ar)K ¼ 1.52 102. After irradiation, samples were loaded into a stainless steel carousel over a double-vacuum resistance furnace and degassed at 550e650  C to remove undesirably large quantities of atmospheric argon. Incremental heating experiments consisted of 5e8 steps between 700 and 1250  C. Each step involves 2 min of increase to a set-point temperature that was maintained for a total duration of 20 min with the gas exposed to a titanium sublimation pump. 5 Additional minutes of gas cleanup were achieved by two SAES 10 GP-MK3 ZreAr getters operated at 400  C. The gas is then attracted during 5 min, into a line sector on an active charcoal

532

H. Guillou et al. / Quaternary Geochronology 6 (2011) 530e538

Fig. 1. Location map of the dated samples.

maintained at liquid nitrogen temperature. After attraction this sector is isolated, and the gas released at room temperature is cleaned up by an air cooled SAES C50 ZreAr getter operated at 250  C during another 5 min. The purified gas is measured using a high-sensitivity noble gas GV5400 instrument operated in ion counting mode. One analytical run consists of 20 peak scans of each

argon isotope with integration times of 1 s (40Ar, 39Ar) or 10 s (36Ar, Ar, 38Ar, baseline), first preceded by a peak centering routine on the five Ar isotopes, upon admission of the sample into the mass spectrometer. Raw argon isotope abundances are regressed back to inlet time using GV software (NG v.2.90) based on linear or polynomial least-squares fit. The instrument was operated at 37

H. Guillou et al. / Quaternary Geochronology 6 (2011) 530e538 Table 2 Raw data from mass spectrometric measurements of samples. IR ¼

40

533

Ar/36Ar isotopic ratio. Subscript S refers to sample, A1 to Air-1 and A2 to Air-2; SE is standard error.

Sample

IRs

SE (%)

IRA1

SE (%)

IRA2

SE (%)

40

CITF-98 CITF-98 CITF-301 CITF-301

283.453 285.740 279.744 278.101

0.066 0.082 0.039 0.031

273.908 273.172 273.166 273.617

0.050 0.074 0.032 0.032

274.147 272.680 272.966 273.511

0.025 0.021 0.026 0.027

3.368 4.398 2.351 1.612

a sensitivity of 8  103 A/Torr. The precision and accuracy of the mass discrimination correction was monitored by periodical measurements of air argon. This monitoring is performed using a dedicated air-calibration system featuring a 6-L tank filled with purified atmospheric argon. This tank is connected to the mass spectrometer vacuum line via two pneumatically- actuated air pipettes of approximately 0.1 and 1.0 cc. This system allows for a 1 cc (e.g. 600 000 counts s1 (cps on 40Ar)) and a 0.1 cc (e.g. 70 000 cps on 40 Ar) atmospheric aliquots to be delivered into the mass spectrometer and permits careful monitoring of the mass discrimination over a wide dynamic range with a precision better than 0.15% (2s; standard deviation for multiple experiments) for any given beam size measured (see online supplement Fig. F1). In principle, the achieved precision allows samples with about 1% 40Ar* or more to be measure with confidence. System blanks were measured prior to step-heating experiments at temperatures between 600 and 1400  C. Blanks were about 9.0  1016 mol of 40Ar and 2.8  1018 mol of 36Ar in nearly atmospheric composition. These values are about 30e100 times lower than the sample signals.

Ar* (%)

SE (%) 2.514 2.447 2.136 2.695

The unspiked K-Ar age of samples CITF-98 and CITF-301 are reported in Table 3, being 33.1  1.8 and 31.6  1.9 ka respectively. The duplicate ages are consistent within the 2 sigma error and the precision is about 6% for the two samples. 4.2.

40

Ar/39Ar results

Plateau ages, isochron regressions and probability of fit estimates were calculated using ArArCalc (Koppers, 2002). We followed the criteria of Sharp and Renne (2005) to build reliable isochron regressions. An isochron includes the maximum number of consecutive steps with a probability of fit 0.1. It consists of at least three or more steps that contain 60% of the 39Ar released and it defines a trapped 40Ar/36Ar ratio not statistically different from 295.5. Retained criteria for an acceptable age plateau are: (1) it must have a minimum of 3 or more consecutive steps that contain 60% or more of the 39Ar released, (2) no resolvable slope at 1s analytical uncertainty, (3) no outliers or age trend within the initial and final steps. Six step-heating experiments were conducted for the 2 samples. Three of them yielded concordant spectra with 100% of the gas defining the age plateaus. The three other plateau ages comprise between 80% and 88% of the gas released (Tables 4 and 5, online supplements Appendixes B1 to B6). The 40Ar* contents range from 1.1% to 8.9%, with typical values of 3e6% for the plateau steps. The 40 Ar/36Ar intercept values defined for the six isochrons are atmospheric and the total fusion ages are similar to plateau or isochron ages (Tables 4 and 5 and Figs. 3 and 4). This indicates that for both samples the effect of argon loss or excess argon is almost negligible. This is also a powerful test of the assumptions required to validate the KeAr ages. Given the low values of 40Ar*, the spread along the isochrons is very limited. In consequence, the uncertainties about the isochron ages which include the analytical precision, peak signals, blanks, mass discrimination, J factor and reactor corrections, are about 16%e33%. Thus, we prefer to use the more precise weighted mean plateau age. The 40Ar/39Ar ages obtained for the two samples, calculated from three independent experiments, are 32.4  1.8 ka (2s) (CITF301) and 31.4  1.7 ka (2s) (CITF-98).

4. Results 4.1. Unspiked KeAr results Unspiked KeAr analysis of each sample involved three independent determinations of potassium and two of argon (Table 3). Based on replicate analysis of material references, the potassium concentrations were determined with an uncertainty of 1% (1s). The potassium concentrations were combined to yield a mean value. Age determinations of each sample were made using this value and the weighted mean of the two independent measurements of 40 Ar*(radiogenic argon). Uncertainties for the Ar data are 1 analytical only, and consist of propagated and quadratically averaged experimental uncertainties arising from the 40Ar (total), and 40Ar* determinations. Uncertainties on the ages are given at 2. Fig. 2 shows the 40Ar/36Ar ratios measured during the replicated analyses of the two samples. This figure highlights the excellent stability during the measurements and evidences the capability, via the unspiked K-Ar technique, to detect relative small amounts (1.5e4%) of 40Ar* with a satisfying precision (2.5%). This is particularly striking when two successive air measurements (AIR1 and AIR-2) are compared. The maximum difference between two consecutive IRA1 and IRA2 is smaller than 0.5 (Table 2). This clearly demonstrates the remarkable reproducibility of the instrument and its ability to measure percentages of 40Ar* lower than 1%.

5. Discussion The reported 40Ar/39Ar ages agree within errors with the KeAr ages, and are of similar precision. Therefore, this study demonstrates that precise ages can be obtained from young volcanic rocks

Table 3 Unspiked KeAr ages of the Teide phonolites. Ages are calculated using the decay constants of Steiger and Jäger (1977). Sample ID Experiment n

Weight molten (g)

K* (wt.%)  1s

40

CITF-98 6807 6832

2.02239 2.41121

3.777  0.038 “......”

CITF-301 6688 6702

1.71730 1.65063

4.284  0.043 “......”

Ar* (10-13 mol/g)

Ar* weighted mean  1s

Age  2s ka

40

40

3.368 4.398

2.304 2.069

2.169  0.056

33.1  1.8

2.351 1.612

2.431 2.213

2.347  0.070

31.6  1.9

Ar* (%)

534

H. Guillou et al. / Quaternary Geochronology 6 (2011) 530e538

CITF-301, analysis 6688

CITF-98, analysis 6807

284.000

295.000

y

sample

y

282.000

Ar/36Ar

=-0.0063x+283.45

sample

=-0.0131x+279.74

280.000

278.000

40

285.000

40

Ar/36Ar

290.000

y

276.000 280.000

=0.0114x+273.91 y Air-1

Air-1

=0.0038x+273.17

274.000

275.000 272.000

y =0.0014x+274.15 y Air-2

270.000 0

10

20

30

40

50

60

70

80

90

0

100

10

20

30

Measurement number

40

50

60

70

80

90

100

70

80

90

100

CITF-301, analysis 6702

CITF-98, analysis 6832

282.000

y

sample

y =-0.0084x+278.09 sample

=-0.005x+285.74 280.000

285.000

278.000

Ar/36Ar

290.000

280.000

=0.0054x+273.17 y Air-1

276.000

y =0.0005x+273.62 Air-1

40

Ar/36Ar

=0.005x+272.97

Measurement number

295.000

40

Air-2

270.000

274.000

275.000

272.000

270.000

y

Air-2

=0.0011x+272.68 y

=0.0013x+273.51 Air-2

270.000

265.000 0

10

20

30

40

50

60

70

80

90

0

100

10

20

30

40

50

60

Measurement number

Measurement number

Figs. 2. 40Ar/36Ar ratio measured during unspiked KeAr analysis of CITF-98 and CITF-301 samples and corresponding air standard. Linear regression gives y-intercept values that are used to calculate moles of radiogenic argon in sample. Complete set of isotope ratio measurements for all samples is in Data Repository.

using the new 40Ar/39Ar facility of the LSCE and confirms the effectiveness of the KeAr unspiked method. The high degree of precision of the unspiked technique is ensured partly by the instrumental procedure. As described above, the gas sample and an aliquot of pure atmospheric argon are sequentially admitted to the mass spectrometer and their 40Ar/36Ar ratios measured directly using a dual collector. Using adjustable volume techniques, Ra (40Ar/36Ar of atmospheric reference) and Ru (40Ar/36Ar of the sample) are measured at the same working gas pressure. With this procedure, any pressure-induced instrumental Table 4 Summary of

40

mass bias cancels out between the reference atmospheric aliquot and the unknown, and the atmospheric correction is made very precisely. As already observed for both samples, plateau and isochron ages are the same at the 2 sigma level, confirming the atmospheric composition of initial Ar in those samples (Tables 4 and 5). Given this, the KeAr ages of these samples are not affected by excess 40Ar* and can be regarded as reliable crystallization ages. Thus, because the 40Ar/39Ar ages and the KeAr ages are equivalent at the 95% confidence level, these data can be pooled to produce more precise

Ar/39Ar data from incremental heating experiments on CITF-98 Teide Phonolite sample (Tenerife, Canary Islands).

Sample site experiment no. wt. (mg) K/Ca (total) Total Fusion Age (ka) Age spectrum Increments used ( C)

39

Isochron analysis Ar (%) Age  2 s (ka) MSWD N

CITF-98, groundmass FG-050 to FG-056 118 1.3 31.9  3.1 800e1200 81.6 FG-058 to FG-063 115 1.9 34.2  4.4 750e1110 88.3 FG-287 to FG-293 124 2.6 30.5  3.9 640e1140 100.0 Weighted mean plateau and composite isochron ages from three experiments Simple mean plateau and isochron ages from three experiments Ages calculated relative to 1.193 Ma Alder Creek Rhyolite sanidine standard.

32.3 31.0 31.1 31.4 31.5

    

3.0 3.4 2.5 1.7 3.0

0.57 0.55 0.27 0.23

6 of 7 5 of 6 7 of 7 18 of 20

Ar/36Ar  2 s Age  2 s (ka) intercept

MSWD

40

0.72 0.71 0.29 0.52

295.3 295.1 295.0 295.1

   

4.6 2.9 2.1 1.6

32.8 31.7 32.1 31.8 32.2

    

11.0 6.6 5.2 4.1 7.6

H. Guillou et al. / Quaternary Geochronology 6 (2011) 530e538 Table 5 Summary of

535

40

Ar/39Ar data from incremental heating experiments on CITF-301 Teide Phonolite sample (Tenerife, Canary Islands).

Sample site experiment no. wt. (mg) K/Ca (total) Total Fusion Age (ka) Age spectrum Increments used ( C)

39

Isochron analysis Ar (%) Age  2 s (ka) MSWD N

CITF-301, groundmass FG-041 to FG-048 118 2.08 32.3  3.7 765e1240 80.2 FG-064 to FG-068 122 1.60 29.9  5.0 750e1140 100.0 FG-280 to FG-286 123 1.34 31.4  3.3 680e1200 100.0 Weighted mean plateau and composite isochron ages from three experiments Simple mean plateau and isochron ages from three experiments

ages (Table 6). When pooled with the KeAr ages, this yields an age of 32.0  1.3 ka for sample CITF-301 and an age of 32.2  1.2 ka for sample CITF-98. The calculated precisions are as low as 4%, which is quite remarkable for young phonolitic lavas.

33.7 32.1 31.4 32.4 32.4

    

3.2 3.3 3.0 1.8 3.2

0.55 0.46 0.02 0.51

6 of 8 5 of 5 7 of 7 18 of 20

Ar/36Ar  2 s Age  2 s (ka) intercept

MSWD

40

0.60 0.05 0.03 0.4

296.9 293.8 295.3 294.9

   

3.9 2.6 4.1 1.9

30.8 36.0 32.0 33.8 32.9

    

11.0 6.7 10.4 5.0 9.4

Given the potential of the coupling of these two dating techniques, we can take advantage of the positive aspects of each of them. In particular, 40Ar/39Ar analyses, although time-consuming, may reveal problems of argon loss or inheritance. However, 39Ar

100

CITF-98 :- 3 experiments Plateau age : 31.4 ± 1.7 ka (2 ) MSWD = 0.41

90 80 70

Age Ka

60 50 40

30 20 10 0 0

10

20

30

40

50

60

70

80

90

100

Cumulative 39Ar Released (%) 0.00345

Age = 31.8±4.1 ka 40 36 Initial Ar/ Ar =295.1±1.6 MSWD = 0.52

0.00335

0.00325

36 40

Ar Ar

0.00315

0.00305

0.00295 0.0

40

39

0.5

1.0

39

1.5

40

2.0

2.5

3.0

3.5

Ar/ Ar

Fig. 3. Age spectra and isochrons depicting Ar/ Ar experimental results from CITF-98 phonolitic flow. Gray filled boxes were rejected for the plateau age and isochron calculations. Uncertainties 2s. Complete set of 40Ar/39Ar measurements for all samples is in Data Repository.

536

H. Guillou et al. / Quaternary Geochronology 6 (2011) 530e538 100

CITF-301 :- 3 experiments Plateau age : 32.4 ± 1.8 ka (2 ) MSWD = 0.51

90

80

70

Age Ka

60

50

40

30

20

10

0 0

10

20

30

40

50

60

70

80

90

100

Cumulative 39Ar Released (%)

0.00345

Age = 33.8±5.0 ka 40 36 Initial Ar/ Ar =294.9±1.9 MSWD = 0.40 0.00335

36

Ar 40 Ar

0.00325

0.00315

0.00305 0.0

0.4

0.8

39 Fig. 4. Age spectra and isochrons depicting calculations. Uncertainties  2s.

40

1.6

2.0

2.4

40

Ar/ Ar

Ar/39Ar experimental results from CITF-301 phonolitic flow. Gray filled boxes were rejected for the plateau age and isochron

recoil can preclude the dating of glassy or very fine-grained lavas. Unspiked K-Ar dating is a sensitive and rapid dating method that does not involve the disadvantage of 39Ar recoil. It can distinguish very small amounts of 40Ar* in late Quaternary lavas. However, the Table 6 Summary of KeAr

1.2

main drawback of this technique is that, within the analysis of a sample, the configuration of the instrument does not allow measurement of the 38Ar isotope without significantly altering the analytical conditions. Therefore, the Achilles’ heel of the method is

40

Ar/39Ar experiments on Teide phonolites.

CITF-301

Age  2s (ka)

40

33.7 32.1 31.4 32.4 32.4 31.6 32.0

Ar/39Ar plateau (FG-041 to FG-048) 40 Ar/39Ar plateau (FG-064 to FG-068) 40 Ar/39Ar plateau (FG-280 to FG-286) Weighted mean simple mean KeAr age KeAr; 40Ar/39Ar pooled age

      

3.2 3.3 3.0 1.8 3.2 1.9 1.3

CITF-98

Age  2s (ka)

40

32.3 31.0 31.1 31.4 31.5 33.1 32.2

Ar/39Ar plateau (FG-050 to FG-056) 40 Ar/39Ar plateau (FG-058 to FG-063) 40 Ar/39Ar plateau (FG-287 to FG-293) Weighted mean simple mean KeAr age

      

3.0 3.4 2.5 1.7 3.0 1.8 1.2

H. Guillou et al. / Quaternary Geochronology 6 (2011) 530e538

537

32000

31000

30000

Radiocarbon Age (yrs)

29000

28000

27000

26000

25000

24000 30000

31000

32000

33000

34000

35000

36000

cal BP yrs (equivalent to K-Ar Ar/Ar pooled age) Fig. 5. Radiocarbon Age vs. Calibrated Age diagram established using the Radiocarbon calibration program: CALIB REV6.0.0. (Copyright, 1986e2010 M Stuiver and PJ Reimer). White circle: Pooled K/Ar 40Ar/39Ar age used as reference to recalculate white square: radiocarbon age.

that the isotopic composition of the initial argon trapped in the samples cannot be verified, that is to say that it has to be assumed that, at the time of formation, the 40Ar/36Ar ratio of the sample was equivalent to the modern atmospheric value (295.5). The approach combining the 40Ar/39Ar and the unspiked KeAr methods allows all the basic assumptions which rule the KeAr clock to be checked via the isochron approach. Then the pooling of a few 40Ar/39Ar and unspiked KeAr ages produces very precise estimates of the ages of the samples. Such an approach has already proven to be very successful to date key events such as geomagnetic field excursion (Guillou et al., 2004; Singer et al., 2009). It also reduces the number of time-consuming 40Ar/39Ar experiments necessary to obtain ages associated with sufficient accuracy. This approach combining two dating methods may also be relevant to discuss the accuracy of 14C ages in a volcanic environment. A charcoal collected within the basal scoriae of phonolitic flow CITF98 has been 14C dated via AMS (Carracedo et al., 2007) at 32 360  800 yrs BP (2s). Because the KeAr and 40Ar/39Ar ages of this sample are in agreement, the pooled age of 32.2  1.2 ka can be retained as a reliable calendar age. Therefore, using the available calibration curves, the accuracy of the radiocarbon age can be evaluated. For this purpose, we have retained the updated calibration curve INTCAL09 14C (Reimer et al., 2009). According to INTCAL09, a calendar age of 32.2 ka (i.e., pooled K-Ar 40Ar/39Ar age of

sample CITF-98) would correspond to a 14C age of about 28.2 ka which is approximately 4 ka younger than the radiocarbon age measured (Fig. 5). There are two main interpretations for this discrepancy between the 14C and KeAr clock derived ages. The first one would be to question the radiocarbon calibration. Given all the stringent criteria adopted by the INTCAL working group to update the calibration curve between 0 and 50 000 years, we discard this first hypothesis. Errors in 14C dates and their possible sources were already documented more than 30 years ago and they were evidenced in several volcanic areas such as in the Eiffel (Bruns et al., 1980), the provinces of Grosseto and Siena (Saupé et al., 1980) and in the Azores (Pasquier-Cardin et al.,1999). In these areas, significant to large 14C depletion may occur in many plants, due to assimilation of 14C endogenous CO2, the consequence of which, as demonstrated by the study of modern plants, is an error in excess of the radiocarbon ages that can reach some ka. We suggest that the apparent old radiocarbon age discussed here, results from the fact that the analyzed charcoal probably derived from a plant which grew close to active volcanic fumaroles, which are sources emitting 14C free CO2. 6. Conclusion 1. This study demonstrates that precise ages can be obtained from young volcanic rocks using the new 40Ar/39Ar facility of the

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LSCE and confirms the effectiveness of the KeAr unspiked method. 2. The approach combining the 40Ar/39Ar and the unspiked K-Ar methods implies that all the basic assumptions which rule the KeAr clock, are checked via the isochron approach. Then the pooling of a few 40Ar/39Ar and unspiked KeAr ages produces very precise ages and constitutes a very promising tool with which to calibrate the Quaternary timescale and to date key events such as geomagnetic field instabilities and also paleoenvironmental and paleoclimatic records. 3. This approach combining two dating methods is also relevant to discuss the accuracy of 14C ages in a volcanic environment. Acknowledgments Acknowledgments are due to J.L. Joron (Lab. P. Süe, CEA Saclay), who performed the irradiations of the samples. The authors are grateful to J.F. Tannau (LSCE) for his constant technical assistance and expertise and to N. Smialkowski who helped with sample preparations. Pauline Agnew revised the English version of the manuscript. I. McDougall and A. Calvert provided constructive and helpful comments. This work was supported by the French Atomic Commission (CEA), CNRS, and also funded by Program AO INSU 2011-CT2 Action Syster. LSCE contribution No. 4513. Appendix. Supplementary data Supplementary data associated with this article can be found in online version at doi:10.1016/j.quageo.2011.03.011. References Ackert, R.A., Singer, B., Guillou, H., Kaplan, M.R., Kurz, M.D., 2003. Cosmogenic 3He production rates from 40Ar/39Ar and K-Ar dated Patagonian lava flows at 47 S. Earth and Planetary Science Letters 210, 119e136. Bruns, M., Ingeborg, L., Münnich, K.O., Hubberten, H.W., Fillipakis, S., 1980. Regional sources of volcanic carbon dioxide and their influence on 14C content of present-day plant material. Radiocarbon 22, 532e536. Carracedo, J.C., Rodríguez Badiola, E., Guillou, H., Paterne, M., Scaillet, S., Pérez Torrado, F.J., Paris, R., Fra-Paleo, U., Hansen, A., 2007. The Teide volcano and the rift zones of Tenerife, Canary islands: eruptive and structural history. Geological Society of America Bulletin 119 (9), 1027e1051. Charbit, S., Guillou, H., Turpin, L., 1998. Cross calibration of K-Ar standard minerals using an unspiked Ar measurement technique. Chemical Geology 150, 147e159. De Vivo, B., Rolandi, G., Gans, P.B., Calvert, A., Bohrson, W.A., Spera, F.J., Belkin, H.E., 2001. New constraints on the pyroclastic eruptive history of the Campanian volcanic Plain (Italy). Mineralogy and Petrology 73, 47e65. Fuhrmann, U., Lippolt, H., Hess, J.C., 1987. HD-B1 Biotite reference material for KeAr chronometry. Chemical Geology 66, 41e51. Guillou, H., Carracedo, J.C., Day, S., 1998. Dating of the upper Pleistocene e Holocene volcanic activity of La Palma using the unspiked KeAr technique. Journal of Volcanology and Geothermal Research 86 (1e4), 137e149. Guillou, H., Singer, B., Laj, C., Kissel, C., Scaillet, S., Jicha, B.R., 2004. On the age of the Laschamp geomagnetic event. Earth and Planetary Sciences Letters 227, 331e343. Hautmann, H.J., Lippolt, H.J., 2000. 40Ar/39Ar dating of central European KeMn oxides, a chronological framework of supergene alteration processes during the Neogene. Chemical Geology 170, 37e80. Hess, J.C., Lippolt, H.J., 1994. Compilation of KeAr measurements on HD-B1 standard biotite. In: Odin, G.S. (Ed.), Phanerozoic Time Scale, vol. 12, pp. 19e23. Bull. Liais. Inform. I.U.G.S., Subcom. Geochronol. Hu, Q., Smith, P.E., Envensen, N.M., York, D., 1994. Lasing in the Holocene: extending the 40Are39Ar laser probe method into the 14C age range. Earth and Planetary Science Letters 123, 331e336. Koppers, A.A.P., 2002. ArAr CALCdsoftware for 40Ar/39Ar age calculations. Computer and Geosciences 28, 605e619.

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