39Ar dating

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ScienceDirect Geochimica et Cosmochimica Acta 141 (2014) 113–126 www.elsevier.com/locate/gca

WA1ms: A 2.61 Ga muscovite standard for

40

Ar/39Ar dating

Fred Jourdan a,⇑, Adam Frew a, Aurore Joly b,1, Celia Mayers a, Noreen J. Evans c a

Western Australian Argon Isotope Facility, Department of Applied Geology & JdL Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia b Centre for Exploration Targeting, University of Western Australia, M 006, 35 Stirling Highway, Crawley, WA 6009, Australia c Department of Applied Geology & JdL Centre, Curtin University, GPO Box U1987, Perth, WA 6845, Australia Received 2 September 2013; accepted in revised form 12 June 2014; Available online 27 June 2014

Abstract The 40Ar/39Ar dating technique requires the use of neutron fluence monitors (standards) to allow calculation of the age of a sample. Precise calibration of these standards is crucial to obtaining accurate ages and decreasing the uncertainties associated with 40Ar/39Ar dates. Few fully intercalibrated 40Ar/39Ar standards with a sufficient total fusion grain-to-grain reproducibility are currently in use in the argon community. For Precambrian samples, only Hb3gr hornblende (1.08 Ga) yields sufficient grain-to-grain reproducibility and has an appropriate age for acceptable argon isotopic ratio measurements. Here, we present chemical and intercalibration results for a new 2.61 Ga standard. WA1ms is a muscovite extracted from an Archaean shear zone in the Lake Johnston greenstone belt, Western Australia. In situ trace element analysis by ELA-ICPMS revealed consistent K contents, subtle zonation and intra-grain and grain-to-grain heterogeneities in Rb, Sr, Ti, and Fe but a lack of mineral inclusions.WA1ms has been investigated over 3 irradiations ranging from 25 to 40 h, in two reactors, with several disc positions and three grains sizes and has been calibrated against FCs and GA1550, and Hb3gr. Overall, we carried out 48 total fusion and 4 step-heating experiments of WA1ms crystals. Flat age spectra and average F-value (40Ar*/39ArK) relative standard deviations ranging from of 0.43% to 0.60% (P = 0.15–0.83) for 47/48 analyses demonstrate the reproducibility of WA1ms and its suitability as a reliable 40Ar/39Ar standard. We calculated R[WA1ms/FCs] = 205.59 ± 0.25, R[WA1ms/GA1550] = 57.25 ± 0.06 and R[WA1ms/Hb3gr] = 3.9713 ± 0.014 (all with P > 0.14) allowing direct comparison between WA1ms and any standards in used in the community, provided that they have been calibrated against any of the three standards used in the calibration and regardless of the age adopted for each of these standards. The recently revised decay constant values and standard ages proposed by Renne et al. (2011) yield a weighted mean age of 2614.2 ± 1.5 Ma (±0.055%; P = 0.8; 1r) for WA1ms against the three standards. When calibrated solely against FCs in order to minimize correlated errors between FCs vs. GA1550 and Hb3gr, we calculate a preferred age of 2613.0 ± 2.4 Ma (±0.09%) which is recommended when using this standard. Additionally, this corresponds to a 40Ar*/40K value of (0.3375 ± 0.0057)  10–1 for WA1ms, which is independent of the value of the decay constant used for calculation and allows recalculation of the age of WA1ms for any preferred set of decay constants and standard ages. Using the constants proposed by Steiger and Ja¨ger (1977) directly calculated from the three R-values, yields an age of 2598.2 ± 3.5 Ma corresponding to an identical, although less precise 40Ar*/40K value of (0.3375 ± 0.0086)  10–1. Benefits of this standard include: (1) an increased relative precision per crystal analyzed relative to other standards for Proterozoic and Archaean samples, (2) no post-irradiation waiting time due to the absence of Ca (no need to wait for 37ArCa decay), and (3) loading of standard grains that are optically different from the unknown (e.g., pyroxene or hornblende) to avoid standard-unknown mixing. WA1ms is freely available to the 40Ar/39Ar community. Ó 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +61 (0)8 9266 2412. 1

E-mail address: [email protected] (F. Jourdan). Now at Aurora Australis, GeoConsulting Pty Ltd, 363 Hay Street, Subiaco WA 6008, Australia.

http://dx.doi.org/10.1016/j.gca.2014.06.010 0016-7037/Ó 2014 Elsevier Ltd. All rights reserved.

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1. INTRODUCTION The 40Ar/39Ar dating technique requires the use of neutron fluence monitors (standards) to allow sample age calculation. Precise calibration of these standards is crucial for accurate age determination and to decrease the uncertainties associated with 40Ar/39Ar dates. Currently, only a few fully intercalibrated 40Ar/39Ar standards are used by the argon community, including: Hb3gr hornblende; TC sanidine; FC sanidine; GA1550 biotite and AC sanidine. All these standards show a consistent total fusion grainto-grain reproducibility, demonstrating for example, an average relative standard deviation of the F-value (40Ar*/39ArK) of 0.38% and 0.59% for FCs and Hb3gr, respectively (Renne et al., 1998; Jourdan and Renne, 2007). Ideally, available standards should span a range of ages along the geological timescale and include a variety of minerals in order to; (1) Minimize the isotopic range between the standard and the unknown (i.e., similar age); (2) Minimize Ca and Cl isotopic interference corrections; (3) Load standard crystals that are optically different from the unknown – a practical and important measure to avoid accidental mixing within the irradiation discs; and (4) Rapidly measure the Ca-free standard grains upon irradiation without the need to wait a few months for 37Ar* to decay. For Precambrian samples, only Hb3gr (1.08 Ga) and NL-25 (2.65 Ga) hornblende standards provide the desired age matching to allow optimal sample age measurement. Whereas Hb3gr showed a good grain-to-grain reproducibility (Jourdan et al., 2006; Jourdan and Renne, 2007), modern mass spectrometric and laser techniques have shown that NL-25 hornblende (Schwarz and Trieloff, 2007) fails to give reproducible single-grain ages with a relative standard deviation of the F-value of 2.3% (Jourdan and Renne, 2007), leaving Hb3gr as the only reliable standard for this time period. Here, we present intercalibration results for a new 2.61 Ga secondary standard and demonstrate the ability of the WA1ms muscovite crystals to yield highly reproducible data over three irradiations with two different durations, carried out at two different reactors, and independent of crystal size. WA1ms is freely available to the argon community upon request. 2. STANDARDS USED IN THE CALIBRATION Available literature provides an extensive description of the FCs, GA1550 and Hb3gr standards. Hereafter, these standards along with WA1ms muscovite are briefly described, but a grouped description of the three latter standards can be found in Jourdan and Renne (2007). 2.1. WA1ms WA1ms is a muscovite extracted from an Archaean shear zone in the Lake Johnston greenstone belt, Western Australia (Joly et al., 2010). The sample B4 that contains WA1ms was extracted underground at level 975 of Maggie Hays mine (32°130 1500 N, 120° 290 4500 E, 375 m in depth). B4 is a mylonitic felsic rock, mainly composed of quartz,

K-feldspar, plagioclase, muscovite, biotite and garnet. Rare retrograde chlorite can be observed. The most common inclusion minerals in the garnet are quartz, plagioclase, ilmenite, rutile, monazite and apatite. The sample B4 displays a well-developed N- to NNWdipping schistosity defined by the preferred orientation of biotite and elongated plagioclases. B4 is characterised by a strongly developed N- to NW-plunging L2LJ stretching lineation, with shallow plunges underlined by elongated aggregates of biotite and muscovite, or pressure shadows around garnets. The deformation is concentrated regularly as quartz mylonitic bands spaced of a few centimetres. Some criteria such as asymmetric pressure shadows around garnet and sigmoidal mineral suggest kinematics towards the SE, in agreement with the D2LJ deformation (Joly et al., 2010). 2.2. FCs The Fish Canyon sanidine (FCs) standard has been extracted from the Fish Canyon volcanic tuff in the San Juan volcanic field, Colorado (Lipman et al., 1997). The FCs standard has been the focus of a large number of studies (Renne et al., 1998; Daze´ et al., 2003; Spell and McDougall, 2003; Jourdan and Renne, 2007; Kuiper et al., 2008; Philips and Matchan, 2013) and acts currently as the chief standard in 40 Ar/39Ar geochronology due to its good documentation by Renne et al. (1998), superior reproducibility, wide distribution in the community and intermediate age range, that allows co-irradiation with both older and younger samples. For these reasons, it served as the main 40Ar/39Ar standard during the calibration of the 40K decay constants against the 238U decay constant (Renne et al., 2010). FCs has been shown to be homogenous in composition through electron microprobe analysis (Daze´ et al., 2003), 40 Ar/39Ar age spectra (Spell and McDougall, 2003) and total fusion grain-to-grain analyses (Renne et al., 1998; Jourdan and Renne, 2007). These studies showed that the relative standard deviations of the F-value (i.e., 40 Ar*/39Ark) ranged from 0.16% to 1.1% with an average of 0.38% attesting to the superior reproducibility of this standard (Renne et al., 1998; Jourdan and Renne, 2007). Several studies have attempted to obtain an accurate and precise age of FCs as a primary standard using K/Ar measurements (Hurford and Hammerschmidt, 1985), i.e., by direct measurement of its age without comparison to another standard. However, K/Ar analyses on sanidine are notoriously difficult due to incomplete degassing upon heating, which is a prerequisite when using the K/Ar method (McDougall and Harrison, 1999). Other studies have attempted to obtain an accurate and precise age of FCs as a secondary standard, i.e., relative to primary standards (Renne, 1994; Renne et al., 1998; Villeneuve et al., 2000; Daze´ et al., 2003; Spell and McDougall, 2003; Jourdan and Renne, 2007) or by calibration to a different chronometer (Renne, 1994; Kwon et al., 2002; Kuiper et al., 2008). Those studies propose a very large variety of absolute ages for FCs. The most widely adopted age for the last 15 years was 28.02 Ma, proposed by Renne et al. (1998) relative to the primary standard GA1550 and against the decay constant of Steiger and Ja¨ger (1977). More

F. Jourdan et al. / Geochimica et Cosmochimica Acta 141 (2014) 113–126

recently a companion study proposed an age of 28.03 Ma (Jourdan and Renne, 2007) obtained using a similar approach and by comparison with four primary standards: GA1550, Hb3gr, NL-25 and GHC-305 and relative to the decay constants of Steiger and Ja¨ger (1977). Recently, in the Geological Time Scale of 2012, the FCs age of 28.201 Ma proposed by Kuiper et al. (2008) was adopted for the time scale (Gradstein et al., 2012). We note however, that Kuiper et al. (2008)’s study measured the absolute age of a standard, but used a decay constant determined by Min et al., (2000) as a re-calculation of radioactivity data used by Steiger and Ja¨ger (1977). As such, as described by Renne (2014), the 28.201 age is only usable in the age range around the age of FCs or one has to pay the price of a very significant large error magnification in the Archean. A recent study by Wotzlaw et al. (2013) seemingly brought support to the age determined by Kuiper et al. (2008), but we note that the FCs zircons have an history of lead loss (Bachmann et al., 2007) and Wotzlaw et al. (2013)’s data define a continuum between ca. 28.64 and 28.19 Ma and do not show any true age convergence. Therefore, there is no mean to differentiate between residence history and minor lead loss (e.g., compare with data from Bachmann et al., 2007) and thus, the results are not yet supportive of any model age for FCs. Perhaps the most significant study about the age of FCs, at least in our view, is the calibration of Renne et al. (2010) of both the age of FCs and the 40K decay constants against the 238U decay constant. These authors compared a series of 40Ar/39Ar–238U/206Pb age pairs measured on instantaneously cooled volcanic rocks and using data pairs distributed over most of the geological timescale. Based on this study, Renne et al. (2011) proposed an age of 28.294 ± 0.036 Ma, to be used in conjunction with a total 40 K decay constant value of (5.5305 ± 0.0135)  1010. This is the age that will be used in this study to provide a recommended age for WA1ms, although we stress that the absolute age is not critical for a calibration study. Far more important are the R-values (a ratio of F-values; Renne et al., 1998) between various well-established standards and WA1ms. This allows recalculation of age of any standard relative to another, independent of the decay constant and age of the primary standard used. Recently, Philips and Matchan (2013) used a new generation of multi-collection noble gas mass spectrometer that achieved significantly higher precision than previous machines. They show that total fusion ages of FCs were indistinguishable at the grain-to-grain level as previously shown (Renne et al., 1998). Step-heating experiments demonstrated that this standard bears some age heterogeneity at the hundreds of thousands of year level. Nevertheless, since FCs is a secondary standard in respect to the U/Pb decay system, the fact that total fusion ages are homogenous is what is important from a 40Ar/39Ar standard point of view. 2.3. GA1550 This biotite has been extracted from a monzonite from New South Wales, Australia (Spell and McDougall,

115

2003). It was originally a primary standard, i.e. the age has been measured by K/Ar technique, independently of any standards. K/Ar ages of 98.5 ± 0.8 Ma and 98.79 ± 0.54 Ma have been reported by Spell and McDougall (2003) and Renne et al. (1998), respectively and more recently, a value of 98.5 ± 0.5 Ma was determined (McDougall and Wellman, 2011), all relative to the decay constant of Steiger and Ja¨ger (1977). The F-values show relative standard deviation values ranging from 0.17% to 0.36% (Renne et al., 1998). In the study by Renne et al. (2010), GA1550 became a secondary standard relative to FCs. Using a R[FCs/ GA1550] value (R-values are theoretically invariant ratios between F-values of two standards) of 0.27803 ± 0.00025 (±0.09%), Renne et al. (2010) calculated an age of 99.769 ± 0.108 Ma (±0.11%). This age has been used for individual age calculation at the time of analysis and is the value used to derive individual apparent ages. Renne et al. (2011) subsequently revised this age to 97.738 ± 0.104 Ma (±0.10%) and this value is the one used in the final age that we proposed for WA1ms. 2.4. Hb3gr The Hb3gr hornblende comes from the Lone Grove pluton in Texas, USA (Zartman, 1964; Turner et al., 1971). Renne (2000), Jourdan et al. (2006) and Jourdan and Renne (2007) demonstrated the compositional homogeneity and 40Ar*/39Ark reproducibility of the Hb3gr standard with a range of F-values relative standard deviations between 0.34% and 1.0% (Jourdan and Renne, 2007) and with average F-value relative standard deviation of 0.49% (Jourdan et al., 2006) and 0.59% (Jourdan and Renne, 2007). Jourdan and Renne (2007) calculated a R[FCs/Hb3gr] value of 0.019276 ± 0.000028 (±0.14%). When using the constants of Renne et al. (2010), this translates into an age of 1080.4 ± 1.1 Ma (±0.10%) which we used at the time of the measurement in this study. Using the slightly revised constants of (Renne et al., 2011), this yields an age of 1081.0 ± 1.2 Ma (±0.11%) used in the final age calculation of WA1ms. 3. ANALYTICAL METHODS 3.1. Sample selection FCs and GA1550 come from the original preparation of these standards (Renne et al., 1998). Hb3gr was sourced from a purified preparation of the original standard from Zartman (1964). This preparation is often named PP-20, but since it is simply a picking of the same crystals, we will use the original name “Hb3gr” in this study. WA1ms comes from a crushed preparation of the B4 sample. The muscovite (Fig. 1b) grains have been hand picked from other minerals (e.g., quartz, feldspars; Fig. 1a). Once the reproducibility of WA1ms was established, muscovite crystals were concentrated and stored in two jars including size fractions of 150–215 and 215–300 lm, at the Western

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Fig. 1. Stereomicroscope microphotographs of crystals extracted from the WA1ms rock. (A) Mix of various unprocessed crystals including quartz, muscovite and hornblende. (B) Concentrate of WA1ms muscovite crystals.

Australian Argon Isotope Facility and largely available for the entire community upon request. 3.2. Irradiations We carried out 47 total fusion measurements and 4 stepheating experiments. WA1ms crystals from several disc positions, and of three grain sizes (125–215, 215–300 and 300–400 lm) have been calibrated against GA1550, FCs and Hb3gr (Tables 1 and 2). Standards were loaded into four small wells in four 1.9 cm diameter and 0.3 cm depth aluminum discs. All standard crystals in a given disc were loaded together in a single pit, thereby eliminating any effect of eventual neutron fluence gradient on the J-value. Irradiation discs are similar to the ones described in Renne et al. (1998). The discs were Cd-shielded (to minimize undesirable nuclear interference reactions) and irradiated over two irradiations of 25 h (75 MWh) at the McMaster reactor, Canada and one irradiation of 40 h (40 MWh) at the Denver TRIGA reactor operated by the USGS. The mean J-values computed from standard grains within the small pits are given in the Appendix in the case of the WA1ms age calculation. Mass discrimination was monitored using an automated air pipette and is calculated relative to an air ratio of

298.56 ± 0.31 (Lee et al., 2006). Mass discrimination values are provided with each analysis in Annex 1. The correction factors for interfering isotopes were (39Ar/37Ar)Ca = 7.30  104 (±11%), (36Ar/37Ar)Ca = 2.82  104 (±1%) and (40Ar/39Ar)K = 6.76  104 (±32%) for McMaster, corresponding to average measurements obtained at the University of Nice and Curtin University and (39Ar/37Ar)Ca = 7.06  104 (±7%), (36Ar/37Ar)Ca = 2.81  104 (±3%) and (40Ar/39Ar)K = 6.76  104 (±10%) (Cosca et al., 2011) for the TRIGA reactor of Denver. Since WA1ms does not contain Ca, the results are largely independent of the interference values arising from this element. However, these values are important for F-value measurements of Hb3gr. The 40Ar/39Ar analyses were performed at the Western Australian Argon Isotope Facility at Curtin University. Each standard crystal was loaded in a single pit of a laser disc. FCs sanidine crystals were accompanied by zero age argon-free dark glass, used to allow the laser to couple with the sanidine and fully fuse the standard. Each standard was fused or step-heated using a 110 W Spectron Laser System, with a continuous Nd-YAG (IR; 1064 nm) laser rastered over the sample during 1 min to ensure a homogenously distributed temperature and full extraction of the argon contained in the standard. The gas was purified in a stainless steel extraction line using two SAES AP10 getters (one hot, one cold) and a SAES GP50 getter. Ar isotopes were measured in static mode using a MAP 215-50 mass spectrometer (resolution of 450; sensitivity of 4  1014 mol/ V) with a Balzers SEV 217 electron multiplier using 9–10 cycles of peak-hopping. The data acquisition was performed with the Argus program written by M.O. McWilliams and run in a LabView environment. The raw data were processed using the ArArCALC software (Koppers, 2002) and the individual step ages have been calculated using the decay constants recommended by Renne et al. (2010). Final ages have been calculated with the slightly adjusted constant of Renne et al. (2011). Blanks were monitored every 3–4 steps and typical 40Ar blanks ranged from 1  1016 to 2  1016 mol. Ar isotopic data corrected for blank, mass discrimination and radioactive decay are given in Annex 1. Uncertainties on individual isotope abundances Annex 1 are given at the 1r level. Results are presented as total-fusion single-grain ages (Figs. 2 and 3) to allow comparison of the data from different irradiations and disc positions. For the step heating experiments (Fig. 4), our criteria for the determination of plateau are as follows: plateaus must include at least 70% of the 39Ar. The plateau should be distributed over a minimum of 3 consecutive steps agreeing at the 95% confidence level and satisfying a probability of fit (P) of at least 0.05. Plateau ages are given at the 2r level and are calculated using the mean of all the plateau steps, each weighted by the inverse variance of their individual analytical error. 3.3. LA-ICPMS The in-situ trace element content of muscovite (grains mounted and polished in epoxy) was determined by

F. Jourdan et al. / Geochimica et Cosmochimica Acta 141 (2014) 113–126

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Table 1 Summary of 40Ar*/39Ark ratios (F-values) and their respective MSWD and P by individual irradiation position. Data have been acquired following three irradiations with two different durations, in two reactors and using different crystal sizes: Fine (f), coarse (c) and very coarse (vc) crystal sizes are indicated. Irradiation Reactor

Duration Disc Standard n°

n n w/o outliers F-value ±r Total

±r (%)

Relative standard MSWD Pdev. value

I9t25 h

McMaster 25 h

A A

WA1ms (f) GA1550

12 6

12 6

337.2 5.912

0.5 0.012

0.15% 0.55% 0.19% 0.60%

0.71 1.60

0.73 0.16

I10t25 h

McMaster 25 h

x x y y x y x y y

WA1ms (c) 4 WA1ms (vc) 5 WA1ms (c) 10 WA1ms (f) 9 GA1550 3 GA1550 9 FCs 4 FCs 10 Hb3gr 9

4 5 10 10 3 9 4 10 9

320.8 322.0 320.9 321.1 5.623 5.590 1.5659 1.5603 80.78

0.9 0.7 0.4 0.5 0.012 0.010 0.0032 0.0024 0.21

0.26% 0.20% 0.14% 0.16% 0.21% 0.17% 0.20% 0.15% 0.25%

0.47% 0.43% 0.58% 0.48% 0.18% 0.43% 0.50% 0.39% 0.46%

0.80 0.37 1.50 0.92 0.23 0.53 1.60 0.54 0.52

0.49 0.83 0.15 0.50 0.79 0.83 0.19 0.84 0.72

I14t40 h

Denver

y y

WA1ms (f) GA1550

7 10

361.1 6.333

0.7 0.010

0.19% 0.59% 0.16% 0.28%

1.40 0.32

0.21 0.97

40 h

7 10

Table 2 Summary of (40Ar*/39ArK)WA1ms/ (40Ar*/39ArK)FCs, GA1550 or Hb3gr ratios (R-values) and their respective MSWD- and P-values for each individual irradiation and irradiation position. R-values are independent of the decay constants and standard ages. F (fine grain, 125-215 lm), c (coarse grain, 215-300 lm), vc (very coarse grain, 300-400 lm). Irradiation

Duration

Disc

Standard

F-value std

±σ

WA1ms (f, c or vc)

F-value WA1

±σ

RWA/x

±σ

I10t25h I10t25h I10t25h I10t25h

25h 25h 25h 25h

x x y y

FCs-x FCs-x FCs-y FCs-y

1.5659 1.5659 1.5603 1.5603

0.0032 0.0032 0.0024 0.0024

WA1ms-c WA1ms-vc WA1ms-c WA1ms-f

320.80 322.00 320.89 321.10

0.85 0.65 0.45 0.50

204.8662 205.6325 205.6592 205.7938

0.6855 0.5907 0.4211 0.4458

205.59

0.25

Weighted Mean (MSWD = 0.45; P=0.72): I9t25h

25h

a

GA1550-1

5.9120

0.0115

I10t25h I10t25h I10t25h I10t25h

25h 25h 25h 25h

x x y y

GA1550-x GA1550-x GA1550-y GA1550-y

5.6230 5.6230 5.5900 5.5900

0.0120 0.0120 0.0095 0.0095

I140t40h

40h

y

GA1550-y

6.3330

0.0195

I10t25h

25h

x

Hb3gr

80.78

ELA-ICP-MS (Resonetics M-50 193 nm ArF excimer laser ablation system coupled to an Agilent 7700s quadrupole ICP-MS) at the GeoHistory Facility, Curtin University. Following a 40 s period of background analysis, samples were spot ablated for 30 s at a 7 Hz repetition rate in an ultrahigh purity He–N2 atmosphere using a 33 lm beam and laser energy of 2.5 J/cm2. High purity Ar was employed as the plasma gas. International glass standard NIST 610 was used as the primary standard to calculate elemental concentrations (using 29Si as the internal standard element) and to correct for instrument drift. The mass spectra were reduced using Iolite (Paton et al., 2011 and references therein). Precision is better than 5% for most elements based on repeated analyses of secondary internal standards. Results are presented in Figs. 5–7 and in Annex 2.

0.20

WA1ms-c WA1ms-vc WA1ms-c WA1ms-f

337.2

0.5

57.0365

0.1395

320.8 322.0 320.9 321.1

0.9 0.7 0.4 0.5

57.0514 57.2648 57.4043 57.4419

0.1941 0.1682 0.1259 0.1324

361.1

0.7

57.0188

0.2075

Weighted Mean (MSWD = 1.6; P=0.14):

57.25

0.06

320.8

3.971

0.014

0.9

4. RESULTS AND DISCUSSION 4.1. Reproducibility of GA150, Hb3gr and FCs All three standards have been previously demonstrated to yield relatively homogenous F-values at the single grain level (Renne et al., 1998; Spell and McDougall, 2003; Jourdan et al., 2006; Jourdan and Renne, 2007). Twentyeight, 14 and 9 analyses of the GA1550, FCs and Hb3gr analyses, respectively, yielded relative standard deviation of the F-values (Table 1) ranging from 0.39% to 0.50% for FCs, 0.18% to 0.60% for GA1550 and a value of 0.46% for Hb3gr. These values are in agreement with previous values. Probability values of all F-values range from 0.16 to 0.97 attesting that each standard yields singlepopulation distributed results for each disc position and

118

F. Jourdan et al. / Geochimica et Cosmochimica Acta 141 (2014) 113–126 Error bars at 2

30

FCs n=28 25

# Analysis

20

15

10

5

2609.3 ± 1.2 Ma (1 ) 0 2560

MSWD = 1.01, P = 0.45 2570

2580

2590

2600

2610

2620

2630

2640

2650

2660

Age (Ma) 50 45

GA1550 n=47

40

# Analysis

35 30 25 20 15 10

2610.8 ± 1.2 Ma (1 )

5

MSWD = 1.0, P = 0.07 0 2560

2570

2580

2590

2600

2610

2620

2630

2640

2650

2660

Age (Ma) 10 9

Hb3gr n=9

8

# Analysis

7 6 5 4 3 2

2612.1 ± 2.2 Ma (1 )

1

MSWD = 0.66, P = 0.73 0 2560

2570

2580

2590

2600

2610

2620

2630

2640

2650

2660

Age (Ma)

Fig. 2. Ages of WA1ms calculated relative to FCs (n = 28), GA1550 (n = 47) and Hb3gr (n = 9) standards using the values recommended by Renne et al. (2010) at the time of the measurements. Weighted mean ages are given for WA1ms relative to all three standards and show internal concordance (P > 0.05) for all three populations.

F. Jourdan et al. / Geochimica et Cosmochimica Acta 141 (2014) 113–126 Error bars at 2

90 80

119

All ages n=84

70

# Analysis

60 50 40 30 20 10

2610.4 ± 0.8 Ma (1 ) MSWD = 1.2, P = 0.17

0 2560

2570

2580

2590

2600

2610

2620

2630

2640

2650

2660

Age (Ma)

Fig. 3. All 84 ages from Fig. 2 plotted together for comparison and demonstrating the internal concordance between FCs and GA1550 and Hb3gr standards and their concordance with the WA1ms standard. The weighted mean age, calculated using the decay constants of Renne et al. (2010) used at the time of the measurements, is given for information only. However, an age for the WA1ms standard cannot be calculated by this approach as the level of correlated uncertainties is too large.

3200

3200

3000

3000

2800

2800

2600

2600

2400

2400

2200

2200

2000 3200

2000 100 0 3200

3000

3000

2800

2800

2600

2600

2400

2400

2200

2200

0

10

20

30

40

50

60

70

80

90

2000 0

10

20

30

40

50

60

70

80

90

2000 0 100

10

20

30

40

50

60

70

80

90

100

10

20

30

40

50

60

70

80

90

100

Fig. 4. Step-heating age spectra of four single-grain WA1ms muscovite crystals showing the perfect reproducibility of this standard.

40

Ar/39Ar homogeneity and

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each irradiation. These results include all total fusion analyses, without the need to remove any outlier analyses.

12.0 11.5

4.2. Composition of WA1ms

4.3. Reproducibility of WA1ms

K (%)

10.5 10.0 9.5 9.0 8.5 8.0 0

5

60

10

15

20

10

15

20

10

15

20

Grain #

50

Sr (ppm)

40

30

20

10

0 0

5

0

5

2000 1800 1600 1400

Ti (ppm)

Single spot ICPMS analyses of nineteen single muscovite grains for one major and nine trace elements show that grains tend to have a slight but resolvable heterogeneous composition at the grain-to-grain level. All trace elements show some greater variability compared to the structural element K (Fig. 5; Annex 2). For instance, analyses of K yielded average and standard deviation values of 10.23% and 0.18%, respectively corresponding to a relative standard deviation of 1.9%. Analyzed trace elements showed relative standard deviation values ranging from 5% for Ga to 17% for Ti. These grains come from a single sample attesting to some pre-existing compositional heterogeneities during the formation of the muscovite crystals. Traverse analyses of four grains (from rim to rim; Figs. 6 and 7) show that muscovite crystals display little K variability with values clustering around 10 wt.%. Trace element variability between grains can be more pronounced, in particular for elements such as Rb and Ti, less so for other elements such as Fe (Fig. 7; Annex 2). All but the biggest crystal (traverse #2) display relatively homogenous compositions. In the case of the largest crystals, Rb and Ti show clear zoning patterns, with values being more depleted and more enriched at the center of the crystals, respectively. These analyses show that, although the variations are minor and the muscovite grains are relatively homogenous, there might be more complexities to the muscovite crystals than petrographic observation alone reveals. However, it is important to note that these intra-grain variations are subtle and continuous and not sharp as it would be expected in the case of different grain populations or the presence of other crystals or inclusions. More importantly, these variations are not associated with any variation in the F-values as we will show in the next section. That means that these heterogeneities are purely compositional and that no inclusions seem to be present in the grains that we have analyzed.

11.0

1200 1000 800 600 400 200

4.3.1. Total fusion analyses Total fusion analyses yielded concordant F-values for each position for 47 over 48 WA1ms crystals analyzed, regardless of the reactor used, irradiation duration and crystal size. One analysis (1A17065D; Annex 1) was rejected as it was slightly younger. F-values relative standard deviation- and P-values range from 0.43% to 0.60% and from 0.15 to 0.83 demonstrating the homogeneity of the F-value of the WA1ms muscovite at the single grain level. In order to compare the reproducibility of all the F-values, we converted the F-values into absolute ages using the decay constants and standard age values given by Renne et al. (2010). Although any combination of decay constants and age for FCs would achieve the same results, R-values in between FCs, GA1550 and Hb3gr must be well calibrated to allow for age comparison between all the

0

Grain #

Fig. 5. LA-ICPMS analyses of 19 single grains of WA1ms for K, Sr and Ti. All analyzed elements are given in Annex 2.

WA1ms analyses against these standards. In this study, we used the R-values given by Jourdan and Renne (2007) and resulting ages calculated by Renne et al. (2010). Each of the three WA1ms age populations (i.e., compared with each one of the three standards) is homogenous and yielded weighted mean ages of 2609.3 ± 1.2 Ma (1r; P = 0.45) relative to FCs, 2610.8 ± 1.2 Ma (P = 0.07) relative to GA150 and 2612.1 ± 2.2 Ma (P = 0.73) relative to Hb3gr (Fig. 2). Each of the P-values demonstrates that

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the apparent age obtained for WA1ms relative to a given standard is identical regardless of the disc position or irradiation. In addition, WA1ms gave three ages (given by each of the standards) that are statistically indistinguishable at the 2r level (P = 0.46) giving further confidence in the age reproducibility of the WA1ms muscovite. The weighted average of the three independent ages gives a global age of 2610.3 ± 0.8 Ma. Another way of comparing the various ages is to compare all of the 84 ages as a single population. This yields a weighted mean age of 2610.4 ± 0.8 Ma and a P-value of 0.17 (Fig. 3). These results demonstrate that that the WA1ms crystals yield consistent ages over several standards, disc positions, irradiations and reactors.

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200 m

4.3.2. Age spectra 40 Ar/39Ar step heating analyses of four 200 lm singlegrain muscovite crystals show flat age spectra including 100% of 39Ar released for all four crystals (Fig. 4). For direct comparison with total fusion age results, apparent ages were calculated using the constant of Renne et al. (2010) as initially adopted at the time of the measurements. The four crystals yielded apparent plateau ages ranging from 2598 ± 21 Ma (P = 0.84) to 2618 ± 20 Ma (P = 0.95). All four ages give a weighted mean age of 2609 ± 8 Ma (P = 0.45) indistinguishable from the total fusion weighted mean ages (cf. above). Inverse isochron are not shown as the data cluster near the radiogenic x-axis and are thus unusable. 4.4. R-value between WA1ms and

40

Ar/39Ar standards

Although calculating an age for WA1ms is particularly useful to compare several sets of results, of critical importance is the R-value between WA1ms and other standards. R-values allow calculation of an age for a given standard relative to any other standard, provided that the age of the first primary standard is known. It also allows propagation of all sources of uncertainties when calculating an age using a secondary or tertiary standard as the uncertainties on the R-value must be accounted for in the calculation of a final age. In this study, WA1ms is, in essence, a secondary (relative to FCs) or even tertiary (relative to GA1550 and Hb3gr) standard when using the approach and decay constants of (Renne et al., 2010, 2011). Here we have calculated a set of R-values for WA1ms compared to each of the three standards used in this study (Table R-values). We obtained weighted average RWA1ms FCs of 205.59 ± 0.25 (±0.12%) (1r; P = 0.72), RWA1ms GA1550 of 57.25 ± 0.06 (±0.11%) (P = 0.14) and RWA1ms of Hb3gr 3.971 ± 0.014 (±0.36%) (Table 2). Note that RWA1ms is Hb3gr based on a single ratio of F-values and no P-value can therefore be calculated. However, the robustness of RWA1ms is demonstrated by the favourable comparison Hb3gr between the individual ages of WA1ms calculated relative to FCs and GA1550. The R-value between WA1ms and other standards not used in this study (e.g., NL-25, MMhb) can be easily calculated following the approach of Renne et al. (1998), as long as a R-value exists between the standard of interest and any intermediate standard (FCs, GA1550 and Hb3gr) used in this study.

Fig. 6. Pictures of WA1ms taken using the LA-ICPMS camera and showing the laser spot positions of the traverse for each of the four grains analyzed.

4.5. 40Ar*/40K and age of WA1ms using the decay constants of Renne et al. (2011) In order to calculate an age for WA1ms, we converted each of the three R-values into an age and calculated the resulting weighted mean age and associated uncertainty. This approach has the advantage of minimising the correlated errors arising from taking into account the J-value uncertainties in the calculation and, more importantly, allows users to easily calculate a preferred age for WA1ms for any given set of decay constants and associated primary standard ages. Fig. 8 shows the various ages of WA1ms for given set of decay constants and primary standard ages. In this study, and at the time of the measurement, we used the decay constants and FCs age proposed by Renne et al. (2010). This yields three individual ages 2609.4 ± 2.4 Ma (relative to FCs), 2611.2 ± 2.1 Ma (relative to GA1550) and 2613.5.2 ± 5.2 Ma (relative to Hb3gr) and a weighted mean age of 2610.7 ± 1.5 Ma (1r; MSWD = 0.30; P = 0.74; Fig. 8a). However, Renne et al. (2011) slightly

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12

210

11.5

190

11.0

170

Traverse 2

Rb (ppm)

10.5 K (%)

Traverse 3 Traverse 4

150

10.0 9.5

130 110 90

9.0

70

8.5

50

8.0 1100

Traverse 1

30 0

2

4

6

8

10

130

0

2

4

0

2

4

6

8

10

6

8

10

120

1000

110

900 Fe (ppm)

Ti (ppm)

100 800 700 600

90 80 70 60

500

50

400

40

300

30 0

2

4

6

8

Analyses

10

Analyses

Fig. 7. K, Rb, Ti and Fe elemental composition obtained by LA-ICPMS traverse analyses for the four WA1ms grains shown in Fig. 6. All analyzed elements are given in Annex 2.

adjusted the value taking into account the constructive comment by Schwarz et al. (2011). Using this new set of decay constants and standard ages, three age values of 2613.0 ± 2.4 Ma (FCs), 2614.8 ± 2.1 (GA1550) and 2616.0 ± 5.3 Ma (Hb3gr) and a weighted mean age of 2614.2 ± 1.5 Ma (± 0.055%; 1r; Fig. 8b) are obtained for WA1ms. A problem that may arise from pooling these three ages together is that the ages calculated for Hb3gr and GA1550 standards are both calculated as tertiary standards relative to FCs (Renne et al., 2010), and therefore GA1550 and Hb3gr are highly correlated to FCs. Therefore, although the two ages obtained for WA1ms relative to Hb3gr and GA1550 offer evidence of the excellent agreement between the four standards used in this study, we conservatively calculate the final proposed age of WA1ms solely relative to FCs at 2613.0 ± 2.4 Ma (±0.09%; 1r) to avoid ambiguous correlation errors. The difference on the age is barely noticeable but this is more appropriate when one needs to include the uncertainty of the decay constant in the calculation of the age of an unknown sample. Renne et al. (2010) proposed an optimization model of the error calculation based on Monte Carlo simulation that minimizes the effect of correlated errors and allows inclusion of all sources of uncertainties. Since the uncertainty calculation model of Renne et al. (2010) is largely based on the age of FCs, the propagation of all sources of uncertainties using this approach (for a sample measured with WA1ms) requires the knowledge of WA1ms standard’s RWA1ms and its uncertainty (205.59 ± 0.25), rather than the FCs absolute age and associated uncertainty of this standard. Once an age is calculated for WA1ms for a given set of constants, we can back calculate a 40Ar*/40K value which in theory, is independent of the 40K decay constants used. An age of 2613.0 ± 2.4 Ma converts to a 40Ar*/40K value of

(0.3375 ± 0.0057)  101. Note however that this value and in particular its uncertainty, is tied to the particular approach used by Renne et al. (2010). The weighted mean age of 2614.2 ± 1.5 Ma calculated above converts to a slightly higher (but largely indistinguishable) 40Ar*/40K value of (0.3378 ± 0.0037)  101, although we do not recommend using this value for the reasons given above. Finally, note that the uncertainty provided here for the age of WA1ms (2613.0 ± 2.4 Ma) does not include the uncertainties on the decay constant since the goal of this study is not to define the absolute age of a given geological event, but rather to provide the user with a new standard. 4.6. 40Ar*/40K and age of WA1ms using the decay constants of Steiger and Ja¨ger (1977) As several laboratories are still using the constants of Steiger and Ja¨ger (1977) independently measured by radioactivity counting (e.g., see discussion by McDougall, 2014), we have also calculated the apparent age of WA1ms using the present R-values, the ages of the standards listed by Turner et al. (1971), Renne et al. (1998) and Jourdan et al. (2007) (i.e., FCs = 28.03 Ma, GA1500 = 98.79 Ma and Hb3gr = 1072 Ma) and the decay constants of Steiger and Ja¨ger (1977). In this particular case, the age of FCs was calculated based on the age of four primary standards namely GA150, Hb3gr, GHC-305 and NL-25 and is also linked to these standards (albeit in a different and weaker way than in the calibration of Renne et al. (2010). This results in ages of 2597.3 ± 4.3 Ma (FCs), 2598.6 ± 7.9 Ma (GA1550) and 2598.9 ± 9.8 Ma (Hb3gr). The weighted mean of the three ages is 2597.8 ± 3.5 Ma (±0.14%; 1r Fig. 8c) which we recommend for geochronologists using the decay constants of Steiger and Ja¨ger (1977). This corresponds to a value of

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2622

WA1ms

2620 2618 2616 2614 2612

GA1550

2610

Hb3gr

FCs

2608 2620

WA1ms

2615 Mean:

2610 2605 2600 2595 2590

FCs

GA1550

Hb3gr

2585

Fig. 8. Apparent ages calculated for WA1ms using the three Rvalues obtained by comparison with FCs, GA1550 and Hb3gr standards. All three R-values provide indistinguishable ages at the 1r level (P  0.05). (A) The apparent ages were calculated using the decay constants and standards ages recommended by Renne et al. (2011), and which results in a weighted mean age of 2614.2 ± 1.5 Ma for WA1ms. Note that our preferred age is calculated solely against FCs, with a resulting age of 2613.0 ± 2.4 Ma. The age WA1ms can be recalculated using any set of decay constants and standard ages by using the 40Ar*/40K value of (0.3375 ± 0057)  101. (B) Apparent ages calculated using the decay constants of Steiger and Ja¨ger (1977) and the standard ages from Turner et al. (1971), Renne et al. (1998) and Jourdan and Renne (2007) (see text for details). 40 Ar*/40K of (0.3375 ± 0.0086)  101, which absolute value is identical to the value calculated using the optimized constant of Renne et al. (2011) but which precision is slightly degraded as a result of the lower precision on the first principle K/Ar age determinations of the primary standards. Calculation of an age for WA1ms using only primary standards (e.g., Renne et al., 1998; Jourdan and Renne, 2007) and thus omitting FCs yields an age of 2599 ± 6 Ma.

4.7. Appropriateness of WA1ms as an international 40Ar/39Ar standard In this study, we have demonstrated that the WA1ms muscovite yielded homogenous F-values regardless of the grain size of the muscovite, the position in the irradiation disc, the length of the irradiation and the reactor used. When calibrated against three well-known 40Ar/39Ar

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standards (FCs, GA1550 and Hb3gr), WA1ms yielded very reproducible ages for each of the standards (Fig. 2) and, furthermore, statistically indistinguishable ages regardless of the standard it was calibrated against. Ultimately, when all WA1ms analyses calibrated against any of the three standards were converted into ages, they yielded a single homogenous age population (P = 0.17; Fig. 3). ELAICPMS analyses showed that small but noticeable intragrain and grain-to-grain compositional heterogeneities exist (Figs. 5 and 7). These heterogeneities are too small or the variation too progressive (in the case of grain traverse analyses) to be associated with the presence of inclusions or several crystal populations. Rather, they correspond to natural variations associated with the geological mode of formation of the shear zone. Compositional variations are not associated with any F-values variations and thus do not have any consequences in the role of WA1ms as a 40Ar/39Ar standard. On the contrary, the fact that despite composition variations, all apparent ages are statistically indistinguishable from each other, indicate that no picking is necessary to obtain homogenous F-values and thus, suggest that WA1ms muscovite crystals are suitable for use as a 40 Ar/39Ar standard. It is likely that the 2.61 Ga age calculated for WA1ms do not represent a crystallization age of the muscovite but rather a cooling age. Homogenous ages (Fig. 3) show that no age gradient exists within the sample from which the muscovite crystals have been extracted. Since WA1ms is a secondary and even tertiary standard relative to FCs and, GA1550 and Hb3gr, respectively, the time–temperature or even geological history of this muscovite is not relevant to the use of WA1ms as a 40Ar/39Ar standard. Based on its grain-to-grain F-value (total fusion) reproducibility, we therefore conclude that WA1ms is a high quality Archaean muscovite, highly suitable for usage as 40Ar/39Ar standard. 4.8.

39

Ar (and

37

Ar) recoil within

40

Ar/39Ar standards?

The 40Ar/39Ar technique requires the activation of 39Ar from 39K via the reaction (39K(n,p)39Ar). Recoil is the process where target nuclei are displaced over a short distance (Turner and Cadogan, 1974). Recoil affects 39Ar, but also 37 Ar for Ca-rich minerals. This effect has been examined for more than forty years using analytical (e.g., Villa, 1997; Paine et al., 2006) and modelling (Onstott et al., 1995; Renne et al., 2005) approaches. Recent studies (Paine et al., 2006; Jourdan et al., 2007; Jourdan and Renne, 2014; Hall, 2014) have focussed on quantifying, via more or less direct measurement methods, the effect of nuclear recoil on standards. Whereas modelling (Onstott et al., 1995) and direct measurements of age vs. grains size for GA1550, Hb3gr and FCs show no effect for grains bigger than ca. 50–70 lm (Paine et al., 2006; Jourdan et al., 2007; Jourdan and Renne, 2014), stepheating measurement and encapsulation experiments may suggest that recoil in fact affect the first few% of the 39Ar released during step heating analyses (Hall, 2014). Whereas theoretical modelling and observations suggest that the expected age of a recoil affected grain should yield older ages, measurement by Hall (2014) show that Hb3gr and

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GA1550 crystals have total fusion ages that are younger than their optimal respective plateau ages suggest. The causes for such discrepancy and counter-intuitive behaviour (i.e., older ages) are open to debate and beyond the scope of this paper. Here, three points are important regarding WA1ms: (1) we showed that several size fractions of WA1ms give the exact same R-values, hence showing that recoil loss by surface ejection does not affect the results, (2) Ages derived from calibration with three standards (supposedly variously affected by recoil) provide in fact a unique age for WA1ms and (3) since we recommend the use of WA1ms as a tertiary standard calibrated against FCs (secondary standard), calibrated in turn to the 238U decay constant and 40K decay counting (Renne et al., 2010), and since the “recoil” effect observed by Hall (2014) is not affecting FCs, then it does not really matter if it affects WA1ms since the true age of a geological event is less important compared to the RWA1ms value between the two FCs standards. The important point in this case, is the reproducibility of the F- and R-values, which is clearly demonstrated in this study. 4.9. Recommended usage Using WA1ms for Proterozoic and Archaean samples (e.g., Craton, shear zones, meteorites) offers significant advantages compared to other 40Ar/39Ar standards available to the community. Compared to excellent standards such as GA1550 (ca. 100 Ma) and FCs (ca. 28 Ma), WA1ms minimizes the dynamic range between standard and a Precambrian unknown. Ultimately, a lower dynamic range allows measurement of standard and unknowns with much closer 40 Ar/39Ar ratios, thus minimizing the uncertainties associated with under- or over-irradiating samples and unknowns (Turner et al., 1971; McDougall and Harrison, 1999). The WA1ms muscovite contains 10 wt.% K2O and hence only a very small quantity is required to produce sufficient quantities of 40Ar* and 39ArK. For example, a single 200 lm in diameter crystal will produce 5  1013 mol of 40Ar* and for a J-value of 0.01, will produce 1.5  1015 mol of neutron-activated 39ArK, resulting in a 40Ar/39Ar ratio of 300 and a theoretical relative error on the absolute age of ca. ±0.19% for each grain measured.2 For comparison, a 300 lm FC sanidine will produce 3.5  1014 mol of 40 Ar* and 2  1014 mol of 39ArK resulting in a relative theoretical uncertainty of ±0.66%. The Hb3gr standard has an age of ca. 1.1 Ga, much more suitable for old samples and showing an F-value reproducibility approximately equal to WA1ms (Jourdan et al., 2006; Jourdan and Renne, 2007). Nevertheless, the principal inconvenience of the Hb3gr hornblende is that it is rich in Ca and Cl. The consequences are threefold. (1) A 250 lm crystal of Hb3gr (the standard average crystal size available) produces only 6  1014 mol of 40Ar* (ten 2 Theoretical error value calculated using the age error calculation spreadsheet provided by P. Renne, Berkeley Geochronology Centre based on the equations provided by McDougall and Harrison (1999).

times less than WA1ms) and irradiation parameters that will results in a J-value of 0.01 will produce 7  1016 mol of 39Ar (two time less than WA1ms); (2) The accuracy and precision of the J-value calculated from this standard are dependent on how well the Ca and, to a lesser degree, Cl interference factors are known. Uncertainties on these ratios combined with a lower 40Ar* and 39ArK ion beam intensity shown in (1), result in a relative error of ±0.43% per grain analyzed; (3) More importantly, following neutron activation, 37ArCa has a half-life of 35 days (Renne and Norman, 2001). This signifies that part of the 37ArCa will decay in the detector of the mass spectrometer, producing an electric current (so called “dark current”) that raises the baseline level of the detector and thus increase the uncertainties of any measurements carried out with this detector. Hence, Ca-rich samples cannot be measured too long after irradiation or interference correction on 36Ar and 39Ar will be impossible to calculate, nor can the samples can be measured too soon after irradiation as this will significantly raise the baseline of the mass spectrometer detector. Even when measuring after few months postirradiation, it is necessary to measure a sufficient enough 37 ArCa signal to allow for interference corrections, but the unavoidable trade off is that these atoms will decay in the detector. Typically, and as a compromise, it is best practice to wait for 3–4 months before analyzing Hb3gr standards, which is not necessary compatible with the schedule of any given laboratory; (4) Having a Achaean muscovite standard allows 40Ar/39Ar users to have access to a range of crystal types which in turn, allows loading of standard grains that are optically different from the unknown, thus avoiding accidental mixing within the irradiation discs. Another Precambrian standard used to analyze planetary science material is NL-25 (Schwarz and Trieloff, 2007). In addition of the cooling time disadvantage mentioned for Hb3gr, this standard has been shown to be largely heterogeneous. Jourdan and Renne (2007) showed that this standard tends to yield unacceptably high F-value relative standard deviation ranging from 1.2% to 4.1%, and questioned its usefulness as a high accuracy 40Ar/39Ar standard. 5. CONCLUSION Flat age spectra and average F-value relative standard deviations ranging from of 0.43% to 0.60% (P = 0.15– 0.83) demonstrate the reproducibility of WA1ms and its suitability as a reliable 40Ar/39Ar standard for Precambrian samples. We calculated R[WA1ms/FCs] = 205.59 ± 0.25, R[WA1ms/GA1550] = 57.25 ± 0.06 and R[WA1ms/ Hb3gr] = 3.9713 ± 0.014 (all with P > 0.14). Using the recently revised decay constant values and standard ages proposed by Renne et al. (2011) and R-values above, we calculated a weighted means age relative to the three different standards of 2614.2 ± 1.5 Ma (±0.055%; P = 0.8; 1r) for WA1ms. As in this case, the age of Hb3gr and GA1550 are already based on the age of FCs, we prefer using the single R[WA1ms/FCs] value to calculate a preferred age of 2613.0 ± 2.4 Ma (± 0.09%; 1r) for WA1ms. 40 This corresponds to a Ar*/40K value of (0.3375 ± 0.0057)  101, which is independent of the value

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of the decay constant used for calculation. Using the constants of Steiger and Ja¨ger (1977) results in an age of 2597.8 ± 3.5 Ma. The 40Ar*/40K ratio provided in this study allows recalculation of the age for this standard for any preferred set of decay constants. WA1ms muscovite largely minimizes the dynamic range between standard and Archaean terrestrial and meteorite samples. Furthermore, it does not require any post-irradiation waiting time due to the absence of Ca in muscovite crystals (no 37Ar decay), and can be analyzed readily upon return from the reactor. Age calculation using WA1ms minimizes the need for Ca-related interference corrections. The large concentration of K (10%) in muscovite crystals allows analysis of minute amounts of muscovite while still yielding a large 40Ar (and 39Ar) argon beam signal, resulting in a better precision for each individual measurement. ACKNOWLEDGMENTS We thank M. Heizler, A. Baksi and an anonymous reviewer for their formal review of this manuscript and C. Hall for his editorial handling. Their constructive suggestions and the interesting debates that they caused were highly appreciated.

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