Age calibration of the Fish Canyon sanidine 40 Ar/ 39 Ar dating standard using primary K–Ar standards Fred Jourdan a,b, * , Paul R. Renne a,b a Berkeley Geochronology Center, 2455 Ridge Rd., Berkeley, CA 94709, USA b Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA Received 9 January 2006; accepted in revised form 6 September 2006 Abstract The 40 Ar/ 39 Ar dating technique requires the use of neutron fluence monitors (standards). Precise calibrations of these standards are crucial to decrease the uncertainties associated with 40 Ar/ 39 Ar dates. Optimal calibration of 40 Ar/ 39 Ar standards should be based on K/ Ar standards having independent isotope dilution measurements of 40 K and 40 Ar*, based on independent isotope tracers (spikes) because this offers the possibility to eliminate random interlaboratory errors. In this study, we calibrate the widely used Fish Canyon sanidine (FCs) standard based on four primary K/Ar standards (GA-1550, Hb3gr, NL-25, and GHC-305) on which K and Ar* concentrations have been determined in different labs with independently calibrated tracers. We obtained a mean age of 28.03 ± 0.08 Ma (1r; neglecting uncertainties of the 40 K decay constants) for FCs, based on the decay constant recommended by Steiger and Ja ¨ger [Steiger R.H., Ja ¨ger. E. 1977. Subcommission on geochronology: convention of the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett. 36, 359–362.]. This age corresponds to a mean 40 Ar*/ 40 K value of (1.6407 ± 0.0047) · 10 3 . We also discuss several criteria that prevent the use of previous calibrations of FCs based on other primary standards (LP-6, SB-3 and MMhb-1). The age of FCs obtained in this study is based on the 40 K decay constants of Steiger and Ja ¨ ger (1977) but we anticipate the imminent need for revision of the value and precision of the 40 K decay constants (representing the main source of uncertainties in 40 Ar/ 39 Ar dating). The 40 Ar*/ 40 K result of FCs obtained in this study allows therefore a rapid calibration of the age of FCs with uncertainties at the 0.29% level but perhaps more impor- tantly this value is independent of any particular value of the 40 K decay constants and may be used in the future in conjunction with revised decay constants. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction The 40 Ar/ 39 Ar dating method is a relative technique whose calibration is based on the ages of neutron fluence monitors (standards). Ideally, the age of the standards is determined by K/Ar dating (e.g. Turner et al., 1971) or other methods such as astronomical calibration (e.g. Renne et al., 1994). However, some minerals, not suitable for K/ Ar dating due to the difficulty of quantitatively extracting 40 Ar* (e.g. sanidine, Webb and McDougall, 1967; McDo- well, 1983), are known to provide more precise and repro- ducible ages when measured by 40 Ar/ 39 Ar dating (e.g. Fish Canyon sanidine (FCs); Renne et al., 1998, Alder Creek sanidine (ACs); Nomade et al., 2005) especially at the sin- gle grain level. Use of these ‘‘secondary’’ standards is also often justified by the fact that their age and composition should be comparable to the unknown and therefore min- imize the range of isotopic ratios to be measured (e.g. Renne et al., 1997). These minerals could be therefore used as secondary [or even higher order; e.g. ACs (Nomade et al., 2005)] standards but their ages should be initially cal- ibrated with primary standards. Ultimately, accuracy and precision on the age of an unknown sample is constrained by the precision and accuracy on the age of the standards. One of the most widely used standards in 40 Ar/ 39 Ar geo- chronology is FCs, mainly because of its high homogeneity and typically superior reproducibility (e.g. Renne et al., 1998; Lanphere and Baadsgaard, 2001; Spell and 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.09.002 * Corresponding author. Fax: +1 510 644 9201. E-mail address: fjourdan@bgc.org (F. Jourdan). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 71 (2007) 387–402