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The new generation of multi-collector mass spectrometers (e.g. ARGUSVI) permit ultra-high precision (<0.1%) 40 Ar/ 39 Ar geochronology of rocks and minerals. At the same time, the 40 Ar/ 39 Ar method is limited by relatively large uncertainties (>1%) in 40 K decay constants and the ages of natural reference minerals that form the basis of the technique. For example, reported ages for widely used 40 Ar/ 39 Ar reference materials, such as the ca. 28 Ma Fish Canyon Tuff sanidine (FCTs) and the ca. 1.2 Ma Alder Creek Rhyolite sanidine (ACRs), vary by >1%. Recent attempts to independently calibrate these reference minerals have focused on K–Ar analyses of the same minerals and inter-comparisons with astronomically tuned tephras in sedimentary sequences and U–Pb zircon ages from volcanic rocks. Most of these studies used older generation (effectively single-collector) mass spectrometers that employed peak-jumping analytical methods to acquire 40 Ar/ 39 Ar data. In this study, we reassess the inter-calibration and ages of commonly used 40 Ar/ 39 Ar reference minerals Fish Canyon Tuff sanidine (FCTs), Alder Creek Rhyolite sanidine (ACRs) and Mount Dromedary biotite (MD2b; equivalent to GA-1550 biotite), relative to the astronomically tuned age of A1 Tephra sanidine (A1Ts), Faneromeni section, Crete (Rivera et al., 2011), using a multi-collector ARGUSVI mass spectrometer. These analyses confirm the exceptional precision capability (<0.1%) of this system, compared to most previous studies. All sanidine samples (FCTs, ACRs and A1Ts) exhibit discordant 40 Ar/ 39 Ar step-heating spectra, with generally monotonically increasing ages (∼1% gradients). The similarity in these patterns, mass-dependent fractionation modeling, and results from step-crushing experiments on FCTs, which yield younger apparent ages, suggest that the discordance may be due to a combination of recoil loss and redistribution of 39 Ar K and isotope mass fractionation. In contrast to our previous inferences, these results imply that the sanidine samples are suitable 40 Ar/ 39 Ar reference materials, provided appropriate corrections are included for differential recoil loss of 39 Ar K and contributions from xenocrysts/antecrysts can be resolved. Relative to an age of 6.943 ± 0.005 Ma for A1Ts, we calculate astronomically tuned ages for FCTs, ACRs and MD2b of 28.126 ± 0.019 (0.066%) Ma, 1.18144 ± 0.00068 (0.058%) Ma and 99.125 ± 0.076 (0.077%) Ma, respectively (95% internal errors). These results are consistent with recent 238 U/ 206 Pb age data from these localities, but are marginally younger (∼0.2%) than previous 40 Ar/ 39 Ar ages inter-calibrated with astronomically tuned tephra from the Mediterranean, and distinctly younger (0.6%) than results optimized against a broad array of 238 U/ 206 Pb zircon ages. Consideration of published and assumed recoil loss 39 Ar K proportions (0.18–0.40%), yields recoil-corrected age estimates of 28.187 ± 0.019 Ma, 1.18404 ± 0.00068 Ma and 99.204 ± 0.076 Ma, respectively. This comparison indicates inherent uncertainties of >0.1% in the 40 Ar/ 39 Ar ages of reference minerals without consideration of recoil artefacts, thus limiting the benefits of high precision multi-collector analyses. Significant improvement to the accuracy of the 40 Ar/ 39 Ar method (<0.1%) will require further inter-laboratory 40 Ar/ 39 Ar studies utilizing multi-collector mass spectrometry, additional constraints on recoil 39 Ar K loss from reference minerals, further resolution of discrepancies between astronomically tuned sedimentary successions and refinement of the 238 U/ 206 Pb zircon age cross-calibration approach. [ABSTRACT FROM AUTHOR]

The rewards from resolving the tempo of magmatic, paleobiologic, paleoclimatic, and tectonic processes at better than the per mil level of precision have spurred a quest within the geochronology community to improve the precision, accuracy, and calibration of several chronometers, including the 40 Ar/ 39 Ar variant of the K-Ar clock. A new generation of multi-collector mass spectrometers for 40 Ar/ 39 Ar geochronology provides the means to address these issues, as well as to evaluate the ages of 40 Ar/ 39 Ar standards and key marker horizons deposited by large volume, caldera-forming supereruptions in the western U.S. at unprecedented temporal resolution. We highlight the utility of a 5-collector Noblesse mass spectrometer that has a sufficiently high mass resolving power to distinguish 36 Ar + H 35 Cl from a 36 Ar + H 35 Cl + 12 C 3 sum peak at m/e 36, maintains a remarkably low and stable background, and thus yields 40 Ar/ 39 Ar dates of exceptionally high precision. We have calibrated a reference gas and developed a simple analytical routine involving one peak hop, both of which are used to evaluate detector-bias correction factors. Because the 40 Ar/ 39 Ar chronometer is a relative dating technique that relies on calibration against mineral standards of known age, precise and accurate determination of the ages of these standards, and verification of isotopic composition and homogeneity, is essential. We present results from both total fusion and incremental heating experiments on single crystals of the widely used Alder Creek (ACs) and Fish Canyon (FCs) sanidine standards. Several recent, independent, measurements of ACs using multi-collector mass spectrometers, including those presented herein, favor an age of 1.1864 ± 0.0003/0.0012 Ma (95% confidence analytical/full external uncertainty), that is > 1% younger than the widely used value obtained by Nomade et al. (2005) using a single collector mass spectrometer. Incremental-heating experiments on single crystals of FCs most commonly do not yield plateau ages, instead many crystals appear to be affected by Ar loss or excess Ar. Our findings, obtained using ion counters, independently confirm those recently obtained using a multiple-Faraday detectors or a combination of Faraday and ion counting detectors. Thus, careful consideration must be given to using FCs in future high-precision 40 Ar/ 39 Ar calibrations of the geologic timescale. Notwithstanding, 40 Ar/ 39 Ar dating of sanidine using the 5- collector Noblesse can now provide leverage critical to interpreting the large range of U–Pb zircon dates commonly found in rhyolitic lavas and tuffs. [ABSTRACT FROM AUTHOR]