INTERCALIBRATION AND AGE OF THE ALDER CREEK SANIDINE 40AR/39AR STANDARD

The accuracy of ⁴⁰Ar/³⁹Ar geochronology depends largely on precise and accurate calibration of the ages and/or K and Ar isotopic compositions of standards. A widely used standard for dating Quaternary samples, the ~1.2 Ma Alder Creek sanidine (ACs), has yielded reported ages spanning a range of ~2%, hindering the precision and accuracy of ages of comparable unknown samples. New measurements of ACs co-irradiated with Fish Canyon sanidine (FCs) and sanidines from astronomically dated Miocene tuffs yield results that enable both direct calibration of ACs relative to FCs and stepwise calibrations between these two standards by employing the Miocene intermediaries. Results are summarized by the parameter R^ACs_FCs , defined as the ratio of (⁴⁰Ar*/³⁹Ar_K) between ACs and FCs, which embodies the fundamental age relationship between these standards that is independent of systematic variables such as decay constants or absolute ages of standards. We present new measurements, executed using three mass spectrometers and various irradiation and analytical protocols, that yield a weighted mean R^ACs_FCs = 0.041707 ± 0.000013 (σ). This result is combined with previously published data to yield a recommended interlaboratory value of R^ACs_FCs = 0.041711 ± 0.000010. The weighted mean age of ACs using this interlaboratory value, based on astronomically-calibrated ages of FCs and the Miocene intermediary sanidines, is t_ACs = 1.1847 ± 0.0007 Ma. Applying this result to the most precise published ⁴⁰Ar/³⁹Ar data for the Matuyama-Brunhes geomagnetic polarity reversal yields an age of t_MBB = 780.0 ± 0.8 ka. Our data support previous conclusions that U-Pb zircon data from the Alder Creek rhyolite record ~13 ka of pre-eruptive residence time.