Ultra-high precision 40Ar/39Ar ages for Fish Canyon Tuff and Alder Creek Rhyolite sanidine: New dating standards required?

The 40Ar/39Ar dating technique is a high precision (<0.1%) method with wide application to geological samples. However, the method is predicated on the availability of natural mineral standards of known age. Widely used 40Ar/39Ar standards include sanidine from the (ca. 28 Ma) Fish Canyon Tuff (FCT) and the (ca. 1.2 Ma) Alder Creek Rhyolite (ACR). Despite common usage, the ages of FCT and ACR sanidine have proven contentious, with reported values varying by >2%; well outside the ±0.1% aspiration of EARTHTIME (www.earth-time.org). The current study presents ultra-high precision, multi-collector 40Ar/39Ar results for FCT and ACR sanidine, using a new generation ARGUSVI mass spectrometer. These analyses demonstrate the significantly higher (by a factor of ∼10) level of precision achievable using the ARGUSVI system, compared to most previous studies. Significantly, 40Ar/39Ar step-heating analyses resolve distinct age gradients for both FCT and ACR sanidine. Possible explanations for these gradients include combinations of recoil loss of 39Ar, instrumental bias, isotopic fractionation during step-heating, thermally induced argon loss and/or extraneous argon contamination. Minor recoil loss of 39Ar and partial retention of inherited argon during extended magma residence, are deemed the most plausible causes of the age discordance. Given this assumption, we calculate revised (maximum?) eruption ages of 28.01 ± 0.04 Ma (2σ) for FCT and 1.178 ± 0.002 Ma (2σ) for ACR, relative to the astronomically calibrated age of A1 Tephra sanidine, Crete [Rivera et al. (2011) Earth Planet Sci. Lett. 311, 420-426]. Nonetheless, the ARGUSVI data demonstrate that FCT and ACR sanidine are non-ideal as high precision 40Ar/39Ar standards, with different heating protocols likely to induce inter-laboratory bias. The current findings account for at least some of the scatter in inter-calibration ages reported previously for various 40Ar/39Ar standards. In broader terms, the results mandate a re-evaluation of the astronomically calibrated ages for these and other sanidine samples, while the new generation of multi-collector mass spectrometers provides the means to further evaluate 40Ar/39Ar ages used to define new decay constants and standard ages.