40Ar/39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain normal-polarity subchron revisited

New ⁴⁰Ar/³⁹Ar age determinations on sanidine from the rhyolite of Alder Creek, California, indicate a 1.186 ±0.006 Ma age for the Cobb Mountain Normal-Polarity Subchron. The new age is statistically older (α = 0.05) than the previously reported K-Ar age (1.12 ±0.02 Ma) and agrees with the age suggested by the astronomical polarity time scale. Incomplete extraction of radiogenic ⁴⁰Ar (⁴⁰Ar*) from the sanidine is the most likely reason for the disparity between the ⁴⁰Ar/³⁹Ar and K-Ar ages. Because the Cobb Mountain subchron is a worldwide, short-duration event, and because no widely used interlaboratory ⁴⁰Ar/³⁹Ar standard younger than 27 Ma exists, we propose that sanidine from the rhyolite of Alder Creek be considered for use as a new Quaternary ⁴⁰Ar/³⁹Ar mineral standard.