Brent D. Turrin, Julie M. Donnelly-Nolan, B. Carter Hearn, Jr.; 40Ar/39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain Normal-Polarity Subchron revisited. Geology ; 22 (3): 251–254. doi: https://doi.org/10.1130/0091-7613(1994)022<0251:AAAFTR>2.3.CO;2
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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.