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

B. D. Turrin·J.M. Donnelly-Nolan·B. C. Hearn
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Abstract

New 40 Ar/ 39 Ar 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 40 Ar ( 40 Ar*) from the sanidine is the most likely reason for the disparity between the 40 Ar/ 39 Ar and K-Ar ages. Because the Cobb Mountain subchron is a worldwide, short-duration event, and because no widely used interlaboratory 40 Ar/ 39 Ar standard younger than 27 Ma exists, we propose that sanidine from the rhyolite of Alder Creek be considered for use as a new Quaternary 40 Ar/ 39 Ar mineral standard.

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B. D. Turrin, J.M. Donnelly-Nolan, B. C. Hearn. 1994. 40Ar/39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain normal-polarity subchron revisited. https://doi.org/10.1130/0091-7613(1994)022%3C0251%3Aaaaftr%3E2.3.co%3B2

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