AR-40 AR-39 AGES FROM THE RHYOLITE OF ALDER-CREEK, CALIFORNIA - AGE OF THE COBB-MOUNTAIN NORMAL-POLARITY SUBCHRON REVISITED

NeW Ar-4-/Ar-39 age determinations on sanidine from the rhyolite of Al der Creek, California, indicate a 1.186 +/- 0.006 Ma age for the Cobb Mountain Normal-Polarity Subchron. The new age is statistically older (alpha = 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 Ar-40 (Ar-40) from the san idine is the most likely reason for the disparity between the AR-40/Ar -39 and K-Ar ages. Because the Cobb Mountain subchron is a worldwide, short-duration event, and because no widely used interlaboratory Ar-40 /Ar-39 standard younger than 27 Ma exists, we propose that sanidine fr om the rhyolite of Alder Creek be considered for use as a new Quaterna ry Ar-40/Ar-39 mineral standard.