Potassium-argon ages of recent rhyolites of the Mono and Inyo craters, California

Abstract

Twenty-two KAr ages were determined for sanidine samples from 10 rhyolite domes of the Mono and Inyo Craters to test the applicability of KAr dating to volcanic rocks of Recent age. Comparison of the results with ‘blank’ and dosed analyses shows that radiogenic 40Ar was detected and was measured to within a factor of two or better. The estimated standard deviation of precision is 12% for analyses containing 5% or more radiogenic 40Ar. A statistical analysis suggests that real differences in apparent age were detected between three of the analyzed domes. The ages, which range from 6400 to 10,200 years for experiments with 5% or more radiogenic 40Ar, are, in general, consistent with ionium ages on 5 of the same samples and with glacial and 14C evidence of age. They suggest that most of the Mono Craters volcanos are on the order of 10,000 years old or less. The results also suggest that the problem of excess 40Ar may not be severe for KAr dating of volcanic rocks.