An 40Ar/ 39Ar age spectrum for a cordierite-bearing rock: Isolating the effects of excess radiogenic 40Ar

Abstract

Cordierite-bearing rocks commonly contain excess radiogenic 40Ar and therefore yield anomalously old conventional K-Ar dates. Mechanical separation of cordierite cannot always be used to circumvent this problem. The 40Ar/ 39Ar incremental release technique was successfully used to isolate the effects of excess radiogenic 40Ar in a cordierite-bearing rock. An 40Ar/ 39Ar age spectrum was obtained for a cordierite-bearing rock whose true cooling age could be presumed by dating equivalent cordierite-free rocks. The initial gas fraction in the age spectrum exhibits an extremely high apparent age. Each successive step shows a decrease in apparent age. The apparent age of the last increment yielding a significant quantity of gas is in reasonable agreement with the presumed cooling age. The release pattern obtained suggests that radiogenic 40Ar associated with cordierite is released primarily at low temperatures. At higher temperatures the 40Ar released is primarily that derived from in-situ decay of 40K in the other minerals present. The 40Ar/ 39Ar incremental release technique may therefore be useful in isolating the effects of excess radiogenic 40Ar in cordierite-bearing rocks. This would enable the determination of true cooling ages for this rocks.