40Ar/ 36Ar analyses of historic lava flows

Abstract

The ratio 40Ar/ 36Ar was measured for 26 subaerial historic lava flows. Approximately one-third of the samples had 40Ar/ 36Ar ratios either higher or lower than the atmospheric value of 295.5 at the 95% confidence level. Excess radiogenic 40Ar in five flows ranged from about 1 × 10 −13 to 1.5 × 10 −12 mol/g. Possible excess 36Ar in three flows was on the order of 10 −16 to 10 −15 mol/g. Upper 95% confidence limits for excess 40Ar in samples with normal 40Ar/ 36Ar ratios are generally less than 3 × 10 −13 mol/g. The origin of the excess 36Ar is unknown but it may be due either to the incorporation of primitive argon that has been stored in the mantle in very low potassium environments or to enrichment in 36Ar as atmospheric argon diffuses into the rocks after they cool.