40Ar/36Ar analyses of historic lava flows

The ratio⁴⁰Ar/³⁶Ar was measured for 26 subaerial historic lava flows. Approximately one-third of the samples had⁴⁰Ar/³⁶Ar ratios either higher or lower than the atmospheric value of 295.5 at the 95% confidence level. Excess radiogenic⁴⁰Ar in five flows ranged from about 1 × 10⁻¹³ to 1.5 × 10⁻¹² mol/g. Possible excess³⁶Ar in three flows was on the order of 10⁻¹⁶ to 10⁻¹⁵ mol/g. Upper 95% confidence limits for excess⁴⁰Ar in samples with normal⁴⁰Ar/³⁶Ar ratios are generally less than 3 × 10⁻¹³ mol/g. The origin of the excess³⁶Ar 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 in³⁶Ar as atmospheric argon diffuses into the rocks after they cool.