40Ar/36Ar analyses of historic lava flows

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Abstract

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.

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