40Ar/36Ar analyses of historic lava flows

G. B. Dalrymple

doi:10.1016/0012-821x(69)90160-5

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40Ar/36Ar analyses of historic lava flows

G. B. Dalrymple

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Abstract

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

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G. B. Dalrymple. 1969. 40Ar/36Ar analyses of historic lava flows. https://doi.org/10.1016/0012-821x(69)90160-5

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