Terrestrial cosmogenic argon

Cosmogenically enriched 38Ar/36Ar ratios are detectable in Ca-bearing terrestrial samples, and are resolvable from a uniform background value of 0.18826±0.00018 for materials lacking nucleogenic or kinetic mass fractionation effects. Relationships between 38Ar/36Ar and cosmogenic 3He in apatite and fluorite are systematic but difficult to reconcile with current estimates of terrestrial production rates for 38Ar and 36Ar from Ca, suggesting that production from Cl and/or other targets is significant, or that the estimates are inaccurate. In either case, the effects of time-varying production of 36Ar from β− decay of cosmogenic 36Cl produced from Ca, K, or Cl on the 38Ar/36Ar ratio must be accounted for.