Terrestrial cosmogenic argon

Cosmogenically enriched Ar-38/Ar-36 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 Ar-38/Ar-36 and cosmogenic He-3 in apatite and fluorite are systematic but difficult to reconcile with current estimates of terrestrial production rates for Ar-38 and Ar-36 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 Ar-36 from beta (-) decay of cosmogenic Cl-36 produced from Ca, K, or Cl on the Ar-38/Ar-36 ratio must be accounted for. (C) 2001 Elsevier Science B.V. All rights reserved.