Terrestrial cosmogenic argon

Cosmogenically enriched Ar-38/Ar-36 ratios are detectable in Ca-bearing ter restrial samples, and are resolvable from a uniform background value of 0.1 8826+/-0.00018 for materials lacking nucleogenic or kinetic mass fractionat ion effects. Relationships between Ar-38/Ar-36 and cosmogenic He-3 in apati te and fluorite are systematic but difficult to reconcile with current esti mates of terrestrial production rates for Ar-38 and Ar-36 from Ca, suggesti ng that production from Cl and/or other targets is significant, or that the estimates are inaccurate. In either case, the effects of time-varying prod uction 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 S cience B.V. All rights reserved.