| Summary: | Cosmogenically enriched ^(38)Ar/^(36)Ar 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 ^(38)Ar/^(36)Ar and cosmogenic ^3He in apatite and fluorite are systematic but difficult to reconcile with current estimates of terrestrial production rates for ^(38)Ar and ^(36)Ar 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 ^(36)Ar from β^− decay of cosmogenic ^(36)Cl produced from Ca, K, or Cl on the ^(38)Ar/^(36)Ar ratio must be accounted for. © 2001 Elsevier Science B.V. Received 28 December 2000; accepted 3 April 2001. We thank the Hawaii Scientific Drilling Project, J.N. Christensen and J.J. Donovan for various 'baseline' samples; P.G. Fitzgerald for the Trans-antarctic samples; B.M. Kennedy, K. Min and K.B. Knight for discussion and suggestions on the manuscript; F.M. Phillips for a preprint of a paper cited herein; D. Lal and an anonymous referee for constructive reviews of the manuscript. We dedicate this paper to the late Prof. John H. Reynolds.[EB]
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