Abstract

Six untreated whole-rock samples of the Stearns Shale with low Sr87/Sr86 and Rb87/Sr86 ratios define a least squares isochron of 371 ± 15 m.y., with an initial Sr87/Sr86 ratio of 0.7091 ± 0.0015, using a 50 b.y. half-life for Rb87. Treatment of the samples with 2N HC1 with constant agitation for 15 minutes produced the following results: 6 samples of insoluble residue with much higher Sr87/Sr86 and Rb87/Sr86 ratios and a Rb-free leach of low Sr87/ Sr86. Least squares analysis of the insoluble residue samples yields a 276 ± 10 m.y. isochron with initial Sr87/Sr86 of 0.7195 ± 0.0005. This age is close to the presumed age of the Lower Permian and suggests that this method may allow the accurate dating of some predominantly allogenic calcareous shales.

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