Five hand-picked, authigenic glauconite concentrations that were analyzed for Rb, Sr, and Sr isotopic composition yielded an isochron age of 68.1 ± 1 m.y. (λ Rb87 = 1.39 × 10−11 yr−1) for Maestrichtian strata in North Carolina. The initial (Sr87/Sr86)0 ratio calculated from the isochron is 0.7070 ± 0.0004 and is in good agreement with previous estimates of the strontium isotopic composition of sea water during Late Cretaceous time. The excellent fit of the isochron age into current age boundaries of the Maestrichtian suggests that the Rb-Sr isochron method could be more widely applied to glauconite.

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