Abstract
Size fractions <5 μm of Fithian illite were treated with 1 N HCl, H+- from cation-exchange resin, and 5% NH4-EDTA-solution, and analyzed for their Rb-Sr isotopic composition. In comparison to the effects of HCl, the ion-exchange resin leached the same amount of Sr, but significantly less Rb and radiogenic 87Sr from poorly-crystallized 1 M illite.An age of 284 ± 8 Ma (I.R. 0.7109 ± 6 is more or less in agreement with the Missourian (Stephanian) age of deposition. The agreement is fortuitous because of the presence of small amounts of detrital 2 M illite and kaolinite, thus the true age of diagenesis might be somewhat less.87Sr86Sr ratio measurements of Tithonian Solnhofen limestone demonstrate that, even though the silicate component constitutes less than 5% of the total, the majority of the radiogenic 87Sr is leached from “broken-bond” surfaces of the nearly pure kaolinite-residues during the HCl treatment. Ion-exchange and EDTA treatment can be useful in this situation to obtain a precise separation of environmental Sr (in carbonate) from radiogenic 87Sr (in silicates), and an adequate spread of RbSr values from argillaceous fine fractions in the construction of isochrons.
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