A Stepwise Dissolution Method Applied to Isotopic Chemostratigraphic and Geochronological Studies of Riphean Carbonate Rocks of the Baykit Uplift

Abstract

The potential of the stepwise dissolution analytical procedure to obtain chemostratigraphic and geochronological information is demonstrated by a case study of Riphean carbonate rocks of the Kamovskaya Group of the Baykit Uplift (Eastern Siberia). The procedures suggested for studying Rb–Sr and U–Pb systems in carbonate rocks included (1) selection of samples with the lowest ⁸⁷Sr/⁸⁶Sr ratio on the basis of preliminary Rb–Sr analysis of the collection using the routine procedure of bulk leaching in 0.1N CH₃COOH; (2) detailed study of the Rb–Sr system in selected samples using two-step dissolution in 0.2N CH₃COOH and derivation of L(Rb–Sr)₁ and L(Rb–Sr)₂ fractions; (3) analysis of Pb isotope composition of the selected samples to derive the preliminary value of their age; (4) detailed study of U–Pb system of carbonate rocks using the stepwise dissolution in 0.5N HBr, resulting in six dissolved fractions L(U–Pb)₁–L(U–Pb)₆ for each of the selected samples; and (5) a final calculation of the Pb–Pb age of the studied rocks, based on the results from the obtained fractions, with the exclusion from the calculation of the initial fractions of this dissolution containing epigenetically altered carbonate material. The values of the primary ⁸⁷Sr/⁸⁶Sr ratio in L(Rb–Sr)₂ fractions of carbonate rocks of the Kamovskaya Group are as follows: limestone of the Madrinskaya Formation, 0.70490; dolomites of the Yurubchenskaya Formation, 0.70495–0.70503; and dolomites of the Kuyumbinskaya and Vingoldinskaya formations, 0.70580 and 0.70521, respectively. These values characterizing the least altered carbonate material correspond to ⁸⁷Sr/⁸⁶Sr ratios in the Early Archean and can be used for chemostratigraphic calculations. Taking into account the Rb–Sr data, the U–Pb age of the Yurubchenskaya Formation carbonate rocks dissolved in six steps was calculated, starting from L(U–Pb)₃. The slope of the resulting isochron in the coordinates ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb corresponds to 1501 ± 23 Ma, which supports the conclusion that the carbonate rocks of the lower part of the Kamovskaya Group of the Baykit Uplift were formed in the Early Riphean.