EarthRef.org Reference Database (ERR) -- Gorokhov et al. 1995

Initial Sr-87/Sr-86 ratios were measured in 51 carbonate rock samples of Riphean, Vendian, and Early Cambrian ages from Siberian sections (Olenek, Turukhansk, and Ura uplifts). According to normal geochemical criteria, the studied rocks seem unaffected by any significant postdepositional alterations. However, all the samples contain a recrystallized carbonate phase that formed by catagenesis and differs from the early diagenetic phase in its Sr isotopic composition. Therefore, published Sr-87/Sr-86 values of Precambrian carbonates contain, at least partially, a systematic error that is dependent on the differences in the Sr concentrations and Sr-87/Sr-86 ratios of both phases. Using a new approach to separate these phases, we measured initial Sr-87/Sr-86 ratios in carbonate successions of the Olenek (Lower and lower Middle Riphean), Turukhansk (upper Middle and lower Upper Riphean), and Ura (uppermost Riphean, Vendian, and Atdabanian Stage of the Lower Cambrian) uplifts. Initial ratios in the Middle and lower Upper Riphean rocks range from 0.7052 to 0.7062 and are well below the values accepted in modem publications. Samples from the uppermost Riphean, Vendian, and Atdabanian successions of Siberia revealed remarkably higher values corresponding to 0.7081 - 0.7110, 0.7084 - 0.7085, and 0.7081 - 0.7085, respectively. New data adds significant detail to the Riphean-Vendian curve of seawater Sr isotopic variations. The inversion points on the curve are correlative with certain Late Proterozoic events.