SR ISOTOPIC COMPOSITION IN RIPHEAN, VENDIAN, AND LOWER CAMBRIAN CARBONATES FROM SIBERIA

Initial Sr-87/Sr-86 ratios were measured in 51 carbonate rock samples of Riphean, Vendian, and Early Cambrian ages from Siberian sections (O lenek, Turukhansk, and Ura uplifts). According to normal geochemical c riteria, the studied rocks seem unaffected by any significant postdepo sitional alterations. However, all the samples contain a recrystallize d carbonate phase that formed by catagenesis and differs from the earl y diagenetic phase in its Sr isotopic composition. Therefore, publishe d Sr-87/Sr-86 values of Precambrian carbonates contain, at least parti ally, a systematic error that is dependent on the differences in the S r concentrations and Sr-87/Sr-86 ratios of both phases. Using a new ap proach to separate these phases, we measured initial Sr-87/Sr-86 ratio s in carbonate successions of the Olenek (Lower and lower Middle Riphe an), Turukhansk (upper Middle and lower Upper Riphean), and Ura (upper most Riphean, Vendian, and Atdabanian Stage of the Lower Cambrian) upl ifts. 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 Atdaba nian successions of Siberia revealed remarkably higher values correspo nding to 0.7081 - 0.7110, 0.7084 - 0.7085, and 0.7081 - 0.7085, respec tively. 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.