Variations of the 238U/235U Isotope Ratio in Metasedimentary Rocks and Evidence of Changes in Sedimentation Conditions during the Ediacarian Period of the Neoproterozoic

Resumo

The isotopic composition of uranium is a new geochemical indicator that facilitates reconstruction of the redox conditions of geological processes. In this paper the results of study of the ²³⁸U/²³⁵U isotope ratio in the Neoproterozoic metasedimentary rocks of the Baikal–Patom fold belt (Northern Transbaikalia) were obtained using the MC-ICP-MS method and ²³⁵U + ²³⁶U double spike. The scale of ²³⁸U/²³⁵U ratio variations (δ²³⁸U = –0.37 to –0.11‰) is within the range of (–0.39 to +0.15‰), typical of metasedimentary terrigenous rocks of marine origin. In terms of the δ²³⁸U values, uranium of the BPB rocks studied is heavier than seawater uranium (δ²³⁸U = –0.41 ± 0.03). There is a correlation between the δ²³⁸U value in the rocks and their position in the stratigraphic section. Variations of the ²³⁸U/²³⁵U ratio in the terrigenous-carbonate sequences of BPB indicate that the sedimentation conditions changed and euxinic conditions were established in the course of evolution of the paleobasin in the Late Ediacarian period.