Abstract:
The presented Rb-Sr systematics of carbonates from the Karatau Group, the Upper Riphean stratotype of southern Urals, elucidates important details of secular variations of 87Sr/86Sr ratios in the Late Riphean seawater, which have been formerly unknown. Samples selected for analysis satisfy all the strict geochemical criteria characterizing the least altered carbonate rocks (Mn/Sr ≤ 0.2, Fe/Sr ≤ 5.0, Mg/Ca ≤ 0.024 in limestones and Mn/Sr ≤ 1.2, Fe/Sr ≤ 3.0, Mg/Ca ≥ 0.608 in dolostones), and they all have been preliminary treated in IN solution of ammonium acetate for a partial removal of epigenetic carbonate phases. A verified curve of secular variations of 87Sr/86Sr ratio in the Late Riphean ocean is plotted based on new data and Sr isotope parameters formerly known for carbonates of the Upper Riphean key sections. As is established, that ratio was nearly constant, ranging from 0.70519 to 0.70566, within the time span of 1030-810 Ma and next rose up to 0.70611 about 775 Ma ago. Afterward, between 765 and 740 Ma, it decreased down to 0.70561-0.70575 and then, within the time span of 740-690 Ma, it ranged from 0.70646 to 0.70686 with a short-term drop down to 0.70620 about 720 Ma ago. At the end of the Late Riphean (660-640 Ma), the ratio lowered to 0.70538-0.70580 to become growing up to 0.70840-0.70860 during the Vendian and initial Cambrian. The established variations of the 87Sr/86Sr ratio in the Late Riphean ocean have been controlled by a combination of geodynamic factors, magmatic events, sea-level oscillations, and compositional changes in provenances, which were subjected to erosion at that time. An additional influence of climatic fluctuations over supercontinent and its fragments is also admissible.
