Sr isotopic composition of Paleoproterozoic 13C-rich carbonate rocks: The Tulomozero Formation, SE Fennoscandian Shield

Abstract Rb–Sr systematics has been studied in 13 C-rich carbonate rocks of the Paleoproterozoic (2.09 ± 0.07 Ga) Tulomozero Formation in the northern Onega Lake area, the SE Fennoscandian Shield. The formation is divided into eight members (A–F) consisting of greenschist-facies-grade, variegated sandstones, siltstones, mudstones, stromatolitic dolostones and subordinate crystalline limestones. Samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the Tulomozero Formation. Prior to isotope analysis, the rocks powders were treated with 1N ammonium acetate for partial removal of the late epigenetic carbonate phases. Major resetting of the Rb–Sr systems in the Tulomozero carbonate rocks appears to take place during the Svecofennian regional metamorphic event, and it was screened by using Mn/Sr, Fe/Sr, Mg/Ca, and 18 O/ 16 O ratios. High Sr content (up to 2080 μg/g in limestones, and 530 μg/g in dolostones) coupled with low Fe/Mn ( 0.40) in dolostones of the upper part of Member B, and of Members G and H is indicative of seawater influxes (sea transgression) into the Tulomozero basin. The 87 Sr/ 86 Sr values in the least altered (Mn/Sr  87 Sr/ 86 Sr ratio in ca. 2.1 Ga seawater is attributable to an increase in hydrothermal flux Sr into the Palaeoproterozoic ocean.