Dolomitization and isotope stratigraphy of the Vazante Formation, São Francisco Basin, Brazil

Abstract

The Vazante Formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized. Samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the Vazante carbonates. Dolomites can be classified, based on petrography and geochemistry, into four generations ranging in crystal size between ∼4 μm (almost micritic) and 3 mm and occurring as both replacements and cements. The Sr/Ca molar ratios, calculated for the dolomitizing fluid (0.0006–0.0138), suggest a contribution from a non-marine, possibly meteoric, water component. The δ18O and δ13C values vary from −0.1 to −14.3‰ (PDB) and 0.2 to −2.3‰ (PDB), respectively. Fluid-inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 °C. A mixing-zone model of dolomitization for Dolomites I–III satisfies the constraints from elemental chemistry, δ18O, 87Sr/86Sr and fluid inclusions. The petrographic and chemical criteria of the latest generation, Dolomite IV, reflect conditions of deep burial environment at temperatures above 120–130 °C. The δ13C values show only small variations at the base of the formation, followed by a major negative plunge (∼4‰) at the top, this plunge correlated with the Sturtian glacial phase. In contrast to C-isotopes, only few samples may have retained their near-primary 87Sr/86Sr values of 0.70614–0.70734. The lowest 87Sr/86Sr value (0.70614), from fibrous cement in the upper part of the formation, correlates with the negative δ13C shift and is also consistent with the Sr-isotope signature proposed for the glacial Sturtian seawater. The δ34S values, obtained from sulfates trapped in carbonates, range between 10.8 and 16.9‰ with a jump to 21.3‰ in the overlying formation. All isotope signals are within the range suggested for the early Neoproterozoic seawater, but the Precambrian baseline is poorly known and additional work is required to confirm this tentative assignment.