Tracing carbonate diagenesis and hydrothermal activity during the opening of the Central South Atlantic: Insights from the Santos Basin (Brazil)

Abstract

The opening of the Central South Atlantic and the consequent formation of the eastern Brazilian continental margin was marked by a complex history of mafic magmatism, carbonate sedimentation, and deposition of a thick salt layer. The carbonates underlying the salt layer (pre-salt carbonates) were formed in restricted lacustrine basins. Here, the timing and fluid sources of deposition, diagenetic, and hydrothermal alterations of the pre-salt carbonate rocks are defined through in-situ U-Pb dating, ⁸⁷Sr/⁸⁶Sr, and trace element analyses of samples from the Santos Basin. The very alkaline nature of the Aptian lake(s) produced characteristically unique and widely distributed carbonate rocks such as Mg-clays with calcite spherulite and calcite crystal shrub limestones transitioning laterally and vertically into travertines formed by hydrothermal pulses during basin evolution. Hydrothermalism caused extensive replacement, dissolution, and calcite cementation. REE+Y PAAS-normalised patterns and ⁸⁷Sr/⁸⁶Sr ratios indicate that deposition/eo-diagenesis of the primary carbonates occurred in a lacustrine environment primarily controlled by evaporation, pH, and continental water source, with 2%–10% hydrothermal fluid input. Trace elements and Sr-isotope of travertines and burial diagenetic phases show that they are produced from a hot mixture of mafic/mantle-derived fluids and dissolution/alteration of older carbonate formations. U-Pb dating indicates that carbonate deposition occurred between 124.8 ± 2.6 Ma and 120.0 ± 1.6 Ma, earlier than previously proposed, followed closely by the circulation of hydrothermal fluids. Replacement and cementation ages range from 120.5 ± 2.4 Ma to 80.4 ± 2.4 Ma.