Unraveling the early South Atlantic records: in situ 87Sr/86Sr isotopic analysis in Aptian carbonates from the conjugated margins of Brazil and Angola.

Abstract

The 87Sr/86Sr ratio stands as a crucial isotopic marker for the paleogeographic and paleoenvironmental reconstruction of Aptian carbonate sequences in the offshore Campos and Santos basins in Brazil, as well as the Namibe basin in Angola. It facilitates the identification of distinct stratigraphic successions and alterations in the basin's source areas, allowing for valuable geological correlations and interpretations on both margins. Furthermore, it can indicate diagenetic processes, hydrothermal events involving chemical mobility, and textural modifications associated with regional tectonic events linked to the evolution of the South Atlantic rift. In isotopic dilution analyses, the quality of the results is ensured by eliminating interferences and ensuring a stable signal during spectrometric reading, where the 87Sr/86Sr ratios have analytical precision between the 5th and 6th decimal place. However, samples with a significant compositional variation, such as laminations, microstructures, microveins, or even with clastic minerals, may yield scrambled distinct signals during the grinding and homogenization processes, making it difficult to recognize subtle variations. Strontium isotope analyses by laser ablation coupled to multi-collector ICP-MS Neptune Plus, obtained through the in situ 87Sr/86Sr method have great potential and broad applicability in cases where the search for gentle isotopic signal variations is relevant, such as in the study of drilling cores. By using integrated petrographic information, it has become possible to perform high-resolution analyses, addressing different components of the crystalline system, with strontium intensities ranging from 200 mg/L to ~ 6000 mg/L. The speed of the analyses, from sampling rock fragments to performing laser-based analyses, provides a large volume of results in a relatively short period of time, facilitating correlations between stratigraphic intervals. The in situ 87Sr/86Sr method has analytical precision between the 4th and 5th decimal place, with sufficient sensitivity to characterize different groups defined by ratios in the ranges of 0.712 - 0.714 (predominantly continental), 0.710 - 0.711 (mixed), and 0.709 - 0.710 (marine influence). Its application in the study of pre-salt carbonates brings a significant advancement in recognizing the environment, and its genetic and diagenetic processes, with great potential to advance our understanding of chemostratigraphy and unravel the transition between continental and marine environments in the early stages of the South Atlantic Ocean's history.