Sr isotopes in complex carbonate matrices by the LA-ICP-MC-MS technique: in situ analysis unraveling the early South Atlantic Records

The 87Sr/86Sr isotopic record is key in the paleogeographic and paleoenvironmental reconstruction of Aptian carbonate sequences in Brazil's offshore Campos and Santos basins and for the connection to the African counterpart, the Kwanza, Benguela and Namibe basin in Angola. The application of strontium isotopes allows identifying distinct stratigraphic successions and changes in the basin's source areas, leading to valuable geological correlations and interpretations for both margins. It also serves as an indicator for diagenetic processes, hydrothermal events with chemical mobility, and textural modifications associated with regional tectonic events linked to the South Atlantic rift evolution.

In isotopic dilution analyses, the quality of the results is ensured by eliminating interferences and guaranteeing a stable signal during data acquisition, allowing 87Sr/86Sr ratios to have analytical precision between the 5th and 6th decimal places. Nevertheless, samples with a significant compositional variation, such as laminations, microstructures, micro veins, or clastic minerals, may yield scrambled distinct signals during the grinding and homogenization processes, making it difficult to recognize subtle variations under micro scale. Strontium isotope analyses by laser ablation coupled to multi-collector ICP-MS Neptune Plus, obtained through the in situ 87Sr/86Sr method developed by LAMIR - UFPR, 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. Using integrated petrographic information makes it 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, facilitating correlations between stratigraphic intervals. It is worth noting the time required for sample preparation in wet chemistry, making conventional analyses expensive, with frequent waiting queues that contrast with the agility needed for exploratory research.

The most problematic aspect of in situ measurements is the corrections required due to isobaric interferents. Monitoring Rb/Sr signal intensities allowed the evaluation of the petrological conditions influencing the 87Sr/86Sr ratio, including the recognition of mixing zones with crystallites of silicates in the matrix or even other perceptible phases, such as authigenic clays and cherts. Applying the in-situ technique in chemostritigraphic surveys makes it possible to recognize parts of the succession free of Rb, which allows the direct acquisition of 87Sr/86Sr initial ratios.

The application of in situ 87Sr/86Sr analysis in Aptian carbonate successions within the Santos and Campos Basins in Brazil and the Namibe Basin in Angola has yielded promising results, achieving a success rate exceeding 80% when considering results within the 4th and 5th decimal place range of analytical precision. This method exhibits remarkable sensitivity, distinguishing distinct isotopic groups characterized by different radiogenic compositions. The results suggest shifts in the source from predominantly continental environments in the base to marine-influenced environments at the top, near the transition to the salt.

The 87Sr/86Sr in-situ application in the pre-salt carbonates examination marks a significant leap forward in understanding environmental settings and their associated genetic and diagenetic processes. Notably, it holds immense potential for advancing our comprehension of chemostratigraphy and elucidating the heterogeneity of the transition between continental and marine environments during the early stages of the South Atlantic Ocean's evolution.