Implementation of U-Pb dating via LA-ICP-MS and its application on carbonates from brittle structures of Jandaíra Formation, Potiguar Basin, NE-Brazil.

U-Pb carbonate dating by LA-ICP-MS is becoming a popular tool to investigate the timing of brittle deformation. In northeast Brazil, the absolute dating of brittle deformation in a series of rift-related intracontinental basins associated with the South Atlantic Rift has been little explored. Carbonates that were previously considered impossible to date, have been dated using a recent and challenging technique of U-Pb dating via LA-ICP-MS. To set up the routine in Multilab UERJ, analytical protocols from several laboratories were compiled to understand their LA-ICP-MS system parameters applied to this method. Preliminary tests combined the use of zircon standards (GJ1, BB, and Plesovice) and NIST 612 glass as Pb isotope mass bias correction with carbonate matrix reference materials WC1, ASH 15, and JT calcite. The final analytical protocol for Multilab UERJ was set combining NIST 612 glass and WC1 carbonate reference material. The analysis sessions on this work took place in sequential rounds, consisting of 40 spots in each round, 6 of which were NIST 612 analyses, 14 analyses of carbonate samples, 6 analyses of WC-1 and 14 analyses of carbonate samples. The laser ablation system in the laboratory is a Photon Machines Analyte Excite ArF 193 nm excimer, and the mass spectrometer is a Thermo Scientific Element 2 SF-ICP-MS. The parameters used were RF power of 1200W, ablation time of the 40s, fluency < 5 J/cm2, spot size 150 μm, and repetition rate of 10 Hz. The data acquired in ICP-MS was treated on Glitter software, exported to an in-house Excel spreadsheet, plotted in Tera Wasserburg diagrams, and lower intercept ages were calculated with IsoplotR. The first application of the method, after implementation and validation in the laboratory, was in carbonate vein samples from Jandaíra limestones, the samples were prepared as chips on epoxy resin and polished slabs. At first, images of the samples were taken by optical cathodoluminescence (CL), to asses internal features for vein type classification. Then, the samples were taken to LA-ICP-MS for random screening to identify regions with high contents (minimum of 10.000 cps) of U. Three samples of carbonate veins from Potiguar basin were successfully dated and yielded ages between 45 and 30 Ma, while WC1 carbonate reference material yielded an age of 251.65 Ma during the analytical sessions. The acquired age interval is coincident with a major magmatic event (Macau-Messejana) in Potiguar Basin, suggesting a causal relationship between magmatism and brittle deformation. Until now, these features' ages (Lower Cretaceous to Middle Miocene) were inferred by field relationship and stratigraphic studies. We validate here the implementation of the U-Pb carbonate dating method in a Brazilian laboratory and its application on brittle deformation in northeast Brazil rift-related sedimentary basins.