Data for 40Ar/39Ar Geochronology of Meso-Cenozoic magmatism and its aqueous alteration, northern Santos Basin, South Atlantic

Description

Two datasets are available: (1) full major and trace element geochemistry data for all samples dated in this study (analytical results produced at ActLabs Canada; (2) complete 40Ar/39Ar results for all sample steps (analyses conducted at UQ-AGES – University of Queensland Argon Geochronology in Earth Sciences – laboratory).

Steps to reproduce

Aliquots (~10 grams) of igneous rock samples previously cleaned with acetone and distilled water were submitted for geochemical analysis (major, minor and trace elements, including C and S) at the Actlabs (Canada). The samples were pulverized in steel mills and split into two aliquots. After quantifying LOI, one aliquot was subsequently fused with lithium metaborate/ tetraborate, digested in weak nitric acid, and analyzed by ICP-OES and ICP-MS (Actlabs 4 Lithoresearch analytical package). A second aliquot was analyzed for C and S using Actlabs’ 4F (C,S) package. Standards and internal replicates were analyzed together with the samples. Further details on analytical approaches are outlined in https://actlabs.com/wp-content/uploads/2023/03/Actlabs-Schedule-of-Services-USD-2023.pdf and complete analytical data are in Electronic Appendix 2. The cored samples or 2-3 mm thick slices of 1-inch diameter sidewall cores were halved, and one half was crushed to 0.2-2 mm diameter fragments. Clean fragments of either whole rock (1-2 mm) or single crystals (2000-100 µm) of amphibole or sanidine were hand-picked for geochronology. The grains were placed into wells in high-purity Al-disks together with Fish Canyon sanidine (FCS) neutron fluence monitors placed in the same disks in a star pattern, as illustrated in Vasconcelos et al. (2002). A stack of 10 sample disks was wrapped in Al-foil, vacuum-encapsulated in a silica ampoule, and irradiated for 14-hours in the CLICIT Facility at the OSU Triga Reactor. After ~ 4-5 months cooling period, 15 grains of FCS were loaded into individual wells in a 145-pit copper disk, placed in the ultrahigh vacuum extraction line, baked to ~ 180 °C overnight, let cool for a day until blank levels reached acceptable levels, and then analysed by the total fusion 40Ar/39Ar method to calculate J-factors for each disk assuming an age of 28.201 ± 0.046. Three total system blanks and one air pipette were analysed before and after each group of 3 FC grains to determine discrimination values (adopting the atmospheric values of Lee et al., 2006) for each period of the run. Single grains/crystals or multiple grain/crystal ensembles were incrementally heated with a 2mm defocussed Verdi diode laser beam. Homogeneous heating was ensured by moving the stage under the laser beam in a cross pattern. After each heating step, the fraction of gas released was cleaned in an extraction line equipped with a cryocooling device (-136 °C) and two C-50 SAES getters before expansion into an MAP-215-50 mass spectrometer equipped with a third C-50 SAES getter and operated in peak-hopping mode. 40Ar, 39Ar, 38Ar, and 37Ar peak tops were quantified 8 times at each cycle, while 36Ar was measured 35 times. For age calculation, measurements were regressed to time zero through either linear or parabolic fits. All ages were calculated using the K-decay constant values of Steiger and Jäger (1977).

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