40Ar/39Ar geochronology of basaltic samples from Bruce Bank and Jane Bank of the Scotia Sea

The datasets refer to the dating of three basaltic samples collected from Bruce Bank and Jane Bank in the southern Scotia Sea. The samples were dredged from depths between 850 -1900 m from steep scrap slopes at two locations (DR.225: 59.927 degrees S, 39.154 degrees W; DR.84: 62.468 degrees S, 039.785 degrees W). The files include full analytical datasets for 40Ar/39Ar analysis of whole rock and plagioclase mineral separates performed at the Open University, UK in June 2022. This project was funded by NERC NC-ALI funding to Geology & Geophysics.

Field methods: Dredge sampling took place at sites along the eastern margin of Bruce Bank and southern margin of Jane Bank where the slopes were interpreted to be the steepest (less than 25 degrees) and therefore likely to have reduced sediment cover. Dredge site DR.225 (c. 850 m) is dominated by mafic volcanic, hypabyssal and plutonic rocks, including gabbro and ultramafic lithologies. Sample DR.225.14 is from a mafic porphyritic lithology interpreted as a probable hypabyssal unit and DR.225.27 is a fine-grained basaltic lava with phenocrysts of olivine and pyroxene. Dredge site DR.84 from Jane Bank yielded a significant return of mafic - silicic volcanic rocks and volcaniclastic lithologies characterised by a thin (few mm) Fe-Mn crust. A fine-grained basaltic lava (DR.84.8) from dredge site DR.84 (c. 1900 m depth) is dated here. Analytical methods: 40Ar-39Ar geochronology 40Ar/39Ar dating was performed at the Department of Earth Sciences, Open University. The samples were crushed using a pestle and mortar and the crushate was sieved and washed repeatedly in de-ionised water to remove dust and clay particles from the surfaces of all the size fractions. Using a binocular microscope whole rock pieces were selected that were free from alteration. The picked separates were cleaned ultrasonically in acetone and de-ionised water, dried using the hot plate, and packaged in aluminium foil packets of c. 10mm x 10mm prior to irradiation. Samples were irradiated at the McMaster Nuclear Reactor (McMaster University, Canada) in reactor position 8E for 120 hours, using cadmium shielding. Neutron flux was monitored using biotite mineral standard GA1550 which has an age of 99.738 +/- 0.104 Ma (Renne et al., 2010). Standards were packed for irradiation, either side of the unknown samples and analysed using the single grain fusion method using a 1059nm CSI fibre laser and a MAP215-50 mass spectrometer. The J Values were then calculated by linear extrapolation between the 2 measured J values, the values for each sample are shown in the data table and a 0.5 percentage error on J is used. The irradiated samples were loaded into an ultra-high vacuum system and a 1059nm CSI fibre laser was focussed into the sample chamber and was used to step-heat basalt. After passing through a liquid nitrogen trap, extracted gases were cleaned for 5 minutes using two SAES AP-10 getters (one at 450 degrees Celsius and one at room temperature), following which the gases were let into a MAP 215-50 mass spectrometer for analysis; the mass discrimination value was measured at 283 for 40Ar/36Ar (using a calibration noble gas mixture of known composition). System blanks were measured before and after every one or two sample analyses. Gas clean-up and inlet is fully automated, with measurement of 40Ar, 39Ar, 38Ar, 37Ar, and 36Ar (relative isotope abudnaces), each for ten scans, and the final measurements are extrapolations back to the inlet time. The system blanks measured before and after every one or two sample analysis were subtracted from the raw sample data. Results were corrected for 37Ar and 39Ar decay, and neutron-induced interference reactions. The following correction factors were used: (39Ar/37Ar)Ca = 0.00065 +/- 0.00000325, (36Ar/37Ar)Ca = 0.000265 +/- 0.000001325, and (40Ar/39Ar)K = 0.0085 +/- 0.0000425; based on analyses of Ca and K salts. Ages were calculated using the atmospheric 40Ar/36Ar ratio of 298.56 (Lee et al., 2006) and decay constants of Renne et al. (2010). All data corrections were carried out using an Excel macro and ages were calculated using Isoplot 4.15 (Ludwig, 2012). All ages are reported at the 2sigma level and include a 0.5 percentage error on the J value. Plateau criteria of at least 50 percent of the 39Ar release in at least 3 consecutive steps were used. For step-heating experiments data is usually presented as a step-heating release spectrum and where the plateau criteria are met, a plateau age can be calculated. The same data can also be plotted on an inverse isochron correlation plot, and the intercepts on this plot allow a calculation of the age of the sample (from the 39Ar/40Ar intercept) and the 40Ar/36Ar ratio of the intercept. The isochron correlation plot allows the 40Ar/36Ar ratio for the sample to be calculated rather than making the assumption that the ratio is atmospheric as is the case in the plateau age (40Ar/36Ar of atmospheric argon 298.56 (Lee et al., 2006). Ideally, the agreement between both calculated ages and an atmospheric ratio for the intercept would provide the most confidence that the age calculated is reliable.

Results were corrected for 37Ar and 39Ar decay, and neutron-induced interference reactions. The following correction factors were used: (39Ar/37Ar)Ca = 0.00065 ± 0.00000325, (36Ar/37Ar)Ca = 0.000265 ± 0.000001325, and (40Ar/39Ar)K = 0.0085 ± 0.0000425; based on analyses of Ca and K salts. Data corrections carried out using an Excel macro. A calibration noble gas mixture was used with known concentration. Neutron flux was monitored using biotite mineral standard GA1550.

Binocular microscope 1059nm CSI fibre laser MAP215-50 mass spectrometer SAES AP-10 getters Data was processed using Isoplot 4.15 software by Ludwig (2012)