Raw data and plots of 40Ar/39Ar age dating of polyhalite from the evaporitic Permian Haselgebirge Formation

The measurements were conducted at the University of Salzburg, Department of Geography and Geology, in the years 2007-2015. Polyhalite samples were manually reduced to small pieces with a hammer. They were washed with destilled water and dried with isopropanol to free them from dust and Cl-ions of halite. Chlorine produces Ar isotopes during irradiation, which may tamper the proportion of Ar isotopes from polyhalite. Grains of 200–250 µm size were selected under the microscope. A sufficient number of grains of each sample were packed into aluminium-foil and put into quartz vials. Details of the analytical 40Ar/39Ar technique is described in Leitner et al. (2014) and Cao et al. (2017). Irradiation was conducted for 16 hours in the Magyar Tudományos Akadémia (MTA) Központi Fizakai Kutato Intézet (KFKI) reactor (Debrecen, Hungary). Flux-monitors were placed between the samples for calculation of the J-values. The distance between adjacent flux-monitors was c. 5 mm. Corrections for interfering isotopes were the same as described earlier: Correction factors were calculated from 45 analyses of co-irradiated Ca-glass samples and 70 analyses of K-glass samples, and are: 36Ar/37Ar(Ca) = 0.000225, 37Ar/39Ar(Ca) = 0.000614, 38Ar/39Ar(K) = 0.0117, and 40Ar/39Ar(K) = 0.0266. Variation in the flux of neutrons were monitored with DRA1 sanidine standard for which a 40Ar/39Ar plateau age of 25.26 ± 0.05 Ma has been reported (van Hinsbergen et al. 2008). 40Ar/39Ar analyses were carried out at the Department of Geography and Geology at the University of Salzburg. The equipment used was the same as described earlier: 40Ar/39Ar analyses are carried out using a ultra high vacuum Ar-extraction line equipped with a combined MERCHANTEKTM UV/IR laser system, and a VG-ISOTECHTM VG-3600 noble gas mass spectrometer. Stepwise heating analyses of samples are performed using a defocused (~1.5 mm diameter) 25 W CO2-IR laser operating in Tem00 mode at wavelengths between 10.57 and 10.63 µm. The laser is controlled from a PC, and the position of the laser on the sample is monitored on the computer screen via a video camera in the optical axis of the laser beam through a double-vacuum window on the sample chamber. Gas clean-up is performed using one hot and one cold Zr-Al SAESTM getter. Gas admittance and pumping of the mass spectrometer and the Ar-extraction line are computer controlled using pneumatic valves. The VG-3600 is an 18 cm radius 60° extended geometry sector field mass analyzer instrument, equipped with a bright Nier-type source operated at 4.5 kV. Measurements are performed on an axial electron multiplier in static mode, peak-jumping and stability of the magnet is controlled by a Hall-probe. For each increment the intensities of 36Ar, 37Ar, 38Ar, 39Ar, and 40Ar are measured, the baseline readings on mass 35.5 are automatically subtracted. Intensities of the peaks are back-extrapolated over 16 measured intensities to the time of gas admittance either by a straight line or a curved fit, depending on intensity and type of pattern of the evolving gas. Inspection of intensities was applied with regard to background, system blanks, interfering isotopes and post-irradiation decay of 37Ar. Calculations of isotope ratios, errors, ages and plateau ages followed suggestions of McDougall and Harrison (1999), Scaillet (2000), Steiger and Jäger (1977) and Ludwig (2012).