Complex 40Ar/39Ar age spectra from low metamorphic grade rocks, Delamerian Orogen, Reid et al

Published: 2 December 2021| Version 1 | DOI: 10.17632/g75hgmypbw.1
Contributors:
Anthony Reid,

Description

40Ar/39Ar data collected by furnace step heating methods from rocks of the Delamerian Orogen, South Australia.

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The 40Ar/39Ar dating technique is adapted from McDougall and Harrison (1999) and described in Forster and Lister (2009). Samples and standards were analysed in the Argon Laboratory at the Research School of Earth Science, The Australian National University, Canberra, Australia using a Thermo Fisher ARGUS-VI multi-collector mass spectrometer. A furnace step-heating technique was used to extract argon isotopes from the samples to ensure 100% release of 39Ar, while the flux monitors crystals (GA1550 biotite) were fused using a CO2 continuous-wave laser; gases extracted from both the samples and standards were analysed in the Argus VI mass spectrometer. Samples had been wrapped in tin foil so as to melt the tin and pump away the gases prior to the sample analysis. The furnace was degassed 4 times at 1,450°C for 15 minutes and the gas pumped away prior to the loading of the subsequent sample. Gas released from flux monitors and each step of sample analyses was exposed to three different Zr-Al getters to remove active gases for 10 minutes and the purified gas was isotopically analysed in the mass spectrometer. Samples were analysed with 30 steps and with temperatures of the overall schedule rising from 450° to 1,450°C. Sample grains were packed into aluminium foils and subjected to neutron irradiation at UC Davies nuclear reactor, USA for 12 hours and 5 minutes along with flux monitors, K2SO4 and CaF2 salts for calculation of J-factors, monitoring corrections factors including 40Ar production from potassium. Biotite standard GA-1550 (99.769 ± 0.108 Ma; Renne et al. 2010) was used as the flux monitor. Stated precisions for 40Ar/39Ar ages include all uncertainties in the measurement of isotope ratios and are quoted at the one sigma level and exclude errors in the age of the flux monitor GA-1550. Reported data have been corrected for system backgrounds, mass discrimination, fluence gradients and atmospheric contamination. 40K abundances and decay constants used are recommended values (Renne et al. 2010). FORSTER M. & LISTER G. 2009 Core-complex-related extension of the Aegean lithosphere initiated at the Eocene-Oligocene transition, Journal of Geophysical Research: Solid Earth, vol. 114, no. B2. MCDOUGALL I. & HARRISON T. M. 1999 Geochronology and thermochronology by the 40Ar/39Ar method. (2nd edition). Oxford University Press, New York. RENNE P. R., et al. 2010 Joint determination of 40K decay constants and 40Ar∗/40K for the Fish Canyon sanidine standard, and improved accuracy for 40Ar/39Ar geochronology, Geochimica et Cosmochimica Acta, vol. 74, no. 18, pp. 5349-5367.

Categories

Geology, Geochronology, Thermochronology in Tectonics

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