Laser Fusion ⁴⁰Ar-³⁹Ar Dating Method Using Multi-Collector Noble Gas Mass Spectrometer Argus Ⅵ and Its Geological Application

The single-collector noble gas mass spectrometer has constrained the further development of the high-precision ⁴⁰Ar-³⁹Ar dating, owing to its relatively low efficiency and precision of measurement. However, the multi-collector noble gas mass spectrometer has been increasingly widely applied in the high-precision ⁴⁰Ar-³⁹Ar dating in recent years, showing great advantage and potentiality. The method of ⁴⁰Ar-³⁹Ar dating by laser total fusion/stepwise heating using the multi-collector noble gas mass spectrometer Argus Ⅵ is reported briefly, and the measurements of air, standard mineral FCs and YBCs, and the K-feldspar from the Kaimuqi granitic pluton in the East Kunlun were performed. Concordant atmospheric ⁴⁰Ar/³⁶Ar ratio and the corresponding mass discrimination factor (MDF) were obtained from 326 air argon analyses during four successive months, indicative of the stability of the machine system. The "model ages" calculated from the laser fusion ⁴⁰Ar-³⁹Ar results of FCs and YBCs suggest that, the level of precision achievable using the Argus Ⅵ for single-grain or minor sample analysis is significantly high and is better than 1‰ (excluding the errors associated with the age of monitors and decay constants). J value and F value (⁴⁰Ar^*/³⁹Ar_K) calculated from the total fusion results of the FCs show similar trend with those from ARGONAUT lab at the University of Salzburg, both suggesting a distinct gradient of the neutron flux of the LVR-15 reactor. The intercalibration factor between YBCs and FCs based on our total fusion analysis of YCs are calculated as: R_FCs^YBCs=1.045 304±0.000 752 (1σ), and the single-grain total fusion age of 29.280±0.086 (1σ) for YBCs is obtained as well. The results are in good agreement with those of previous studies within the range of uncertainty. Stepwise heating analyses of the K-feldspar from the Kaimuqi granitic pluton in the East Kunlun yield a plateau age of 229.9±0.2 Ma (1σ, MSWD=1.59), and an inverse isochron age of 229.8±0.4 Ma, MSWD=1.71). In combination with published geochronology data of zircon U-Pb, and biotite and K-feldspar ⁴⁰Ar-³⁹Ar in this area, it is suggested that the ⁴⁰Ar-³⁹Ar age of the K-feldspar from the Kaimuqi granitic pluton represents a rapid cooling-exhumation event in the East Kunlun during Late Triassic.