Single grain Rb-Sr isotopic analysis of GA-1550 biotite, LP-6 biotite and Bern-4M muscovite 40Ar-39Ar dating standards

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  • LI QIU-LI
    Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences Laboratory for Radiogenic Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences
  • CHEN FUKUN
    Laboratory for Radiogenic Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences
  • LI XIANG-HUI
    Laboratory for Radiogenic Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences
  • WANG FEI
    State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences
  • HE HUAI-YU
    State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences

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Three standard minerals GA-1550 biotite, LP-6 biotite and Bern-4M muscovite used for K-Ar and 40Ar-39Ar dating have been investigated by the single grain Rb-Sr technique. Compared to the K-Ar and 40Ar-39Ar results previously reported, these mica standards nevertheless show complex characteristics in the Rb-Sr isotopic system. Analysis on eight individual grains of GA-1550 biotite gives a Rb-Sr isochron age of 98.7 ± 1.9 Ma, consistent with previous dating results of the K-Ar and stepwise heating and laser ablation 40Ar-39Ar methods. Although evidence of the laser ablation 40Ar-39Ar analyses in many previous studies demonstrates that LP-6 biotite is considerably inhomogeneous in the K-Ar isotopic system, Rb-Sr isotopic analyses on ten individual biotite grains show achievement of Sr isotopic equilibrium during crystallization. A Rb-Sr isochron age of 128.2 ± 2.2 Ma calculated from the analytical results is concordant with those previously reported K-Ar and 40Ar-39Ar ages obtained from c. 30 mg aliquots of biotite grains. Excellent age reproducibility on Bern-4M muscovite was previously achieved by K-Ar and 40Ar-39Ar dating by total fusion in c. 30 mg aliquots of muscovite grains, but single grain Rb-Sr analytical results in this study demonstrate considerable disequilibrium of Sr isotopic system during crystallization of muscovite. The discordance in the Rb-Sr and K-Ar isotopic systems of two standard minerals indicates complex factors that can influence isotopic disequilibrium of both isotopic systems.

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