ABSTRACT

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.