ABSTRACT

A new mineral standard for K-Ar dating, SORI93 biotite, has been prepared from the Sori Granodiorite in Gunma Prefecture, Japan. This is intended as a substitute of the existing standard, SORI biotite, which is well calibrated against international standards. The K₂O and radiogenic ⁴⁰Ar contents for SORI93 biotite have been determined to be 8.16 ± 0.04 (1σ) wt % and (25.0 ± 0.1) × 10^-6 (1σ) cm³STP/g, respectively. The calculated K-Ar age is 92.6 ± 0.6 Ma, which is about 1 million years older than that of SORI biotite. Chloritization up to 10% partly occur along cleavages of SORI93 biotite. In order to certify the homogeneity, K₂O contents have been repeatedly analyzed for 100, 50, 20 and 10 mg aliquots. The analytical result of 100 mg SORI93 biotite shows a relative standard deviation similar to that of SORI biotite and the geochemical standards, JG-1 and JB-1. The standard deviations of aliquots less than 100 mg are about twice those of the three standards. This inhomogeneity may be caused by the presence of partially chloritized biotite. It is suggested to use 100 mg to ensure the homogeneity of subsamples.