SORI93 BIOTITE - A NEW MINERAL STANDARD FOR K-AR DATING

A new mineral standard for K-Ar dating, SORI93 biotite, has been prepa red from the Sori Granodiorite in Gunma Prefecture, Japan. This is int ended as a substitute of the existing standard, SORI biotite, which is well calibrated against international standards. The K2O and radiogen ic Ar-40 contents for SORI93 biotite have been determined to be 8.16 /- 0.04 (1 sigma) wt % and (25.0 +/- 0.1) x 10(-6) (1 sigma) cm(3)STP/ 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 fo r 100, 50, 20 and 10 mg aliquots. The analytical result of 100 mg SORI 93 biotite shows a relative standard deviation similar to that of SORI biotite and the geochemical standards, JG-1 and JB-1. The standard de viations of aliquots less than 100 mg are about twice those of the thr ee standards. This inhomogeneity may be caused by the presence of part ially chloritized biotite. It is suggested to use 100 mg to ensure the homogeneity of subsamples.