ZBH-25 biotite, as K-Ar age geostandard of China, were separated from Fangshan granodiorite complex in the southwest of Beijing, which occupies an area of 47 km(2). The granodiorite consist of biotite, hornblende, feldspar, quartz, and minor epidote, zircon, sphere ete. About 5800g of pure ZBH-25 biotite was obtained at the 0.25 - 0.63mm fraction. The purity is about 99.8%. ZBH-25 biotite belong to the kind of normal biotite mineral according to the chemical composition, X-fluorescenee spectrum infra-red absorption spectrum, X-ray diffraction and DAT curve. Following mixing, the ZBH-25 biotite was divided into 20 jars about 290g each. Each jar sample was further subdivided into 20 aliquots about 14.5g each (total 400 aliquots). One aliquot (prefixed C) was retained for control analysis of K and 40Ar *, the other 19 aliquot of the same jar remain to be used fo interlabortory work. 40Ar/39Ar age spectrum of ZBH-25 biotite indicats that the biotite has remained undisturbed since its crystallisation and cooling. The plateau age (4 - 16steps) and total gases age (1 - 17steps) are 133.0 +/- 0.3Ma and 132.1 +/- 0.6Ma. The argon extraction occurs mainly between 600 - 1300 degrees C, and about 95.7% radiogenie 40Ar can be released at that temperature rang. From the data of 4 - 16 steps heating the isochron age and the initial 40Ar/Ar-16 ration of the biotite are 133.1 +/- 1.3Ma and 293.5 +/- 1. 6, respectively. The correlation coefficient and MSWD in the isoehron diagram are 0. 99999 and 0. 4, respectively. The homogeneity tests and the certification analyses of 40Ar* and K in ZBH-25 biotites were performed according to the rules from the International Standard Organization(ISO) Guid 35( 1985) and primary referenc material technique of China (1990). During the homogeneity tests the analyses of K and 40Ar * in 77600 hornblende, B600 biotite, GL-0 glauconite, PL-6 biotite and GA1550 biotite were done and the results are almost coincident with their mean value published. The homogeneity tests were performed in Ar isotopic laboratory of Institute of Geology and Geophysics, Chinese Academy of Science ( IGGCAS). The sample weights are 15 mg for 40Ar * and 60 mg for K. Statistical estimate of analytical data show that F values of K and 40Ar* are less than their critical F values. The test results indicate that the uncertainty caused by homogeneity of the sample is 0.28 % for radiogenic 40Ar * and 0.08 % for K, respecitively. A good reproducibility of analysis has been got in IGGCAS. Accordingly, the mininum weight of about 3.35 mg of recommended sample should be proper. Fourteen laboratories, accepted the invitation to participate in calibrations of 40Ar * and K of ZBH-25 biotite. K-calibrations have been done with flame photometry and atomic absorption using 40Ar- 39Ar technique. According to Grubs test, the K from laboratory 14 is an outlier which does not join the calculation of certificate value. The calibrations of 40Ar * were performed by using isotopic dilution method, " Ar spike and 40Ar '9Ar techniques. The Grubbs and Dixon tests indicate that there is no outlier in all data from 14 laboratories. The calculations of skewness and kurtosis coefficients, and the Cochran isoprecision indicate that all data of K and4o Ar *, except the K value from laboratory 14, obey a normal distribution. The certificate values for 40Ar and K of ZBH-25 biotite are alculated as follows( with 95 % confidence level).. 40Ar* = 1.817 +/- 0. 013 (10(-9) mol/g), K = 7.60 +/- 0.02 (%). The K-Ar age, from X, = 0. 581 x 10(-10)/a, lambda(beta) =4. 962 x 10(-10)/a and (40) K/K = 1. 167 X 10(-4) ( atom/atom), was calculated to be 132.9 +/- 1.3 Ma (2 sigma). About results suggests that ZBH-25 biotite is a better reference material for K-Ar and Ar-Ar dating.
