Determination of BSP—1 Hornblende for ~(40)Ar—~(39)Ar Method International Standard Samples

BSP—1 hornblende was collected from a hornblendite dike which occurs at Laoniugou area of Huadian County, Jilin Province, China. The hornblendite is very fresh. About 1.63 kg of pure hornblende were obtained at the 60-80 mesh fraction after ten times magnetic separations. The purity is about 99.70％. The hornblende was divided into 16 jars about 101g each. Each jar sample was further subdivided into aliquots about 6.3g each. All samples are 256 aliquots. The ~(40)Ar-~(39)Ar age spectrum of the BSP-1 hornblende (Fig.2)indicates the hornlende has remained undisturbed since last cooling. The plateau age and the total gaese age are 2054±2Ma and 2055±18Ma, respectively. Its ratio of K/Ca and C1/K are stable at middle-high temperatures. The isochon age from all data of step-heating experiments and initial ~(40)Ar/~(39)Ar ratio of the hornblende are 2053Ma and 294.2, respectively. The correlation cofficient in the isochon diagram is 0. 99998(Fig.3). Thirteen laboratories accepted the certificate analyese of ~(40)Ar and K of BSP-1 hornblende. The certificate values of the ~(40)Ar and K are calculated as follows (with 95％ confidence level): K=0.729±0.003(％), 40Ar =109.06±0.26(10~(-6)ccSTP/g). The K—Ar age was calculated to be 2060±8Ma. It is quite necessary to prepare an old—age standard for flux monitor of the ~(40)Ar—~(39)Ar method, especially for geochronological research on Early Precamhriam rocks, meteorites, cosmic dusts and low—K minerals.