^(40)Ar—^(39)Ar法国际标准样BSP—1角闪石的测定

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

利用RGA—10质谱计和析氢仪联合装置对BSP—1角闪石进行了K—Ar及^(40)Ar—^(39)Ar年龄测定,坪年龄为2054±2Ma;谱线平坦,^(40)Ar^(?)保存性很好,K/Ca和CI/K比值稳定;^(40)Ar—^(39)Ar等时年龄为2053Ma,^(40)Ar/^(36)Ar初始值为294.2,与大气Ar丰度一致。国内外13个实验室参加了该角闪石K和^(40)Ar^(?),的定值分析,其结果为:K=0.725±0.003(%),^(40)Ar^(?)=109.06±0.26(10^(-6)cc-STP/g),由此求出的K—Ar年龄为2060±8Ma,与坪年龄、等时年龄吻合。这些结果表明,BSP—1角闪石结晶时未捕获大气Ar,结晶以来未受过热扰动,保持着良好的^(40)K、^(40)Ar^(?)封闭体系,是很理想的^(40)Ar—^(39)Ar法的国际标准样。

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.