K-Ar法年龄标准物质ZGC粗面岩K和~(40)Ar含量及年龄的定值结果

为了满足K-Ar和Ar-Ar法定年的需要,年代学工作者研制了一个用于中新生代定年的K-Ar法年龄标准物质——ZGC粗面岩。该粗面岩采自广东省南海市走马营第三纪火山岩。~(40)Ar/~(39)Ar定年结果表明,~(40)Ar~*在矿物晶格中保存均匀稳定,年龄谱平坦,~(39)Ar析出量高达97.9%。证明该粗面岩结晶以后未受过热扰动,完好地保持了~(40)K-~(40)Ar~*同位素计时的化学封闭体系。坪年龄为52.8±0.3 Ma,总气体年龄为52.9±0.8 Ma。~(36)Ar/~(40)Ar-~(39)Ar/~(40)Ar反等时线年龄为52.5±0.4 Ma,~(40)Ar/~(36)Ar初始值为296.6±4.2,此值与(~(40)Ar/~(36)Ar)。大气氩丰度比(295.5±0.5)一致,表明样品不含过剩氩。国内12个实验室参加了ZGC粗面岩K含量和~(40)Ar~*含量的定值分析。经统计学方法检验,结果显示全部定值数据服从正态分布并具等精度。当置信概率为0.95时,~(40)Ar~*和K含量的相对标准偏差都小于1%。~(40)Ar~*和K含量分析的认定值和不确定度分别为:~(40)Ar~*=4.199±0.022×10~(-10)mol/g,K=4.576±0.028%,由此计算得K-Ar年龄为52.2±0.5 Ma。根据国家一级标准物质技术规范的相关要求,经统计学方法检验在0.05显著性水平下,K和~(40)Ar~*的F分布小于F临界值,说明该标准物质是均匀的。t检验法表明,该标准物质具有良好的稳定性,~(40)Ar~*和K含量在有效期内不会发生显著性变化。ZGC粗面岩粒度为0.3～0.7mm,重量为850 g,缩分成最小样品单元100瓶,每瓶8.5 g。可供K-Ar和Ar-Ar法实验室使用43年。

The certified results of K and 40 Ar* and the age of ZGC tracphyte reference material for K-Ar and40 Ar-39 Ar datings in China

ZGC tracphyte，as K Ar age references material of China，were separated from Zoumayin Tertiary Period volcaic rocks area in Nanhai city，Guangdong Province. About 850 g of the tracphyte was obtained at the 030～070 mm fraction. Folowing complete mixing，the tracphyte was divided into 10 jars about 85 g each. Each jar sample was further subdivided into 10 aliquots about 85 g each(total 100 aliquots). 40 /39 Ar age spectrum of ZGC tracphyte indicats that the tracphyte has remained undisturbed since its last crystallisation. The plateau age(4～14 steps)and total gases age(1～14 steps)are 528±03 Ma and 529±08 Ma，respectively. The argon extraction occurs mainly between 700～1 400 ℃，and about 98％ radiogenic 40 Ar can be released at that temperature rang. From the data of 4～14 steps heating，the isochron age and the initial 40 Ar/36 Ar ration of the tracphyte are 525±04 Ma and 2966±42，respectively. The homogeneity tests were performed in Ar isotopic geochronlogy laboratory of Institute of Geology and Geophysics，Chinese Academy of Science(IGGCAS). The sample weights are 30 mg for 40 Ar* and 40 mg for K. Statistical estimate of analytical data show that F values of K and 40 Ar* are less than their critical F values. The test results indicate that the uncertainty caused by homogeneity of the sample is 143％ for radiogenic 40 Ar* and 044％ for K，respecitively. A good reproducibility of analysis has been got in IGGCAS. Accordingly，the mininum weight of about 13 mg of recommended sample should be proper. Twelve laboratories accepted the invitation to participate in calibrations of K and 40 Ar* of ZGC tracphyte. The certificate values for K and 40 Ar* of the tracphyte are calculated as follows(with 95％ confidence level)： 40 Ar * =4199±0022×10-10mol/g，K=4576±0028％. The K Ar age，from λe=0581×10-10/a，λβ=4962×10-10/a and 40K/K=1167×10-4(atom/atom)，was calculated to be 522±05 Ma(2σ).