用于早前寒武纪岩石Ar-Ar法定年的K-Ar法年龄国家二级标准物质——ZMT04白云母的定值结果

摘要

为了满足Ar-Ar快中子活化法定年的需要,经过十几年的连续测定与检验,一个用于早前寒武纪矿物定年的K-Ar法年龄标准物质—ZMT04白云母已被研制成功.它采自内蒙古自治区卓资县天皮山.40Ar-39 Ar定年结果表明,40Ar*在矿物晶格中保存均匀稳定,年龄谱平坦,39 Ar析出量为99.1％,40Ar/39 Ar坪年龄为1823±15Ma,说明该白云母结晶以来未受热扰动,40K-40Ar*同位素计时体系封闭良好.40Ar/39Ar等时年龄为1824±15Ma,与坪年龄一致,代表了伟晶岩中白云母的结晶时间.(40Ar/36Ar)0初始值为288±16,此值与(40Ar/36Ar)a大气氩丰度比(295.5±0.5)处于同一范围,表明该白云母不含过剩氩.根据国家一级标准物质技术规范(JJG 1006-1994)的有关规定,对ZMT04白云母进行了一系列分析检验.均匀性检验结果表明,在0.05显著性水平下,K和40Ar*的F分布值小于F临界值,证明该白云母是均匀的.t检验法和稳定性检验与分析表明,ZMT04白云母完全满足国家一级标准物质技术规范对稳定性的要求.国内8个实验室参加了ZMT04白云母K和40 Ar*含量的定值分析,正态性和等精度检验结果显示,全部定值分析数据服从正态分布并具等精度.当置信概率为0.95时,该白云母40Ar*和K含量的相对标准偏差都小于1％.经过严格计算最后获得40 Ar*和K含量及年龄的认定值和不确定度分别为:40Ar*=4.449±0.060×10-8mol/g(2σ),K=8.28±0.04％(2σ),K-Ar年龄=1804±21Ma(2σ).此标准物质纯度达100％,总重量为1080g,缩分为196瓶,每瓶5.51g.可供我国K-Ar和Ar-Ar法年代学实验室至少使用54年.此标准物质于2009年3月4日被国家质量监督检验检疫总局审核批准为国家二级标准物质(见国质检量函[2009] 89号文),国家质检总局认定的ZMT04白云母制造计量器具许可证编号为GBW (E) 040019.%In the 40Ar-39Ar dating technique,the irradiation with a high integrated neutron dose is very desirable for any relatively old sample.In that occasion,if the neutron flux monitor is much younger than the unknown age sample,the 40 Ar/39Ar ratio in standard may become very small,approaching a smaller limit value (≈3).Therefore,it is quite necessary to prepare an old age standard for the flux monitor of the 40 Ar-39 Ar method,especially for isotopic geochronological research on Early Precambrian rocks and meteorites.ZMT04 muscovite as K-Ar age geostandard of China,were separated from a pegmatite dike which occurs at Tianpishan area,in the southwest of Zhuozi County,Inner Mongolia of China.The pegmatite consists of muscovite,quartz and minor zircon,sphere etc.About 1080g of pure ZMT04 muscovite was obtained at the (10 × 1 × 0.02)mm fraction.The purity is about 100％.Following mixing,the ZMT04 muscovite was divided into 14 jars about 77g each.Each jar sample was further subdivided into14 aliquots about 5.51g each (total 196 aliquots).One aliquot (prefixed C) was retained for control analysis of K and 40Ar*,the other 13 aliquot of the same jar remain to be used fo interlabortory work.40Ar-39 Ar age spectrum of ZMT04 muscovite indicats that the muscovite has remained undisturbed since its crystallisation.The plateau age age is 1823 ± 15Ma.Thirteen stepwise hating Ar isotopic data of the muscovite give a 40Ar/39Ar isochron age of 1824 ± 15Ma and the initial 40Ar/36Ar ration of 288 ± 16,and the MSWD in the 40Ar/36Ar-39Ar/36Ar isochron diagram is 2.10.This age may represent the crystallization time of the muscovite in the pegmatite dike.The homogeneity tests and the certification analyses of 40Ar* and K in ZMT04 muscovite were performed according to the rules from the International Standard Organization (ISO) Guid 35 (1985) and primary referenc material technique of China (1994).During the homogeneity tests the analyses of K and 40Ar* in 77-600 hornblende,Hb3gr hornblende and BSP-1 hornblend were done and the results are almost coincident with their mean value published.The K-Ar and 40Ar-39Ar dating,homogeneity tests were performed in Ar isotopic laboratory of Institute of Geollogy and Geophysics,Chinese Academy of Science (IGGCAS).The sample weights are 30mg for 40Ar* and 40mg 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 0.18％ for radiogenic 40Ar* and 0.24％ for K,respecitively.A good reproducibility of analysis has been got in IGGCAS.Accordingly,the mininum weight of about 4mg of recommended sample should be proper.Eight laboratories accepted the invitation to participate in calibrations of 40Ar* and K of ZMT04 muscovite.K-calibrations have been done with flame photometry and atomic absorption.The calibrations of 40Ar* were performed by using isotopic dilution method,38 Ar spike and 40 At/39 Ar techniques.The Grubbs and Dixon tests indicate that there is no outlier in all data from 8 laboratories.The calculations of skewness and kurtosis coefficients,and the Cochran isoprecision indicate that all data of K and 40Ar*,obey a normal distribution.The results of the certificate values for 40 Ar* and K of ZMT04 muscovite are calculated as follows:40Ar * =4.449 ± 0.060× 10-8mol/g (2σ),K =8.28 ±0.04％ (2σ).The K-Ar age,from λe =0.581 × 10-10/a,λβ =4.962 × 10-10/a and 40K/K =1.167 × 10-4 (atom/atom),was calculated to be 1804 ±21Ma (2σ).