K-Ar法地质年龄国家一级标准物质ZBH-25黑云母的研制

Intercalibration of ZBH-25 biotite reference material untilized for K-Ar and ^(40)Ar-^(39)Ar age determination

ZBH-25黑云母采自北京房山花岗闪长岩体。它的^(40)Ar/^(39)Ar 阶段加热实验结果表明,^(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96%,^(40)K-^(40)Ar~*同位素体系封闭良好,证明该黑云母结晶以后未受过热扰动。坪年龄为133.0±0.3Ma,总气体年龄为132.1±0.6Ma;^(36)Ar/^(40)Ar-^(39)Ar/^(40)Ar 反等时线年龄为133.1±1.3 Ma,^(40)Ar/^(36)Ar 初始值为293.5±1.6,与大气氩丰度比(295.5±0.5)处于同一范围,表明样品不含过剩氩。激光显微探针单颗粒坪年龄为132.8±0.2Ma,年龄谱和 K/Ca 谱线平坦,激光熔样总气体年龄为131.3±0.2Ma(图6)。这几个 Ar-Ar 年龄的一致性,说明样品具有良好的均匀性和稳定性,是理想的 K-Ar 和 Ar-Ar 法年龄标准物质。对均匀性检验数据进行统计学方法检验,在0.05显著性水平下,证明 K 和^(40)Ar~*的 F 分布值小于 F 临界值,说明该样品是均匀的。国内外14个实验室参加了 K 含量的定值分析,15个实验室参加了^(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都服从正态分布并具等精度。在置信概率为0.95时,^(40)Ar~*和 K 含量的相对标准偏差都小于0.5%。两个特性量值定值分析结果的一致值和不确定度分别为:^(40)Ar~*=1.817±0.013×10^(-9)mol/g,K=7.60±0.02%,K-Ar 年龄(标准值)=132.9±1.3Ma(2σ).此标准物质纯度为98.8%,粒度为0.25-0.63mm,总重量为5800g,缩分成最小样品单元共400瓶,每瓶重量为14.5g。可供我国 K-Ar 和 Ar-Ar 法同位素年代学实验室使用120年。

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 etc. 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-fluorescence 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 ℃, and about 95.7% radiogenic ^40Ar can be released at that temperature rang. From the data of 4 - 16 steps heating the isochron age and the initial ^40Ar/^36Ar ration of the biotite are 133.1 ± 1.3Ma and 293.5 ± 1.6, respectively. The correlation coefficient and MSWD in the isochron 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, 11600 biotite, GL-O 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 is0.28 % 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, ^38Ar spike and ^40Ar ^39Ar 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 and ^40Ar^＊ , except the K value from laboratory 14, obey a normal distribution. The certificate values for ^40Ar^＊ and K of ZBH-25 biotite are calculated as follows（ with 95% confidence level） ： ^40Ar^8 = 1. 817 ＋ 0. 013 （ 10 -9 mol/g）, K = 7.60 ＋ 0.02 （ % ）. 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 132.9 ± 1. 3Ma（2σ）. About results suggests that ZBH-25 biotite is a better reference material for K-Ar and Ar-Ar dating.