摘要
BSP-1角闪石选自中国吉林省桦甸县老牛沟的角闪石岩。^(40)Ar-^(39)Ar分析和均匀性检查表明:此角闪石结晶以后未受过热扰动,^(40)Ar~*保存性好,谱线平坦,K/Ca和Cl/K比值稳定。定值结果显示,在置信概率为0.95时,^(40)Ar~*和K含量的相对偏差均<5‰,是目前最理想的Ar-Ar和K-Ar法国际标准物质,可供全世界Ar-Ar实验室使用80年以上。
1. The irradiation with a high integrated neutron dose is very desirable for any relatively old sample. In that occasion, if the flux monitor (age standard)is much younger than the unknown sample, the ^(40)Ar/^(39)Ar ratio in the standard may become very low, approaching a lower limit value (～3). Therefore, 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 Precambrian rocks, low-K minerals and meteorites.2. BSP-1 hornblende was separated from a hornblendite dike which occurs at Laoniugou area, Jilin Province, Northeast China (42°55′N, 127°23′E). The dike is a part of a layered ultramafic-mafic complex with a K-Ar age of about 2 Ga, whereas the granulite, as the country rock, has a zircon U-Pb age of about 2.5 Ga. The hornblendite is very fresh, without visible trace of alteration in hand specimen. Microscopically, it is composed mainly of dark-green hornblende, with crystals of millimetric size, and accessory minerals include magnetite, apatite and sphene.3. About 1.6 kg of pure hornblende was obtained at the 60～80 mesh fraction after ten times magnetic separations. The purity is about 99.7％ (Table 3) and the main impurity is intergrowth apatite. From chemical composition, X-fluorescence spectrum, infra-red absorption spectrum, X-ray diffraction and DTA curve(Table 1,2,3), the BSP-1 hornblende belongs to magnesio-hastingsite of calcic amphibole series. Following complete mixing, the BSP-1 hornblende was divided into 16 jars about 100g each. Each jar sample was further subdivided into 16 aliquots about 6 g each. One aliquot (prefixed C) was retained for control analysis of K and ^(40)Ar, the other 15 aliquots of the same jar remain to be used for inter-labortory work.4. The ^(40)Ar-^(39)Ar spectrum of BSP-1 horblende (Fig.4) indicates that the hornblende has remained undisturbed since its last cooling. The plateau age and the total gases age are 2054±4 Ma (2σ) and 2055±32 Ma (2σ), respectively. The ratio of K/Ca and C1/K are stable at middle-high temperatures. The pre-heating at less than 200℃ in vacuum on the extraction line is possible without affecting argon reieantion. The argon extraction occurs mainly between 1000℃ and 1300℃. and about 95％ radiogenic ^(40)Ar can be released at that temperature range (Fig.6).From all data of step-heating, the isochron age and the initial ^(40)Ar/^(36)Ar ratio of the hornblende are 2053 Ma and 294.2, respectively (Fig.6). The correlation coefficient in the isochron diagram is 0.9999.5. The homogeneity tests and the certification analyses of ^(40)Ar~* and K in BSP-1 hornblende were performed according to the rules from the International Standard Organization (ISO) Guide 35 (1985).During the homogeneity tests, the analyses of ^(40)Ar~* and K in MMHB-1 hornblende, GL-O glauconite, P-207 muscovite, KA83 K-feldspar and GA 1550 biotite were done (Table 5,6), and the results are almost coincident with their mean value published. The homogenity teats were performed in University of Sao Paulo and the Institute of Geology, Academia Sinica (IGAS). The sample weights are 25 nag for ^(40)Ar~* and 100 mg for K. Statistical estimate of analytical data shows that F values of ^(40)Ar~* and K are less than their critical F values. The test results (Table 7) indicate that the uncertainty caused by homogeneity of the sample is 0.23％ for radiogenic ^(40)Ar~* and 0.48％ for K,respectively. A good reproducibility of ^(40)Ar, analysis has been got in IGAS. Accordingly, the minimum weight of about 3.5 mg(Table 9) of recommended sample should be proper.6. Thirteen laboratories (Table 10), including eight ones in China, accepted the invitation to participate in calibrations of ^(40)Ar^8 and K of BSP-1 hornblende. K-calibrations have been done with flame photometry and atomic absorption using ^(40)Ar/~^(39)Ar technique (Table 12). According to Grubbs test and Dixon test, the K from laboratory 4 is an outlier which does not join the calculation of certificate value.The calibrations of ^(40)Ar~* were performed by using isotopic dilution me- thod, ^(38)Ar spike and ^(40)Ar-^(39)Ar techniques. The Grubbs and Dixon tests indicate that there is no outlier in all data from 13 laboratories.The calculations of skewness and kurtosis coefficients, and the Cochran isoprecision test indicate that all data of K and ^(40)Ar~*, except the ^(40)Ar~* value from laboratory 4,obey a normal distribution.The certificate values for ^(40)Ar~* and K of BSP-1 hornblende are calculated as follows (with 95％ confidence level, Table 15):^(40)Ar~*=109.06±0.26 (10^(-6)ccSTP/g)K=0.729±0.003(％)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 2060±8 Ma.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
1992年第2期103-107,共5页
Acta Petrologica Sinica
基金
中国国家自然科学基金
巴西圣保罗大学资助项目
关键词
标准物质
角闪石
氩-氩定年
定值
^(40)Ar-^(39)Ar dating
Reference material
Hornblende
Homogeneity
Certification
