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The Certified Results of ZMT04 Muscovite Reference Material Utilized for 40Ar/39Ar Isotopic Geochronological Research on Early Precambrian Rocks in China
详细信息   
摘要
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 40Ar/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 40Ar-39Ar 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-39Ar 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, 38Ar spike and 40Ar/39Ar 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 40Ar* and K of ZMT04 muscovite are calculated as follows: 40Ar* = 4. 449 ± 0. 060 × 10-8 mol/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).

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