| 我国K-Ar法标准样40Ar-40K和40Ar-39Ar年龄测定及放射成因40Ar的析出特征 |
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| 引用本文： | 王松山.我国K-Ar法标准样40Ar-40K和40Ar-39Ar年龄测定及放射成因40Ar的析出特征[J].地质科学,1983,0(4):315-323. |
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| 作者姓名： | 王松山 |
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| 作者单位： | 中国科学院地质研究所 |
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| 摘 要： | 同位素地质年龄标准样的建立,是一项重要的基础工作,它在检查和比较各种实验流程,衡量各种定年方法的可信度,标定同位素稀释剂等方面,都是必不可少的。从1977年初开始,我国同位素地质标准样小组着手建立K-Ar法标准样。为了外检,笔者曾将ZBH-25黑云母、ZBJ角闪石带到澳大利亚国立大学地球科学院进行测定。这两个样品的K-Ar稀释法年龄、40Ar/39Ar坪年龄及40Ar/39Ar等时年龄的一致性表明,这些矿物结晶以后,未受过热的挠动,也不存在过剩氩,是作标准样的理想样品。本文还介绍样品中放射成因40Ar的析出特征,并指出40Ar可能有两种存在状态,它们各具不同的晶格能。
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| 关 键 词： | , |
| 收稿时间： | 1981-11-04 |
| 修稿时间： | 1981-11-04; |
AGE DETERMINATIONS OF 40Ar-40K, 40Ar-39Ar AND RADIOGENIC 40Ar RELEASED CHARACTERISTICS ON K-Ar GEOSTANDARDS OF CHINA |
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| Wang Sungshan.AGE DETERMINATIONS OF 40Ar-40K, 40Ar-39Ar AND RADIOGENIC 40Ar RELEASED CHARACTERISTICS ON K-Ar GEOSTANDARDS OF CHINA[J].Chinese Journal of Geology,1983,0(4):315-323. |
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| Authors: | Wang Sungshan |
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| Institution: | Institute of Geology, Academia Sinica, Beijing |
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| Abstract: | ZBH-25 biotite and ZBJ hornblande, as K-Ar age geostandards of China, were sepera-ted from Fangshan granodiorite complex in the southwest of Beijing, which occupies an area of about 47 km2. The granodiorite consists of biotite, hornblande, feldspar, quartz, and minor zircon, sphene etc. A total of about 50 kg of biotite was seperated from 2.5 tons of whole rock, of which 20 kg are 0.25-0.63mm(i.e. ZBH-25 biotite), 30 kg is 0.63-1.0 mm(i.e. ZBH-63 biotite), while about one hundregd of 0.15-0.30mm hornblande(i.e. ZBJ hbl)was also seperated for comparison, both of 0.25-0.63 mm biotite and hornblande are 99% pure. They belong to the kind of normal minerals according to the chemical composition, X-rays diffraction, differential thermal analysis, thermal gravity and infra-red spectrum.The potasium contents of ZBH-25 biotite and ZBJ hornblande are 7.597±0.030% and 1.083±0.018%, respectively on the basis of the analyses by using the ion exchange and conventional analytical technique.All the isotopic analyses of argon were made on a greatly modified AEI mass spectrometer operating in the static mode. Peak hopping was employed and all data were taken digitally on-line with a HP 21 MX computer in which all the data reduction was done. The recently recommended values for the decay constants and abundance of 40K are used in calculations. The biotite and hornblande were measured by the conventional K-Ar method, using isotope dilution of argon, and 40Ar/39Ar incremental heating techniques. For the 40Ar/39Ar measurements, irradiations were performed in HIFAR reactor of the Australian Atomic Energy Commission. Samples were shieled with 0.2 mm of Cd during the irradiation, which had a duration of 100 h. Intra-laboratory standard, the GA 1550 biotite of Australian National University, with a K-Ar age of 97.9 m.y., was used as the flux monitor.ZBH-25 biotite yields an average content of radiogenic Ar* of 1.813×10-9 mol/g and an average K-Ar age of 132.7±1.2 m.y.. An 40Ar/39Ar incremental heating experiment performed on this biotite yields a remarkably simple pattern. Apparent ages from step 6-16, comprising over 90 percent of the release of 39Ar, are virtually concordant, and give a plateau age of 132.7±0.1 m.y.. When these same data are plotted on an isochron diagram, an age of 133.6±0.7 m.y. is obtained, using the method of York(1969).An average radiogenic 40Ar* content of 0.2594×10-9 mol/g and an average K-Ar age 132.8± 2.4 m.y. are obtained for ZBJ hornbland. The 40Ar/39Ar incremental heating experiment on this mineral yields a virtually concordant set of ages over 60 percent of the release of 39Ar with a plateau age of 132.8±0.1 m.y.. The age obtianed from the isochron plot, using the same 6 data points, is 133.8±1.2 m.y., indistingui-shable from the plateau age. The concordancy of all the age spectra is regarded as a good evidence that the two samples have remained undisturbed since crystallisation and cooling.The curves of radiogenic 40Ar* released show that ZBH-25 biotite and ZBJ hornblande both have two 40Ar* release peaks corresponding to 780° and 1050℃, respectively. The existence of two 40Ar* release peaks suggests that each of the two minerals possibly has two kinds of 40Ar* lattice energy |
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