一种新的K-Ar定年方法:峰值比较法  

A NEW METHOD OF K-Ar DATING: PEAK COMPARISON METHOD

作　　者：刘玉琳[1] 

机构地区：[1]北京大学造山带与地壳演化教育部重点实验室和地质学系,北京100871

出　　处：《地球科学进展》2004年第2期312-315,共4页Advances in Earth Science

基　　金：国家重点基础研究发展规划项目"大型剪切成矿系统与金矿成矿作用"(编号 :2 0 0 1CB40 980 7)资助

摘　　要：K Ar法定年技术经历了体积法和稀释法等不断趋于成熟 ,但对于年轻火山岩进行定年时 ,由于同位素质量分馏的影响 ,上述方法都具有不可克服的缺陷。随着质谱仪稳定度和精度的提高 ,直接建立氩同位素含量与峰强度之间的定量关系 ,从而不再使用稀释剂 (38Ar)成为可能 ,这就是K Ar定年的新方法 :峰值比较法 (或无稀释剂法 )。该方法可以消除质量分馏作用带来的误差 ,对年轻火山岩定年具有重要意义。近代火山物质都含有初始氩 ,且有不同程度的质量分馏 ,其同位素比值落在质量分馏线上 ,这表明质量分馏作用影响K Ar定年的准确性 ,尤其是非常年轻的样品。通过精确测定样品的38Ar/ 36 Ar比值 ,可以准确扣除样品中的初始4 0 Ar ,从而得到放射成因4 0 Ar含量。这种测量方法可以用于较老岩石、矿物样品的年龄测定 ,其结果与K Ar稀释法以及4 0 Ar - 39Ar法相一致 ;更重要的是可以用于年轻样品的年龄测定 ,可以测 2～ 5ka的岩矿样品 ,对富钾矿物的测量误差小于 1ka ;对老于 0 .1Ma的样品 ,相对误差不大于 1.5 %。介绍了峰值比较法的基本原理和测试过程 。From volumetric method to isotope dilution method, K Ar dating has got its period of maturity. But traditional K Ar dating techniques could not overcome the errors coming from the influence of isotope mass discrimination. It is possible to establish quantitative relationship between peak height and abundance of Ar. A new method of K Ar dating with peak comparison method has been produced, in which the spike ( 38 Ar) has not been used again. With this method, the errors coming from mass discrimination could be eliminated. This is especially important to young volcanic rocks dating. Recent volcanic substances contain initial argon, which underwent mass fractionation in some extent, and their isotopic ratios fell on the mass fractionation line. This implies that the fractionation influence the K Ar dating of very young samples. According to accurate measurement of 38 Ar/ 36 Ar ratio of samples, initial 40 Ar can be precisely subtracted from the total Ar and the radiogenic 40 Ar obtained. This method can be used in dating old rock and mineral samples, with their results corresponding well with that of K Ar dilute method and 40 Ar 39 Ar method, and more importantly, used in very young samples. This method can be used to date samples as young as 2 ～ 5ka, where the error is less than 1ka for high potassium samples and the relative error is not more than 1.5% for samples older than 0.1 Ma. This paper introduces the basic principles and analysis process of peak comparison method and at the same time the requirements and range of application are also discussed.

关 键 词：K—Ar定年法 同位素质量分馏 峰值比较法 年轻火山岩定年 

分 类 号：P597[天文地球—地球化学]