K-Ar、^(40)Ar/^(39)Ar定年中尼尔值和初始氩比值  

Nier value and initial argon isotope ratio of K-Ar,^(40)Ar/^(39)Ar dating technique

作　　者：权伍勋[1] 季建清[1] 周晶[1] 

机构地区：[1]北京大学地球与空间科学学院造山带与地壳演化教育部重点实验室,北京100871

出　　处：《地质科学》2015年第1期213-221,共9页Chinese Journal of Geology(Scientia Geologica Sinica)

基　　金：中国地质调查局地质调查项目(编号:12120113013700)资助

摘　　要：K-Ar、^(40)Ar/^(39)Ar定年中,用以计算囚禁^(40)Ar绝对量的^(40)Ar/^(36)Ar,称为初始氩比值。现代大气中的初始氩比值叫尼尔值,为295.5。K-Ar、^(40)Ar/^(39)Ar定年的表观年龄都是在假设初始氩比值与现代大气中的尼尔值一致的基础上得到的。事实证明,地球样品的初始氩比值不总是尼尔值,因此这种假定不可靠,尤其在对年轻样品进行K-Ar、^(40)Ar/^(39)Ar定年时,会带来错误的表观年龄结果。对于K-Ar、^(40)Ar/^(39)Ar精细年代学,特别对于年轻样品和低钾含量样品,只有初始氩比值才对定年研究有意义,而初始氩比值只能通过^(40)Ar/^(39)Ar等时线法求得。所以严格地讲,只有等时线年龄才是可靠的。结合实验流程,本文分析了^(40)Ar/^(39)Ar定年中为什么大多数等时线获取的初始氩比值接近尼尔值,一是因为常规^(40)Ar/^(39)Ar坪年龄的计算方法,二是因为吸附氩的干扰。初始氩比值的一致性,被用来证明样品同位素同源和封闭特性,对年龄数据的精度及可靠性有非常大的影响,尤其是年轻样品。因此,在^(40)Ar/^(39)Ar定年中要选取氩同位素初始比值一致的同源样品,并在实验流程中尽量克服样品吸附气体。本文对传统^(40)Ar/^(39)Ar方法中坪年龄、年龄坪的意义提出了质疑,对提高^(40)Ar/^(39)Ar定年的精度具有重要意义,并使之能够应用到极年轻的火山岩定年中。In K-Ar and ^40Ar/^39Ar dating,the ratio of ^40Ar/^36Ar,which is calculated for the absolute amount of 40Ar,is called initial argon isotope ratio.Initial argon isotope ratio of modern atmosphere,called Nier value,is 295.5.An important hypothesis in K-Ar and ^40Ar/^39Ar dating is that initial argon isotope ratio is consistent with Nier value.So the apparent age are calculated with Nier value.Research shows that the initial argon isotope ratio is not always equal to Nier value.Therefore,this assumption is not reliable.It even brings wrong apparent age results in K-Ar and ^40Ar/^39Ar dating for young samples.For precise K-Ar and ^40Ar/^39Ar chronology,especially for young and low potassium content samples,only the initial argon isotope ratio is significant,which is only calculated through ^40Ar/^39Ar isochron method.Strictly speaking,only the isochron age is credible.Combined with experimental process,this paper analyzes why the initial argon isotope ration gets close to Nier value in previous ^40Ar/^39Ar dating using mathematical method.One reason is the calculation of conventional ^40Ar/^39Ar plateau age method.The other reason is interference of adsorbed argon.Consistent initial argon isotope ratio can be used to demonstrate that the argon isotope was from the same source and there was no fractionation.And it also has a great influence on the accuracy and reliability of the dating results,especially for young earth samples.Thus,we should pay attention to the following two aspects in ^40Ar/^39Ar dating：Choosing samples with uniform initial argon isotope ratio and the same source,removing the adsorbed gas from samples in the experimental process.These ideas are important on improving the accuracy of ^40Ar/^39Ar dating,and have important significance on the application of young volcano ^40Ar/^39Ar dating.

关 键 词：尼尔值 初始氩比值 K-Ar定年 40Ar/39Ar定年 

分 类 号：P597[天文地球—地球化学]