用84Sr作稀释剂时样品中87Sr/86Sr比值的计算

用Rb-Sr法测定地质年龄,对地质样品中的锶往往要进行两种同位素分析:非稀释分析以测定⁸⁷Sr/⁸⁶Sr比值;稀释分析以测定样品中⁸⁶Sr的含量。随着稀释剂⁸⁴Sr丰度的提高,Wasserburg et al.(1964),Van schmus(1965),Wetherill与Bickford(1965)采用了以一代二的办法——用稀释分析既测定含量又测⁸⁷Sr/⁸⁶Sr比值。由于稀释剂中⁸⁴Sr高度富集,⁸⁷Sr/⁸⁶Sr受到的影响小,在当时的技术条件下,这些作者没有对一些数据处理进行讨论。但随着计算技术的应用,大大提高了测量精度,稀释剂对⁸⁷Sr/⁸⁷Sr的影响则必须加以考虑。

THE CALCULATION OF 87Sr/86Sr RATIO VALUE IN 84Sr-SPIKED SAMPLES

The Rb-Sr dating needs first of all a precise isotopic analysis of Sr in natural materials.Generally,the ⁸⁷Sr/⁸⁶Sr ratio value is always normalized by (⁸⁷Sr/⁸⁶Sr)_N=(⁸⁷Sr/⁸⁶Sr)_M·(⁸⁷Sr/⁸⁶Sr)_M+0.1194/2×0.1194 This formula (I),however,is not valid for the Sr-isotope spiked analysis.It requires some modifications dependent on the dilution degree.This paper shows that an infinite series analysis gives a simple formula for calculating the (⁸⁷Sr/⁸⁶Sr) ratio value in a spiked sample without the modifications of formula (I).After the geological samples are diluted by 84Sr spike,the ⁸⁷Sr/⁸⁶Sr ratio value can be normalized by the formula (I) as it does in unspiked analysis.Then the (⁸⁷Sr/⁸⁶Sr) ratio value in the sample can be calculated from the normalized ratio as follow: (⁸⁷Sr/⁸⁶Sr)_sample=(⁸⁷Sr/⁸⁶Sr)_N·1/1-δ where δ is calculated by δ=0.0033c/c'·(⁸⁴Sr/⁸⁶Sr)_M-d/c / d'/c'-(⁸⁴Sr/⁸⁶Sr)_M where c,d and c’,d’ are isotopic abundance of ⁸⁶Sr and ⁸⁴Sr in the samples and in the spike respectively,and constant 0.0033 is calculated from the isotopic abundance of the spike and the common strontium.Besides,the δcan also be given from δ-(⁸⁴Sr/⁸⁶Sr)_M plot.