~4He同位素含量测试技术研究

Study on the ~4He Content Measurement

4He同位素含量的准确测定是(U—Th)/He同位素定年方法的关键,因为4He同位素是气体,所以其含量测定难度是比较大的,国内其他实验室还没有建立4He同位素含量的测定方法。本文详细介绍了国外(U—Th)/He同位素定年实验室4He同位素含量的测试技术和本实验室基于现有仪器条件建立的4He同位素含量的测试方法。笔者等利用了仪器原来配置的体积为0.1mL的校准小管中的空气,实现了对样品中4He同位素含量的准确校准。为了验证方法的准确性,笔者等用国外Durango磷灰石标样对测试方法进行了验证,得到3份Durango磷灰石标样的4He同位素含量的平均值是1.2472×10-8mol/g,与Sole(2005)得到的该磷灰石的4He同位素含量一致,为了更进一步验证方法的准确性,笔者等还对该磷灰石中的238U、232Th和147Sm也进行了测定,得到该磷灰石标样的(U—Th—Sm)/He同位素年龄为30.1Ma,与McDowell(2005)获得的该磷灰石的(U—Th—Sm)/He年龄31.02±1.01Ma(1σ)在误差范围内一致,说明本项研究采用的4He同位素含量的测试方法是可行的。

How to analyze 4He content in minerals accurately is the key to the （U—Th）/He dating. Because 4He is gas,it is relatively hard to measure its content.There are no lab except ours in China can analyze the 4He content now. This paper introduces the 4He content analyzing method in foreign （U—Th）/He lab and the method our lab uses based on the exiting noble gas Mass Spectrometer. We use the 0.1mL air in the pipette equipped with the original instrument to calibrate the 4He content in the sample. In order to prove our method is accurate,we use this method to measure the 4He content of Durango apatite which is used as analytical standard in （U—Th）/He lab. The mean 4He content of three Durango apatite aliquots is 1.2472×10-8mol/g which is in agreement with that of Sole （2005）.We also measured the 238U、232Th and 147Sm in the Durango apatite and get its（U—Th—Sm）/He age of 30.1Ma which is within error of 31.02±1.01 Ma（1σ）obtained by McDowell（2005）.The above results show our method of measuring 4He content is feasible.