| 摘要: |
| 电子自旋共振（ESR）法是一种用于直接测量和研究含有未成对电子的顺磁性物质的分析方法。ESR测年是应用这种分析方法对沉积物最后一次埋藏以来的年代进行测定。相比于其他方法，它具有测年范围广，测试的样品种类多，样品可以重复测量，测量周期短等优势。20世纪80年代以来我国开始进行沉积物ESR测年与研究，但是在冰碛物ESR测年的制样和测量过程中存在一些问题。本文介绍冰碛物ESR测年的过程：样品野外采集时剖面的选取及其注意事项；详细描述提纯冰碛物中石英颗粒的前处理过程；石英颗粒经过人工辐照后在顺磁波谱仪上测试ESR信号并拟合古剂量过程中的具体方法和需要注意的问题。从实验中得到，用0.063 —0.125 mm粒径的组分提纯石英砂比较困难，前处理过程中试剂和时间消耗量大，因此建议ESR测年时：在样品充足的情况下选用细颗粒组分（0.063—0.125 mm），样品不充足的情况下选用中颗粒部分（0.125—0.25 mm）。待测样品中存在其他矿物会对石英的ESR信号有影响，因此在提纯石英过程中用重液分选提高石英纯度并可以增强测量的ESR信号的稳定性。经过氢氟酸刻蚀后的样品，石英的纯度要达到95%—99%或以上。对于冰碛物ESR测年过程中ESR信号的选择需要参照样品自身的特点，综合考虑多次尝试才能确定用常温Ge心信号还是低温的Al心、Ti心作为样品中石英的ESR测年信号。通过规范的采样、制样和ESR信号测量流程，使样品的定年能够被不同的人重复，并能够进行不同区域之间的对比。 |
| 关键词: ESR测年技术 实验流程 石英矿物 冰碛物 |
| DOI：10.7515/JEE182055 |
| CSTR：32259.14.JEE182055 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41230523）；中国科学院战略性先导科技专项（XDA20070102） |
| 英文基金项目:National Natural Science Foundation of China (41230523); Strategic Priority Research Program of Chinese Academy of Sciences (XDA20070102) |
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| Basic procedures of electron spin resonance (ESR) signal measurement for glacial tills |
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BI Weili, YANG Haijun, YI Chaolu
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1. Institute of Tibetan Plateau Research, Chinses Academy of Sciences, Beijing 100101, China
2. Department of Chemistry, Tsinghua University, Beijing 100084, China
3. CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China
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| Abstract: |
| Background, aim, and scope Electron spin resonance (ESR) as a microwave technology, is an analytical method to detect and research the paramagnetic substance with unpaired electrons. ESR dating is an application of this technique to date sediments since their last burial. Compared to other dating methods, ESR dating has advantages in large time span, different kinds of samples, repeated measurements and the short measurement period. Since 1980s ESR dating has been applied to sediments, however, there exist some problems in sample preparation and ESR signal measurement. The purpose of the paper is to introduce the experimental procedures for those who are interested in ESR dating of glacial till. Materials and methods Sample collection: the quartz was one of the best minerals for detecting ESR signals. In the field, we selected the moraine profiles that had rich in quartz and were not retrofitted. It was better to collect silt and fine sand samples through subglacial grinding or being exposed to the sun. The samples must avoid to the direct sunshine from the field to the laboratory. We gathered samples 2—3 kg and made detailed records and environment descriptions. And we also collect sample using sealed aluminum boxes for water content. If there was not gamma spectrometer for measuring environmental dose rate in situ, the contents of uranium, thorium, potassium from the samples were measured using neutron activation analysis (NAA). The rigorous sampling procedure and detailed field records were of benefit to analysis and comparison of the data. Extraction and purification of quartz from the sediment: we sieved out 0.063—0.25 mm of purified quartz grains from samples for ESR dating. At first, we removed carbonate with hydrochloric acid and organic matter with hydrogen peroxide. Next, we eliminated magnetic mineral with permanent magnet. Then, we separated minerals of different specific gravities with the heavy liquid. Heavy mineral, quartz, feldspar and mica were separated and pure quartz was obtained. Lastly, the residual feldspar and quartz surface damaged by alpha-ray was etched by hydrofluoric acid. After that, we removed fluoride with 10% hydrochloric acid, cleaned and dried the samples in an oven at 40℃. Through hydrofluoric acid etching, the purity of quartz, which is checked under the binocular microscope, should reach at least 95%, and 99% or higher would be better. The purification of quartz could exclude the effects of other minerals for accurate and repeatable measurements of ESR signals. Additional irradiation of purified quartz samples: we divided the quartz into aliquots with 250 mg or 300 mg. One aliquot was not irradiated and others were irradiated at different doses using 60Co gamma-ray source. There were two methods to erasure short life signal. One way is to heat samples at a certain temperature. Another is to let samples sit for 7—10 days or more. In the experiment, the corresponding doses were set according to the response of sample’s ESR signal to additional irradiation. Measurement of ESR signal: the ESR signals for dating glacial tills could be chosen germanium (Ge) center measured at room temperature or aluminum (Al) center and titanium (Ti) center at low temperature. The area under the absorption curve represents the peak intensity of ESR signals. The peak to peak height of the signal differential could be instead with peaks of the same line and width. To prevent the difference among the sample separations, the position of samples and the centre of the resonant cavity should be coincide. Results The results showed that: (1) The grain-size fraction of 0.125—0.25 mm should be selected for quartz purification. (2) The process of heavy liquid separation could improve sample purity effectively and reduce signal interference from other minerals. (3) The choice of different ESR signal centers could result in the dating difference. Discussion The grain size of samples had an impact on ESR signals. Fine grains would be better for dating glacial tills because ESR signal of quartz in them is more likely reset. However, it is difficult to obtain enough purified quartz from the grain-size fraction of 0.063—0.125 mm extracted from glacial tills. Therefore, we suggest that the grain-size fraction of 0.125—0.25 mm is selected for quartz purification. The sample purity also affects ESR signals. For poorly sorted glacial tills, the process of heavy liquid separation could improve sample purity effectively and reduce signal interference from other minerals. The choice of different ESR signal centers could result in the dating difference. The spectral line widths might increase at room temperature and the resolution might reduce. The small power was needed at low temperature. Conclusions The accurate and stable signals required standard experimental procedures and better signal selection. Recommendations and perspectives Establishment of standard procedures in sampling, sieving, extraction of quartz, quartz purification, additional irradiation and measurement of ESR signal can make the data be duplicated in different laboratories so as to make sure that the dating data can be compared with each other. |
| Key words: the technique of ESR dating experimental procedures quartz glacial tills |
