基于分步热解技术的沉积岩形态汞同位素分析方法

Analysis of Specie-specific Isotopic Compositions of Mercury in Sedimentary Rocks Based on the Thermo-Desorption Technique

沉积岩中的汞及其同位素常被用于指示大规模火山活动、海洋氧化还原演变等古环境演化事件,但现有研究主要聚焦总汞含量及相应的总汞同位素组成,缺少形态汞含量及其同位素组成数据,因此无法精准判别古环境中汞的来源、沉积环境和沉积过程。本研究开发了基于热解技术的沉积岩中不同形态汞的分离和同位素分析方法。首先通过沉积岩中典型形态汞(有机质、硫化物和粘土矿物结合态汞)单一标准建立热解温度曲线,然后将上述单一标准物质混合,通过调整升温速率以及形态汞采集的温度范围和时间优化热解分离方法,收集不同温度下析出的气态单质汞进行同位素分析。结果表明,分离的有机质结合态及硫化物结合态汞的δ^(202)Hg和Δ^(199)Hg值同参考值的平均差值分别小于0.20‰和0.03‰,因此该分离方法能有效分离上述形态汞并进行汞同位素组成分析。上述方法被进一步应用于分析天然标准物质(黄红壤、水系沉积物、海相沉积岩)的汞形态及其同位素组成,验证了该方法在自然样品中的广泛适用性。

Mercury(Hg)and its isotopes in ancient sedimentary rocks are often used to rebuild the volcanic activities and the redox changes in paleoenvironment,which are linked to the evolution of life,but all current studies on Hg and its stable isotopes in sedimentary rocks are based on bulk analysis that measures the whole sample.The studies on Hg speciation in sediments are scarce and no data have been reported on the isotope compositions of different Hg forms in sedimentary rocks,which hinder the application of Hg isotopes to trace the source and transformation of Hg in the paleoenvironment.In this study,we develop a novel method for the separation and isotope analysis of different Hg forms in sedimentary rocks based on the thermo-desorption technique.Firstly,the thermo-desorption curves of typical Hg forms in sedimentary rocks(Hg bound to organic matter,sulfide and clay minerals)were determined.Then the above-mentioned single-mineral Hg compounds were physically mixed to test their separation by stepwise thermo-desorption.Based on these results,the optimal values for the heating rate,temperature range and time of collection were selected for the separation of different Hg forms in mixed Hg compounds.The average difference in δ^(202)Hg and Δ^(199)Hg between each Hg form and their origin reference values was less than 0.20‰and 0.03‰,respectively,suggesting that this method can retain the original isotopic composition of the above Hg forms.Finally,this method was applied to determine the Hg forms and their isotope compositions in various reference materials(yellow red soil,stream sediment,marine sedimentary rock),which further validates the applicability of this method in actual samples.