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| 陨石汞的预富集方法与同位素组成研究 | |
| 其他题名 | mercury preconcentration method and isotope composition of meteorites |
| 刘德泽 | |
| 出版年 | 2018 |
| 学位类型 | 硕士 |
| 导师 | 赵哈林 |
| 学位授予单位 | 中国科学院大学 |
| 中文摘要 | 汞同位素作为一种新兴的非传统稳定同位素体系近年来已逐渐受到国内外学者的重视。相对于其他金属稳定同位素体系，汞同位素具有多项独特性质，如金属元素中最低的50%冷凝温度（252K），已知的唯一具有非质量分馏的重金属稳定同位素，这些特征决定了对汞同位素的研究可以对认识早期太阳系挥发分丢失与分布、类地行星与小行星内部的热演化等科学问题起到重要帮助。然而，部分实验方法、样品选择问题困扰着陨石汞同位素的研究者，如低汞陨石难以测定同位素组成、发现型陨石汞同位素研究的可行性等，这些问题的解决将有助于拓宽用于陨石汞同位素研究的可选样品范围。铁陨石是了解天体内部演化的重要研究对象，其汞同位素组成对认识母体内部的演化过程具有重要的指示意义，为此我们测定了铁陨石的汞同位素组成，报道了其汞同位素组成存在异常的现象并进行了讨论。了解陨石汞的赋存相对认识汞在母体的迁移行为具有重要意义，因此我们以CV陨石Allende为研究对象，探讨了其中汞的赋存相形式以及汞在母体中的迁移。本论文主要完成以下工作：（1）采用高温释放—回收预富集方法对陨石样品进行预富集，在参考前人方法基础上制定了适用于陨石样品的参数。该方法通过两阶段马弗炉的阶段加热提取出陨石中的汞，并使其富集于后续吸收装置中。高温释放—回收预富集方法可以克服消解法无法有效富集低汞陨石样品的问题，拓宽了可用于汞同位素测定的陨石样品范围，并降低来自铂和铅等元素对汞同位素的信号干扰。在此方法为基础上，我们讨论了样品粒度对回收率的影响，并探讨了陨石汞在较低加热温度下（＜90℃）的释放情况。（2）评估了发现型陨石汞同位素研究的可行性。分别测定了11块采集自南极格罗夫山的球粒陨石样品与11块沙漠球粒陨石样品的总汞含量与汞同位素组成，通过将南极陨石，沙漠陨石与降落型陨石汞同位素组成的比较研究，我们发现南极陨石和沙漠陨石的同位素组成与降落型陨石相比更为偏重，而这种差异极有可能来自于风化污染。南极陨石、沙漠陨石的Δ199Hg也与降落型陨石不同，降落型球粒陨石的Δ199Hg显示出偏正的特征，而南极陨石和沙漠陨石则明显偏负，且Δ199Hg/Δ201Hg的比值也与降落型陨石不同：降落型陨石Δ199Hg/Δ201Hg的比值接近1.3，而南极的Δ199Hg/Δ201Hg的比值接近1.0，沙漠陨石的线性关系较差，但趋势也与降落型陨石不同。这些差异表明南极陨石与沙漠陨石受到了后期风化污染的影响。因此，我们认为南极陨石和沙漠陨石等发现型陨石不是进行地外物质汞同位素研究的最佳研究对象。（3）发现铁陨石的汞同位素异常。部分铁陨石与橄榄陨铁样品具有明显不同于降落型陨石、发现型陨石甚至常见自然样品的汞同位素组成，具体表现为：汞同位素组成较轻（δ202Hg＜-10‰），且Δ201Hg极负（＜-9‰）。异常的汞同位素组成难以由核体积效应，磁同位素效应解释。考虑到铁陨石具有长期的宇宙射线暴露年龄和较高的金，铂含量，我们认为是宇宙射线诱导下发生的核反应生成了宇宙成因汞，造成了198Hg过量，从而出现了铁陨石异常的汞同位素组成。（4）讨论了汞在Allende中的载体相，并探讨了汞在母体内部的迁移行为。通过将Allende与吸附汞以及常见汞化合物的热释放剖面的对比，我们认为Allende陨石汞的主要赋存相为吸附汞（200℃释放），且具有的HgS（HgSe）形式的赋存相（250~350℃释放）。在此基础上，结合前人对Allende陨石母体的热演化模型，我们讨论了汞在Allende母体中的迁移机制。 |
| 英文摘要 | As a kind of non-traditional stable isotope, mercury isotope has been an important tool to be used in earth and planetary sciences. Considering the unique characteristics of mercury isotope, for example, the lowest 50% condensation temperature among all metal stable isotopes, significant mass-dependent fractionation and mass-independent fractionation, mercury isotope has been a useful and necessary tool which can be used to solve some important problems in cosmochemistry and planetary sciences, such as the volatiles depletion during the early solar system and the thermal evolution of terrestrial planets and asteroids. However, some problems including experimental methods and sample selection should be solved before the investigation of mercury isotope in extraterrestrial materials. The first problem is the pre-concentration method which can pre-concentrate mercury from meteorites with low content. The second problem is the feasibility that meteorite finds (Antarctic meteorites and hot desert meteorites) have the potential to research mercury isotope. Iron meteorites are the fragments of Fe-Ni core of planetesimals, the investigation of mercury isotope of iron meteorites will be helpful to know the evolution of planetesimals and terrestrial planets. The carreier phase of mercury in meteorites is another question which need to be focused, the investigation of the carrier phase of mercury will give insight to the migration path and mechanisms of mercury in their parent bodies. This thesis will focus the problems mentioned above. The main conclusion was summarized as follows:(1) Traditional digestion methods are unavailable for pre-concentrating mercury from meteorites with low content. To solve this problem we establish a high-temperature release–recovery method for pre-concentrating mercury from meteorites with two-stage tube furnaces. The method can be used to determine the isotope composition of meteorites with the limited content of mercury (i.e.,<50 ng/g). Also, this new method can avoid the problem coming from the isotopic measurement. The recovery rate reaches 96.9±6.6 % with the method, which meets the requirement of isotope measurement.(2)We measured the mercury isotope composition of 11 Antarctic meteorites collected from Grove Mountains and 11 meteorite samples from hot desert. Comparing the mercury isotope composition of the Antarctic meteorites, hot desert meteorites with observed fall meteorites and polar environmental materials, we found both Antarctic and hot desert meteorites are with negative Δ199Hg, while observed fall meteorites are with positive Δ199Hg; The Δ199Hg/Δ201Hg ratio among meteorite finds and falls are totally different: The ratio of falls is nearly 1.3, and the ratio of Antarctic meteorites is nearly 1.0. It indicates that both the mercury isotope composition in the Antarctic and hot desert meteorites have been changed by terrestrial weathering. Their mercury isotope composition can’t represent their primitive records.(3)We report the anomalous mercury isotope composition in iron meteorites and pallasite. The mercury isotope composition is different from meteorites falls and finds, even most natural samples (snow, soil, coals, etc.). The main characteristics including extremely negative δ202Hg, significant mass-independent fractionation including odd-number and even-number mercury isotope. Such mercury isotope composition can’t be caused by nuclear volume effect, magnetic isotope effect and photochemistry self-shielding. We suggest that this anomalous mercury isotope composition is the result of galactic cosmic ray-induced nuclear reactions (neutron capture and decay). This process produces excessive 198Hg which cause extremely negative δ202Hg, Δ199Hg, and Δ200Hg. (4)We measured the thermal release profile of Allende (CV) and the mercury isotope composition at different temperature (from 100 to 1100℃). We found that adsorbed mercury and HgS (HgSe) are the main carrier phase in Allende. We suggest that the adsorbed mercury comes from their parent body rather than atmospheric mercury. We proposed a hypothesis of the transport mechanism of mercury in the parent body of Allende. |
| 中文关键词 | 陨石 ; 汞同位素 ; 风化作用 ; 赋存相 |
| 英文关键词 | meteorites mercury isotope terrestrial weathering carrier phase |
| 语种 | 中文 |
| 国家 | 中国 |
| 来源学科分类 | 地球化学 |
| 来源机构 | 中国科学院地球化学研究所 |
| 资源类型 | 学位论文 |
| 条目标识符 | http://119.78.100.177/qdio/handle/2XILL650/288088 |
| 推荐引用方式 GB/T 7714 | 刘德泽. 陨石汞的预富集方法与同位素组成研究[D]. 中国科学院大学,2018. |
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