【作者】 李杰；

【导师】 许继峰； 梁细荣； 徐义刚；

【作者基本信息】 中国科学院研究生院（广州地球化学研究所） ， 地球化学， 2007， 博士

【摘要】 本文简要介绍了Re-Os同位素体系及其地球化学研究进展，回顾和评述了Re-Os同位素分析测试技术，包括化学制备和质谱测定方法；报道了我们在本实验室建立的利用多接收器电感耦合等离子体质谱（MC-ICPMS）测定天然岩石样品中的Re和Os含量以及¹⁸⁷Os／¹⁸⁸O同位素比值的方法；并将Re-Os同位素体系应用在峨眉山木里地区二叠纪苦橄岩研究中，以对地幔柱的源区物质组成进行约束。本论文主要得到以下几方面的认识和进展：1、Re-Os同位素地球化学研究进展。随着N-TIMS质谱测试技术的不断发展和MC-ICPMS在Re-Os同位素分析测试中的应用，以及化学分离和纯化方法的改进，极大的提高了Re-Os同位素测试数据的精确度。由于该同位素体系具有不同于其它常用同位素体系（如Sm-Nd、Rb-Sr和U-Th-Pb）的特性，同益受到地球化学研究者的关注，并逐步成为进行定年和地球化学示踪的重要工具。多接收器电感耦合等离子体质谱（MC-ICPMS）开始应用到Re-Os同位素测定方面，并显示出巨大的发展潜力。2、建立了Re-Os同位素样品的化学分离和纯化方法以及MC-ICPMS质谱测定方法：岩石样品的Re和Os化学分离方法是采用Carius管溶样法，结合四氯化碳萃取以及氢溴酸反萃取的方法分离出Os，最后是通过微蒸馏的方法纯化Os；利用阴离子交换树脂的方法分离纯化Re。Re的MC-ICPMS测定方法，采用膜除溶雾化器（Aridus）作为进样系统，使样品以“干”的方式进入等离子体，仪器测试灵敏度提高了15-20倍，成功的解决了低含量Re样品测试精度低的问题；在被测的Re样品中加入适量的Ir标准溶液，进行在线校正仪器的质量分馏，解决了采用常规N-TIMS测定方法无法进行内部质量分馏校正的问题，使测试准确度和灵敏度提高了一个数量级；单柱分离后的Re样品可以直接进行质谱测定无须过二次柱，简化了化学分离流程。Os的MC-ICPMS测定方法，采用常规进样方式，利用5％的盐酸和乙醇的混合溶液作为样品溶液和进样系统的清洗溶液，成功地解决了Os的“记忆效应”问题；同时采用多离子计数器静态同时接收测定Os同位素，大大提高了仪器的灵敏度和测试结果的精确度。利用该方法测定了峨眉山苦橄岩Re和Os含量以及¹⁸⁷Os／¹⁸⁸Os同位素比值。同时也在日本国家海洋科学与技术研究中心（JAMSTEC）利用N-TIMS进行了测定，两种方法得到的结果在误差范围内一致。3、报道了峨眉山木里地区二叠纪苦橄岩中的Sr、Nd和Re-Os同位素分析结果。木里地区苦橄岩具有低的Re含量和高的Os含量，Re和Os的含量受到熔融程度和结晶分异的控制，低的Re和Os的含量和Re／Os，以及高的Os含量支持高程度的地幔熔融导致熔体中的硫不饱和，同时源区可能有石榴子石残留。根据Ti／Y和γOs值，我们将木里苦橄岩分为两组，一组为低Ti／Y型苦橄岩，这组苦橄岩以低的Ti／Y（小于520）、γOs（-4.1～-0.2）以及εNd_（t）（+3.2～+6.6）为主要特征：第二组为高Ti／Y型苦橄岩，这组苦橄岩以高的Ti／Y（大于520）、γOs（+7.6～+11.5）以及εNd_（t）（+5.9～-+7.7）为主要特征。木里高Ti／Y型苦橄岩具有高的γOs值以及εNd_（t）值，与全球地幔柱成因的岩浆岩石（OIB）的初始Os同位素比值（γOs=+5～+10）一致，表明它们形成于地幔柱源区，而且它们几乎没有受到地壳或是陆下岩石圈地幔（SCLM）的混染。木里低Ti／Y型苦橄岩具有低的γOs（-4.1～-0.2）和高εNd_（t）（+3.2～+6.6），它们也未受到地壳的混染，Nd-Os同位素二元混合模型表明它们可能是由地幔柱起源的苦橄质原始岩浆混合了15％或更多的SCLM物质形成的；与其共生的低钛玄武岩具有低的γOs（＜0）和低的εNd_（t）值，它们可能是由地幔柱起源的玄武质原始岩浆混合了15％或更多的SCLM的熔融物质形成的。更多还原

【Abstract】 This Ph.D thesis firstly reviews the progresses of Re-Os isotopic system in past years and its applications on geochemical field, and summarizes the techniques used for Re-Os analysis including chemical separation procedures and mass spectrometer measurements. Then the thesis reports a new analytical method of Re-Os concentration and ¹⁸⁷Os/¹⁸⁸Os ratio in a natural rock sample by multi-collector inductively coupled plasma mass spectrometry. Moreover, this Re-Os isotopic measurement technique has been used to study the Permian picrites from Emeishan LIPs. The main conclusions of this thesis are as following:1. The progresses of Re-Os isotopic system in instrumental measurement and chemical separation method. With the development of N-TIMS and application of MC-ICPMS techniques, both TIMS and MC-ICPMS can provide higher and higher sensitivity and precision in Re-Os measurements at present. On the other hand, Re-Os isotopic system has unique geochemical behavior, i.e., Os is preferentially retained in a mantle mineral facies during a partial melting process in contrast to all other radiogenic isotopic systems in common use （e.g. Rb-Sr, Sm-Nd, Lu-Hf, U-Th-Pb） where both parent and daughter elements are strongly concentrated in a melt. As a result, the Re-Os isotopic system has been recognized of particularly useful in tracing the geochemical evolution of mantle rocks and in defining the chronology of mantle differentiation. In addition, our works also indicate that MC-ICPMS technique shows a good potential on Re-Os isotopic measurement.2. A new method for the determination of low Re-Os abundances and ¹⁸⁷Os/¹⁸⁸Os in a natural rock sample by MC-ICP-MS has been established in our laboratory. Rock samples were digested in inverse aqua regia in sealed Carius tube for 24 hours at 230°C, then Os was separated from the matrix by CCl₄ solvent extraction, and further purification by microdistillation. For Re chemical procedure, Re was separated through anion exchange chromatography. Re separation with single ion exchange column appears to suffice for measurement therefore it is not necessary for the twice separation, which shows relatively less preparative chemistry to be required. During analyzing Re by MC-ICPMS, a sample solution is introduced to the plasma through an Aridus desolvating introduction system. This system can remove water and acid in the sample solutions by passing them through a desolating membrane in 160°C, thus providing a "dry" sample to the plasma, increasing ionization efficiency by a factor of 15～20 times over conventional sample introduction system. Therefore a Re sample with very low concentration can be measured; meanwhile element Ir is added into Re sample solutions for in-run Re isotopes mass fractionation correction. For Os measurement by MC-ICPMS, an Os sample solution is introduced into ICP through a conventional introduction system, Os memory effect may be eliminated by using 5% HCl-EtOH mixture solution and 5% HC1 to clean inlet system alternately. ¹⁸⁷Os, ¹⁸⁸Os, ¹⁹⁰Os and ¹⁹²Os ions were detected simultaneously by four ion-counters in order to increase the sensitivity and precision. Using this Re-Os measurement method, Re-Os abundances and ¹⁸⁷Os/¹⁸⁸Os of the picrite samples from Emeishan have been measured. The results were consistent with those measured by N-TIMS at JAMSTEC Os isotopic laboratory in Japan.3. Re-Os and Sr, Nd isotopic data have been reported for Permian picrites which were collected from Muli area in Emeishan LIPs. The picrites have low Re and high Os concentrations, which are probably controlled by the degree of partial melting and crystallization fractionation. The low Re ,Yb and Re/Os ratios of the picritic lavas support models of high degree partial melting with residual garnet in their mantle source during the early stages of partial melting, and without residual sulphide. Based on Ti/Y and Os isotope data, the picrites in Muli area can be classified into two subgroups: （1） a low Ti/Y type that exhibits low Ti/Y （<520）、γOs （-4.1～-0.2） andεNd_t （+3.2～+6.6） ;（2） a high Ti/Y type that has Ti/Y （>520）、γOs （+7.6～+11.5） andεNd_t （+5.9～+7.7） . The initial Os and Nd isotopes of high-Ti/Y type picrites are consistent with those of magmatic rocks related to plume, indicating that they were derived from a mantle plume and experienced little contamination with a continental crust and SCLM, therefore, most probably representing Re-Os composition of a primary melt. On the other hand, the low-Ti/Y type picrites and low-Ti basalts were also probably derived from the same plume source, but magmas of the low-Ti/Y type picrites most likely experienced a mixture with more than 15% bulk SCLM material; and magams of the low-Ti basalts were probably contaminated with over 15% melt from SCLM.更多还原