【作者】 祁昌实；

【导师】 李献华；

【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， 2006， 博士

【摘要】 本文回顾了Lu-Hf同位素地球化学研究进展、报道了MC-ICPMS（多接受器电感耦合等离子体质谱）技术下的岩石与锆石（ZrSiO4）的Hf分离纯化和¹⁷⁶Hf／¹⁷⁷Hf比值测定方法、以及Lu-Hf同位素体系在华南古元古代变质岩和中生代花岗岩成因研究中的应用。具体包括：1、Lu-Hf同位素地球化学研究进展。MC-ICPMS技术的出现后，Hf同位素比值分析技术瓶颈（高电离能带来的难电离问题）被打破，Lu和Hf分离纯化流程日趋简化，样品用量显著减少。该部分综述Lu-Hf同位素体系基本地质地球化学特征、岩石矿物现有分离纯化以及测定方法、以及Lu-Hf同位素体系在地质年代学和地球化学示踪上的应用。研究表明，MC-ICPMS的出现导致了可供分析的地质样品种类显著增多且Hf分离纯化条件显著降低。Lu-Hf体系现在已逐步成为了进行定年和地球化学示踪体系的重要手段。2、报道一个改进的单柱Ln-spec提取色谱萃取分离Hf方法。用该方法可以有效地分离出岩石样品的基体元素、REE和干扰元素等，并使Hf的回收率＞90％、本底＜5×10^-11g。用该方法对一些国际和国家岩石标样进行了Hf分离，采用单聚焦和双聚焦两种型号的MC-ICPMS测定了Hf同位素组成，国际标样的结果与文献值在误差范围内完全一致，国家标样具有的Hf同位素结果具有很好的外部重现性。3、报道一个锆石Hf分离纯化方法及锆石标样的¹⁷⁶Hf／¹⁷⁷Hf比值的测定结果。Hf分离纯化采用改进的高温高压氢氟酸溶解和其后的一个离子交换分离流程实现，整个流程Hf的回收率高于90％。标准样91500、Temora和We-1¹⁷⁶Hf／¹⁷⁷Hf比值测定的平均值分别为0.282310±0.000034、0.282706±0.000020和0.281534±0.000009。上述每个结果包括至少7组独立的锆石样晶的溶解、分离和测定，表明标准样91500、Temora和We-1是Hf同位素组成均一的，可作为LAM-MC-ICPMS（激光剥蚀多接受器电感耦合等离子体质谱）的原位微区锆石¹⁷⁶Hf／¹⁷⁷Hf比值分析过程中外部校正及监控标准样。4、报道中国东南部华夏板块1.77Ga的斜长角闪岩的Lu-Hf同位素分析结果。这些斜长角闪岩有异常高的εHf（t）比值（+15.2～+23.3），且εHf（t）比值与已发表的εNd（t）比值表现出良好的正相关（r²=0.81）。这表明这些斜长角闪岩的Sm-Nd同位素体系没有受到后期变质作用影响，形成过程中受到少量地壳物质混染。混染程度最低的样品（拥有最高Nb／La比值～13）有全球同时期岩石最高的εNd（t）比值（约+8.5）和εHf（t）比值（+22），意味着在古元古晚期华夏古陆下部存在一个极度亏损地幔。该地幔可能形成于晚太古代大规模壳幔分异事件中。Lu-Hf和Sm-Nd的联合调查表明该地幔组成为含少量石榴石（～1％）的尖晶石橄榄岩。新形成岩石圈地幔在冷却过程中，位于石榴石-尖晶石相过渡带的石榴石将周围橄榄岩中的Lu运移到晶格中，导致了华夏斜长角闪岩非常高的Lu／Hf和高放射成因的Hf同位素组成。5、报道桂东南大容山-十万大山花岗岩带浦北岩体（东北带）、旧州岩体（中部带）和台马岩体（西南带）全岩的主、微量元素、Sr-Nd同位素和锆石的LAM-MC-ICPMS原位Hf同位素分析结果。岩石学及元素地球化学结果显示：上述三个岩体为典型S型花岗岩；高Ⅰ_Sr（＞0.721）和低εnd（t）（-9.9～-13.0）意味着它们可能来自古老地壳的重熔。岩浆结晶（～230 Ma）锆石的εHf（t）主要集中在-11～-9，相应T_DM2为1.9～1.8 Ga，少数结晶锆石的εHf（t）逐渐升高到-4.5，相应地T_DM2降低到1.5 Ga。捕获锆石（1681～384 Ma）的的εHf（t）分布在-17.1～+3.4，T_DM2主要集中在2.4 Ga、1.9 Ga和1.5 Ga。岩浆结晶锆石εHf（t）值与根据全岩εNd（t）和“地壳Hf-Nd相关”预测值基本一致，表明T_DM2为1.9 Ga的地壳为最重要的物源区。部分岩浆锆石与捕获锆石具有相同的T_DM2～1.5Ga，表明平均地壳存留年龄为1.5Ga的物源区参与了该花岗岩带的形成；由于缺少T_DM2＞2.0Ga的岩浆锆石，少量平均地壳存留年龄为2.4Ga的再循环地壳物质参与了该花岗岩带的形成。因为缺少显著幔源特征的高εHf（t）值锆石，本文认为地幔物质基本没有参与该S型花岗岩带的形成。更多还原

【Abstract】 This thesis reviews the progress of Lu-Hf isotopic system and its applications, and establishes a rapid chemical separation method of Hf from whole-rock samples and zircon minerals and ¹⁷⁶Hf/¹⁷⁷Hf ratios determination by MC-ICPMS. This new isotopic systems has been applied to investigations of Paleoproterozoic metamorphic rocks and Mesozoic granitoids from South China. The contents include:1. The progress of Lu-Hf isotopic study. Widespread application of Lu-Hf isotopic systems has long been hampered due to the very high ionization potential of hafnium until the advent of MC-ICP-MS technique since 1990s. Meanwhile, a number of methods of separation and purification of Hf have been developed, which significantly reduce the sample mass required for Hf isotopic analysis. This part reviews the geochemical characteristic of Lu-Hf isotopic system, and development of separation and purification methods of Lu and Hf, Hf isotope ratio determination using MC-ICPMS, and the applications of this Lu-Hf isotopic system as a new tool of geochronological dating technique and a geochemical tracer in geological studies.2. A modified single-column Ln-Spec extraction chromatography method has been developed for separation of Hf from rock samples. Using this method, hafnium can be effectively separated from the major and rare earth elements as well as the interference elements in rock samples, with recovery of >90% and whole procedure blank of <5×10¹¹ g for Hf. A number of international and national rock standards were analyzed for Hf isotopic compositions using both single- and double-focus multiple collector-inductively coupled mass spectrometers. While the analytical results for the international rock standards are identical with those reference values within analytical errors, the external precision for the national rock standards is in good agreement.3. A method that Hf in zircon was purified from other interferential elements was developed and the Hf isotope ratios were determined by using a Micromass IsoProbe MC-ICPMS for two international zircon standards 91500 and Temora and one in-house zircon standard We-1. Zircons were digested by HF in high-pressure bombs. For the reason that Zr does not affect Hf isotopic determinations in the simulative experiment, Hf was separated using a modified one-step ion-exchange chromatography. The overall recovery of Hf is >90%. Calibrated by an external standard solution JMC-475 Hf, ¹⁷⁶Hf/¹⁷⁷Hf ratios of these standard zircons were precisely determined. The average ¹⁷⁶Hf/¹⁷⁷Hf values of 91500 and Temora and we-1 were 0.282310±0.000034,0.282706±0.000020 and 0.281534±0.000009, respectively. It is concluded that the ¹⁷⁶Hf/¹⁷⁷Hf ratios of 91500, Temora and We-1 were homogeneous, because the aforementioned Hf isotopic results consist of at least seven separated performances of aliquots for each zircon sample. These results have significant implications for zircon Hf isotopic determination by using LA-MC-ICPMS.4. New Lu-Hf isotopic data were reported for the 1.77 Ga amphibolites in the Cathaysia Block of South China. These amphibolite samples have very high initial eHf（t） values of+15.2 to +23.3 and the eHf（t） values are tightly correlated with eNd（t） values. This suggests that the amphibolites were derived from a depleted mantle source with minor crustal contamination, and their Sm-Nd isotope system was not disturbed during later metamorphism. The least-contaminated samples possess exceptionally high eNd（t） value of ca.+8.5 andεHf（t） value of ca.+22, which are the highest values ever reported for the same-aged rocks worldwide, indicating there once existed a ultra-depleted mantle underlying the Cathaysia Block in the late Paleoproterozoic. This mantle source was likely formed by large scale crust/mantle differentiation in the Neoarchean. Lu-Hf and Sm-Nd isotopic investigations demonstrate that this ultra-depleted mantle is spinel peridotite with minor amount of （～1%） garnet. Garnets in spinel-garnet phase transition removed Lu from peridotites vicinity to their nucleations during cooling of the newly-formed lithospheric mantle, resulting in very high Lu/Hf ratio and strongly radiogenic Hf isotopic compositions found in the studied Cathaysian amphibolites. There existed great isotopic heterogeneity of depleted mantle in the late Paleoproterozoic.5. Detailed petrological, Sr-Nd isotope and in-situ Zircon Hf isotope data are reported for Pubei intrusive, Jiuzhou intrusive and Taima intrusive, which consist of main body of Darongshan-Shiwandashan granitoid belt. Petrological and element geochemistry show that these intrusives are typical S-type granites. High I_Sr （>0.721） and low eNd（t） （-9.9 to -13.0） indicate they originated from remelting of old crust. After corrected by their each own SHRIMP Zircon U-Pb age （Deng et al., 2004）, most of zircons which age are～230 Ma have focused eHf（t） value of -11 to -9 and the corresponding T_dm2 age between～1.9 and～1.8 Ga while the rest have higher eHf（t） value of-4.5 and corresponding to T_dm2 age of～1.5 Ga. Inherited zircons in age of 1681～384 Ma haveεHf（t） of-17.1 to +3.4 and their T_DM2 a]ge focused at～2.4 Ga,～1.9 Ga and～1.5 Ga. Considering that most～230 Ma magmatic zircons have eHf（t） values consistent with the estimated values in terms of Hf-Nd isotopic array, we interpret that the crust with average residential time of 1.9Ga is the dominant material source for the studied granites. Some magmatic and inherited zircons have nearly the same T_dm2 ages of ¹.5Ga, indicating that the crust with average residential time of 1.5Ga is also involved in formation of these granitoids. Given that no igneous zircon with T_dm2 older than 2.0Ga have been detected, minor amount of recycled crustal materials with average residential time of 2.4Ga could also be involved. Because new mantle inputs have not been identified in terms of zircon Hf isotopes, mantle-derived magmas might not have been involved in formation of this S-type granitoid belt.更多还原