La-Ce Isotopic System: Application Status and Advances
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摘要: 铈在自然界中属变价元素, 可导致地质样品La-Ce与Sm-Nd同位素体系之间发生脱偶现象.除了在同位素年代学中的应用外, 放射成因Ce同位素地球化学在地壳物质再循环示踪、地球表层演化过程中的氧化—还原事件和成矿作用等研究领域具有独特优势.总结了La-Ce同位素体系目前的应用现状及其尚存在的问题, 并对实验技术与应用的发展趋势进行了分析.介绍了采用无机酸进行La-Ce同位素样品化学制备的实验流程及应用该流程对部分国际标样和扬子克拉通核部辉长岩样品进行Ce同位素分析的结果.
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关键词:
- La-Ce同位素体系 /
- 新化学流程 /
- USGS标样 /
- 地球化学
Abstract: Element Ce shows variable valences in the nature, which accounts for the decoupling between La-Ce and Sm-Nd isotopic system. In addition to a radiometric application, the La-Ce system thus sheds new light on a set of studies such as crust recycling in mantle components, oxidation events during outer sphere evolution of the Earth and metallogenesis. This paper consists of two parts. The first part summarizes the application status of the La-Ce system, its analytical problem faced as well as developing trends in experimental technique and geological application. The second half describes a new chemical procedure for La-Ce separation and purification. Analytical results for some USGS standards and some gabbroic rocks from the Neoproterozoic Huangling complex, interior of the Yangtze craton, are also reported.-
Key words:
- La-Ce isotopic system /
- new chemical procedure /
- USGS standards /
- geochemistry
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图 1 不同LREE组成条件下的εCe、εNd演化趋势示意(箭头示随时间演化方向)(据Hayashi et al., 2004)
Fig. 1. schematic diagram for explaining the relationship between LREE patterns of rocks derived from CHUR and the direction of their isotopic growth of Ce and Nd isotope ratio in εCe and εNd values with time
图 2 MORB、OIB、IAB和大陆地壳的εCe-εNd关系(据Dickin, 1987; Tanaka et al., 1988; Makishima and Masuda, 1994; Lee et al., 2001)
Fig. 2. εCe-εNd correlation plot for MORB, OIB, IAB and continental rocks
图 3 不同稀土元素组成岩石的Ce、Nd同位素理论增长线(Tanaka et al., 1987)
Fig. 3. Theoretically calculated variation of 138Ce/142Ce and 143Nd/144Nd values for the rocks with various REE patterns
图 4 西格林兰太古宙黑云母片麻岩和花岗质片麻岩的εNd-εCe组成(Lee et al., 2001)(圆圈表示黑云母片麻岩, 方块表示花岗质片麻岩, 空心符号表示ε(0), 实心符号表示ε(t))
Fig. 4. εNd-εCe diagram of the present (ε (0) open symbols) and initial isotopic compositions (ε(t), solid symbols) for the Archean biotite (circle) and granitic gneisses (rectangle) in the western Greenland
图 5 仪器标样JMC304的138Ce/142Ce比值测量值
Fig. 5. Measured 138Ce/142Ce ratios of JMC304 by this study
图 7 黄陵杂岩体辉长-闪长岩球粒陨石标准化REE配分模式
Fig. 7. Chondrite normalized REE patterns for the Huangling gabbroic suite
图 8 黄陵杂岩体辉长-闪长岩样品εCe-εNd演化趋势
空心符号表示ε(0)；实心符号表示ε(t)
Fig. 8. εCe-εNdcorrelation plot for the Huangling gabbroic suite
表 1 138La衰变常数
Table 1. Documented 138La decay constants
参考文献 分析方法 λβ(10-12 a-1) λEC(10-12 a-1) Sato and Hirose, 1981 γ射线计数法 2.29 4.44 Tanaka and Masuda, 1982 文献加权平均 2.35 3.98 Norman and Nelson, 1983a γ射线计数法 2.17 4.39 Norman and Nelson, 1983b 文献加权平均 2.24 4.41 Tanaka et al., 1987 Sm-Nd对比法 2.58 4.59 Tanaka et al., 1988 Sm-Nd对比法 2.68 — Masuda et al., 1988 Sm-Nd对比法 2.77 4.44 Makishima and Masuda, 1993 Sm-Nd、Rb-Sr对比法 2.33 4.42 Tanimizu and Tanaka, 2002 Sm-Nd对比法 2.32 4.41 
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表 2 USGS标样BCR-2、AGV-2、BHVO-2和La-Ce法国家标样(EQB)的Ce同位素比值测量值
Table 2. Measured Ce isotopic ratios of BCR-2, BHVO-2, AGV-2 and Emei basalt standards (EQB)
样号 测量次数 138Ce/142Ce 误差(2σ) 测量平均值(138Ce/142Ce, 2σ) 1 0.022 582 2 0.000 001 1 2 0.022 578 4 0.000 001 8 BCR-2 3 0.022 580 0 0.000 002 0 0.022 580 4±0.000 001 5 4 0.022 582 4 0.000 001 3 5 0.022 579 1 0.000 001 3 1 0.022 582 9 0.000 002 0 AGV-2 2 0.022 581 7 0.000 003 3 0.022 584 2±0.000 002 4 3 0.022 588 1 0.000 002 0 BHVO-2 1 0.022 576 1 0.000 000 9 0.022 578 1±0.000 001 1 2 0.022 576 8 0.000 000 6 3 0.022 576 7 0.000 001 4 4 0.022 578 9 0.000 001 1 5 0.022 579 1 0.000 000 9 6 0.022 581 9 0.000 001 4 7 0.022 578 1 0.000 001 2 8 0.022 577 0 0.000 001 0 EQB 1 0.022 582 1 0.000 001 2 0.022 574 3±0.000 001 0 2 0.022 574 5 0.000 001 0 3 0.022 570 7 0.000 001 1 4 0.022 569 4 0.000 000 9 5 0.022 574 2 0.000 001 3 6 0.022 576 0 0.000 001 0 7 0.022 576 2 0.000 001 1 8 0.022 573 6 0.000 000 7 9 0.022 574 2 0.000 000 7 10 0.022 572 5 0.000 000 8 注：Ce同位素比值在中国地质大学(武汉)国家重点实验室用Triton测定.
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表 3 黄陵杂岩体辉长-闪长岩Ce、Nd同位素组成
Table 3. Ce and Nd isotopic compositions for the Huangling gabbroic suite
序号 样号 岩性 年龄(Ga) 143Nd/144Nd 2σ(10-7) 138Ce/142Ce 2σ(10-7) εNd(0) εNd(t) εCe(0) εCe(t) Ce/CeNd* 1 ZG07-2 闪长岩 0.82 0.512 649 11 0.022 578 0 47 +0.21 +3.85 +2.56 +0.28 0.68 2 ZG07-3 辉长岩 0.82 0.512 467 11 0.022 572 2 9 -3.34 +4.07 +0.02 -0.10 0.97 3 ZG07-24-3 闪长岩 0.82 0.512 335 2 0.022 575 9 11 -5.91 +2.15 +1.64 -0.04 0.76 4 ZG07-25-1 辉长岩 0.82 0.512 949 10 0.022 569 7 47 +6.07 +6.57 -1.11 -6.89 0.43 5 ZG07-27 辉长岩 0.82 0.513 059 13 0.022 592 0 31 +8.21 +7.75 +8.77 +7.58 0.76 6 ZG07-28-2 辉长岩 0.82 0.512 548 2 0.022 576 4 23 -1.76 +4.02 +1.88 -1.29 0.61 7 ZG07-34 辉长岩 0.82 0.512 922 2 0.022 572 8 36 +5.54 +5.47 +0.25 -1.81 0.68 8 ZG07-35 辉长岩 0.82 0.512 992 2 0.022 573 4 13 +6.91 +5.68 +0.55 -6.35 0.39 注：Ce/CeNd*=w(Ce)N/[(1/3)×w(La)N +(2/3)×w(Nd)N]；Nd、Ce同位素比值在中国地质大学(武汉)国家重点实验室用Triton测定.
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