High-Precision Pb Isotope Ratio Determination of Zircon By Multi-Ion Counter TIMS with Multi-Dynamic Collection Method
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摘要: 采用装配多离子计数器系统的TRITON Plus热电离质谱仪（thermal ionization mass spectrometer，TIMS），建立了多离子计数器动态多接收锆石Pb同位素（以205Pb为稀释剂）测定方法.相对多离子计数器静态多接收方法，该方法完全消除了不同离子计数器间增益差异对锆石Pb同位素测定的影响.相对传统的单个二次电子倍增器（secondary electron multiplier，SEM）五次跳峰的锆石Pb同位素测定方法，该方法两次跳峰即可测定全部Pb同位素比值，Pb同位素离子流接收效率提高2.5倍，同时，降低了离子流稳定性对Pb同位素分析结果的影响.为验证方法的可靠性，对加入205Pb稀释剂的NIST981 Pb标准和标准锆石清湖（Qinghu）进行了测定.对5×10-11 g 205Pb-NIST981 Pb混标，207Pb/206Pb测定精度达到0.079%（2RSD，n=20）；对清湖标准锆石，获得的年龄结果为159.51±0.11 Ma（2SE，n=7；MSWD=1.1），与文献报道值在误差范围内一致.Abstract: A high precision zircon Pb isotope ratio measurement method (using 205Pb as spike) was developed with a TRITON Plus thermal ionization mass spectrometer (TIMS) equipped with a multiple ion-counting (MIC) system. With the method, the Pb isotopes were measured with multi-ion counters using a multi-dynamic ion collection method. Compared to the traditional static multi-collection MIC method, the multi-dynamic MIC method can completely eliminate the influence of gain bias between different ion counters on the Pb isotope analytical results. Compared to the traditional zircon ID-TIMS Pb isotopic measurement method using a single secondary electron multiplier (SEM) with five peak jumps, the multi-dynamic MIC method can determine all the Pb isotope ratios of zircon in two peak jumps. Thus, the ion collection efficiency for the Pb isotopes were improved by 2.5 folds. In the meantime, the influences of ion beam stability on the analytical results of Pb isotopes was also reduced. To verify the reliability of the method, NIST981 Pb standards doped with 205Pb spike and Qinghu standard zircons were measured. The analytical precision of 207Pb/206Pb for 5×10-11 g of 205Pb-NIST981 Pb mixture can achieve 0.079% (2RSD, n=20). The analytical 206Pb/238U age result for Qinghu standard zircon was 159.51±0.11 Ma(2SE, n=7; MSWD=1.1), which was consistent with the previously reported values within analytical error.
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Key words:
- ID-TIMS U-Pb /
- multiple ion-counters(MIC) /
- zircon /
- Pb isotope analysis /
- 205Pb spike /
- geochemistry
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图 1 TRITON PLUS热电离质谱仪多离子计数器系统配置示意
Fig. 1. Schematic diagram of multi-ion counter array for TRITON PLUS TIMS
图 2 TRITON PLUS离子计数器系统不同测定方式NIST981 207Pb/206Pb测定结果对比
*图中实线代表平均值, 虚线代表平均值±2SD的范围
Fig. 2. Comparison of analytical results for 207Pb/206Pb of NIST981 with different measurement methods using TRITON PLUS TIMS MIC system
表 1 TRITON PLUS热电离质谱仪多离子计数器Pb同位素分析接收器配置
Table 1. TRITON PLUS TIMS MIC configuration for Pb isotopic measurement
Center Mass IC3 A (SEM) IC2 L5 (SEM) RPQ/IC1 B
(SEM)IC5
(CDD)Integration Time(s) Idle Time(s) Main 222.93 205Pb 206Pb 207Pb 208Pb 4 1 Second 221.86 204Pb 205Pb 206Pb 207Pb 4 1 
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表 2 多离子计数器Yield测试接收器配置
Table 2. MICC on figuration for Yield test of multiple ion counters
Line IC4⁃L5 IC3 A IC2 L5 RPQ/IC1 B IC5 L4 C Integration Time(s) Idle Time(s) 1 208Pb 4 3 2 207Pb 208Pb 224.00 4 3 3 208Pb 225.08 4 3 4 208Pb 226.13 4 3 5 208Pb 227.20 4 3 6 204Pb 205Pb 206Pb 207Pb 208Pb 222.93 4 3
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表 3 MIC动态多接收205Pb⁃NSIT981混合溶液Pb同位素比值测定结果
Table 3. Analytical results for Pb isotope ratio of 205Pb⁃NSIT981 mixed solution using MIC multi⁃dynamic method
跳峰接收方式
SampleMIC两次跳峰动态接收 207Pb/206Pb 2RSE(%) 208Pb/206Pb 2RSE(%) 204Pb/206Pb 2RSE(%) 1 0.913 54 0.046 2.162 4 0.048 0.059 14 0.075 2 0.913 60 0.031 2.160 8 0.034 0.058 85 0.053 3 0.913 93 0.052 2.161 6 0.052 0.059 15 0.093 4 0.913 35 0.036 2.162 6 0.039 0.058 90 0.063 5 0.912 97 0.031 2.160 9 0.032 0.058 94 0.045 6 0.913 07 0.028 2.150 7 0.030 0.059 11 0.051 7 0.913 94 0.048 2.155 8 0.050 0.059 11 0.070 8 0.913 50 0.036 2.154 9 0.037 0.059 16 0.058 9 0.913 32 0.038 2.155 1 0.041 0.059 15 0.073 10 0.913 72 0.039 2.156 0 0.042 0.059 18 0.065 11 0.913 99 0.043 2.156 1 0.044 0.059 13 0.070 12 0.914 04 0.042 2.159 1 0.045 0.059 13 0.062 13 0.913 19 0.059 2.157 7 0.061 0.059 17 0.079 14 0.913 30 0.061 2.152 0 0.063 0.059 24 0.094 15 0.912 86 0.026 2.157 3 0.027 0.058 99 0.044 16 0.912 98 0.025 2.160 0 0.027 0.059 00 0.039 17 0.913 31 0.049 2.152 8 0.051 0.059 14 0.074 18 0.913 81 0.042 2.156 8 0.043 0.059 06 0.076 19 0.913 50 0.045 2.156 4 0.046 0.059 09 0.068 20 0.913 66 0.045 2.150 9 0.046 0.059 09 0.070 Mean 0.913 48 ± 0.000 72(2SD) 2.157 0± 0.007 3(2SD) 0.059 09 ± 0.000 20(2SD) 注：RSE:相对标准误差；SD:标准偏差，下同；(RSE:relative standard error; SD:standard deviation，the same below).
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表 4 清湖锆石年龄测定结果
Table 4. Analytical results for Qinghu standard zircon
分析点 Pbc
(pg)Pb*/
PbcTh/
U206Pb/
204Pb208Pb/
206Pb同位素比值 年龄 206Pb/
238U2σ
(%)207Pb/
235U2σ
(%)207Pb/
206Pb2σ
(%)206Pb/
238U
(Ma)2σ
(%)207Pb/
235U
(Ma)2σ
(%)207Pb/
206Pb
(Ma)相关误差 QH⁃1 5.5 13.1 0.40 812 0.170 4 0.025 03 0.16 0.170 5 1.0 0.049 43 0.23 159.37 0.16 159.86 0.92 168 0.08 QH⁃2 6.6 5.7 0.38 364 0.216 9 0.025 05 0.24 0.169 8 2.2 0.049 19 0.22 159.48 0.24 159.3 2.0 157 0.05 QH⁃3 5.2 19.5 0.48 1 173 0.183 3 0.025 04 0.17 0.170 94 0.69 0.049 52 0.12 159.46 0.17 160.23 0.64 173 0.15 QH⁃4 3.3 41.9 0.42 2 551 0.146 3 0.025 05 0.16 0.171 22 0.38 0.049 59 0.13 159.51 0.16 160.47 0.35 176 0.23 QH⁃5 5.0 4.7 0.44 300 0.256 1 0.025 07 0.28 0.171 0 2.7 0.049 48 0.36 159.61 0.28 160.3 2.5 171 0.05 QH⁃6 4.3 10.6 0.41 660 0.184 2 0.025 09 0.20 0.170 4 1.2 0.049 26 0.21 159.76 0.20 159.7 1.1 160 0.11 QH⁃7 3.6 5.6 0.35 361 0.210 8 0.025 10 0.24 0.168 9 2.3 0.048 81 0.61 159.81 0.24 158.4 2.1 139 0.04 注：(1)Pbc代表普通铅, 本文认为其来自实验室本底，*Pb代表放射性成因铅；(2)采用实验室长期Pb本底同位素比值测定结果: 206Pb/204Pb=17.78±0.50 (2σ)，207Pb/204Pb =15.31±0.34 (2σ), 进行本底扣除计算；(3)205Pb⁃235U稀释剂采用NIST981 Pb及GBW04205 U3O8配制的标准溶液标定，稀释剂U/Pb比标定误差~0.18%(2RSE).
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