High Spatial Resolution U-Pb Dating of Low-Uranium Calcite Using LA-MC-ICP-MS
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摘要:
碳酸盐矿物特别是方解石分布广泛，作为原生和次生矿物在多种地质环境中形成，结合U-Pb同位素测年体系，能为地球科学应用提供直接的时间约束，具有广阔的应用前景。但方解石矿物的铀含量通常较低(小于5μg/g)，测年难度大，限制了该方法的发展。激光剥蚀多接收电感耦合等离子体质谱仪(LA-MC-ICP-MS)具有高灵敏度和高精度的特点，已成功应用于锆石、方解石等副矿物的高空间分辨率U-Pb测年。本文针对LA-MC-ICP-MS的载气和氮气流速、屏蔽炬状态等实验参数进行了详细优化，建立了适用于低铀方解石矿物的高空间分辨率U-Pb测年方法。为提升仪器灵敏度和等离子体状态，详细讨论了锥组合、屏蔽炬接地状态、N2引入量和Ar载气流速对U、Pb信号强度及氧化物产率(UO/U)的影响。结果表明，在Jet+X锥组合、屏蔽炬接地、8mL/min N2引入量和0.9L/min载气Ar流速条件下，Pb灵敏度达到最高，同时氧化物产率(UO/U)低于1%。为验证该方法的准确性，采用三个低铀(0.04～0.63μg/g)方解石U-Pb定年参考物质进行测试，使用高铀含量的WC-1标准物质作为校正238U/206Pb的外标，同时减小束斑大小和剥蚀频率，保持标准物质和待测样品的剥蚀坑纵横比一致。实验结果表明，在44μm束斑条件下，LD-5、PTKD-2和TARIM的U-Pb年龄测试值分别为73.20±0.56Ma、152.7±2.5Ma和206.2±1.3Ma，与ID-TIMS/ID-MC-ICP-MS的定值结果在误差范围内一致，验证了LA-MC-ICP-MS高空间分辨率测定低铀方解石U-Pb年龄的可靠性。
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关键词:
- 低铀方解石 /
- U-Pb测年 /
- LA-MC-ICP-MS /
- 高空间分辨率 /
- 参数优化
Abstract:The combination of carbonate minerals with the U-Pb isotope dating system provides direct temporal constraints for geoscientific applications. LA-MC-ICP-MS with its high sensitivity and accuracy, has been successfully applied to the high spatial resolution U-Pb dating of zircon, calcite and other minerals. The experimental parameters of LA-MC-ICP-MS were optimized in detail, and a high spatial resolution U-Pb dating method was established for low-uranium calcite minerals. The effects of cone combination, GE mode, N2 introduction and Ar carrier gas flow rate on the U and Pb signal intensity and oxide yield (UO/U) were discussed in detail. The results showed that the highest sensitivity of Pb was achieved under the conditions of Jet+X cone combination, guard electrode (GE) grounded state, 8mL/min N2 introduction and 0.9L/min Ar carrier gas flow rate, while the oxide yield (UO/U) was lower than 1%. Three low uranium (0.04−0.63μg/g), calcite U-Pb dating reference materials were used to validate the reliability of the method. Samples LD-5, PTKD-2 and TARIM yielded a lower intercept age of 73.20±0.56Ma, 152.7±2.5Ma and 206.2±1.3Ma, respectively, under the condition of 44μm beam spot, which were in agreement with the calibrated results of ID-TIMS/ID-MC-ICP-MS within the error range. The BRIEF REPORT is available for this paper at
http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202403160057 .-
Key words:
- low-uranium calcite /
- U-Pb dating /
- LA-MC-ICP-MS /
- high spatial resolution /
- parameter optimization
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图 4 氧化物产率(UO/U)低于1%条件下，不同锥组合(a. S+H，b. S+X和c. Jet+X)、屏蔽炬状态和引入氮气流量条件下206Pb最高信号强度
Figure 4.
图 6 方解石参考物质LD-5在(a) 60μm和(b) 44μm束斑条件下的LA-MC-ICP-MS U-Pb年龄Tera-Wasserburg图和测试点的U元素含量分布不同激光参数下WC-1的未校正Tera-Wasserburg图
Figure 6.
图 7 方解石参考物质PTKD-2在(a) 60μm和(b)44μm束斑条件下的LA-MC-ICP-MS U-Pb年龄Tera-Wasserburg图和测试点的U元素含量分布不同激光参数下WC-1的未校正Tera-Wasserburg图
Figure 7.
图 8 方解石参考物质TARIM在(a) 60μm和(b) 44μm束斑条件下的LA-MC-ICP-MS U-Pb年龄Tera-Wasserburg图和测试点的U元素含量分布不同激光参数下WC-1的未校正Tera-Wasserburg图
Figure 8.
表 1 硅酸盐玻璃和方解石标准物质
Table 1. Silicate glass and calcite reference materials
标准物质编号 样品类型 铀含量
(μg/g)ID-TIMS或
ID-MC年龄
(Ma)ID-TIMS或ID-MC
测得初始铅
207Pb/206PbLA-(MC)-ICP-MS
年龄(Ma)参考文献 NIST612 玻璃 37.38 − − − Jochum等 (2011)［28］ NIST614 玻璃 0.823 − − − Jochum等 (2011)［28］ WC-1 海相方解石 2～8 254.4±6.4 0.85 254.4±1.6 Roberts等 (2017)［29］ LD-5 矿区方解石 0.2～5 72.36±0.27 0.863 72.5±1.5 昆士兰大学地球与环境科学学院，
放射性同位素实验室内部标准物质PTKD-2 矿区方解石 约0.16 153.7±1.7 0.85 153.8±0.8 Nguyen等(2019)［26］ TARIM 沉积碳酸盐地层
方解石巨晶0.03～1.53
(0.48)208.8±0.6 0.86 208.0±0.4
(3.2)Zhang等(2022)［27］ 
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表 2 仪器设备及测试参数
Table 2. Instrumental operating conditions
杯结构设置 检测器 IC4-CDD IC5-CDD IC3-SEM IC2-SEM IC1-SEM IC6-CDD IC7-CDD 色散
电压
（V）同位素 202Hg 204Pb+204Hg 206Pb 207Pb 208Pb 232Th 238U 1 MC-ICP-MS 仪器型号 NEPTUNE Plus 射频功率 1200W 屏蔽矩 ON, OFF 等离子体气流速 16.0L/min 辅助气流速 0.80L/min 样品载气流速 0.35~1.45L/min 氮气 0~8mL/min 锥类型 三套Ni锥组合：标准采样锥+H截取锥（S+H）；标准
采样锥+X截取锥（S+X）；Jet采样锥+X截取锥（Jet+X）数据采集参数 采集模式 静态 检测器 离子计数器 杯结构 202Hg (IC4-CDD), 204Pb+204Hg (IC5-CDD),
206Pb (IC3-SEM), 207Pb (IC2-SEM), 208Pb (IC1-SEM), 232Th (IC6-CDD), 238U (IC7-CDD)分辨率 低分辨 积分时间 0.262s 信号采集量 1 Block 200 cycles 激光设置 激光类型 ArF 脉冲宽度 15ns 波长 193nm 载气及流量 氦气，0.60L/min 剥蚀模式 单点模式 能量密度 3~5J/cm2 束斑大小 32~120μm 激光频率 2~10Hz
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表 3 不同激光参数下NIST614玻璃校正WC-1的U-Pb年龄结果
Table 3. The U-Pb age results of WC-1 with different laser parameters corrected by NIST614 glass
激光束斑
(μm)激光频率
(Hz)剥蚀坑纵横比［38］ 未校正年龄
(Ma)不确定度
(Ma)外标NIST614玻璃的
激光束斑和频率32 7 0.79 308.7 3.4 44μm，10Hz 44 10 0.82 309.0 1.5 32 4 0.45 273.4 2.4 60μm，7Hz 60 7 0.42 273.62 0.91
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