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
要点(1)优化质谱测量参数最高可将U、Pb的灵敏度提高至7.4倍和5.8倍，N2的引入和Jet+X锥组合分别对Pb和U的增敏效果更为明显。
(2)本实验中激光剥蚀束斑和频率不同，剥蚀坑纵横比相同时，“down-hole”分馏行为一致。
(3)通过优化实验参数，建立的方法可实现低铀含量(0.04～0.63μg/g)方解石参考物质高空间分辨率(44μm)U-Pb年龄的准确测定。
HIGHLIGHTS(1) The sensitivity of U and Pb can be enhanced up to 7.4 and 5.8 times after optimization of the mass spectrometer parameters, and the introduction of N2 and the combination of Jet+X cone were maximized the sensitivity of Pb and U, respectively.
(2) In this study, the "down-hole" fractionation behavior was consistent when depth to diameter ratios (aspect ratio) of the ablation crater was the same, although beam spot and frequency were different.
(3) By optimising the experimental parameters, accurate determination of U-Pb ages with high spatial resolution (44μm) for calcite reference materials with low U content (0.04-0.63μg/g) was achieved by LA-MC-ICP-MS.
Abstract:Carbonate minerals combination with the U-Pb isotope dating system can provide 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. In this paper, the experimental parameters of LA-MC-ICP-MS are optimised in detail, and a high spatial resolution U-Pb dating method is 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) are discussed in detail. The results showed that the highest sensitivity of Pb was achieved under the conditions of Jet+X cone combination, GE on mode, 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 yield a lower intercept age of 73.20±0.56Ma, 152.7±2.5Ma and 206.2±1.3Ma, respectively, under the condition of 44μm beamspot, which are 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.
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Keywords:
- low-uranium calcite /
- U-Pb dating /
- LA-MC-ICP-MS /
- high spatial resolution /
- parameter optimization
BRIEF REPORTSignificance: Carbonate minerals，especially calcite，are widely distributed and formed as primary and secondary minerals in a variety of geological environments［1］. The U-Th-Pb dating technique is one of the most classical isotopes dating methods，applicable to almost the entire geologic time scale，and calcite can be bound to uranium during its formation，making it a potentially suitable timer for U-Pb and U-Th geochronology［7-8］. Therefore，calcite geochronology can provide direct temporal constraints for many geoscientific applications and has a very broad application prospect. However，carbonate minerals usually have relatively low U contents，which have hindered the development of this method. With its high sensitivity and high accuracy，MC-ICP-MS has been successfully applied to high spatial resolution U-Pb dating for many high U content auxiliary minerals.LA-MC-ICP-MS with high sensitivity and accuracy has been successfully applied to high spatial resolution U-Pb dating of a wide range of minor minerals include carbonate minerals. Zhang et al ［22］ carried out a calcite U-Pb age methodology using mixed collectors (FCs+ICs) and multiple ions counting collectors (MICs) analytical mode. And achieved accurate U-Pb age determinations for PKC-1 (uranium content of 6.2-74.18μg/g)，Duff Brown Tank，JT and ASH-15 standards for accurate U-Pb age determination by optimising the mass spectrometer and laser parameters at 33μm，90μm，75μm and 110μm beam spots，respectively. Xie et al［21］ achieved accurate U-Pb age determinations of low-uranium (down to 0.03μg/g) calcite at 85μm beam spot conditions using a mixed collectors (FCs+ICs) mode based on optimised instrumental parameters. However，the previous U-Pb age analysis studies have not investigated the influence of the oxide yield and the guard electrode modes of the MC-ICP-MS instrument in detail. In addition，the U-Pb age analyses have only been carried out on carbonate samples with high uranium content (Duff Brown Tank，ASH-15，etc.，with ≥2 μg/g uranium) with U-Pb ages below 50μm beam spot. The spatial resolution of U-Pb dating of low uranium carbonate samples (＜1μg/g) needs to be further improved.
Methods: The samples used in the experiments were NIST glass standards NIST612 and NIST614［28］，and the calcite reference materials WC-1，PTKD-2，LD-5，and TARIM，listed in Table 1. NIST612 was used to optimise the instrumental parameters，NIST614 and WC-1 were used as standards to calibrate 207Pb/206Pb and 238U/206Pb ratios，and PTKD-2，LD-5，and TARIM were unknown samples with low uranium contents. Glass and calcite samples were fixed in epoxy resin to prepare the target and polished with 7000 grit sandpaper until the sample surface was exposed. Sample surfaces were polished smooth with 0.3μm Al2O3 polishing powder and ultrasonically cleaned with ultrapure water prior to each test.
The Neptune Plus multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS，ThermoFisher Scientific) and the GeoLas Pro excimer laser ablation system (LA，Coherent) were used in this experiment. In order to achieving the highest sensitivity，multiple ions counting collectors (MICs) analytical mode was used to receive all U and Pb elements，and the specific cup configuration and instrumental parameter settings are shown in Table 2.
The data were processed using the two-step calibration method of the previous work［29］. The raw data were first imported into the Iolite software［30］ and processed using X_U-Pb_Geochro4. After blank subtraction，the 207Pb/206Pb and 238U/206Pb ratios of the calcite standard WC-1 and the unknown samples were calibrated using NIST 614 as an external standard sample and the uncertainty of the ratios was derived with a single-point internal precision of 2 SE. The 238U/206Pb ratio of the unknown sample was further corrected with matrix-matched WC-1，and the 207Pb/206Pb，238U/206Pb of WC-1 was plotted on a Tera-Wasserburg plot using IsoplotR［31］，and the ratio of the obtained lower intercept to the reference lower intercept (26.43) is the correction factor. This correction factor includes matrix effects，laser induced fractionation and fractionation between U/Pb elements associated with ICP and is used to further correct the 238U/206Pb ratio of the unknown sample. Finally，the 207Pb/206Pb and 238U/206Pb ratios of the samples resulting from the two-step correction were plotted on a Tera-Wasserburg plot using IsoplotR to calculate the lower intersection age and age uncertainty (2SE).
Data and Results: The effects of the experimental parameters (including the cone combination，the amount of nitrogen introduced，whether the guard electrode is open，and the Ar carrier gas flow rate) on the U and Pb sensitivity and the plasma state (oxide yield，UO/U) were systematically discussed in order to realize the best analysis instrument parameters. The Jet+X cone combination and the introduction of N2 had significant sensitising effects on both U and Pb. The Jet+X cone combination combined with the introduction of N2 increased the sensitivities of U and Pb to a maximum of 7.4 and 5.8 times，respectively，under GE on conditions. See Fig.1 and Fig.2 for details. The high-sensitivity Jet+X cone combination and the introduction of N2 have obvious sensitizing effects on U and Pb. The sensitivity of U and Pb was enhanced up to 7.4 and 5.8 times under the GE on mode，the Jet+X cone combination and with the introduction of N2. In which the sensitizing effect of the high-sensitivity cone combination on Pb is more obvious，and that of the introduction of N2 on U is more obvious.
Fig.3 shows the effect of the instrumental conditions on the oxide yield，the oxide yield (UO/U) in the GE on mode is usually higher than that in the GE off condition，and the oxide yield becomes rapidly higher with the increase of the Ar carrier gas flow rate. In addition，the optimisation of the experimental conditions is aimed at increasing the sensitivity and decreasing the oxide yield，in other word，the conditions with the highest sensitivity are chosen at lower oxide yields. Fig.4 shows the maximum signal intensity of 206Pb for different instrumental conditions for oxide yields (UO/U) within 1% and the Ar carrier gas flow rate corresponding to the maximum signal intensity. The best experimental conditions were achieved using the Jet+X cone combination，N2 flow rate of 8 mL/min and the GE on mode，corresponding to an Ar carrier gas flow of 0.9 L/min.
Under optimised experimental conditions，the same depth to diameter ratios (aspect ratio) of the ablation crater is guaranteed，and the “down-hole” fractionation behavior of WC-1 is similar under different laser spot sizes and frequencies. Based on the optimized experimental parameters，the method was validated using three low-uranium (0.04-0.63μg/g) calcite U-Pb dating reference materials. High-uranium WC-1 was used for 238U/206Pb correcting，the spot size and frequency were altered on the premise of crater aspect ratios of the standard and samples keeping consistent. In our experiment，the calcite samples LD-5，PTKD-2 and TARIM yield a lower intercept age of 73.20±0.56Ma，152.7±2.5Ma and 206.2±1.3Ma，respectively，which were in agreement with the calibrated results from the ID-TIMS/ID-MC-ICP-MS within uncertainties.
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从地球表面的土壤到与热液流体相关的地球深部，碳酸盐矿物无处不在。方解石是最稳定和最常见的碳酸钙形式，与其他碳酸盐矿物(文石、白云石、菱镁矿等)一起，作为原生和次生矿物普遍存在于洞穴、土壤、海洋、热液埋藏和断裂带等各种环境中［1-2］。U-Th-Pb定年技术是经典的同位素年代学方法之一［3-5］，几乎适用于整个地质时间尺度，方解石在形成过程中可与铀结合，使其可能成为U-Pb和U-Th地质年代的合适计时器［6–8］。因此，方解石地质年代学能为众多地球科学应用提供直接的时间约束，具有十分广阔的应用前景［9-11］。
与锆石、磷灰石等含U副矿物的U-Pb测年方法类似，最早的方解石U-Pb数据主要依赖化学消解的方式，结合同位素稀释法用TIMS或MC-ICP-MS(ID-MC-ICP-MS/ID-TIMS)测得［12–14］。尽管同位素稀释法提供了准确的数据，但其操作繁琐、耗时，且对实验环境洁净度要求极高，同时空间分辨率远低于激光原位分析技术。此外，相较于传统的含铀副矿物(如锆石的铀含量通常高于100μg/g)，碳酸盐矿物的铀含量通常非常低(0.01～10μg/g)，这意味着溶液方法需要的样品量较大，更容易获得同位素组成的平均值，各数据点母子体同位素比值(238U/206Pb)相差不大，难以拟合出理想的线性关系。目前，ID-MC-ICP-MS/ID-TIMS方法主要应用于超高精度U-Pb测年需求或标定潜在方解石参考物质的年龄和初始铅组成。激光剥蚀联用质谱仪效率高、成本低，可以通过大量测试筛选合适的测年位置和数据，极大地提高了方解石U-Pb测年的成功率和分析效率。另外，激光剥蚀(LA)作为微区原位技术，不仅可以提供微区尺度的年龄信息，区分矿物不同期次和生长环带的年代，还可以将U-Pb年龄同图像及其他地球化学信息(元素、同位素组成)等结合起来，能够为解读年龄数据提供更多的背景。常用于联用激光剥蚀系统进行方解石U-Pb测年的质谱仪有四极杆质谱仪(Q-ICP-MS)［15-16］、扇形磁场单接收质谱仪(SF-ICP-MS)［17–19］和多接收杯质谱仪(MC-ICP-MS)［20–25］。LA-Q-ICP-MS相较另两种方法灵敏度较低，通常需要使用～200μm的束斑采样［15］，才能获得较为可靠的U-Pb年龄。LA-SF-ICP-MS灵敏度较高，更适用于低铀副矿物U-Pb年龄测试。吴石头等［19］通过Jet+X锥组合和N2增敏技术进一步将LA-SF-ICP-MS的分析灵敏度提高了10倍左右，在50μm束斑条件下实现了石灰岩Duff Brown Tank(铀含量11.35～13.79μg/g)U-Pb年龄的准确测定，在110μm束斑条件下实现了标准物质JT(铀含量0.01～5μg/g)和方解石标准物质ASH-15(铀含量0.43～9.94μg/g)U-Pb年龄的准确测定。
LA-MC-ICP-MS不仅灵敏度高，还可以同时检测所有的待测U、Pb同位素，进一步提高了分析精度。另外，MC-ICP-MS有常规法拉第杯、高阻法拉第杯和高灵敏度离子计数器三种检测器，可以根据样品的铀含量选用不同的检测器组合，适用于超低到较高铀含量的方解石U-Pb测年［23］，已经成为应用最为广泛的方法。Zhang等［22］分别使用法拉第杯和离子计数器混合使用以及全离子计数器模式进行了方解石U-Pb年龄方法研究，通过优化质谱仪和激光参数，分别在33μm、90μm、75μm和110μm束斑下实现了石灰华PKC-1(铀含量6.2～74.18μg/g)、Duff Brown Tank、JT和ASH-15 标准物质U-Pb年龄的准确测定。谢博航等［21］使用法拉第杯和离子计数器混合使用的杯结构，基于优化后的仪器参数，在85μm束斑条件下实现了低铀(最低至0.03μg/g)方解石U-Pb年龄的准确测定。氧化物产率的高低可以表征等离子体的状态，屏蔽炬(Guard Electrode, GE)的接地状态对灵敏度和氧化物产率也有影响，这些指标关乎分析结果的准确度和精密度。但前人的U-Pb年龄分析研究中并未对MC-ICP-MS仪器的氧化物产率和屏蔽炬的影响作详细探讨，并且仅针对铀含量较高的碳酸盐样品(石灰岩Duff Brown Tank、方解石ASH-15等，铀含量≥2μg/g，)进行了低于50μm束斑的U-Pb年龄测试，对于低铀含量(＜1μg/g)碳酸岩样品U-Pb测年的空间分辨率还需要进一步提高。
本文旨在实现低铀方解石样品的高空间分辨率(小束斑)U-Pb测年，较低的铀含量和较高的空间分辨率均对分析方法的灵敏度提出了挑战。为了保证最佳灵敏度，使用全离子计数器接收U、Pb同位素，同时详细讨论了实验参数对U、Pb灵敏度和等离子体状态(氧化物产率)的影响，以寻求最佳的测试条件。另外，最常用的标准物质WC-1铀含量较高，离子计数器线性范围窄，需要通过改变剥蚀WC-1的激光参数降低其信号强度，参数改变是否会影响测试结果也需要实验评估。实验中，选取了三个低铀方解石参考物质LD-5、PTKD-2［26］和TARIM［27］对方法的可靠性进行验证。
1. 实验部分
1.1 仪器和设备
本文所有实验均在山东省地质科学研究院自然资源部金矿成矿过程与资源利用重点实验室和山东省金属矿产成矿地质过程与资源利用重点实验室完成。使用的仪器为Neptune Plus多接收电感耦合等离子体质谱(MC-ICP-MS，美国ThermoFisher公司)和GeoLas Pro准分子激光剥蚀系统(LA，美国Coherent公司)。Neptune Plus配备有大功率接口泵(泵速可达100m3/h)，可以使用标准和Jet采样锥以及H和X截取锥的任意组合。MC-ICP-MS的动态变焦系统将质量色散扩大到17%，配备了9个法拉第杯和7个离子计数器(3个SEM、4个CDD)，可以实现Pb、Th和U同位素的同时接收。GeoLas Pro准分子激光剥蚀系统的波长为193nm(深紫外)，脉冲宽度是15ns，剥蚀孔径大小为5～160µm，最高能量为200mJ，最高脉冲频率为20Hz。另外，激光剥蚀系统配置了信号平滑装置，以便在低频率的剥蚀下获得光滑的分析信号。
1.2 实验样品和样品处理
实验使用的样品有美国国家标准与技术研究院(NIST)的玻璃标准物质NIST612和NIST614［28］，以及各同行实验室开发的方解石U-Pb年龄参考物质WC-1、PTKD-2、LD-5和TARIM，详见表1。其中，NIST612用来优化仪器参数，NIST614和WC-1作为标准物质校正方解石年龄，PTKD-2、LD-5和TARIM作为待测样品。PTKD-2、LD-5和TARIM的铀含量较低，能充分验证本文方法测试低铀方解石样品的有效性和可靠性。
表 1 硅酸盐玻璃和方解石标准物质Table 1. Silicate glass reference materials and calecite 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 ～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］ 玻璃和方解石样品固定在环氧树脂中制备成靶，使用7000目的砂纸抛光至露出样品表面。每次测试之前均使用0.3μm的Al2O3抛光粉将样品表面打磨光滑，并用超纯水超声清洗。
1.3 U-Pb同位素测试及数据处理
为了提高检测低铀方解石样品的灵敏度，本文实验均采用全离子计数器模式接收所有的同位素信号。离子计数器的背景噪音和工作电压采用仪器自带的PCL脚本测定，背景噪音小于0.03cps，IC1~IC7的操作电压分别为2500、2100、2450、2100、2075、2050和1950V。实验讨论了不同的样品锥和采样锥组合(S+H、S+X、Jet+X)，样品载气Ar流速，辅助气N2流速和屏蔽炬接地状态对同位素信号的影响，以获取最佳的测试参数。所用到的激光剥蚀束斑有32、44、60和120μm，剥蚀频率为2～10Hz，能量密度为3～5J/cm2。数据采集均在静态接收模式下进行，每个数据有90个cycles，积分时间为0.524s，包括背景信号约10s、数据采集信号30s和背景冲洗时间7s。具体的杯结构和仪器参数设置见表2。每隔10～14个样品数据点间插一个NIST614、两个WC-1，每个样品进行20～50次点分析。
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 数据处理采用前人的两步校正法［29］。先将原始数据导入Iolite软件中［30］，采用X_U-Pb_Geochro 4进行处理，进行空白扣除后，用NIST614作为外标样品校正方解石标准物质WC-1和未知样品的207Pb/206Pb、238U/206Pb比值，并得出比值的不确定度，不确定度为单点的内部精度2SE。再用基体匹配的WC-1进一步校正未知样品的238U/206Pb比值，使用IsoplotR［31］将WC-1的207Pb/206Pb、238U/206Pb绘制成Tera-Wasserburg图，获得的下交点与参考下交点(26.43)的比值即为校正因子。该校正因子包括了基体效应、激光诱导分馏以及与ICP相关的U/Pb元素间的分馏，用于进一步校正未知样品的238U/206Pb比值。最后，使用IsoplotR将经两步校正所得样品的207Pb/206Pb、238U/206Pb比值绘制成Tera-Wasserburg图，计算出下交点年龄和年龄不确定度(2SE)。
2. 结果与讨论
2.1 质谱测量参数的优化
为了实现低铀方解石样品U-Pb年龄在高空间分辨率下准确测定，首先要对质谱仪灵敏度进行优化。本文详细地探讨了锥组合、Ar载气流速、辅助气N2的引入和屏蔽炬状态等条件对U、Pb信号强度和氧化物产率的影响，所有条件实验均使用NIST612标准物质在相同的激光剥蚀条件下进行。
2.1.1 灵敏度随测试条件的变化
图1和图2是仪器条件对质谱仪的灵敏度的影响，其中a、b、c分别是S+H锥、S+X锥和Jet+X锥组合条件下，238U和206Pb的信号强度随N2的引入和Ar载气流速的变化。在未引入N2且屏蔽炬处于接地状态时，相较于标准的S+H锥组合，采用S+X锥和Jet+X锥组合分别将U元素的最佳灵敏度提升至1.7倍和2倍，而对于Pb元素，最佳灵敏度分别提高至4.6倍和3.8倍。屏蔽炬未接地的状态下，U、Pb的最佳灵敏度与接地状态相近，但锥组合带来的灵敏度增强效应略有减弱。相较于S+H锥组合，S+X锥和Jet+X锥组合将U的最佳灵敏度提高至1.4倍，Pb的最佳灵敏度分别提高至3.4倍和2.3倍。屏蔽炬处于接地状态下，N2的引入显著增强了不同锥组合下U的最佳信号强度，但对Pb的增敏效果不明显。当N2流量为4mL/min时，S+H锥和S+X锥组合的U、Pb信号强度最高，相较于无N2条件，S+H锥组合的U、Pb信号分别增强至2倍和1.2倍，S+X锥组合的U、Pb信号分别增强至2.8倍和1.2倍，随着N2引入量的增加，U的信号强度保持稳定，Pb的信号强度反而有所下降。当N2的引入量增至6～8mL/min时，Jet+X锥组合的U、Pb信号达到最优，相较于无N2条件分别提高至3.8和1.5倍；相较于引入N2的S+H锥组合，Jet+X锥组合的U、Pb信号分别提高至3.8倍和5倍。
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图 1 锥组合a(S+H)、b(S+X)和c(Jet+X)时，238U的信号强度在屏蔽炬是否接地和引入氮气量条件下随Ar载气的变化情况Figure 1. The 238U signal intensity of three cone combinations a (S+H), b (S+X), and c (Jet+X) with GE on or off and different amounts of nitrogen![]()
图 2 锥组合a(S+H)、b(S+X)和c(Jet+X)时，206Pb的信号强度在屏蔽炬是否接地和引入氮气量条件下随Ar载气的变化Figure 2. The 206Pb signal intensity of three cone combinations a (S+H), b (S+X), and c (Jet+X) with GE on or off and different amounts of nitrogen相比之下，屏蔽炬在未接地的状态下，N2的增敏效果并不明显，尤其是使用S+H锥组合时，N2的引入反而降低了U、Pb的信号强度。然而，对于S+X锥和Jet+X锥组合，引入2mL/min的N2时，U和Pb的信号强度最高，分别提高至无N2引入的1.7倍和1.3倍。
高灵敏度的Jet+X锥组合和N2的引入对U、Pb均有明显的增敏效果，屏蔽炬处于接地状态时，Jet+X锥组合结合N2的引入最高可将U、Pb的灵敏度分别提高至7.4倍和5.8倍。值得注意的是，锥组合对Pb的增敏效果更为显著，这可能与Jet+X锥组合具有更大的锥孔直径和锥长度有关［32-33］。相比较重的U+，Pb+在锥界面的位置更易受空间电荷效应影响而被排斥在离子束外围，而Jet+X锥组合更大的锥孔直径和锥长度使得更多位于离子束外围的离子能够进入检测器，从而更大程度地提高了Pb元素的灵敏度。此外，N2的引入对U的增敏效果更显著，这可能与元素的氧化物解离能大小有关，氧化物解离能越大，N2对元素的增敏效果越明显［34-35］。
2.1.2 氧化物产率(UO/U)随测试条件的变化
氧化物产率是影响元素、同位素分析结果准确性的重要因素，其产率的高低则受等离子体状态、载气流速和质谱接口状态的共同影响。图3是仪器条件对氧化物产率的影响，其中，a、b、c分别是S+H锥、S+X锥和Jet+X锥组合条件下，氧化物产率(UO/U)随N2的引入和Ar载气流速的变化。如图3所示，屏蔽炬在接地状态下的氧化物产率(UO/U)通常高于屏蔽炬不接地的状态，另外，氧化物产率随Ar载气流速的增加迅速变高。而U、Pb灵敏度峰值通常出现在较高的载气流速区间，这意味着在最高灵敏度的载气流速下，仪器往往伴随着较高的氧化物产率，并不是最优的测试条件。为了降低测试过程中的氧化物产率，需要降低载气流速，或采用屏蔽炬未接地的状态［25］，这两种方式都会不可避免地降低各元素的灵敏度。另外，引入N2会降低最佳灵敏度对应的Ar载气流速，进而降低测试过程中的氧化物产率［35-36］。
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图 3 锥组合a(S+H)、b(S+X)和c(Jet+X)时，氧化物产率(UO/U)在屏蔽炬是否接地和引入氮气量条件下随Ar载气的变化Figure 3. The Oxide yield UO/U of three cone combinations a (S+H), b (S+X), and c (Jet+X) with GE on or off and different amounts of nitrogen2.1.3 最优测试条件的选择
在本文中，优化实验条件的目的在于提高灵敏度并降低氧化物产率，即在较低的氧化物产率下选取灵敏度最高的条件。2.1.1节中，随着仪器参数的改变，Pb和U最佳信号强度对应的仪器参数不一致。鉴于本实验采用全离子计数器接收所有的U、Pb同位素信号，且放射性成因Pb含量低于铀含量，因此在选择实验条件时给予Pb信号强度的优化以更高的优先级。图4展示了氧化物产率(UO/U)在1%以内的条件下，不同锥组合、N2引入量和屏蔽炬状态时206Pb的最大信号强度，以及最大信号强度对应的Ar载气流量。当使用Jet+X锥组合、N2引入量为8mL/min、屏蔽炬处于接地状态时，测试条件最佳，此时对应的Ar载气流量为0.9L/min。
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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。其中，蓝色和黄色柱状图对应主Y轴(左侧)，分别代表屏蔽炬接地状态(GE ON)和未接地状态(GE OFF)时206Pb最高信号强度；蓝色和黄色线对应次Y轴(右侧)，分别代表屏蔽炬在接地状态(GE ON)和未接地状态(GE OFF)时206Pb最高信号强度的Ar载气流速Figure 4. The Highest 206Pb signal intensity under conditions of oxide yield (UO/U) below 1%, with different cone combinations (a. S+H, b. S+X, and c. Jet+X), GE mode, and nitrogen flow rates. The blue and yellow bars correspond to the primary Y-axis (on the left), representing the highest 26Pb signal intensities when the GE is grounded (GE ON) and ungrounded (GE OFF), respectively. The blue and yellow lines correspond to the secondary Y-axis (on the right), depicting the argon carrier gas flow rates corresponding to the highest 26Pb signal intensities when the GE is in the grounded (GE ON) and ungrounded (GE OFF) states, respectively2.2 激光剥蚀参数
难熔元素U和挥发性元素Pb之间的分馏效应是影响U-Pb年龄准确测定的重要因素［36-37］，随着激光单点剥蚀的进行，剥蚀坑纵横比增大，Pb/U比值也逐渐增加，即“down-hole”分馏效应。消除或减小“down-hole”分馏的影响最有效且常见的方法是通过数学模型进行校正，当标准物质和待测样品的分馏模式一致时，可以获得准确的分析结果。即选用基体匹配的标准物质，保持相同的激光剥蚀参数对待测样品进行校正。也有研究表明，通过同时调整激光的剥蚀束斑和剥蚀频率，保持一致的剥蚀坑纵横比，也可获得准确的分析结果，但是需要在测试之前进行条件实验，验证结果的可靠性［18，21，38］。
WC-1是目前表征最充分、应用最为广泛的方解石U-Pb测年标准物质，大部分实验室均使用WC-1作为238U/206Pb校正的外标样品。但是本实验中使用的离子计数器线性范围较窄，而WC-1中的铀含量较高，在与待测样品使用同样的激光束斑和频率参数时，U的计数往往超出离子计数器使用的安全范围。因此，本实验尝试在测试中减小剥蚀WC-1的束斑和频率，使其保持相似的剥蚀坑纵横比，NIST614的剥蚀条件则保持不变(表3)。图5 是不同束斑和频率下WC-1的Tera-Wasserburg图，不同剥蚀条件、剥蚀坑纵横比相似的WC-1年龄在误差范围内一致，其中的纵横比是根据前人报道的剥蚀速率计算［38］。同时，剥蚀坑的纵横比越大，WC-1测定的年龄越老，与参考值相差越大，这与前人的研究结果一致［18］。
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 ![]()
图 5 不同激光参数下NIST614玻璃校正WC-1的Tera-Wasserburg图Figure 5. The Tera-Wasserburg plots of WC-1 with different laser parameters corrected by NIST614 glass2.3 低铀方解石参考物质的分析结果
在最优的质谱仪条件下，本实验用60μm-7Hz和44μm-10Hz的剥蚀条件对LD-5、PTKD-2和TARIM的U-Pb年龄进行了分析。三种样品均为同行实验室近年开发、使用同位素稀释法(ID-TIMS/MC-ICP-MS)、测得初始铅比值(207Pb/206Pb)和Tera-Wasserburg图下交点年龄的方解石，U-Pb年龄均一，铀含量较低(均低于5μg/g)，适合作为验证低铀方解石U-Pb年龄分析方法可靠性的参考物质。玻璃标准物质NIST614的剥蚀参数与待测参考物质一致，方解石WC-1的剥蚀参数分别对应为32μm-4Hz、32μm-7Hz。
2.3.1 LD-5的U-Pb年龄
图6是两种束斑条件下LD-5样品的Tera-Wasserburg图和测试位置的铀含量分布图，测试区域的普通铅比例变化范围较小，因此在进行年龄拟合时固定上交点初始铅207Pb/206Pb为0.863。60μm束斑条件下得到年龄为71.84±0.55Ma，铀含量分布于0.04～0.63μg/g(图6a)，其中0.04～0.10μg/g的点数最多，对应了Tera-Wasserburg图中单点误差较大的数据。在44μm束斑条件下得到年龄为73.20±0.56Ma，铀含量分布于0.04～0.61μg/g(图6b)，其中0.21～0.27μg/g的点数最多。两种条件下测得的年龄均与ID-TIMS的年龄数据72.36±0.27Ma在误差范围内一致。
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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. The Tera-Wasserburg plots of LA-MC-ICP-MS U-Pb ages and distribution of elemental U content at laser ablation sites for the calcite reference material LD-5 at (a) 60μm and (b) 44μm spot size2.3.2 PTKD-2的U-Pb年龄
图7是两种束斑条件下PTKD-2样品的Tera-Wasserburg图和测试位置的铀含量分布图，该样品普通铅比例变化范围很大，24个测试点中207Pb/206Pb在0.03～0.78之间均有分布，因此在进行年龄拟合时未固定上交点。60μm束斑条件下得到年龄为155.1±1.9Ma，初始铅(207Pb/206Pb)c为0.8135±0.0099，铀含量分布于0.09～0.16μg/g(图7a)，其中0.12～0.13μg/g的点数最多。44μm束斑条件下得到年龄为152.7±2.5Ma，初始铅(207Pb/206Pb)c为0.802±0.006，铀含量分布于0.09～0.18μg/g(图7b)，其中0.14～0.15μg/g的点数最多。两种条件下测得的年龄均与ID-MC-ICP-MS的年龄数据153.7±1.7Ma在误差范围内一致，但是初始铅(207Pb/206Pb)c均比参考值(0.846 ± 0.0125)偏低，这种现象在前人研究中也有报道［19］。
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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. The Tera-Wasserburg plots of LA-MC-ICP-MS U-Pb ages and distribution of elemental U content at laser ablation sites for the calcite reference material PTKD-2 at (a) 60μm and (b) 44μm spot size2.3.3 TARIM的U-Pb年龄
图8是两种束斑条件下TARIM样品的Tera-Wasserburg图和测试位置的铀含量分布图，测试区域的普通铅比例变化范围最小，且该样品的普通铅占比很低，大多数测试点的207Pb/206Pb分布在0.1以下，在进行年龄拟合时固定上交点初始铅207Pb/206Pb为0.86。在60μm束斑条件下得到年龄为210.4±1.3Ma，铀含量分布于0.04～0.21μg/g(图8a)，其中0.10μg/g附近的点数最多。在44μm束斑条件下测得的年龄为206.2±1.3Ma，铀含量分布于0.07～0.14μg/g(图8b)，同样是0.10μg/g的点数最多。在44μm束斑条件下测得的年龄比ID-TIMS的年龄数据208.5±0.6Ma偏低，但考虑到激光测年的误差较大，总体上年龄在误差允许范围内一致。
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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. The Tera-Wasserburg plots of LA-MC-ICP-MS U-Pb ages and distribution of elemental U content at laser ablation sites for the calcite reference material TARIM at (a) 60μm and (b) 44μm spot size3. 结论
本文实现了LA-MC-ICP-MS对低铀方解石U-Pb年龄的高空间分辨率准确测定。通过实验详细优化了仪器参数，在屏蔽炬接地状态下，Jet+X锥组合结合N2的引入将U、Pb的灵敏度最高提高至7.4倍和5.8倍。在本实验条件下，保证剥蚀坑纵横比一致，不同激光剥蚀束斑大小和剥蚀频率条件下WC-1的“down-hole”分馏行为类似。采用三个低铀(0.04～0.63μg/g)方解石U-Pb定年参考物质对方法进行验证，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72.36±0.27Ma、153.7±1.7Ma和208.5±0.6Ma在误差范围内相吻合。
方解石U-Pb年代学应用潜力巨大，方解石样品通常较低的铀含量是限制U-Pb年龄准确测定的主要因素之一，LA-MC-ICP-MS具有超高的灵敏度和同时检测能力，是方解石U-Pb年龄测试的有力工具。需要指出的是，放射成因的Pb同位素相比U含量更低，使用全离子计数器接收U、Pb同位素的信号，以及环境中较高的Pb背景，使得Pb成为限制方法空间分辨率的最短板，因此本文还需要在提高Pb的信噪比方向做进一步尝试，从而继续提高该分析方法的空间分辨率。
致谢：感谢昆士兰大学赵建新教授、中国地质大学(北京)张亮亮副研究员和中国地质科学院矿产资源研究所侯可军研究员分享的方解石参考物质。
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图 1 锥组合a(S+H)、b(S+X)和c(Jet+X)时，238U的信号强度在屏蔽炬是否接地和引入氮气量条件下随Ar载气的变化情况
Figure 1. The 238U signal intensity of three cone combinations a (S+H), b (S+X), and c (Jet+X) with GE on or off and different amounts of nitrogen
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图 2 锥组合a(S+H)、b(S+X)和c(Jet+X)时，206Pb的信号强度在屏蔽炬是否接地和引入氮气量条件下随Ar载气的变化
Figure 2. The 206Pb signal intensity of three cone combinations a (S+H), b (S+X), and c (Jet+X) with GE on or off and different amounts of nitrogen
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图 3 锥组合a(S+H)、b(S+X)和c(Jet+X)时，氧化物产率(UO/U)在屏蔽炬是否接地和引入氮气量条件下随Ar载气的变化
Figure 3. The Oxide yield UO/U of three cone combinations a (S+H), b (S+X), and c (Jet+X) with GE on or off and different amounts of nitrogen
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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。其中，蓝色和黄色柱状图对应主Y轴(左侧)，分别代表屏蔽炬接地状态(GE ON)和未接地状态(GE OFF)时206Pb最高信号强度；蓝色和黄色线对应次Y轴(右侧)，分别代表屏蔽炬在接地状态(GE ON)和未接地状态(GE OFF)时206Pb最高信号强度的Ar载气流速
Figure 4. The Highest 206Pb signal intensity under conditions of oxide yield (UO/U) below 1%, with different cone combinations (a. S+H, b. S+X, and c. Jet+X), GE mode, and nitrogen flow rates. The blue and yellow bars correspond to the primary Y-axis (on the left), representing the highest 26Pb signal intensities when the GE is grounded (GE ON) and ungrounded (GE OFF), respectively. The blue and yellow lines correspond to the secondary Y-axis (on the right), depicting the argon carrier gas flow rates corresponding to the highest 26Pb signal intensities when the GE is in the grounded (GE ON) and ungrounded (GE OFF) states, respectively
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图 5 不同激光参数下NIST614玻璃校正WC-1的Tera-Wasserburg图
Figure 5. The Tera-Wasserburg plots of WC-1 with different laser parameters corrected by NIST614 glass
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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. The Tera-Wasserburg plots of LA-MC-ICP-MS U-Pb ages and distribution of elemental U content at laser ablation sites for the calcite reference material LD-5 at (a) 60μm and (b) 44μm spot size
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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. The Tera-Wasserburg plots of LA-MC-ICP-MS U-Pb ages and distribution of elemental U content at laser ablation sites for the calcite reference material PTKD-2 at (a) 60μm and (b) 44μm spot size
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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. The Tera-Wasserburg plots of LA-MC-ICP-MS U-Pb ages and distribution of elemental U content at laser ablation sites for the calcite reference material TARIM at (a) 60μm and (b) 44μm spot size
表 1 硅酸盐玻璃和方解石标准物质
Table 1 Silicate glass reference materials and calecite 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 ～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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