微区原位锆石Hf同位素比值测试在武汉上谱分析科技有限责任公司利用激光剥蚀多接收杯等离子体质谱（LA-MC-ICP-MS）完成。激光剥蚀系统为Geolas HD （Coherent，德国）, MC-ICP-MS为Neptune Plus（Thermo Fisher Scientific，德国）。分析过程同时配备了信号平滑装置以提高信号稳定性和同位素比值测试精密度（Hu et al. 2015）。载气使用氦气，并在剥蚀池之后引入少量氮气以提高Hf元素灵敏度（Hu et al. 2012）。分析采用Neptune Plus新设计高性能锥组合。前人研究表明，对于Neptune Plus的标准锥组合，新设计的X截取锥和Jet采样锥组合在少量氮气加入的条件下能分别提高Hf、Yb和Lu的灵敏度5.3倍、4.0倍和2.4倍。激光输出能量可以调节，实际输出能量密度为~7.0 J/cm2。采用单点剥蚀模式，斑束固定为44 μm。详细仪器操作条件和分析方法可参照(Hu et al.2012)。 采用LA-MC-ICP-MS准确测试锆石Hf同位素的难点在于176Yb和176Lu对176Hf的同量异位素的干扰扣除。研究表明，Yb的质量分馏系数(βYb)在长期测试过程中并不是一个固定值，而且通过溶液进样方式测试得到的βYb 并不适用于激光进样模式中的锆石Hf同位素干扰校正（Woodhead et al. 2004）。βYb 的错误估算会明显地影响176Yb对176Hf的干扰校正，进而影响176Hf/177Hf比值的准确测定。在实际中，我们实时获取了锆石样品自身的βYb用于干扰校正。179Hf/177Hf =0.7325和 173Yb/171Yb=1.132685（Fisher et al. 2014）被用于计算Hf和Yb的质量分馏系数βHf和βYb。使用176Yb/173Yb =0.79639（Fisher et al. 2014）来扣除176Yb 对 176Hf的同量异位干扰。使用176Lu/175Lu =0.02656（Blichert-Toft et al. 1997）来扣除干扰程度相对较小的176Lu对 176Hf的同量异位干扰。由于Yb和Lu具有相似的物理化学属性，因此在本实验中采用Yb的质量分馏系数βYb来校正Lu的质量分馏行为。分析数据的离线处理（包括对样品和空白信号的选择、同位素质量分馏校正）采用软件ICPMSDataCal（Liu et al. 2010）完成。 为确保分析数据的可靠性，Plešovice、91500和GJ-1三个国际锆石标准与实际样品同时分析，Plešovice用于进行外标校正以进一步优化分析测试结果。91500和GJ-1作为第二标样监控数据校正质量。Plešovice、91500和GJ-1的外部精密度（2SD）优于0.000020。测试值与推荐值确保在误差范围内一致。同时为了监控高Yb/Hf比值锆石的测试数据，采用国际常用的高Yb/Hf比值标样Temora 2监控高Yb/Hf比值锆石的测试数据,。以上标样推荐值请参考Zhang et al. (2020）。
21.2In situ Hf isotope ratio analysis of zircon by LA-MC-ICP-MS
Experiments of in situ Hf isotope ratio analysis were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas HD excimer ArF laser ablation system (Coherent, Göttingen, Germany) that was hosted at the Wuhan Sample Solution Analytical Technology Co., Ltd, Hubei, China. A “wire” signal smoothing device is included in this laser ablation system, by which smooth signals are produced even at very low laser repetition rates down to 1 Hz (Hu et al. 2015). Helium was used as the carrier gas within the ablation cell and was merged with argon (makeup gas) after the ablation cell. Small amounts of nitrogen were added to the argon makeup gas flow for the improvement of sensitivity of Hf isotopes (Hu et al. 2012). Compared to the standard arrangement, the addition of nitrogen in combination with the use of the newly designed X skimmer cone and Jet sample cone in Neptune Plus improved the signal intensity of Hf, Yb and Lu by a factor of 5.3, 4.0 and 2.4, respectively. All data were acquired on zircon in single spot ablation mode at a spot size of 44 μm. The energy density of laser ablation that was used in this study was ~7.0 J cm-2. Each measurement consisted of 20 s of acquisition of the background signal followed by 50 s of ablation signal acquisition. Detailed operating conditions for the laser ablation system and the MC-ICP-MS instrument and analytical method are the same as description by Hu et al.(2012). The major limitation to accurate in situ zircon Hf isotope determination by LA-MC-ICP-MS is the very large isobaric interference from 176Yb and, to a much lesser extent 176Lu on 176Hf. It has been shown that the mass fractionation of Yb (βYb) is not constant over time and that the βYb that is obtained from the introduction of solutions is unsuitable for in situ zircon measurements (Woodhead et al. 2004). The under- or over-estimation of the βYb value would undoubtedly affect the accurate correction of 176Yb and thus the determined 176Hf/177Hf ratio. We applied the directly obtained βYb value from the zircon sample itself in real-time in this study. The 179Hf/177Hf and 173Yb/171Yb ratios were used to calculate the mass bias of Hf (βHf) and Yb (βYb), which were normalized to 179Hf/177Hf =0.7325 and 173Yb/171Yb=1.132685 (Fisher et al. 2014) using an exponential correction for mass bias. Interference of 176Yb on 176Hf was corrected by measuring the interference-free 173Yb isotope and using 176Yb/173Yb =0.79639 (Fisher et al. 2014) to calculate 176Yb/177Hf. Similarly, the relatively minor interference of 176Lu on 176Hf was corrected by measuring the intensity of the interference-free 175Lu isotope and using the recommended 176Lu/175Lu =0.02656 (Blichert-Toft et al. 1997) to calculate 176Lu/177Hf. We used the mass bias of Yb (βYb) to calculate the mass fractionation of Lu because of their similar physicochemical properties. Off-line selection and integration of analyte signals, and mass bias calibrations were performed using ICPMSDataCal (Liu et al. 2010). In order to ensure the reliability of the analysis data, three international zircon standards of Plešovice, 91500 and GJ-1 are analyzed simultaneously with the actual samples. Plešovice is used for external standard calibration to further optimize the analysis and test results. 91500 and GJ-1 are used as the second standard to monitor the quality of data correction. The external precision (2SD) of Plešovice, 91500 and GJ-1 is better than 0.000020. The test value is consistent with the recommended value within the error range. At the same time, in order to monitor the test data of the high Yb/Hf ratio zircon, the internationally used high Yb/Hf ratio standard sample Temora 2 is used to monitor the test data of the high Yb/Hf ratio zircon. The Hf isotopic compositions of Plešovice,91500 and GJ-1 have been reported by Zhang et al. (2020）. References Hu, Z.C., Zhang, W., Liu, Y.S., Gao, S., Li, M., Zong, K.Q., Chen, H.H. and Hu, S.H., 2015. “Wave” Signal-Smoothing and Mercury-Removing Device for Laser Ablation Quadrupole and Multiple Collector ICPMS Analysis: Application to Lead Isotope Analysis. Analytical Chemistry, 87(2), 1152–1157. Hu, Z.C., Liu, Y.S., Gao, S., Liu, W., Yang, L., Zhang, W., Tong, X., Lin, L., Zong, K.Q., Li, M., Chen, H. and Zhou, L.,, Improved in situ Hf isotope ratio analysis of zircon using newly designed X skimmer cone and Jet sample cone in combination with the addition of nitrogen by laser ablation multiple collector ICP-MS, Journal of Analytical Atomic Spectrometry, 2012, 27, 1391–1399. Woodhead, J., Hergt, J., Shelley, M., Eggins, S. and Kemp, R., 2004. Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation. Chemical Geology, 209(1-2): 121-135. Fisher, C.M., et al., 2014, Guidelines for reporting zircon Hf isotopic data by LA-MC-ICPMS and potential pitfalls in the interpretation of these data, Chemical Geology, 363, 125-133. Blichert-Toft, J., Chauvel, C. and Albarède, F., Separation of Hf and Lu for high-precision isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS, Contributions to Mineralogy and Petrology, 1997, 127, 248–260. Liu, Y.S., Gao, S., Hu, Z.C., Gao, C.G., Zong, K.Q. and Wang, D.B., 2010. Continental and oceanic crust recycling-induced melt-peridotite interactions in the Trans-North China Orogen: U-Pb dating, Hf isotopes and trace elements in zircons of mantle xenoliths. Journal of Petrology, 51(1–2): 537–571. Zhang W , Hu Z , Spectroscopy A . Estimation of Isotopic Reference Values for Pure Materials and Geological Reference Materials[J]. Atomic Spectroscopy, 2020, 41(3):93-102.
