微區原位鋯石Hf同位素比值測試在武漢上譜分析科技有限責任公司利用激光剝蝕多接收杯等離子體質譜（LA-MC-ICP-MS）完成。激光剝蝕系統為Geolas HD （Coherent，德國）, MC-ICP-MS為Neptune Plus（Thermo Fisher Scientific，德國）。分析過程同時配備了信號平滑裝置以提高信號穩定性和同位素比值測試精密度（Hu et al. 2015）。載氣使用氦氣，并在剝蝕池之后引入少量氮氣以提高Hf元素靈敏度（Hu et al. 2012）。分析采用Neptune Plus新設計高性能錐組合。前人研究表明，對于Neptune Plus的標準錐組合，新設計的X截取錐和Jet采樣錐組合在少量氮氣加入的條件下能分別提高Hf、Yb和Lu的靈敏度5.3倍、4.0倍和2.4倍。激光輸出能量可以調節，實際輸出能量密度為~7.0 J/cm2。采用單點剝蝕模式，斑束固定為44 μm。詳細儀器操作條件和分析方法可參照(Hu et al.2012)。 采用LA-MC-ICP-MS準確測試鋯石Hf同位素的難點在于176Yb和176Lu對176Hf的同量異位素的干擾扣除。研究表明，Yb的質量分餾系數(βYb)在長期測試過程中并不是一個固定值，而且通過溶液進樣方式測試得到的βYb 并不適用于激光進樣模式中的鋯石Hf同位素干擾校正（Woodhead et al. 2004）。βYb 的錯誤估算會明顯地影響176Yb對176Hf的干擾校正，進而影響176Hf/177Hf比值的準確測定。在實際中，我們實時獲取了鋯石樣品自身的βYb用于干擾校正。179Hf/177Hf =0.7325和 173Yb/171Yb=1.132685（Fisher et al. 2014）被用于計算Hf和Yb的質量分餾系數βHf和βYb。使用176Yb/173Yb =0.79639（Fisher et al. 2014）來扣除176Yb 對 176Hf的同量異位干擾。使用176Lu/175Lu =0.02656（Blichert-Toft et al. 1997）來扣除干擾程度相對較小的176Lu對 176Hf的同量異位干擾。由于Yb和Lu具有相似的物理化學屬性，因此在本實驗中采用Yb的質量分餾系數βYb來校正Lu的質量分餾行為。分析數據的離線處理（包括對樣品和空白信號的選擇、同位素質量分餾校正）采用軟件ICPMSDataCal（Liu et al. 2010）完成。 為確保分析數據的可靠性，Plešovice、91500和GJ-1三個國際鋯石標準與實際樣品同時分析，Plešovice用于進行外標校正以進一步優化分析測試結果。91500和GJ-1作為第二標樣監控數據校正質量。Plešovice、91500和GJ-1的外部精密度（2SD）優于0.000020。測試值與推薦值確保在誤差范圍內一致。同時為了監控高Yb/Hf比值鋯石的測試數據，采用國際常用的高Yb/Hf比值標樣Temora 2監控高Yb/Hf比值鋯石的測試數據,。以上標樣推薦值請參考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. 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