发布时间 : 星期日 文章中国地质大学(武汉)原位微区锆石Hf同位素比值测试方法描述更新完毕开始阅读e6eba6553868011ca300a6c30c2259010302f313
原位微区锆石Hf同位素比值测试
原位微区锆石Hf同位素比值测试在中国地质大学(武汉)地质过程与矿产资源国家重点实验室(GPMR)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 2005 (Lambda Physik，德国), MC-ICP-MS为Neptune Plus (Thermo Fisher Scientific，德国)。该系统配备了本实验室自主研发的信号平滑装置。采用该装置即使激光脉冲频率降到1Hz，还可以获得平稳的信号(Hu et al., 2012a)。对于193nm的激光，在给定的仪器条件下，使用氦气作为载气比使用氩气的信号灵敏度提高了2倍(Hu et al., 2008a)。我们研究还表明，少量氮气的引入还可进一步提升大部分元素的灵敏度(Hu et al., 2008b)。相对于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5.3J/cm2。采用单点剥蚀模式，斑束固定为44μm。详细仪器操作条件和分析方法可参照Hu et al. (2012b)。
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βY 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。在本次试验中，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 。179Hf/177Hf 和173Yb/171Yb的比值被用于计算Hf (βHf) and Yb (βYb)的质量偏差。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。
In situ Hf isotope ratio analysis of zircon by LA-MC-ICP-MS
Experiments were conducted using a Neptune Plus MC-ICP-MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 excimer ArF laser ablation system (Lambda Physik, G?ttingen, Germany) that was hosted at the state Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences in Wuhan. 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., 2012a). The energy density of laser ablation that was used in this study was 5.3 J cm-2. Helium was used as the carrier gas within the ablation cell and was merged with argon (makeup gas) after the ablation cell. As demonstrated by our previous study, for the 193 nm laser a consistent 2-fold signal enhancement was achieved in helium than in argon gas (Hu et al., 2008a). We used a simple Y junction downstream from the sample cell to add small amounts of nitrogen (4 ml min-1) to the argon makeup gas flow (Hu et al., 2008b). 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 in this study. 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. (2012b).
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 normalised to
179
Hf/177Hf =0.7325 and 173Yb/171Yb=1.132685 [Fisher et al., 2014] using an exponential correction for mass bias.
176
Interference of
176176
Yb on
176
Hf was corrected by measuring the interference-free
173
Yb isotope and using
Yb/173Yb =0.79639 [Fisher et al., 2014] to calculate 176Yb/177Hf. Similarly, the relatively minor interference of Lu 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).
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