应用激光拉曼光谱研究锆石LA-ICP-MSU-Pb定年中的α通量基体效应

Laser Raman Spectroscopic Study and LA-ICP-MS U-Pb Dating of Zircons from the Badaling Granitic Complex

α通量基体效应是由标准锆石与样品锆石之间的晶体损伤(以α通量表示)不同引起的激光剥蚀速率和坑下分馏行为的差异,已被证实是导致锆石LA-ICP-MS U-Pb定年结果存在系统偏倚的重要原因。α通量越高,剥蚀速率越快,坑下分馏越明显,然而α通量基体效应的校正尚未引起足够的重视。本文采用激光拉曼光谱和LA-ICP-MS对八达岭花岗杂岩样品进行研究,结果表明:实际α通量(D_α~P)≤0.75×1018g-1的锆石样品,蜕晶化程度较弱,其定年结果存在的α通量基体效应可以忽略;D_α~P>0.75×1018g^(-1)的锆石样品,蜕晶化程度较高,其定年结果受α通量基体效应的影响明显,依据年龄差-D_α~P经验方程如y=347.8×exp(0.260×10^(-15)x)进行校正可获得准确的年龄结果。本研究认为:采用激光拉曼光谱和半高宽(FWHM)-D_α~P校准曲线获得目标锆石D_α~P,利用年龄差-D_α~P经验方程估计LA-ICP-MS系统偏倚,是校正α通量基体效应的可行途径。

Alpha dose matrix effect is the discrepancy of ablation rate and down-hole fractionation induced by the difference of crystal damage degree （ alpha dose ） between standard zircons and sample zircons. It has been proved to be the cause of systematic bias of zircon U-Pb dating by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry （ LA-ICP-MS ）. Alpha dose is closely related to ablation rate and down-hole fractionation behaviour of zircon. The higher the alpha dose , the faster the ablation rate and more obvious is the down-hole fractionation. However , the correction of alpha dose matrix effect has received little attention thus far. In this study,Laser Raman Spectroscopy and LA-ICP-MS were used to analyze zircons from the Badaling Granitic Complex rocks. Results show that the metamictization degree is low when the actual retained alpha dose （Dα^P ） is no more than 0.75×1018g^-1 and the matrix effect is negligible.However, when the actual retained alpha dose （Dα^P） is larger than 0. 75 ×1018g^-1 , metamictization degree is high and the dating results are obviously affected byalpha dose matrix effect. For this reason,U-Pb data of zircons need to be corrected using the ‘ Age offset-Dα^P’ empirical equation such as y=347.8×exp（0.260×10^-15x）. In conclusion, it was found that this is a feasible method to correct alpha dose matrix effect by using Laser Raman Spectroscopy and a full width at half maximum （ FWHM）-Dα^P correction curve to obtain values of analyzed zircons by using age deviations-Dα^P empiric formula to determine the systematic deviations of LA-ICP-MS.