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

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 (D_α^P) is no more than 0.75×10¹⁸ g^-1 and the matrix effect is negligible. However, when the actual retained alpha dose (D_α^P) is larger than 0.75×10¹⁸ g^-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-D_α^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)-D_α^P correction curve to obtain D_α^P values of analyzed zircons by using age deviations-D_α^P empiric formula to determine the systematic deviations of LA-ICP-MS.