激光剥蚀束斑大小及深度对锆石U-Pb定年准确度的影响

Assessing the Effects of Aperture and Depth of Hole Ablated by Laser Beam on the Accuracy of Zircon U-Pb Dating

激光剥蚀电感耦合等离子体质谱锆石U-Pb定年是目前矿物微区原位定年中使用最为广泛的技术。激光诱导的Pb/U分馏效应是影响锆石U-Pb定年准确度的重要因素。本文简要总结了激光剥蚀锆石过程中,不同的激光剥蚀束斑和深度对Pb/U元素分馏的影响程度,并探讨了激光束光学聚焦和深部剥蚀特征,指出由于剥蚀束斑的减小和深度的增加,气溶胶的颗粒分布和组成会发生变化,这是造成钻孔分馏效应的主要原因。在实践中,采用水蒸气辅助、激光束动态聚焦或者景深内剥蚀等手段可以有效改善分馏效应,而采取大的直径/深度剥蚀仍然是减小元素分馏效应的关键。

The zircon U-Pb dating by using laser ablation inductively coupled plasma mass spectrometry is currently the most widely used technique for in-situ micro-beam mineral dating.The Pb/U fractionation effect induced by laser ablation is an important factor affecting the accuracy of zircon U-Pb dating.In this paper,we have briefly reviewed the effects of different apertures and depths of laser ablation holes on Pb/U elemental fractionation during laser ablation on zircons,discussed the optical focusing and deep ablation characteristics of laser beams,and pointed out that downhole fractionation effect was mainly caused by the changes of particle distribution and composition of aerosol with the reduction of aperture and the increase of depth of laser ablation hole.In practice,the application of some methods,such as the water vapor-assisted ablation,active focus laser ablation,or ablation within depth of field,could effectively improve the Pb/U fractionation effect.However,the large diameter/depth of laser ablation hole is still the key factor for reducing the elemental fractionation.