U-Pb定年锆石标准~(176)Hf/~(177)Hf比值的多接收器电感耦合等离子体质谱测定

利用MicromassIsoprobe型多接收器电感耦合等离子体质谱(MCICPMS)测定UPb定年国际锆石标准91500、Temora和本实验室锆石工作标准样We1的176Hf/177Hf比值。对添加Zr的室内Hf标准溶液HfGIG的模拟实验表明:Zr/Hf≤120时,Zr对Isoprobe型MCICPMS的Hf同位素比值测定值没有明显的影响,所以锆石Hf分离纯化流程采用改进的高温高压氢氟酸溶解和其后的一个离子交换分离流程实现,整个流程Hf的回收率高于90%。176Hf/177Hf比值测定采用JMC475Hf标准溶液(176Hf/177Hf=0.282160)进行外部校正并监控仪器漂移。标准样91500、Temora和We1平均的176Hf/177Hf比值测定结果分别为(0.282310±0.000034)、(0.282706±0.000020)和(0.281534±0.000009)。上述每个结果包括至少7组独立的锆石样品的溶解、分离和测定,表明标准样91500、Temora和We1是Hf同位素组成均一的。本实验结果对于激光熔蚀多接收器电感耦合等离子体质谱(LAMCICPMS)锆石原位176Hf/177Hf比值测定校正方法的检验具有重要意义。

High-precision Measurement of Hf Isotopic Reference Values for the U-Pb Geochronology Standard Zircons by Multi-collector Inductively Coupled Plasma-Mass Spectrometry

The Hf isotope ratios were determined by a Micromass Isoprobe multiple collector-inductively coupled plasma mass spectrometry (MC-ICP-MS) for two international zircon standards 91500 and Temora and one in-house zircon standard We-1. Zircons were digested by HF in high-pressure bombs. For the reason that Zr did not change Hf isotopic ratios in the simulative experiment, Hf was separated using a modified one-step ion-exchange chromatography reported by Arnard et al.. The overall recovery of Hf was better than 90%. Calibrated by an external standard solution JMC-475 Hf, 176Hf/ 177Hf ratios of these standard zircons were precisely determined. The average 176Hf/ 177Hf values of 91500 and Temora and We-1 were (0.282 310±0.000 034), (0.282 706±0.000 020) and ( 0.281 534±0.000 009), respectively. Considering that the Hf isotope results above consist of at least seven separated performances of aliquots for each zircon sample, the 176Hf/ 177Hf ratios of 91500, Temora and We-1 were homogeneous. These results have significant implications for testing the calibration method in zircon Hf isotopic determination by laser-ablation multiple collector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS).