LA-ICP-MS独居石的U(Th)-Pb年龄精确测定方法及地质意义探究

LA-ICP-MS Monazite U(Th)-Pb Dating and Geological Applications

独居石[(LREE, Th)PO4]在各类地质样品中广泛存在且具有Th含量高和普通铅低的成分特征,可作为U(Th)-Pb定年的合适矿物。本文利用中国地质大学(北京)元素地球化学实验室的激光等离子体质谱仪(LA-ICP-MS),建立了以独居石(44069)为外标的LA-ICP-MS独居石U(Th)-Pb年龄测定方法,通过对捷克南部Bohemian Massif麻粒岩地块长英质伟晶岩(脉)中独居石的对比测定,讨论了不同检测条件(积分时间、激光剥蚀取样方式(点剥蚀和线扫描)、激光束斑直径(25μm和12μm)、数据处理软件参数)对独居石U-Th-Pb同位素体系质量分馏的影响。同位素比值和年龄结果对比表明,25μm点垂直取样和25μm线扫描取样的分析精度最优;Th-Pb同位素体系的质量分馏略高于U-Pb同位素体系,但误差明显小于锆石,提出独居石的^(208)Pb/^(232)Th年龄与^(206)Pb/^(238)U年龄结果误差范围一致,可作为含高U锆石地质体定年的新途径。对东昆仑清水泉麻粒岩带变质岩中独居石和锆石年龄结果的对比分析,揭示独居石年代学研究可有效地约束缺少锆石生长的沉积变质岩系变质作用时限。LA-ICP-MS独居石U(Th)-Pb定年方法在区域变质、沉积成岩作用、S型花岗岩的精细研究中有广泛应用前景。

Monazite[(LREE,Th)PO4],widespread in most geological samples,has high Th and low common lead contents,and can thus be used for U(Th)-Pb dating.In this paper,monazite LA-ICP-MS U(Th)-Pb age determination method with monazite(44069)as external standard was established,using laser ablation inductively coupled plasma mass spectrometer(LA-ICP-MS)of the Element Geochemistry Laboratory at the China University of Geosciences(Beijing).Monazite from the Bohemian Massif granulite were selected to analyze,aiming to address the mass fractionation of monazite U(Th)-Pb isotope system under different conditions,such as the integration time,denudation conditions(point denudation and line scanning),beam spot diameter(25μm and 12μm)and parameters of data processing software.Isotopic ratio and age results show that the analytical accuracy is the best under 25μm vertical and 25μm line scanning mode,and that the mass fractionation of Th-Pb isotope system is slightly higher than that of U-Pb isotope system.The mass fractionation between Th-Pb and U-Pb system of monazite,however,is significantly smaller than that of zircon.We suggest that the monazite ^(208)Pb/^(232)Th age is consistent with the ^(206)Pb/^(238)U age within error,which can be used as a new way to date geological bodies with high-U zircon.Based on the age results of monazite and zircon from metamorphic rocks of the Qingshuiquan granulite belt in East Kunlun,it is revealed that monazite geochronology can effectively constrain the metamorphic age,especially in sedimentary-metamorphic rocks that lack zircon.LA-ICP-MS monazite U(Th)-Pb dating method will be widely used in determining regional metamorphism,sedimentary diagenesis and S-type granite intrusion.