In situ U-Pb dating of titanite by LA-ICPMS

Titanite is an ideal mineral for U-Pb isotopic dating because of its relatively high U,Th and Pb contents.Here,we developed a technique for U-Pb dating of titanite using the 193 nm ArF laser-ablation system and Agilent 7500a Q-ICP-MS.Standards of titanite (BLR-1 and OLT-1) and zircon (91500 and GJ-1) were dated using single spot and line raster scan analytical methods.To check the matrix effect,titanite (BLR-1) and zircon (91500) standards were analyzed as the external standards.The weighted mean 206 Pb/238 U ages of OLT-1 titanite are 1015±5 Ma (2,n=24) and 1017±6 Ma (2,n=24) by single spot and line raster scan analyses,respectively,using BLR-1 titanite as the external standard.These ages are consistent with its reference age of about 1014 Ma.However,using 91500 zircon as the external standard,the weighted mean 206 Pb/238 U ages are 917±4 Ma (2,n=24) and 927±5 Ma (2,n=24) for BLR-1 titanite,and 891±4 Ma (2,n=24) and 901±5 Ma (2,n=24) for OLT-1 titanite by single spot and line raster scan analyses,respectively.It is evident that these ages are ～12% younger than their reference values.Our results reveal that significant matrix effect does exist between different kinds of minerals during LA-ICPMS U-Pb age determination,whereas it is insignificant between same minerals.Therefore,same mineral must be used as the external standard for fractionation corrections during in situ LA-ICPMS U-Pb age analysis.In addition,we determined U-Pb ages for titanites from the Early Cretaceous Fangshan pluton,which indicates a rapid cooling history of this pluton.