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

Abstract

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 206Pb/ 238U ages of OLT-1 titanite are 1015±5 Ma (2σ, n=24) and 1017±6 Ma (2σ, 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 206Pb/ 238U ages are 917±4 Ma (2σ, n=24) and 927±5 Ma (2σ, n=24) for BLR-1 titanite, and 891±4 Ma (2σ, n=24) and 901±5 Ma (2σ, 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. © 2012 The Author(s).