LI Qiu-li

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

Abstract: "High-U effect" is used to describe a phenomenon during SIMS zircon U-Pb dating that the apparent Pb/U ages of high-U zircons show positive correlation with their high U contents. In this paper, based on reviewing many investigations on radiogenic damage effect on the lattice of zircons by the U-Pb decay, and the structural change of damaged zircon caused by the thermal annealing effect, and the efficiency differences between Pb and U ionization of zircons by using SIMS, it is believed that the "High-U effect" could be mainly caused by the mixture state of crystalline crystals of ZrSiO4, ZrO2 and amorphous state of SiO2 for the radiogenic damaged zircons,as the radiogenic Pb is in favor to combine with the amorphous SiO2, while the U is in favor to enter into the lattice of ZrO2, resulting in the efficiency differences between Pb and U ionization during SIMS analysis. In addition, based on the statistics, the high-U zircon not only has "High-U effect", but also Pb-depletion effect due to the radiogenic damage, resulting in the relatively big uncertainties for the U-Pb dating results. Therefore, it is believed that the high-U zircon is not suitable and should be avoided for precise SIMS U-Pb dating. It is recommended that some other minerals, such as monazite, xenotime, and titanite, could be good substitutes of the high U zircon for SIMS U-Pb dating.

Keywords: SIMS zircon U-Pb dating high U effect radiogenic damage

E-Mail: liqiuli@mail.iggcas.ac.cn

Bulletin of Mineralogy, Petrology and Geochemistry Vol.35, No.3, 2016, page 405-412