Optimization of standard zircon U–Pb dating: insights into high-temperature thermal annealing

Abstract

The difference in radiation damage retained in unknown and reference zircons, caused by the α-dose matrix effect, has been considered as a great influence on the accuracy and precision of zircon U–Pb dating as found from LA-ICP-MS and SIMS analysis. This study aims to explore the effect of high-temperature thermal annealing (>1300 °C) on the optimization of the U–Pb dating of standard zircons, such as Sri Lanka, Plešovice, and Qinghu zircons, with heterogeneous radiation damage. Raman spectroscopy was employed to investigate the structural state of Sri Lanka zircons that were annealed at 100 °C to 1300 °C at various times (isochronal and isothermal annealing), indicating that annealing at 1400 °C for 96 h can result in a nearly complete crystalline state for zircons with a low-moderate degree of radiation damage. SIMS U–Pb isotopic analyses of unannealed and annealed Sri Lanka zircon grains revealed that the homogeneity of sample SL-3 annealed at 1300 °C with a ²⁰⁶Pb/²³⁸U age of 572.4 ± 4.4 Ma (2σ, MSWD = 0.08) is greater than those of the sample SL-2 annealed at 850 °C with ²⁰⁶Pb/²³⁸U age of 575.0 ± 4.5 Ma (2σ, MSWD = 0.57) and sample SL-1 unannealed with ²⁰⁶Pb/²³⁸U age of 575.3 ± 4.3 Ma (2σ, MSWD = 0.30). Furthermore, the results of LA-ICP-MS U–Pb ages and trace element concentrations for Plešovice and Qinghu zircons indicate that high-temperature thermal annealing can reduce the distribution of trace element composition and the scatters of the ²⁰⁶Pb/²³⁸U ages with MSWD decreasing by 82% and 71%, respectively, despite the annealed zircons yielding slightly younger ²⁰⁶Pb/²³⁸U ages than unannealed zircons. This approach provides new insights into acquiring homogeneous U–Pb ages of standard zircons with heterogeneous radiation damage, which would be useful to calibrate and obtain unknown zircon U–Pb dating with better precision and accuracy.