Reverse age zonation of zircon formed by metamictisation and hydrothermal fluid leaching

Understanding the stability of radiation-damaged zircon is crucial to the application of zircon to U-Pb geochronology, especially for zircons rich in radioactive nuclides. Electron microprobe and laser Raman spectral analyses show that zircons from the Ejinao sodalite syenite in Southeast China are composed of cores without zonation and oscillatory rims. The cores are rich in uranium and have experienced increased metamictisation. An internal sieved structure and radial cracks were formed by radiation damage and radiation-induced volume expansion. Infrared spectral analyses show that zircon from the Ejinao sodalite syenite contains large amounts of OH and H ₂O. This finding suggests that the zircon crystals experienced hydrothermal alteration.The Pb loss of zircons is ascribed to their interaction with a hydrothermal fluid derived from the Ejinao syenite. Radiogenic Pb ⁴⁺ situated in α-recoil damaged sites has been significantly reduced to Pb ²⁺ by the hydrothermal fluid, becoming incompatible in the crystal lattice and highly mobile in solution; consequently, it has been leached out from the zircon. This Pb loss leads to diverse ²⁰⁶Pb/ ²³⁸U ages from 134Ma to 147Ma, and severe Pb loss from the inner core even produces reverse age zonations, with cores approximately 5Ma younger than the rims. The real crystallisation age for zircons from the Ejinao sodalite syenite should be approximately 145Ma, considering the entire range of analytical data obtained by both the SHRIMP and LA-ICPMS methods.