COMPARATIVE-STUDY OF U-PB AND U-XE ISOTOP E SYSTEMS OF METAMICT ZIRCONS UNDER HIGH-TEMPERATURES AND PRESSURES

A comparative laboratory hydrothermal study of migration properties of Pb and Xe has been conducted for metamict zircon 9-A from Uros lake m etasomatites (Central Karelia). Even starting natural sample clearly s howed discordant and too young U-Pb age (Pb-206/U-238 - 2.1 Ga, and Pb -206/U-238 - 2.3 Ga) as compared to true age of this zircon - 2.5 Ga. Five Xe(s)-Xe(n) age spectra of initial and hydrothermally treated zir con were obtained. Two samples - initial one and treated under the ext remely low conditions (1 kbar pressure of 2M NaCl solution vapor, temp erature 300-degrees-C, 72 hours) showed Xe(s)-Xe(n) age 2.34 +/- 0.16 and 2.73 +/- 0.36 Ga close to the true age. Supposition has been made that U-Xe isotope system is preserved better under the definite condit ions than U-Pb one.