Retention of Actinides in Natural Pyrochlores and Zirconolites

Natural pyrochlore and zirconolite undergo a crystalline-aperiodic transformation caused by alpha-decay of 232Th and 2380 at dose levels between 2 X 1014 and 3 X 1017 a/mg. The principal effects of the transformation are volume expansion and microfracturing, providing potential pathways for fluids. Geochemical alteration of the minerals may occur under hydrothermal conditions or in low temperature, near surface environments, but Th and U usually remain immobile and can be retained for time scales up to 109 years. However, the Th-U isotope systematics of a zirconolite-bearing vein and dolomite host rock may provide evidence for disequilibrium between 230Th, 234U and 238U.