Retention of actinides in natural pyrochlores and zirconolites

Abstract

Natural pyrochlore and zirconolite undergo a crystalline-aperiodic transformation caused by alpha-decay of 232Th and 238U at dose levels between 2X1014 and 3X1017 α/mg. The principal effects of the transformation are volume expansion and micro-fracturing, 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 10 9 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. © 1994 R. Oldenbourg Verlag, München