Theoretical and practical aspects of uranium geology - The geochemical cycle of uranium

If the uranium content of the mantle is based on data from basalts, a homogeneous mantle would contain about 10 parts U/109. This gives a similar mass of uranium in mantle and crust. Sea floor processes may enrich the surface layers of basalt in uranium but do not significantly change the mass of uranium subducted in basalts. If pelagic sediments are subducted, they contribute a significant mass of uranium to the mantle. Present rates of andesite production do not seem adequate to return all subducted uranium to the crust but andesites may make a significant contribution to uranium which eventually appears in granitic rocks. Uranium transport and accumulation in sediments may be dominantly biochemical. The problem of uranium distribution in the mantle during evolution of the Earth is unresolved. The main models of crustal evolution suggest continual depletion, early depletion, or early depletion followed by slow replenishment; a constant mantle composition seems unlikely.