Mechanisms of uranium and thorium transfer to the crust
A variety of transfer mechanisms can be conceived which are compatible with the evidence. It is assumed that the initial condition of all three radioelements was one of more or less homogeneous dispersion. If the earth accreted from solid fragments or dust, homogeneous dispersion is not to be expected; but it would be expected if the earth consolidated from gases. Broadly, matter can be transferred in three states: solid, liquid, or gas. Five mechanisms involving the three states are:
Bodily transfer of large masses without melting, by creep.
Bodily transfer of large masses by total melting.
Transfer of elements selected at depth by partial melting and concentrated at depth into a differentiated magma.
Gas and/or Liquid
Upward volatile diffusion of selected elements or ions without significant rock melting. These elements would remain dispersed and low in concentration. They would appear in the’crust as pervasive, widespread disseminations.
Upward transfer of large masses of gases or fluids selectively volatilized and collected and concentrated at depth. These would appear in the crust as concentrated hydrothermal fluids.
At the outset of any of the transfer processes, the three radioelements might be considered to act similarly and together because their dispersions, abundances, and chemistry would be reasonably comparable. However, the fact that this similarity of action has not persisted into the crust is apparent because of the different degrees of enrichment in different rocks. In the crust, uranium and thorium continue to act similarly to produce trace-element disseminations through the magmatic stage, but potassium concentrates sufficiently at an early stage to become a rock-forming constituent. Eventually, uranium and thorium separate completely to produce independent deposits in the low-temperature realm.