Marikos and Hansman assembled a group of papers in early, 1987 on previously held views that radionuclide distributions in nature are determined by the often complex interplay of chemical transformations and nuclear transmutations. Chemical transformations occur at rates controlled by kinetic factors; equilibrium considerations serve as a feedback mechanism controlling the extent to which a reaction approaches completion. Nuclear transmutations are also controlled by kinetics, but the attendant "equilibria" are a result of the kinetic rates, not a control over them.
Chemical equilibrium is determined by the chemical and physical environments and the species present; therefore, the chemical kinetics are also controlled by these factors. On the other hand, the kinetics of radioactive decay are, for all practical considerations, independent of chemical or physical environments and depend only on the identity of the decaying nuclide.
At any particular time, a radioactive atom may decay to the next nuclide in its decay chain and thereby drastically change its geochemical behavior. In a perfectly closed system or a well-constrained open system, continual changes in chemical content would be predictable. In nature, however, there are few, if any, closed systems, and not many more that can be adequately constrained by our present knowledge. Because of the differences in behavior (particularly solubility) between the various members of a decay chain, considerable fractionation of the chain may result. As Wanty et al. (this volume) point out, chemical equilibrium forces (thermodynamics) therefore often oppose the establishment of secular equilibrium in the open systems of nature.
[Read on, an interesting assembly of views on natural radioactivity.]
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