Does diffusion change the rare earth patterns of igneous rocks?

Abstract

We examined the possibility of diffusion-controlled fractionation of the rare earth elements during igneous processes on the basis of lattice diffusion, by using available sets of diffusivity data on olivine and melilite. During partial melting of olivine, it is shown with a simple model that Fe and Mn are more enriched in the early stage liquid phase, while Ca, with lower diffusivity, is less enriched in the liquid. This implies that, during partial melting of olivine, the heavy rare earth elements with ionic radius similar to that of Mn can be more enriched in the early stage liquid phase relative to the light rare earth elements with ionic radius similar to that of Ca, contrary to what would be expected from equilibrium partition. Low inter-diffusivity of Mg + Si and Al + Al pairs in akermanitic melilite suggests that diffusivities of the rare earth elements in olivine are low, and that the effect of diffusion would continue for a longer period of time for trivalent rare earth elements than for divalent cations. Eu2+ also possibly behaves quite differently from trivalent rare earth elements during diffusion, and thus may produce Eu anomalies that are not expected by equilibrium partition. Diffusion can produce widely different distributions of the rare earth and other trace elements even in a single system depending on the time scale of the diffusion process. However, since natural systems are much more complicated than models, it is not clear whether the calculated results have practical significance in explaining the distribution of trace elements in nature.