Contributions of isotope research to the knowledge of the distribution coefficients of rare earth elements between natural aluminosilicates and their molten phases

The distribution coefficients D of yttrium and the lanthanides between solid and liquid natural aluminosilicates can be described in terms of an extension of Schuetze's incremental method for calculating oxygen isotope effects in silicate systems: log D = k + S x ΔI, where D is the ratio of the concentration range in the solid phase to the concentration in the liquid phase, ΔI the difference of the 18O indices of the liquid and the solid phase and k and S coefficients to be determined empirically. The coefficient k accounts for the effect of the aggregational state, particularly the water content, S accounts for the effect of the degree of linking of the (Si, Al)O4 tetrahedrons in the phases being in equilibrium on the distribution coefficients. The dependence of D, k and S on the atomic number can be understood in terms of the radii of the hydrated and non-hydrated lanthanide ions. Conclusions as to the structural properties of magmas are discussed. (author)