New data on Cs and Rb distribution between potassium feldspar and alkalinefluid: A study of the "trapping effect"

The effect of trapping a trace element, i.e., increasing element partitioni ng to the solid phase due to interaction of its atoms with crystal lattice defects, is an important phenomenon in trace-element behavior in geochemica l systems. We have experimentally determined the coefficients of Rb and Cs cocrystallization with K in the K-feldspar-alkaline hydrothermal fluid syst em at 500 degreesC and I kbar using highly sensitive methods with high-reso lution for the analysis of solid phases (ICP-MS and ion microprobe). The fl uid was sampled with a specially designed technique. Numerical modeling of Rb and Cs capture by K-feldspar crystal dislocation defects was based on re al crystal structure data determined from X-ray powder diffraction measurem ents. Theoretical and experimental results show that, unlike Rb, Cs is accu mulated in dislocation defects, and, at a low Cs content in K-feldspar, the Cs partition coefficient significantly increases. An inhomogeneous Cs dist ribution at a microscopic scale at a generally uniform Rb distribution and some earlier experimental data suggest that a significant amount of Cs in K -feldspar is confined to dislocations, and the trapping effect is important for this element at < similar to 10(-4) mol % CsAlSi3O8 in K-feldspar. Thi s tendency to increasing co-crystallization coefficients is also observed a t larger CsAlSi3O8 concentrations of >0.4 mol %. New special experiments ar e needed to estimate the errors caused by the nonisothermal sampling of flu id or using residual liquids for modeling of the high-temperature fluid com position. Such experiments should also demonstrate the applicability of the analytical methods used for studying solid phases with very low trace-elem ent concentrations.