NEW DATA ON CS AND RB DISTRIBUTION BETWEEN POTASSIUM FELDSPAR AND ALKALINE FLUID: A STUDY OF THE "TRAPPING EFFECT"

Abstract

The effect of trapping a trace element, i.e., increasing element partitioning to the solid phase due to interaction of its atoms with crystal lattice defects, is an important phenomenon in trace-element behavior in geochemical systems. We have experimentally determined the coefficients of Rb and Cs cocrystallization with K in the K-feldspar-alkaline hydrothermal fluid system at 500°C and 1 kbar using highly sensitive methods with high-resolution for the analysis of solid phases (ICP-MS and ion microprobe). The fluid was sampled with a specially designed technique. Numerical modeling of Rb and Cs capture by K-feldspar crystal dislocation defects was based on real crystal structure data determined from X-ray powder diffraction measurements. Theoretical and experimental results show that, unlike Rb, Cs is accumulated in dislocation defects, and, at a low Cs content in K-feldspar, the Cs partition coefficient significantly increases. An inhomogeneous Cs distribution 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 <~10-4 mol % CsAlSi3O8 in K-feldspar. This tendency to increasing co-crystallization coefficients is also observed at larger CsAlSi3O8 concentrations of >0.4 mol %. New special experiments are needed to estimate the errors caused by the nonisothermal sampling of fluid or using residual liquids for modeling of the high-temperature fluid composition. Such experiments should also demonstrate the applicability of the analytical methods used for studying solid phases with very low trace-element concentrations.