THE TRACE-COMPONENT TRAPPING EFFECT - EXPERIMENTAL-EVIDENCE, THEORETICAL INTERPRETATION, AND GEOCHEMICAL APPLICATIONS

Experimental data indicating increase of crystal-melt (fluid) partitio n coefficients in the range of microconcentrations of trace elements a re reviewed and analyzed in detail. This concentration dependence of p artition coefficients has been referred to as either deviations from H enry's law or the trace-component trapping effect. A critical review o f a variety of models proposed to explain this phenomenon is also give n. It is shown that the most reasonable and developed of these models relate changes in trace element partition coefficient at low concentra tions to interactions between the trace element ions and metastable la ttice defects (i.e., linear and planar defects) at low temperatures or intrinsic point defects of thermal origin at higher temperatures. The mechanism of interaction between trace element substituent atoms and intrinsic defects is considered in detail, with particular considerati on given to the creation of pair associates, coupled substitutions, an d the influence of other impurities on the trace element dissolution. The models developed are fit to the available experimental data to pro vide descriptions of the dependence of partition coefficients on compo sition and to estimate the concentrations and free energies of formati on of the intrinsic defects (i.e., vacancies and interstitial atoms) i n a matrix crystal. Some probable geochemical applications and manifes tations of the trapping effect are discussed. This leads to the conclu sion that there is an urgent need for further consideration of the pro blem. Copyright (C) 1998 Elsevier Science Ltd.