EXPERIMENTAL NA-K DISTRIBUTION BETWEEN AMPHIBOLES AND AQUEOUS CHLORIDE SOLUTIONS, AND A MIXING MODEL ALONG THE RICHTERITE-K-RICHTERITE JOIN

The cation exchange equilibrium (1) Na - richterite + KCl(aq) = K - ri chterite + NaCl(aq) has been investigated by hydrothermal experiments at 700 and 800 degrees C at 200 MPa. To avoid equilibration problems o f conventional exchange experiments, we synthesized amphiboles with an excess fluid allowing exchange between solid and fluid during the exp eriment. The exchangeable cations Na and K were provided as excess 1 t o 2n chloridic solution. These exchange syntheses can be described by the reaction equation [GRAPHICS] with ((ag)) For hydroxides and chlori des in aqueous solutions and ((s)) and ((p)) = start and product fluid . The amphiboles grew in presence of the exchange fluid and adjusted t heir stoichiometry in equilibrium with the fluid phase. The solid prod ucts consist of more than 99% amphibole (Na,K-richterite(55)) with tra ces of diopside and quartz. The amphiboles are up to 1 mm long and oft en approximate to 40 mu m thick. Detailed EMP- and HRTEM-observations show that they are chemically homogeneous and structurally wellordered . The experimental results give consistent phase relations in the reci procal ternary system Na-richterite-K-richterite-NaCl-KCl. We analysed the product fluid with AAS- and ICP-methods. The Na-K distribution co efficients between fluid and amphiboles of the richterite-K-richterite join are close to unity at 700 degrees C and 800 degrees C at 200 MPa . Small systematic deviations are explained by a symmetric solution mo del for the A-position of the amphiboles. Using ideal mixing for H2O-N aCl-KCl fluids, a mixing model for the system richterite-K-richterite is presented. We suggest that the composition of richterite solid solu tions can be used as a sensor for NaCl/KCl-ratios in metamorphic fluid s.