Experimental study on the sorption of rare-earth elements and other trace elements during rhyolite-hydrothermal water interactions

The behaviour of the rare-earth elements (REE), Sc and Ba during rhyolite-hydrothermal water interactions has been investigated experimentally. Both leaching and sorption experiments were performed using various solutions (i.e., pure water, dilute HCl solution, the solution doped with REE, Sc and Ba). Most runs were at 250°C and 1 kbar. The mobility of these elements in our hydrothermal reactions was found to be dependent on the concentration of HCl (or pH) in the solution. In near-neutral solutions, REE showed considerable mobility but no relative fractionation, whereas fractionation did occur between REE and Sc. In dilute HCl solution REE behaviour is quite complex: in sorption experiments, REE remained in 2 N HCl solution, whereas in 0.1–0.6 N HCl solutions REE were readily incorporated into the solid. In contrast, Ba showed strong affinity for the dilute HCl solution. A large fractionation between light and heavy REE was observed in an intermediate case between high and low HCl concentrations. The mechanisms of these trace-element behaviours are not clearly understood, but ion exchange properties of a thin surface layer may be important.