Experimental Study of the Reaction Pyrope + Enstatite = Phlogopite in the Presence of H 2 O–KCl Fluid at 5 GPa in Application to the Problem of Modal Mantle Metasomatism

In order to model phlogopite formation in the upper-mantle garnet peridotites, it was performed an experimental study of the reaction En + 1/3Prp + [2/3KCl + 1/3H2O] = 1/3Phl + 1/3Cl-Phl in the presence of a H2O–KCl fluid with XKCl = 0.05–0.4 at pressure of 5 GPa and temperatures of 900–1250 °C. Experiments showed that the decrease of the Al content in orthopyroxene and the increase of the Cl content in phlogopite are good indicators of the KCl activity in the fluid. These relationships can be used to quantify the KCl activity during the metasomatic processes in the upper mantle.