PHLOGOPITE AND COESITE EXSOLUTION FROM SUPER-SILICIC CLINOPYROXENE

Abstract

We document the exsolution of phlogopite and coesite/quartz from pre-existing super-silicic clinopyroxene in dolomite marble from the Kokchetav massif, northern Kazakhstan. The exsolution texture was formed by clinopyroxene decomposition through the reaction (3enstatite + 2KalSi2O6)cpx = phlogopite + 4coesite. Phlogopite exsolution must have occurred at pressures less than 8.0 GPa (at 1000°C), where the garnet + super-silicic clinopyroxene + phlogopite assemblage was stable, based on experimentally defined phase relations. Observations described in this report suggest that the precursor clinopyroxene was stable at pressures higher than 8 GPa (>1000°C), implying that the dolomite marble was subducted to mantle depths greater than 240 km. Such pro-found subduction could be an important mechanism to transport abundant H2O and potassium into the deep Earth.