Phlogopite and coesite exsolution from super-silicic clinopyroxene

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 + 2KAlSi(2)O(6))(cpx) = phlogopite + 4coesite. Phlogopite exsolution must have occurred at pressures less than 8.0 GPa (at 1000degreesC), 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 (>1000degreesC), implying that the dolomite marble was subducted to mantle depths greater than 240 km. Such profound subduction could be an important mechanism to transport abundant H2O and potassium into the deep Earth.GeologySCI(E)41ARTICLE9831-8364