Phase relations of phlogopite with and without enstatite up to 8 GPa : implication to potassic magmatism and mantle metasomatism

Stability relations of phlogopite and phlogopite+enstatite systems without additional water have been experimentally determined at pressures of 4 - 8 GPa and at temperatures of 1200 - 1500˚C by using a uniaxial split-sphere apparatus. The phlogopite gradually dissociates into pyrope and Al 2O3-deficient phlogopite at pressures above 5 GPa. The Al2O3 content in the phlogopite decreases from 14.4 wt% at 6 GPa to 12.9 wt% at 8 GPa and concurrently the mode of pyrope increases from <5 modal% at 5.GPa to 20 - 30 % at 8 GPa, in the phlogopite system. Thus increase in pressure enhances two cation substitutions, Al+Al=Mg+Si and Mg+2Al= []+2Si, in the phlogopite structure. The solidi of the phlogopite and phlogopite+enstatite systems reach maximum temperatures of 1350˚C and 1300˚C, respectively, at about 5 GPa. Pyrope is a residual phase under hypersolidus conditions in both the systems above 5 GPa. The stabilities of phlogopite determined in the present study indicate that phlogopite can be stable in garnet harzburgite in the subcratonic lithosphere down to 210 km depth. The observed decomposition of the phlogopite into an assemblage of garnet and fluid or hydrous silicate melt suggests that phlogopite can be secondarily formed by a reaction of garnet and upwelling metasomatic agents near the base of continental lithosphere.