PHASE-RELATIONS OF NATURAL PHLOGOPITE WITH AND WITHOUT ENSTATITE UP TO 8 GPA - IMPLICATION FOR MANTLE METASOMATISM

Phase relations of natural phlogopite and phlogopite + enstatite compo sitions were determined experimentally at pressures of 4-8 GPa and tem peratures of 1200-1500 degrees C using a uniaxial split-sphere apparat us. Phlogopite in both compositions is stable in the subsolidus up to 5 GPa, and it breaks downs into pyrope, Al2O3-deficient phlogopite and fluid at pressures above 5 GPa. The Al2O3 content in phlogopite decre ases from 14.4 wt% at 6 GPa to 12.9 wt% at 8 GPa, and the mode of pyro pe increases concurrently from < 5 vol.% at 5 GPa to 30% at 8 GPa, in the phlogopite composition. Thus, an increase in pressure enhances two cation substitutions, Al + Al = Mg + Si and Mg + 2Al = V (vacancy) 2Si, in the phlogopite structure. The solidi of the two compositions r each maximum temperatures of 1350 degrees C and 1300 degrees C, respec tively, at about 5 GPa. Pyrope is a residual phase under suprasolidus conditions in both compositions above 5 GPa. These results indicate th at phlogopite can be stable in garnet harzburgite in subcratonic litho sphere down to 200 km in depth. The observed decomposition of phlogopi te into an assemblage of garnet and either fluid or hydrous silicate m elt, suggests that phlogopite can be formed secondarily by a reaction between garnet and upwelling metasomatic agents near the base of the c ontinental lithosphere.