STABILITY OF PHLOGOPITE IN GRANITIC MELTS, AN EXPERIMENTAL INVESTIGATION

Water-saturated and water-undersaturated experiments (a(H2O) = 1.0 and 0.5) were performed in the temperature range 780-1040 degrees C at 2 and 5 kbar in order to determine the upper thermal stability of phlogo pite in granitic melts. Starting compositions were: (A) subaluminous m ixtures of 20 wt% synthetic phlogopite and 80 wt% synthetic anhydrous haplogranitic glass; (B) peraluminous mixtures (normative corundum = 4 %) of 20 wt% synthetic phlogopite and 80 wt% synthetic anhydrous peral uminous haplogranitic glass. The molar quartz: albite: orthoclase rati o of the glasses of the 2 kbar runs was 35:39:26 and that of the 5 kba r runs 30:42:28. In the subaluminous system, phlogopite is stable up t o 820 degrees C at a(H2O) = 1.0 and up to 780 degrees C at a(H2O) = 0. 5. At higher temperatures, it is replaced by enstatite. In the peralum inous system phlogopite has a remarkably higher thermal stability (up to 1000 degrees C at 5 kbar and a(H2O) = 1.0) and there is a temperatu re interval of 80 degrees C at a(H2O) = 1.0, and 90-100 degrees C at a (H2O) = 0.5 between the first appearance of enstatite and the disappea rance of phlogopite. In the peraluminous system, phlogopite is a solid solution (ss) of phlogopite, muscovite, talc and eastonite components . The crystalline product of the phlogopite,, breakdown reaction is an aluminous enstatite. The MgO-content of the melt depends on the norma tive corundum content of the starting material and the run temperature . It is independent of pressure. In the subaluminous system, the MgO-c ontent ranges between 0.05 and 0.3 wt% in the temperature interval 780 -880 degrees C at both investigated water activities. The MgO-content of the peraluminous melts at a(H2O) = 1.0 ranges between 0.4 and 1.7 w t% and at a(H2O) = 0.5 between 0.2 and 1.4 wt% in the temperature rang e 780-980 degrees C.