EXPERIMENTAL-STUDY OF A BIOTITE-BEARING GRANITIC SYSTEM UNDER WATER-SATURATED AND WATER-UNDERSATURATED CONDITIONS

Phase relationships in haplogranitic melts containing ferromagnesian minerals were investigated in the temperature range 760-850°C, at Ptotal=2 kbar, {Mathematical expression}=0.25 1.00, and at NNO buffer conditions. The compositions of coexisting biotite, orthopyroxene, magnetite and granitic melt were determined by electron microprobe. Biotite was the only Fe-Mg mineral in the starting material. It disappeared in water-undersaturated melts at temperatures above 830°C and in melts saturated with water above 810°C. Magnetite was present in all runs. Orthopyroxene became stable between 780 and 800°C. The Mg/(Mg+Fe) ratios of biotite (0,63-0,72) and orthopyroxene (0,63-0,71) are similar in water-saturated and undersaturated melts at comparable temperatures and increase slightly with increasing temperature. The solubility of Mg in haplogranitic melts is very low (0,07-0,24 wt% MgO). It increases slightly with temperature and is independent of the prevailing water activity. The solubility of Fe is low (0,91-1,37 wt% FeO); it also increases with increasing temperature, and it is higher in water-saturated than in water-undersaturated melts. © 1990 Springer-Verlag.