THE EFFECT OF FELDSPAR ON QUARTZ-H2O-CO2 DIHEDRAL ANGLES AT 4 KBAR, WITH CONSEQUENCES FOR THE BEHAVIOR OF AQUEOUS FLUIDS IN MIGMATITES

An investigation was made of the effect of trace amounts of feldspar (Na and/or K) on dihedral angles in the quartz-H2O-CO2 system at 4 kbar and 450-1050 degrees C. Quartz-quartz-H2O dihedral angles in feldspar-bearing quartz aggregates are observed to be the same as those in pure quartz aggregates at temperatures below 500 degrees C. Above this temperature, they decrease with increasing temperature until the solidus. The final angle at the inception of melting is about 65 degrees or microcline-quartz-H2O and microcline-albitequartz-H2O, and much less than 60 degrees (the critical value for formation of grain-edge fluid channels in an isotropic system) for the albite-quartz-H2O system. CO2 was observed to produce a constant quartz-quartz-fluid dihedral angle of 97 degrees in feldspar-bearing quartz aggregates at all temperatures studied. Also examined were the dihedral angles for the two co-existing super-solidus fluids in quartz aggregates. In all systems the quartz-volatile fluid angle is greater than 60 degrees, whereas the quartz-melt angle is lower than 60 degrees. Both super-solidus angles decrease with increasing temperature. The transition from nonconnected to connected porosity with increasing temperature observed in the quartz-albite-H2O system some tens of degrees below the solidus (termed a permeability transition), if a common feature of rocks near their melting points, will play an important role in controlling the permeability of high-grade rocks to aqueous fluids.