The effect of feldspar on quartz-H2O−CO2 dihedral angles at 4 kbar, with consequences for the behaviour of aqueous fluids in migmatites
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- Holness, M.B. Contr. Mineral. and Petrol. (1995) 118: 356. doi:10.1007/s004100050020
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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°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°C. Above this temperature, they decrease with increasing temperature until the solidus. The final angle at the inception of melting is about 65° for microcline-quartz-H2O and microcline-albite-quartz-H2O, and much less than 60° (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° in feldspar-bearing quartz aggregates at all temperatures studied. Also examined were the dihedral angles for the two co-existing supersolidus fluids in quartz aggregates. In all systems the quartz-volatile fluid angle is greater than 60°, whereas the quartz-melt angle is lower than 60°. Both supersolidus 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.