Temperature and pressure dependence of quartz-aqueous fluid dihedral angles: the control of adsorbed H 2O on the permeability of quartzites

The dihedral angles of H ₂O sbnd CO ₂ fluids in quartz aggregates have been determined experimentally at 450-1080°C and 1-9.5 kbar. At 4 kbar, the quartz-quartz-H ₂O dihedral angle increases linearly from values close to 60° at 450°C to about 81° at 600°C. Further increase in temperature has little effect on the angle until 900°C, whereupon the dihedral angle decreases at an increasing rate until values below 60° are reached close to the melting point at 1098°C. Pure CO ₂ fluids have a constant dihedral angle of 98° in the temperature range 600-1080°C. An equimolar H ₂O and CO ₂ fluid has a constant dihedral angle of 92° in the temperature range 600-900°C which decreases with increasing temperature at approximately the same rate as the quartz-quartz-H ₂O angle until 70° is attained at 1080°C. The pressure at which the quartz-quartz-H ₂O dihedral angle maximum occurs shows a positive correlation with temperature. It is shown that the observed temperature and pressure dependence of quartz-aqueous fluid dihedral angles may be due to the presence of an adsorbed layer of H ₂O on the quartz-fluid interface with an adsorption density of about 8.5 molecules/nm ², and a thicker layer on the quartz grain boundary with an adsorption density of 0.04 moles/cm ³. The quartz-quartz-H ₂O dihedral angle is contoured as a function of P and T, showing the metamorphic regimes for which quartz-rich rocks will be permeable to pervasive grain-edge flow of H ₂O. Windows of permeability exist at temperatures near the melting point and at much lower temperatures such as those at which infiltration-driven hydration reactions occur during retrogression.