TEMPERATURE AND PRESSURE-DEPENDENCE OF QUARTZ AQUEOUS FLUID DIHEDRAL ANGLES - THE CONTROL OF ADSORBED H2O ON THE PERMEABILITY OF QUARTZITES

The dihedral angles of H2O-CO2 fluids in quartz aggregates have been determined experimentally at 450-1080-degrees-C and 1-9.5 kbar. At 4 kbar, the quartz-quartz-H2O dihedral angle increases linearly from values close to 60-degrees at 450-degrees-C to about 81-degrees at 600-degrees-C. Further increase in temperature has little effect on the angle until 900-degrees-C, whereupon the dihedral angle decreases at an increasing rate until values below 60-degrees are reached close to the melting point at 1098-degrees-C. Pure CO2 fluids have a constant dihedral angle of 98-degrees in the temperature range 600-1080-degrees-C. An equimolar H2O and CO2 fluid has a constant dihedral angle of 92-degrees in the temperature range 600-900-degrees-C which decreases with increasing temperature at approximately the same rate as the quartz-quartz-H2O angle until 70-degrees is attained at 1080-degrees-C. The pressure at which the quartz-quartz-H2O 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 H2O on the quartz-fluid interface with an adsorption density of about 8.5 molecules/nm2, and a thicker layer on the quartz grain boundary with an adsorption density of 0.04 moles/cm3. The quartz-quartz-H2O 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 H2O. 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.