Physics
Scientific paper
Jun 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993e%26psl.117..363h&link_type=abstract
Earth and Planetary Science Letters, Volume 117, Issue 3-4, p. 363-377.
Physics
35
Scientific paper
The dihedral angles of H2O-CO2 fluids in quartz aggregates have been determined experimentally at 450-1080°C and 1-9.5 kbar. At 4 kbar, the quartz-quartz-H2O 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 CO2 fluids have a constant dihedral angle of 98° in the temperature range 600-1080°C. An equimolar H2O and CO2 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-H2O angle until 70° is attained at 1080°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.
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