Solution mechanisms of silicate in aqueous fluid and H2O in coexisting silicate melts determined in-situ at high pressure and high temperature

摘要

The structure of H2O-saturated silicate melts, coexisting silicate-saturated aqueous solutions, and supercritical silicate liquids in the system Na2O4SiO2–H2O has been characterized with the sample at high temperature and pressure in a hydrothermal diamond anvil cell (HDAC). Structural information was obtained with confocal microRaman and with FTIR microscopy. Fluids and melts were examined along pressure-temperature trajectories defined by the isochores of H2O at nominal densities, qfluid, (from EOS of pure H2O) of 0.90 and 0.78 g/cm3. With qfluid = 0.78 g/cm3, water-saturated melt and silicate-saturated aqueous fluid coexist to the highest temperature (800 C) and pressure (677 MPa), whereas with qfluid = 0.90 g/cm3, a homogeneous single-phase liquid phase exists through the temperature and pressure range (25–800 C,0.1–1033 MPa). Less than 5 vol% quartz precipitates near 650 C in both experimental series, thus driving Na/Si-ratios of melt + fluid phase assemblages to higher values than that of the Na2O4SiO2 starting material.Molecular H2O (H2O) and structurally bonded OH groups were observed in coexisting melts and fluids as well as in supercritical liquids. Their OH/(H2O)-ratio is positively correlated with temperature. The OH/(H2O) in melts is greater than in coexisting fluids. Structural units of Q3, Q2, Q1, and Q0 type are observed in all phases under all conditions. An expression of the form, 12Q3 + 13H2O2Q2 + 6Q1 + 4Q0, describes the equilibrium among those structural units. This equilibrium shifts to the right with increasing pressure and temperature with a DH of the reaction near 425 kJ/mol.