Abstract
Experiments to determine silicate structural species in silicate-saturated aqueous fluids in equilibrium with silica polymorphs (quartz and coesite), enstatite, and enstatite+forsterite in the SiO2-H2O and MgO-SiO2-H2O systems have been carried out in situ in the 0.4-5.4GPa and 700-900 degrees C pressure and temperature ranges, respectively. MicroRaman spectroscopy was the structural probe. In the SiO2-H2O system (1.6-5.4GPa/700-900 degrees C), the detected silicate species are Q(0) (SiO44-), Q(1) (0.5 Si2O76-), and Q(2) (SiO32-). The expression 2Q(1)Q(0)+Q(2) describes the equilibrium among these species with H and V values from the isochoric temperature and isothermal pressure dependence of its equilibrium constant, K=XQ0XQ2/(X-Q1)(2), range from -23 to -69kJ/mol and -1 to -2cm(3)/mol, respectively. In the system MgO-SiO2-H2O the calculated silica solubility, using literature algorithms, is approximately 50% of that in the SiO2-H2O system at similar temperature and pressure. Only Q(1) and Q(0) species were detected in the MgO-SiO2-H2O fluids, whether in equilibrium with enstatite+forsterite (P<3GPa) or enstatite only (P>3GPa). The temperature and pressure dependence of the equilibrium constant, K=X-Q1/X-Q0, for this system yields average values of H=405kJ/mol and V=-2.30.4cm(3)/mol. The speciation of silicate in aqueous fluids resembles that in hydrous melts as a function of temperature and pressure at deep crustal and upper mantle temperature and pressure conditions, and they become increasingly similar with depth. As the silicate speciation and solubility in the aqueous fluid depend on silicate composition, the pressure and temperature at which complete miscibility occurs will also vary with silicate composition. The structural similarity between fluids and melts will also lead to fluid/melt element partition coefficients trending toward 1 and mineral/fluid partition coefficients trending toward mineral/melt values in the upper mantle as the silicate-H2O systems approach complete miscibility with increasing temperature and pressure.