Structural and dynamical properties of supercritical H2O–SiO2 fluids studied by ab initio molecular dynamics

• We modeled supercritical H2O–SiO2 fluid, as a proxy to high-silica fluids in the continental wedge of subduction zones.
• Network breaking and forming frequently occurred according to reaction SiOSi + H2O = 2 SiOH.
• Reaction constant K = [OH−]2/[H2O][O2 −] was calculated for 3000 K and 2400 K and compared to experimental extrapolation.
• The Si–O bond autocorrelation function shows the mean Si–O bond lifetime as 26 ps at 3000 K, rocketing to 200 ps at 2400 K.
• Comparison of 192-atom and 96-atom models demonstrates a crucial influence of model size on the amount of molecular water.

In this study we report the structure of supercritical H2O–SiO2 fluid composed of 50 mol% H2O and 50 mol% SiO2 at 3000 K and 2400 K, investigated by means of ab initio molecular dynamics of models comprising 192 and 96 atoms. The density is set constant to 1.88 g/cm 3, which yields a pressure of 4.3 GPa at 3000 K and 3.6 GPa at 2400 K. Throughout the trajectories, water molecules are formed and dissociated via the network modifying reaction 2 SiOH = SiOSi + H2O. The calculation of the reaction constant K = [OH-]2/[H2O][O2-] is carried out on the basis of the experimentally relevant Qn species notation and agrees well with an extrapolation of experimental data to 3000 K. After quench from 3000 K to 2400 K, the degree of polymerization of the silicate network in the 192-atom models increases noticeably within several tens of picoseconds, accompanied by release of molecular H2O. An unexpected opposite trend is observed in smaller 96-atom models, due to a finite size effect, as several uncorrelated models of 192 and 96 atoms indicate. The temperature-dependent slowing down of the H2O–silica interaction dynamics is described on the basis of the bond autocorrelation function.