Compressibility of hydrated and anhydrous Na2O‐2SiO2 liquid and also glass to 8 GPa using Brillouin scattering

Abstract

Brillouin scattering measurements have been carried out on an aqueous solution of Na2O‐2SiO2 and anhydrous Na2O‐2SiO2 glass and liquid under in situ pressures to 8 GPa in the temperature range from 300 to 850 K. This temperature range spans through the glass transition of both the hydrous and anhydrous forms of Na2O‐2SiO2. The “modified” platelet scattering geometry has allowed us for the first time to determine VP independently from refractive index and hence the adiabatic compressibility and density of liquids as a function of pressure and temperature. The observed increase in density of the melt and glass phases formed at high‐pressure‐temperature conditions is likely associated with structural effects. There is a marked change in pressure derivative of adiabatic bulk modulus (K′S = dKS/dP) between the two phases. The large values of K′S of the liquid phase illustrate that the means of compaction of the liquid differ substantially from those of the glass and that the liquid is able to access a wider range of compaction mechanisms. The measured bulk modulus of the aqueous sodium silicate solution is highly temperature‐dependent but is much more similar to values of silicate melts than to that of end‐member water, particularly at high pressures. Thus water‐rich silica‐bearing solutions present at depth are likely to be difficult to seismically distinguish from anhydrous silicate melts solely on the basis of their sound velocities. The data on water‐bearing compounds allow the pressure dependence of the partial molar volume of water to be assessed to upper mantle depths. These results are in excellent agreement with prior determinations and illustrate the utility of Brillouin experiments coupled with the externally heated diamond anvil cell in determining partial molar properties of liquids at high pressures.