Sodium Silicates at the Boundary between the Transition Zone and the Lower Mantle: Compositional and Structural Patterns

The new results of experimental study of the Na 2 MgSiO 4 –Mg 2 SiO 4 section of the SiO 2 –MgO–Na 2 O model system at 22–24 GPa and 1600–1900°C are discussed in this paper. An association of Na-bearing ringwoodite (up to 0.42 wt % Na 2 O) and the phase with the composition Na 2 MgSiO 4 with a cristobalite-type structure was obtained at 22 GPa. Incorporation of Na in ringwoodite occurs through the mechanism Mg 2+ → 2/3Na + + 1/3Si 4+ and results in an increase in the cell parameter in relation to that of pure Mg 2 SiO 4 ringwoodite. A paragenesis of Na-bearing bridgmanite (up to 0.67 wt % Na 2 O), periclase, and anhydrous sodium phase X (Na 2 Mg 2 Si 2 O 7 ) is formed at 23–24 GPa. Based on the data obtained, high-pressure Na–Mg silicates, Na 2 MgSiO 4 and Na 2 Mg 2 Si 2 O 7 , may be considered as mineralogically probable mantle phases within the locally metasomatized mantle regions, although their finds as inclusions in diamond crystals are unlikely.