Decaying shock studies of phase transitions in MgO-SiO₂ systems: Implications for the super-Earths' interiors

We report an experimental study of the phase diagrams of MgO, MgSiO₃, and Mg₂SiO₄ at high pressures. We measured the shock compression response, including pressure-temperature Hugoniot curves of MgO, MgSiO₃, and Mg₂SiO₄ between 0.2–1.2 TPa, 0.12–0.5 TPa, and 0.2–0.85 TPa, respectively, using laser-driven decaying shocks. A melting signature has been observed in MgO at 0.47 ± 0.04 TPa and 9860 ± 810 K, while no such phase changes were observed either in MgSiO₃ or in Mg₂SiO₄. Increases of reflectivity of MgO, MgSiO₃, and Mg₂SiO₄ liquids have been detected above 0.55 TPa (12760 K), 0.15 TPa (7540 K), 0.2 TPa (5800 K), respectively. In contrast to SiO₂, melting and metallization of these compounds do not coincide, implying the presence of poorly electrically conducting liquids close to the melting lines. This has important implications for the generation of dynamos in super-Earth's mantles.