Density contrast between silicate melts and crystals in the deep mantle: An integrated view based on static-compression data

Mineral-physics data of SiO2, MgSiO3, and MgO under static compression, together with the physical theories of melting and structural changes in liquid, suggest that the density of silicate melts does not exceed that of crystalline assemblages with the same chemical composition at the pressure range of the deep mantle. Because drastic structural changes accompanied by the increase in the coordination number of silicon are completed in the shallow mantle, SiO2 component becomes stiff and the compressibility of magnesium silicate melts decreases with SiO2 content in the deep mantle while it increases with SiO2 content in the shallow mantle. If the ideal mixing of melts with respect to volume is valid in the deep mantle, as is the case for the shallow mantle, the density contrast is significant in a composition range close to MgSiO3. Therefore, magmas in a wide composition range would be buoyant in the deep mantle. Chemical compositions of magmas which are neutrally buoyant in the deep mantle have been estimated with a simplified model in (Mg,Fe)O–SiO2 system.