The density of volatile bearing melts in the Earth's deep mantle: The role of chemical composition

Density of silicate melts in the Earth's deep interior is important to understand the material circulations in the mantle. We study the role of chemical composition on melt density based on the ideal-mixing model and the third-order Birch–Murnaghan equation of state (EOS). Using this EOS, the role of composition can be understood through its effect on three EOS parameters: room-pressure density, room-pressure bulk modulus, and the pressure derivative of bulk modulus at room pressure. The dependences of these parameters on compositional variables such as Mg# (molar MgO/(MgO + FeO) × 100) of the melt, SiO2 content in the dry part of the melt, and H2O content in the melt are estimated from available experimental data in the literature using the ideal-mixing model. Results show that H2O content and Mg# of the melt are the most important factors that control the melt density at high pressure. Calculated densities of silicate melts are compared to the density of the ambient upper mantle from the PREM model near 410 km depth where dehydration-induced melting may occur. The relative density between the melt and the surrounding solid minerals is sensitive to the water content, Mg# and temperature at the conditions of melting. For an Mg# that is consistent with the pyrolite model, the conditions for density crossovers are marginally satisfied for a system when water (hydrogen) is the only volatile component to promote melting.