Effect of H2O on the density of silicate melts at high pressures: Static experiments and the application of a modified hard-sphere model of equation of state

Density of ultramafic silicate melts was determined using the sink/float technique at high pressures. Seven melt compositions were studied, among which three were dry compositions with different Mg#’s (molar MgO/(MgO+FeO)×100) and the other four were hydrous compositions synthesized by adding 2–7wt.% H2O to the anhydrous ones. Experimental conditions range from 9 to 15GPa and from 2173 to 2473K. The sinking and floatation of density markers were observed for all melt compositions. Melt density data were analyzed by applying the Birch–Murnaghan equation of state and a newly developed equation of state for silicate melts based on the model of hard sphere mixtures. The presence of water can significantly reduce the density of melts due to its small molecular mass. On the other hand, water makes hydrous silicate melts more compressible than anhydrous melts and therefore the effect of H2O on melt density is less significant at high pressures. The density of hydrous melts was then calculated as a function of H2O content at the conditions of the bottom of the upper mantle, and was compared with the density of the dominant upper mantle minerals. Results show that the conditions for a negatively buoyant melt that coexists with a pyrolite mantle atop the 410km discontinuity are marginally satisfied if H2O is the only volatile component to facilitate melting, but such conditions will be satisfied by a broader range of conditions when other heavier volatile elements (C, K, etc.) are also present.