Density and thermal expansion of a peridotite melt at high pressure

Density of a FeO-rich peridotite (MA, a model composition of Martian mantle) melt was investigated by high pressure experiments conducted at 6.3-18.0 GPa and 1800-2570°C. We adopted the 'sink-float' method, i.e., the sinking and flotation of a certain crystal (density marker) in the silicate melt is judged from its position in the run product recovered from high pressure and high temperature conditions. The density marker moves in a vertical direction driven by the density difference between the melt and the marker. We used olivine and diamond for the density marker. The third-order Birch-Murnaghan equation of state was applied for a FeO-rich peridotite melt, and its thermal expansion coefficients at 1980°C and bulk moduli at 1980°C and 2330°C were determined from the constraints obtained by sinking and flotation of olivine and diamond. Assuming that the pressure derivative of bulk modulus (K′) equals 10.1 as the same value as that of fayalite melt (Agee, 1992a), the thermal expansion coefficients at 1980°C are 6.2 ± 2.0 × 10^-5 K^-1 at 8.1 GPa and 2330°C. The density relations between the Martian mantle melt and the liquidus crystals obtained in the present study suggest the enrichment of olivine at 600 km depth and of majorite at the bottom of the mantle in the crystallizing Martian magma ocean.