Thermal expansion of mantle and core materials at very high pressures

The thermal expansivities (α) of MgO and high-pressure phases of CaO, CaMgSi_2O_6, and Fe at ultrahigh pressure are obtained by comparing existing shock compression and temperature measurements to 300 K compression curves constructed from ultrasonic elasticity and static compression data. For MgO, α can be represented by: α = ρ_oγ_oC_V(ρ_o/ρ)^(0.5±0.5)/K_T where γ is the Grüneisen parameter, C_V is the constant volume specific heat, K_T is the isothermal bulk modulus, and ρ is the density. Using this expression, the thermal expansivity of MgO is 28-32×10^(−6)K^(−1) at the pressure of the top of the lower mantle and 10-16×10^(−6)K^(−1) at its base (at 2000 K). New data for α of ε-Fe, together with an inner core temperature of 6750 K, constrain the density of the inner core to be 5±2% less than the density of ε-Fe, implying the inner core contains a light element.