THERMAL-EXPANSION OF MANTLE AND CORE MATERIALS AT VERY HIGH-PRESSURES

The thermal expansivities (alpha) of MgO and high-pressure phases of CaO, CaMgSi2O6, 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, alpha can be represented by: alpha = rho0gamma0C(V)(rho0/rho)0.5+/-0.5/K(T) where gamma is the Gruneisen parameter, C(V) is the constant volume specific heat, K(T) is the isothermal bulk modulus, and rho is the density. Using this expression, the thermal expansivity of MgO is 28-32 x 10(-6) K-1 at the pressure of the top of the lower mantle and 10-16 x 10(-6) K-1 at its base (at 2000 K). New data for alpha of epsilon-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 epsilon-Fe, implying the inner core contains a light element.