Thermoelastic models of minerals and the composition of the Earth's lower mantle

A thermoelastic model for polycrystalline solids is presented extending our previous work. The model describes consistently thermoelastic properties s uch as the equations of state and the compressional and shear velocities v( p) and v(s) of MgSiO3 and CaSiO3 perovskites and (Mg, Fe)O magnesiowustite under high pressure and/or high temperature. The values of the parameters i n the model are determined from experimental data. A number of the model co mpositions of the (upper) mantle have been proposed by several researchers where the compositions are specified by the wt.% of five major oxides, SiO2 , MgO, FeO, Al2O3, and CaO. From these model compositions, we have derived the model aggregates for the lower mantle assuming that the lower mantle co nsists of a homogeneous three-component aggregate of (Mg1-x-u, Fe-x, Al-u)( Si1-u, Al-u)O-3 perovskite, (Mg1-y, Fe-y)O magnesiowustite, and CaSiO3 pero vskite. Using our model, we have calculated density, bulk and shear moduli, and v(p) and v(s) of these aggregates under lower mantle conditions as a f unction of depth z. The temperature T (z) is calculated assuming that the l ower mantle is adiabatic and T (670 km) = 1873 K and the nonadiabatic effec t is incorporated perturbatically following the work of Shankland and Brown . We have found that the chondritic model and the lower mantle model both b y Anderson and Bass are in excellent agreement with preliminary reference e arth model (PREM). Our results show that the elastic relaxation effect due to long-time-scale mantle convection is relatively small in the lower mantl e. (C) 2001 Elsevier Science B.V. All rights reserved.