Density and constitution of the mantle

Abstract

Available experimental data for some rock-forming minerals indicate that, for the prediction of density in the upper mantle, it may be preferable to use an empirical relation between bulk sound velocity and density rather than Birch's relation between the velocity of compressional waves and density. An empirical relation between bulk sound velocity and density is applied to the prediction of density distribution in the mantle to a depth of 1000 km. In the lower mantle and the core, the density changes are computed by the use of the Adams-Williamson relation. One of the models generated this way is modified in order to fit the relevant geophysical data, which include the periods of earth's free oscillations, phase and group velocities of Rayleigh and Love waves along great-circle paths, and the total mass and moment of inertia of the earth. A model that satisfies all these data well within the observational uncertainties is found. Density in the upper 170 km of the mantle is about 3.3 g/cm3; this density distribution lends support to the current petrological theory that the upper mantle may be composed of some varieties of peridotite. The mean atomic weight for the lower mantle is about 21.3–21.5, suggesting the possibility that the entire mantle may have a uniform iron content.