A Reference Earth Model–compatible Hydrogen-rich Chemical Model of Inner Earth

Abstract A database of densities and compressional and shear sound velocities as function of pressure up to 360 GPa is calculated from first principles for 51 mineral phases. Plotting velocities versus densities (Birch diagram) allows the identification of likely components of the various zones revealed by geophysical analysis (REM1D, a recent update of the Preliminary Reference Earth Model): solid inner core, liquid outer core, D” zone, central lower mantle, transition zone, upper mantle, lower crust, upper crust, and ocean. Average properties of 33 concentric layers of a discretized spherical model of Earth are computed with volume fractions and melting ratios of minerals as free parameters. An objective function describing the distance between between simulated and layer-by-layer-averaged REM1D profiles is minimized. A hydrogen-rich solution of this problem is presented, matching the Earth global composition predicted by the “photophysical” model of chemical differentiation of planets, provided that the latter is corrected for its deficit in iron and oxygen. Such corrections assume magnetic aggregation in the early stages of accretion, and addition of oxidized chondrites during the Late Heavy Bombardment. The density deficit of the inner core is very well explained by a composition associating 87% solid NiH face-centered cubic and 13% partially melted FeH.