The identity and quantity of the light matter on each side of the Earth's inner core boundary

Abstract

Light elements in the iron-rich core of the Earth are important indicators for the evolution and dynamics of our planet. However, there is a longstanding controversy surrounding the identity and quantity of the light elements. The theory of tricritical phenomena has been recently employed as a precise theoretical framework to study solidification at the high pressures and temperatures of the Earth. When combined with the most reliable iron melting data and the seismic data, the theory provides the solidification temperature at the inner core boundary (ICB) pressure for both pure iron and for the alloy of iron and light elements in the actual core melt. Here the theory is used further to find a value of about 5 mol.% for the amount of light matter in the core melt at ICB and to calculate the density of liquid pure iron at its melting temperature. These give the density of the light matter consistent with it being MgSiO 3, and this identification is supported by sound velocity against density analysis. In addition, using the pure iron density and volume contraction obtained from the same theory, one finds the density of pure solid iron at its melting temperature. The sound velocity against density analysis for different iron crystal structures and various light compounds possibly present in the solid at the ICB shows that the most likely structure here is iron in its bcc δ phase with 4 wt.% Mg 2SiO 4 as light impurity. This finding is close to the conclusion of recent shear velocity calculations for Fe with Mg. The light matter identified here challenges the commonly assumed exclusion of mantle material in the core and this issue is discussed.