Effect of Pressure on the Composition of the Lower Mantle End Member Fe x O

Abstract

The composition limits of Fe_xO are sensitive to both pressure and temperature. Earlier studies have shown that Fe_xO becomes highly nonstoichiometric when in equilibrium with metallic iron above 10 gigapascals, which is difficult to reconcile with available thermodynamic data. Experiments with a uniaxial split-sphere apparatus demonstrate that the iron content of Fe_xO increases continuously at high pressure in excellent agreement with quantitative models and suggest that there is a discontinuity in the elastic properties of Fe_xO above x = 0.95. On the basis of these results, it is inferred that the Earth's present lower mantle is not in equilibrium with metallic iron.