Effect of Pressure on the Composition of the Lower Mantle End Member FexO.

The composition limits of Fe(x)O are sensitive to both pressure and temperature. Earlier studies have shown that Fe(x)O 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(x)O increases continuously at high pressure in excellent agreement with quantitative models and suggest that there is a discontinuity in the elastic properties of Fe(x)O 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.