Radiative conductivity in the Earth's lower mantle
Résumé
Iron in crustal and mantle minerals adopts several possible oxidation states: this has implications for biogeochemical processes 1, oxygenation of the atmosphere 2 and the oxidation state of the mantle 3,4. In the deep Earth, iron in silicate perovskite, (Mg0.9Fe0.1)SiO3, and ferropericlase, (Mg0.85Fe0.15)O, influences the thermal conductivity of the lower mantle and therefore heat flux from the core. Little is known, however, about the effect of iron oxidation states on transport properties. Here we show that the radiative component of thermal conductivity in the dominant silicate perovskite material of Earth's lower mantle is controlled by the amount of ferric iron, Fe3+. We obtained the optical absorption spectra of silicate perovskite and ferropericlase at pressures up to 133 GPa, corresponding to pressures at the core-mantle boundary. Absorption spectra of ferropericlase up to 800 K and 60 GPa exhibit minimal temperature dependence. The results on silicate perovskite show that optical absorption in the visible and near-infrared spectral range is dominated by O-Fe3+ charge transfer and Fe3+-Fe2+ intervalence transitions, whereas a contribution from the Fe2+ crystal-field transitions is substantially smaller. The estimated pressure-dependent radiative conductivity, krad, from these data is 2-5 times lower than previously inferred from model extrapolations, with implications for the evolution of the mantle, such as generation and stability of thermo-chemical plumes in the lower mantle.