Reduced radiative conductivity of high and low spin FeO6 octahedra in the Earth's lower mantle

Abstract

The ability of Earths mantle to conduct heat by radiation is determined by optical properties of mantle phases. Optical properties of mantle minerals at high pressure are accessible through diamond anvil cell experiments, but because of the extensive thermal radiation at T above 1000 K such studies are limited to lower temperatures. Particularly uncertain is the temperature-dependence of optical properties of lower mantle minerals across the spin transition as the spin state itself is a strong function of temperature. Here we use laser-heated DACs combined with a pulsed bright supercontinuum laser probe and a synchronized time-gated detector to examine optical properties of high and low spin ferrous iron at 45-73 GPa and to 1600 K in FeO6, one of the most abundant building blocks in the mantle. Siderite (FeCO3) is used as a model for FeO6-octahedra as it contains no ferric iron and exhibits a sharp optically apparent spin transition at 44 GPa, simplifying data interpretation. We find that the optical absorbance of low spin FeO6 is substantially increased at 1000-1200 K due to the partially lifted Laporte selection rule. The temperature-induced low to high spin transition, however, results in a dramatic drop in absorbance of the FeO6-unit. The absorption edge (Fe-O charge transfer) red-shifts (~ 1 cm-1/K) with increasing temperature and at T above 1600 K becomes the dominant absorption mechanism in the visible range, suppressing radiative conductivity. This implies that the radiative conductivity of analogous FeO6-bearing minerals such as ferropericlase, the second most abundant mineral in the Earths lower mantle, is substantially reduced approaching the core-mantle boundary conditions. Finally, our results emphasize that optical properties of mantle minerals probed at room temperature are insufficient to model radiative thermal conductivity of planetary interiors.