Thermal conductivity of lower-mantle minerals

Geodynamic models of heat transport and the thermal evolution of Earth's interior require knowledge of thermal conductivity for high-pressure phases at relevant temperatures and pressures. Here we present new data on radiative and lattice heat transfer in mantle materials determined from optical spectroscopy and time-resolved optical radiometry. The pressure dependence of optical absorption in ferropericlase (Mg,Fe)O, and silicate perovskite (Mg,Fe)SiO₃, has been determined in the IR through UV regions up to 133 GPa. Whereas (Mg,Fe)O exhibits a strong pressure dependence of absorption and spectral changes associated with the high-spin (HS) to low-spin (LS) transition of Fe²⁺ [Goncharov, A.F., Struzhkin, V.V., Jacobsen, S.D. 2006. Reduced radiative conductivity of low-spin (Mg,Fe)O in the lower mantle. Science 312, 1205-1208], the pressure dependence of optical absorption in (Mg,Fe)SiO₃ is relatively weak. We observe a moderate increase in absorption with pressure for (Mg,Fe)SiO₃ in the visible and infrared spectral range due to a red-shift of absorption in ultraviolet, however the crystal-field transitions of Fe²⁺ become weaker with pressure and disappear above 50 GPa as a result of the HS-LS transition in (Mg,Fe)SiO₃. Intervalence charge-transfer transitions in silicate perovskite shift to higher energies with pressure. The temperature dependence of the optical absorption of (Mg,Fe)O measured up to 65 GPa and 800 K is moderate below 30 GPa and weak above 30 GPa. Thus, the temperature correction of the radiative conductivity is insignificant. The estimated total pressure-dependent radiative conductivity (in approximation of a large grain size) is lower than expected from the pressure extrapolation of the ambient and low-pressure data [Hofmeister, A.M., 1999. Mantle values of thermal conductivity and the geotherm from phonon lifetimes. Science 283, 1699-1706; Hofmeister, A.M., 2005. Dependence of diffusive radiative transfer on grain-size, temperature, and Fe-content: implications for mantle processes. J. Geodyn. 40, 51-72]. A new method has been developed to measure thermal diffusivity of mantle materials at high P-T using time-resolved radiometry combined with a pulsed-IR source. Here, the technique is tested on MgO to 32 GPa and used to obtain a functional pressure dependence of thermal diffusivity and calculated thermal conductivity of the lower mantle.