LAUSR

Abstract The heat flux across the core-mantle boundary ( Q CMB ) is the key parameter to understand the Earth's thermal history and evolution. Mineralogical constraints of the Q CMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity ( k rad ) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ∼3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ∼0.8 W/m/K at 1000 km to ∼0.35 W/m/K at the CMB, the latter being ∼30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ∼8.5 TW, on the lower end of previous Q CMB estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.