  The heat flux across the core-mantle boundary (QCMB) is the key parameter to
understand the Earth/s thermal history and evolution. Mineralogical constraints
of the QCMB 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 (krad) 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, at odds with
present 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.
