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Abstract:
Temperature is a key parameter that governs the physical behavior of subducting slabs: their buoyancy, rheology, phase transitions, and dehydration reactions. Upon their descent into the mantle, subducting slabs are heated by diffusion, which is controlled by the total thermal conductivity. In mantle minerals, two microscopic mechanisms contribute to total conductivity: lattice and radiative heat transport. The crystal structure of a mineral determines its lattice conductivity, whereas the optical absorption coefficient determines the radiative component. Both depend on P, T, composition, and phase polymorphs. Nevertheless, in thermal evolution models of slabs, lattice component is often assumed constant while radiative is usually ignored, because its variation with P, T, phase, and composition is generally unconstrained. Here, we measured the optical absorption coefficient of olivine at T up to 1000 K and P up to 10 GPa to infer its radiative thermal conductivity. We found that radiative contribution is as important as lattice contribution for diffusing heat transport, representing 40-50% of olivine´s total conductivity. We further modelled slab thermal evolution with 2D finite-difference method. Accounting for radiative heat transport produces slabs that are ~200 K warmer than the models with only the lattice contribution. Our models show extensive dehydration of the upper 10 km of the slab at depths of ~140-280 km. The released water fails to reach the MTZ, and it might be responsible for the intermediate-deep seismicity. Water delivery into the MTZ is therefore only possible within the cold slab core (~30 km) or inside fast sinking slabs (>10 cm/year).
