Thermal diffusivity of olivine single crystals and a dunite at high temperature: Evidence for heat transfer by radiation in the upper mantle

The thermal diffusivity of San Carlos olivine single crystals is measured at elevated temperatures to differentiate lattice and radiative contributions to the thermal diffusivity. Thermal diffusivity was determined with a transient apparatus which allows direct observation of radiative processes in the samples. The method allows the separation of non-conductive radiative processes and purely diffusive mechanisms, i.e., intrinsic heat transport by phonons and radiation. The contributions of heat transfer by phonons and photons are separated by their different temperature dependencies. Thermal diffusivity D by phonons and photons are approximated by Dphonon = 1/(A + BT) and Dphoton = CT3, respectively. A strong increase in the thermal diffusivity by photons is observed with the increasing sample lengths which were varied from 4.5 to 13.1 mm. In the small samples, only an apparent thermal diffusivity is measured. For the larger samples, the radiative contribution to the thermal diffusivity tends to the intrinsic value of an infinite sample. For these samples, the heat transfer by radiation contributes up to 60% to the total thermal diffusivity at 850 °C. Based on the numerical modelling of heat transfer by radiation in semi-transparent media, the present measurements are extrapolated in order to model large volumes like the mantle. Furthermore, the behaviour of single crystals and a fine-grained dunite are compared to evaluate the influence of grain boundaries on the thermal transport properties of polycrystalline aggregates. All the experiments point to a significant radiative transfer in olivine single crystals and polycrystalline aggregates at high temperature. Thus, heat transfer by radiation should strongly enhance conductive heat transport in the upper mantle.