Heat transport in olivine single crystals and upper mantle rocks

Heat transfer is a key process for the upper mantle dynamics. In the lithospheric mantle, heat transport by conduction is expected to be the dominating heat transport mechanism. Thermal diffusivity is thus the key parameter that controls the temperature distribution as a function of time and, indirectly, through the temperature dependence of the rheology, the deformation patterns in the lithospheric mantle. However, a close analysis of thermal diffusivity data for mantle materials shows a large scatter of values. In particular, single crystal thermal diffusivities are systematically higher than those measured on polycrystalline samples. Another poorly constrained parameter is the role of radiative heat transport in the mantle. The aim of the present study is to investigate these two points. An especially designed transient apparatus, which allows direct observation of radiative transfer at high temperature, has been used to measure thermal diffusivity in both San Carlos olivine single crystals and natural dunites up to 850°C. The measured components of the thermal diffusivity tensor of olivine at ambient conditions along the [100], [010] and [001] directions are 2.70, 1.72, and 2.48 mm2/s, respectively. Measurements on dunites at ambient conditions, as a function of structural direction, are in good agreement (in both absolute values and anisotropy) with petrophysical models based on the Lattice Preferred Orientation of minerals (measured by the EBSD method) and the measured single crystal tensor. High-temperature measurements highlight a significant radiative transport in both olivine single crystals and fine-grained dunites (about 25 percent of the total thermal diffusivity at 850°C), suggesting that grain boundaries do not hinder the heat transfer by photons. This suggests that thermal transport by phonons and photons is not significantly affected by natural imperfections, such as defects, grain boundaries or closed microcracks.