Thermal Conductivity and Thermal Diffusivity of Ferrosilite under High Temperature and High Pressure

Abstract

Orthopyroxene is an important constitutive mineral in the crust and the upper mantle. Its thermal properties play a key role in constructing the thermal structure of the crust and the upper mantle. In this study, we developed a new method to synthesize polycrystalline ferrosilite, one end-member of orthopyroxene, via the reaction of FeO + SiO₂ → FeSiO₃. We found that the P-T condition of 3 GPa and 1 273 K is suitable to synthesize dense ferrosilite samples with low porosity. We employed the transient plane-source method to investigate the thermal conductivity κ and thermal diffusivity D of synthetic ferrosilite at 1 GPa and 293–873 K, of which, κ = 1.786 + 1.048 × 10³T⁻¹ − 9.269 × 10⁴T⁻² and D = 0.424 + 0.223 × 10³T⁻¹ + 1.64 × 10⁴T⁻². Our results suggest phonon conduction should be the dominant mechanism at P-T conditions of interest since the thermal conductivity and the thermal diffusivity of ferrosilite both decrease with increasing temperature. The calculated heat capacity of ferrosilite at 1 GPa increases with temperature, which increases with increasing temperature with about 10% per 100 K (<500 K) and 4% per 100 K (>500 K). Iron content of an asteroid significantly influences its thermal evolution history and temperature distribution inside. It is expected that the mantle temperature of the Fe-rich asteroid will be higher and the Fe-rich asteroid’s cooling history will be longer.