Thermal diffusivity and conductivity of olivine, wadsleyite and ringwoodite to 20 GPa and 1373 K

We present results of lattice thermal diffusivity measurements on (Mg0.9Fe0.1)2SiO4 olivine and its high-pressure polymorphs wadsleyite and ringwoodite under mantle conditions. We used the Ångström method on cylindrical samples in multianvil apparatus at pressures up to 20 GPa and temperatures up to 1373 K. Because of the fine polycrystallinity of the specimens (∼30−40 μm or less), there is strong scattering/absorption of light and suppression of radiative transport so that the lattice vibrational component is the dominant heat transfer mode. Lattice thermal conductivities were calculated from the thermal diffusivity results using heat capacity and equation of state data. Olivine thermal conductivities are consistent with previous results obtained at 1 atm [e.g. J. Am. Ceramic Soc. 38 (1954) 107; J. Geophys. Res. 77 (1972) 6966; Science 283 (1999) 1699]. Thermal conductivity increases by approximately 30% at the transition from olivine to wadsleyite (corresponding to the 410 km discontinuity) and a further, but smaller, increase may occur at the transition from wadsleyite to ringwoodite. For each of these phases, lattice conductivity closely follows a T−1/2 dependence on temperature T [Phys. Rev. 119 (1960) 507; J. Geophys. Res. 79 (1974) 703; Brown, 2002]. If such a dependence applies to other silicates and complex crystals, there should be a useful way to estimate conductivities at high temperatures from room temperature measurements.