2.19 - Properties of Rocks and Minerals - Thermal Conductivity of the Earth

This chapter reviews measurements and theory of heat transport properties (thermal conductivity, k, and thermal diffusivity, D) of electrical insulators, for example, mantle minerals. Recent developments regarding the highly accurate, contact-free laser-flash method for measuring D permit isolation of the lattice contribution (D lat) for partially transparent silicate and oxide minerals from spurious (direct or boundary-to-boundary) radiative transfer effects. Laser-flash measurements, which provide absolute values, give D lat near room temperature for hard minerals that is ∼20% higher than methods involving multiple contacts, and ∼10% higher than single-contact methods. Contact methods underestimate D lat near 298K due to loss of heat at interfaces. Use of long cylinder geometries mixes the directional dependences of D lat for anisotropic samples, due to differences in coupling between the longitudinal and transverse components of the lattice vibrations with the transverse electromagnetic field of the propagating phonons. As temperature (T) increases, data obtained using contact methods are increasingly contaminated with spurious radiative transfer, sometimes even at 298K. Pressure determinations share these problems, but numerous studies of olivine and NaCl set constraints. Picosecond transient grating spectroscopy also underestimates D lat, possibly due to electronic-vibronic coupling. Reliable laser-flash data exist for few minerals (olivine, garnet, and quartz). We provide new results for MgO and MgAl2O4. Trustworthy, verified data agree with the damped harmonic oscillator model. This model and laser-flash data of structural analogs set constraints on D for high-pressure phases. Diffusive radiative transport (k rad,dif), as occurs in the mantle, is not constrained by laboratory experiments, and is therefore calculated. The latest model accounts for grain-size and emissivity. Spectral data needed for model input are inadequate, so results for olivine are generalized to other mantle phases at the expense of accuracy. For all phases, krad,dif depends nonlinearly on T, grain-size, and Fe content.In summary, not only has k lat been grossly underestimated at room temperature, but also the high-temperature responses of both the lattice and radiative components for silicates and oxides have been misrepresented to varying degrees in mineral physics experiments and calculations. Heat transport in the core is uncertain because available measurements on Fe and Ni metals at low pressure do not agree with the Weidemann-Franz model used to extrapolate k to core conditions. Improved measurements of diverse types of Earth materials are needed, which will improve our understanding of microscopic processes, and will impact lithospheric studies, estimation of the global power, and the outcome of geodynamic models.