Anisotropic lattice thermal diffusivity in olivines and pyroxenes to high temperatures.

摘要

The anisotropic lattice thermal diffusivity of three olivines (Fo ₀, Fo₇₈, and Fo₉₁), one orthopyroxene (En ₉₁), and one clinopyroxene (Di₇₂He₉Jd₃Cr ₃Ts₁₂) have been measured via impulsive stimulated light scattering, permitting the calculation of their lattice thermal diffusivity tensors to high temperatures. For Fo₀ olivine, measurements extend from room temperature to 600°C, for Fo₇₈ to 900°C, and for Fo₉₁ to 1000°C, all in steps of 100°C. The orthopyroxene also was taken in steps to 1000°C, while the clinopyroxene was measured at room temperature. A limited set of room-temperature measurements to 5 GPa on a fourth olivine (Fo₈₉) is also included. Diffusivities have been combined with calculations of density and specific heat to determine the lattice thermal conductivity tensors. An earlier theory that explains the observed behavior in terms of a positive lower bound on the phonon mean free path is discussed, and the data are used to constrain a model of thermal conductivity at high temperature. The relative contributions of optic and acoustic modes are evaluated from analysis of published dispersion curves. Five conclusions are reached: First, the anisotropy of lattice thermal conductivity remains essentially unchanged over the observed range of temperatures, indicating that anisotropy remains significant under upper-mantle conditions, and, in regions displaying preferred alignment, may account for observed lateral variations in the geotherm. Second, thermal conductivity departs significantly from earlier predictions of its temperature dependence; this may be understood in terms of a phonon mean free path that cannot diminish below 1.75 times the mean interatomic spacing. Third, for olivine, the optic modes have group velocities that are approximately one-third those of the acoustic modes, and do not dominate lattice conduction despite their greater number. Fourth, impurity scattering is significant along the olivine Fe-Mg solid solution series, but is not appreciable near the endpoints and therefore likely does not play a major role in the upper mantle. Fifth, the historic underestimation of lattice thermal conductivity at temperature has led to an overestimation of radiative conductivity; radiative transport, although significant, plays an even smaller role in the upper mantle than has heretofore been assumed.