HR: 0830h
AN: S21E-0363    [PDF]
TI: The Effect of Critical Points in Radiative Thermal Conductivity (With Grain Size and Temperature) on the Transition Zone and Lower Mantle
AU: Yanagawa, T K
EM: tomo@geo.kyushu-u.ac.jp
AF: Dept. Earth and Planet. Sci, Kyushu University, Fukuoka, 812-8581 Japan
AU: * Hofmeister, A M
EM: hofmeist@levee.wustl.edu
AF: Dept. Earth and Planet. Sci, Washington U., St.Louis, MO 63130 United States
AU: Yuen, D A
EM: davey@krissy.geo.umn.edu
AF: Dept. Geol. and Geophys., University of Minnesota, Minneapolis, MN 55455 United States
AB: When temperature (T) exceeds 2000 K inside the Earth, transfer of heat by diffusion of photons is important. However, the effective thermal conductivity (k$_{rad}$) associated with this process does not follow a T$^{3}$ law. The dependence of scattering and emission spectra on grain size (d), and the non-linear dependence of absorption and emission spectra on frequency, result in a complex dependence of k$_{rad}$ on T and d. Specifically, for large grains, k$_{rad}$ rises to a local maximum near 1300 K, followed by a local minimum near 2000 K, followed by a gentle rise towards high temperature. For small grains, k$_{rad}$ depends quadratically on T. Above ~2000 K, k$_{rad}$ is largest for d near 1 mm. Irrespective of possible grain sizes, k$_{rad}$ has a minimum for the temperatures expected near 670 km: this critical point must impact convection above 670 km, as negative dk/dT iis destabilizing. The effect of a critical point in the transition zone on mantle convection is being investigated through geodynamic models. Below 670 km, radiative transport dominates over phonon scattering and is stabilizing. To compare the relative importance of k$_{rad}$ to that of viscosity, numerical simulations were made of constant k or of k$_{rad}$ strongly depending on T, each with a range of lateral viscosity contrasts due to T spanning from 10$^{2}$ to 10$^{6}$. The lower bound is characteristic of what is expected in the lower mantle due to the high background temperature in the Arrhenius argument of the viscosity. We found that krad exerts greater control. In particular, for the low viscosity contrast and high krad expected for the lower mantle, convection is substantially weakened. Based on this result, we interpret the low heterogeneity inferred from the tomographic images of the middle of lower mantle as the signature of a stagnant layer.
DE: 5139 Transport properties
DE: 7260 Theory and modeling
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8130 Heat generation and transport
SC: Seismology [S]
MN: 2003 Fall Meeting