HR: 0800h
AN: MR11A-0894    [Abstracts]
TI: The Threshold Dependencies of Thermal Conductivity and Implications on Mantle Dynamics
AU: * Hofmeister, A M
EM: hofmeist@levee.wustl.edu
AF: Washington U., Dept. of Earth and Planetary Sciences, 1 Brookings Dr., St. Louis, MO 63130 United States
AU: Yuen, D A
EM: davey@krissy.geo.umn.edu
AF: U. Minnesota, Dept. of Geology and Geophysics and Minnesota and Supercomputing Institute, 310 Pillsbury Dr. S.E., Minneapolis, MN 55455 United States
AB: New experimental measurements of thermal diffusivity using a laser-flash apparatus show that the lattice component of thermal conductivity for minerals becomes independent of temperature within experimental uncertainty above roughly 1200 to 1900 K. This behavior is seen in MgO, Al$_{2}$O$_{3}$, several olivines, diopside, albite, microcline, SrTiO$_{3}$ perovskite, rutile, garnet, quartz, various ceramics, rhyolite, and basalt. Recent revision of the formulation for an effective thermal conductivity due to radiative transfer shows that a different type of threshold dependence on the grain-size with greater complexity in the temperature dependence for large grains, and a dependence on Fe content. Similar behavior is seen for krad calculated from visible spectra of olivine and of (MgFe)SiO$_{3}$ perovskite, which shows that the particulars of the absorption spectra are unimportant. The combination of these threshold effects in thermal conductivity have ramifications on the style of mantle convection, since radiative transfer acts to stabilize the flow, whereas the lattice contribution tends to destabilize the boundary layer. However, radiative transfer can make convection chaotic and time-dependent as the sign of dk$_{rad}$/dT is sometimes negative, e.g., for large, Fe-rich grains. The specific domains in thermal conductivity suggest sluggish lower mantle flow, but a strongly time-dependent pattern for the upper mantle-transition zone. That radiative transfer decreases as Fe/(Fe+Mg) increases beyond 0.1 suggests that thermo-chemical plumes can form at the base of the lower mantle through a positive feedback involving chemical enrichment, thermal conductivity, and viscosity. This mechanism would be particularly effective in stagnant points of the upwelling. The above cross-scaling connection of microscopic processes in solids with macroscopic transport mechanisms in the mantle plays an important role in differentiation of the Earth.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8130 Heat generation and transport
DE: 5139 Transport properties
DE: 3994 Instruments and techniques
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting