HR: 09:15h
AN: U31A-04 INVITED     [Abstracts]
TI: Thermal Conductivity in the Deepest Mantle
AU: * Hofmeister, A M
EM: hofmeist@levee.wustl.edu
AF: Washington U. St. Louis, 1 Brookings Dr Dept. EPSc, St. Louis, MO 63130 United States
AB: The assumed dependence of thermal conductivity (k) on temperature strongly influences results from mantle convection models due to feedback in the temperature equation. To estimate k of post-perovskite, analogues are used. Experimental measurements of thermal diffusivity (D)using a laser-flash apparatus show that the lattice component(k = D x density x heat capacity) for minerals becomes independent of temperature above ca 1200-1900 K. This behavior is seen in 10 crystal structures, glasses, and lavas. Independent of phase, D asymptotes to ca 0.7 mm²/s. Heat capacity of 3R and density from PREM give k_lat near 1 W/m-K at deep mantle temperatures. Theoretically, k_lat(P,T)= [1+K'P/K]k_lat(T) where K is bulk modulus, providing ca 5 W/m-K near the core mantle boundary. Radiative transfer is important at high T. To first order, this process is pressure independent because absorption and emission characteristics roughly cancel. Important parameters are temperature, grain-size (d) due to scattering, and the absorption coefficient (A). Spectra of real materials are frequency dependent with intensities proportional to Fe content. As post-perovskite spectra are unknown, asymptotic limits are considered for expected d = 1 mm. For small, weakly absorbing grains with A roughly constant, k_grey = (16/3)d² An²ST³ where S is the Stefan-Boltzmann constant and n is index of refraction. For mantle garnets, A nears 0.2/mm in the visible, which should proxy for the similar site in post-perovskite, providing k_rad about 2 W/m-K at 2500 K for a transparent mantle (low Fe content, any spin state). For dark grains (large dA, e.g., high Fe content), k_grey =(16/3)n²ST³/A. Estimating A as 10/mm, gives 1 W/m-K. Calculations based on spectral data have a weaker T dependence as neither the UV nor IR spectral regions participate, but other factors increase k, and thus our asymptotic limits constrain k for the deepest mantle with small grain size. Although the values of k_lat and k_rad are similar, the strong temperature dependence of radiative transfer means that this process controls mantle convection, and formation of large plumes. In particular, radiative transfer is relatively impeded in Fe-rich regions, leading to locally warmer temperatures and possibly upwellings.
DE: 3924 High-pressure behavior
DE: 3999 General or miscellaneous
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8130 Heat generation and transport
DE: 8147 Planetary interiors (5430, 5724)
SC: Union [U]
MN: 2005 Joint Assembly