HR: 1340h
AN: MR23A-0180    [Abstracts]
TI: The Role Played by Radiative Heat-Transfer in Earth's Thermal History
AU: * van den Berg, A P
EM: berg@geo.uu.nl
AF: Dept. of Geophysics,Institute of Earth Sciences, Utrecht University, Budapestlaan 4, Utrecht, 3508 TA Netherlands
AU: Yuen, D A
EM: davey@msi.umn.edu
AF: Dept. of Geology and Geophysics and University of Minnesota Supercomputing Institute,University of Minnesota, 108 Pillsbury hall 310 Pillsbury Dr SE, Minneapolis, MN 55455-0219 United States
AU: Rainey, E S
EM: emma@gps.caltech.edu
AF: Planetary Sciences Division, Caltech, Code-150-21, Pasadena, CA 91125 United States
AB: For the last 30 years mantle heat transfer was thought to be governed by convective mantle circulation, where heat-transfer is operative by means of a thermal-boundary type of law in which classical scaling works well. In the last 5 years there has been increasing evidence that radiative heat-transfer may play an important role, especially in the deep mantle (Badro et al., 2004). Consequently we have studied the role played by radiative heat-transfer in conjunction with a core-coupling thermal history model. Our model results are based on a 2-D cartesian domain geometry and do not include the effects of phase transitions, at 670 km and 2750 km. We have focussed on varying the strength of radiative thermal conductivity by means of a single parameter $f$. This prefactor $f$ is applied to the the radiative part of the Hofmeister composite conductivity model (Hofmeister, 1999). We have neglected the effects of water, grain-size and Fe on the radiative thermal conductivity. Our results show a clear impact of the scaling parameter $f$. Small values of $f$ representing models which are dominated by lattice conductivity show a significant delay of 1-2 Gyr in planetary secular cooling compared to corresponding uniform conductivity models. This appears to be due to a low conductivity zone (LCZ) produced at shallow depth by these variable temperature and pressure dependent models. Increasing $f$ from 0 to 10 produces a less pronounced LCZ. As a result the thermal resistance of the thermal boundary layer decreases and the rate of secular cooling increases with $f$. Our results dominated by the temperature and pressure sensitive LCZ illustrate the shortcomings of purely pressure dependent monotonic conductivity profiles for thermal history models. Heat flow from the core also depends strongly on the radiative conductivity in our models, including thermal coupling between mantle and core. Strong variations of some 100%, increasing with $f$, were observed in the core heat flux. The recently discovered post-perovskite phase transition with a highly positive Clapeyron slope will have a destabilizing effect and impact on core/mantle coupling. However, this would interact strongly with the radiative thermal conductivity at the D" and is likely to be stabilizing (Matyska et al., 1994). Results of ongoing modelling experiments with this phase transition included will be shown. The observed strong fluctuations in core heat output, resulting from the natural variability of the non-linearly coupled mantle and core systems, may have impacted the operation of the geodynamo through the Earth's history. Badro, J., Rueff, J-P., Vanko, G., Monaco, G., Fiquet, G., and F. Guyot, Electronic transitions in perovskite: possible nonconvecting layers in the lower mantle, Science, 305, 383-386, 2004. Hofmeister, A.M., Mantle values of thermal conductivity and the geotherm from phonon lifetimes, Science, 283, 1699-1706, 1999. Matyska, C., Moser, J. and D.A. Yuen, The potential influence of radiative heat transfer on the formation of megaplumes in the lower mantle, Earth Planetary Sci Lett., 125, 255-266, 1994.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8125 Evolution of the Earth
DE: 8130 Heat generation and transport
DE: 5134 Thermal properties
DE: 3999 General or miscellaneous
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting