Other
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufmmr23a0180v&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #MR23A-0180
Other
8120 Dynamics Of Lithosphere And Mantle: General, 8125 Evolution Of The Earth, 8130 Heat Generation And Transport, 5134 Thermal Properties, 3999 General Or Miscellaneous
Scientific paper
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.
Rainey E. S.
van den Berg Arie P.
Yuen Dave A.
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