A sensitivity study of 3-D spherical mantle convection at 10exp8 Rayleigh number: Effects of depth dependent viscosity, heating mode and an endothermic phase change

Mantle convection is influenced simultaneously by a number of physical effects: brittle failure in the surface plates, strongly variable viscosity, mineral phase changes, and both internal heating (radioactivity) and bottom heating from the core. Here we present a systematic study of three potentially important effects: depth-dependent viscosity, an endothermic phase change, and bottom vs. internal heating. We model 3-D spherical convection at Rayleigh Ra = 10$^8$, thus approaching the dynamical regime of the mantle. An isoviscous, internally heated reference model displays point-like downwellings from the cold upper boundary layer, a blue spectrum of thermal heterogeneity and small but rapid time variations in flow diagnostics. A modest factor 30 increase in lower mantle viscosity results in a planform dominated by long, linear downwellings, a red spectrum, and great temporal stability. Bottom heating has the predictable effect of adding a thermal boundary layer at the base of the mantle. We use a Clapeyron slope of $\gamma$ = -4MPa/$^o$K for the 670 km phase transition, resulting in a phase buoyancy parameter of P = -0.112. This phase change causes upwellings and downwellings to pause in the transition zone, but has little influence on the inherent time-dependence of flow and only a modest reddening effect on the heterogeneity spectrum. Larger values of P result in stronger effects but our choice of P is likely already too large to be representative of the mantle transition zone. Combinations of all three effects are remarkably predictable in terms of the single-effect models, and the effect of depth-dependent viscosity is found to be dominant.

@article{id5,
  author = {Bunge, H.-P. and Richards, M.A. and Baumgardner, J.R.},
  journal = {J. Geophys. Res.},
  language = {en},
  pages = {11991-12007},
  title = {A sensitivity study of 3-D spherical mantle convection at 10exp8 Rayleigh number: Effects of depth dependent viscosity, heating mode and an endothermic phase change},
  url = {http://www.geophysik.uni-muenchen.de/Members/bunge/download/sensitivity.pdf
http://www.geophysik.uni-muenchen.de/Members/bunge/download/sensitivity.ps.gz},
  volume = {102},
  year = {1997},
}

%O Journal Article
%A Bunge, H.-P.
%A Richards, M.A.
%A Baumgardner, J.R.
%J J. Geophys. Res.
%G en
%P 11991-12007
%T A sensitivity study of 3-D spherical mantle convection at 10exp8 Rayleigh number: Effects of depth dependent viscosity, heating mode and an endothermic phase change
%U http://www.geophysik.uni-muenchen.de/Members/bunge/download/sensitivity.pdf
http://www.geophysik.uni-muenchen.de/Members/bunge/download/sensitivity.ps.gz
%V 102
%D 1997