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# 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

Bunge, H.-P., M.A. Richards, and J.R. Baumgardner (1997), 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, J. Geophys. Res., 102, 11991-12007.

Abstract
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.
Further information
BibTeX
@article{id5,
author = {H.-P. Bunge and M.A. Richards and J.R. Baumgardner},
journal = {J. Geophys. Res.},
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}},
volume = {102},
year = {1997},
}
EndNote
%0 Journal Article
%A Bunge, H.-P.
%A Richards, M.A.
%A Baumgardner, J.R.
%D 1997
%V 102
%J J. Geophys. Res.
%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