HR: 10:35h
AN: T42A-02    [Abstracts]
TI: Plume Heat Flux, Core Heat Flux, and the Style of Mantle Convection
AU: * Zhong, S
EM: szhong@anquetil.colorado.edu
AF: University of Colorado, Dept. of Physics, University of Colorado, Boulder, CO 80309 United States
AB: The style of mantle convection (i.e., whole mantle convection versus layered mantle convection) is an important question that affects nearly every aspect of the Earth's evolution. Although geochemical observations have long suggested a layered mantle with more enriched mantle material in the bottom layer to provide a significant amount of heat to the top layer, the nature of such a layering remains unclear. An important observation that may help constrain the style of mantle convection is the plume heat flux [Davies, 1999]. Plume heat flux is estimated as $\sim$ 3.5 TW, or 10% of the surface heat flux, and which is also believed to be roughly equal to the heat flux out of the core [Davies, 1988; Sleep, 1990]. However, Labrosse [2002] argued that plume heat flux may only account for a small fraction of the core heat flux based on 3-D Cartesian isoviscous thermal convection models. Here by formulating 3-D spherical mantle convection with different depth- and temperature-dependent viscosity, we examine the dependence of ratios of plume heat flux to the surface heat flux and bottom heat flux on internal heating rate, mantle rheology, and Rayleigh number. We found that without a depth-dependent viscosity, mantle convection is required to be driven mostly ($>$50%) by basal heating, in order to produce the observed ratio (10%) of plume heat flux to surface heat flux. A large fraction of the bottom heat flux is consumed to heat the cold slabs that reach the bottom boundary and does not lead to plume heat flux, consistent with Labrosse [2002]. Since it is unlikely that the core can supply this amount of heat flux ($>$18 TW), these calculations with no depth-dependent viscosity support the layered mantle convection in which both the core and the bottom layer of the mantle can provide this amount of heat for the top layer. With a depth-dependent viscosity (X30 jump at 670 km depth plus X10 gradual increase), both the ratios of plume heat flux to surface heat flux and bottom heat flux increase for a given amount of internal heating rate. Plume heat flux is more comparable to core heat flux. Therefore, to use plume heat flux as a constraint on core heat flux and the style of mantle convection is critically dependent on mantle viscosity structure.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8130 Heat generation and transport
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting