Controls on plume heat flux and plume excess temperature

Abstract

Plume heat flux and plume excess temperature in the upper mantle inferred from surface observations may pose important constraints on the heat flux from the core and mantle internal heating rate. This study examined the relationship between plume heat flux Qp, core‐mantle boundary (CMB) heat flux Qcmb and plume excess temperature ΔTplume in thermal convection using both numerical modeling and theoretical analysis. 3‐D regional spherical models of mantle convection were computed with high resolution and for different Rayleigh number, internal heat generation rate, viscosity structures and dissipation number. An analytic model was developed for variations in Qp and ΔTplume with depth. The results can be summarized as following. (1) Mantle plumes immediately above the CMB carry nearly 80%–90% of the CMB heat flux. (2) Qp and ΔTplume decrease by approximately a factor of two for plumes to ascend from near the CMB to the upper mantle depth. (3) Our analytic model indicates that the decrease in Qp and ΔTplume is mainly controlled by the steeper adiabatic gradient of plumes compared with the ambient mantle and the reduction ratios for Qp and ΔTplume due to this effect depend upon the dissipation number and the distance over which plumes ascend. (4) The subadiabatic temperature also contributes to the reduction of Qp and ΔTplume, but its contribution is only 20% to 30%. Subadiabatic temperature from our models with >50% internal heating rate ranges from 35 K to 170 K for CMB temperature of 3400°C. (5) Our results confirms that ∼70% internal heating rate for the mantle or Qcmb of ∼11 TW is required to reproduce the plume‐related observations.