Constraints on Thermochemical Mantle Convection From Plume Excess Temperature, Plume Heat Flux and Upper Mantle Temperature

Mantle convection is responsible for releasing about 36 TW heat from the mantle. However, how much of the 36 TW mantle heat flux is from the core (i.e., Qcmb), mantle radiogenic heating and mantle secular cooling remains poorly understood. This question is further complicated if the mantle is compositionally heterogeneous or layered as suggested by seismic and geochemical observations. The current study seeks to constrain Qcmb and mantle contributions to the total mantle heat flux for whole-mantle mantle and layered mantle models by using observations of plume excess temperature, plume heat flux, and upper mantle temperature. 3-D spherical models of whole-mantle and layered mantle convection are computed for different Rayleigh number, internal heat generation, buoyancy number, and bottom layer thickness for layered mantle models. The model results show that these observations are controlled by internal heating rate in the layer overlying the thermal boundary layer from which mantle plumes are originated. To reproduce the observations, internal heating rate needs ~65% for whole-mantle convection, but for layered mantle models, the internal heating rate for the top layer is ~60% for averaged bottom layer thickness ~600 km. A direct inference from these results is that Qcmb is ~12.6 TW for whole-mantle convection, while for layered mantle, an upper bound on Qcmb is ~14.4 TW. The required mantle contributions pose constraints on radiogenic heating rate and secular cooling rate. For whole-mantle convection, the required 23.4 TW from the mantle is consistent with mantle radiogenic heating from the Bulk Silicate Earth (BSE) model (i.e., ~13 TW) and heating from mantle secular cooling of 80 K/By (i.e., ~10 TW). For layered mantle models, if the cooling rate is 80 K/By, the top layer is required to have radiogenic heating rate of ~2.8x10-12 W/kg that is >3 times of that for the depleted MORB sources. If mantle radiogenic heating rate of the BSE model is taken as the upper bound for the MORB source, then the thickness of the bottom layer is constrained to be <350 km. Our calculations also show: 1) plume temperature in the upper mantle is ~0.55 of the CMB temperature and is mainly controlled by the CMB temperature and for whole-mantle convection, and is ~0.65 of the temperature at the density interface for layered mantle models, 2) plume heat flux accounts for ~75% of Qcmb right above the CMB for whole-mantle convection, but it decreases by a factor of ~3 when plumes reach the upper mantle due to adiabatic and diffusive cooling and slight subadiabatic temperature (~180 K).