Constraining Thermo-Chemical Mantle Convection from Observations of Mantle Plumes and Upper Mantle Temperature

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Recent seismic observations indicate that the bottom part of the lower mantle is compositionally distinct from the overlying mantle, suggesting a layered mantle model that may be consistent with geochemical observations. However, the composition and volume of each layer of the layered mantle and consequences of the layering to surface observations remain poorly understood. This study seeks to constrain the thermochemical convection of the mantle by using observations of plume heat flux, plume excess temperature, and the upper mantle temperature. 3-D regional spherical models of isochemical (i.e., whole mantle convection) and thermochemical (i.e., layered mantle convection) convection are formulated to examine controls on plume heat flux, plume excess temperature, and the upper mantle temperature. It is hypothesized that the controlling parameter on these observables is internal heating rate for the layer (i.e., the top layer of a layered mantle) overlying the thermal boundary layer from which mantle plumes are derived, while other parameters including Rayleigh number play secondary roles. This hypothesis is supported by the investigator's preliminary modeling calculations. Model calculations over a large model parameter space are to be performed to test this hypothesis. These model calculations determine the internal heating rate that is required to reproduce all these three observations for isochemical models and thermochemical models with different layering depths. This study explores the implications of the required internal heating rate for the top layer for a range of geophysical and geochemical problems including mantle secular cooling, distribution of radioactive elements, mantle composition, core heat flux and cooling, and mantle layering.