Because there is little material transport across the core-mantle boundary, heat transfer must occur by conduction, with adiabatic gradients above and below this boundary.
2.
In stars, the convection process is known to be highly efficient, with a typical \ frac { d \ ln T } { d \ ln P } that only exceeds the adiabatic gradient by 1 part in 10 million.
3.
Thus, the geothermal gradient within the bulk of Earth's mantle is of the order of 0.5 kelvin per kilometer, and is determined by the adiabatic gradient associated with mantle material ( peridotite in the upper mantle ).
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