Excess temperature of mantle plumes: The role of chemical stratification across D''

The temperature increase across the thermal boundary layer at the base of the Earth's mantle (D '') may be as high as 1000-1300 degrees C, while the excess temperature of mantle plumes, inferred from petrological studies, is ca. 200-300 degrees C. These two estimates are generally considered separately, although this should not be the case if plumes do originate from D ''. This paper investigates the evolution of plume excess temperature from the source region in the deep mantle to lithospheric depths. A finite element model in axisymmetric geometry is used to explore the effects of a number of model variables. The results show that if the source region is only a thermal boundary layer, the plume excess temperature are unrealistically high, since entrainment of surrounding mantle does not cool significantly a large plume head rising through the mantle. I then investigate the role of a chemically denser layer at the base of D '', considering a range of values for its thickness and excess density. The presence of a 30 km thick layer, 5% chemically denser than the surrounding mantle, buffers the excess temperature of plumes td plausible values. The results suggest that chemical stratification across D '' may govern the excess temperature of mantle plumes.