How deep is the source region for mantle plumes – upper mantle or lower mantle?
The 670 km seismic discontinuity is the seismic signature of the boundary between less dense rock of the upper mantle from the denser perovskite of the lower mantle. The old conventional model (Don Anderson and others) held that this density difference is great enough to prevent any material from the upper mantle from sinking into the lower mantle or any material from the lower mantle from rising into the upper mantle. The upper and lower mantle must cool via separate upper mantle and lower mantle convection systems. So plumes (if they exist) must originate in the upper mantle. But a number of observations argue that plumes originate in the lower mantle. incompatible elements: Many of the minor elements will substitute into the crystal lattice structures of the common silicate minerals in the Earth’s mantle. Certain elements with high ionic charges and/or very large or very small ionic sizes have difficulty substituting into the common silicates. They are called incompatible elements (
