D’’ layer

• at the bottom of the mantle, difference in density wrt overlaying mantle: 200-300 km + 200-300 km;

• ultra-low-velocity zone, ULVZ (5-40 km);

• ‘very’ conductive layer at bottom of the mantle?

  • possibly infiltration of iron along grain boundary,
  • chemical heterogeneities, segregation of denser material from the overlying mantle;

• important lateral heterogeneities:

  • large scale: 2000-3000 km, related to
    • down-going slabs (subduction),
    • dregs swept upward (start of plume),
    • heterogeneities re-circulated in the layer;
  • small scale: 10-70 km, non-metallic/metallic.