D" layer

CMB, mantle side : D"
 
A strong change of density characterizes the core-mantle interface, it looks very 
abrupt so that this boundary acts as a quite perfect reflector for the seismic waves.
There is, additionally, at the bottom of the lower mantle a layer called D" where 
an important change in the speed of P waves appears. That layer is a thermal and 
chemical boundary layer showing important lateral variability.


D" = 200-300 km thick ± 200-300 km !

local heterogeneities of 10-70 km in dimension:  
non-metallic/metallic heterogeneities

global heterogeneities of 2000-3000 km determined:
for some of them: vertical continuity  <=> down-going slab (subduction)
 
even: 
	some heterogeneities ("dregs") swept upward (starting point for plume, 
     		 plume-like instabilities and volcanic centers called hotspots)
	some recirculated within D" 

infiltration of iron along grain boundaries 
vigourous chemical reactions of liquid iron and crystalline oxides at high pressure
 fluid infiltration and reaction -> very rapid (103-106 yr)  
 dispersal of the reaction products -> longer

D" ultra-low-velocity layer
recently discovered low-velocity layer of 5-40 km thick 
(ultra-low-velocity zone ULVZ)


no evidence for anisotropy but potentially downwellings and upwellings with strong 
shear flows may produce anisotropy in D"
 
segregation of dense material from the overlying mantle 
partial melt in mantle of denser material



CMB topography depends strongly on the D" role and mantle fluxes 




D" implication magnetic field

 compostional variations and partial melt
 => heterogeneities in the conducting layer at CMB
 => screening effect of the magnetic field

 iron enrichment of lower most boundary
 => enhancement of electric conductivity of the mantle side