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.
no evidence for anisotropy but potentially downwellings and upwellings with strong
shear flows may produce anisotropy in D"
- 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)
- 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)
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