Refinement of seismic wave data has shown number of discontinuities (Fig. 8):
These zones are:
VELOCITY ZONE: from below lithosphere to about 200-250 km. Not always
present. Asthenosphere has S-wave
attenuation . . small amount of liquid, perhaps
ca 1% melting?
(2) MINOR DISCONTINUITY around 350 km.
(3) MAJOR DISCONTINUITY at 400 km.
(4) MAJOR DISCONTINUITY at 650 km.
(5) Between 900 and 2700 km no major discontinuities, but some smaller ones. In general
increase in seismic velocities and density explained by self-compression of homogenous material.
The following explanations have been proposed to explain these discontinuities (Fig. 9):
350 km. Pyroxene forms a complex solid solution with pre-existing garnet in which one-quarter of silicon atoms are octahedrally co-ordinated, leading to 10% increase in density of pyroxene component:
Mg3(MgSi)Si3O12 & Ca3(CaSi)Si3O12
400 km. Olivine transforms to beta-Mg2SiO4 which has SPINEL structure and is 8% denser than olivine.
500-550 km. Calcium silicate CaSiO3 component of garnet transforms to extremely dense PEROVSKITE structure. Also beta-Mg2SiO4 transforms to gamma-Mg2SiO4 with 2% increase in density.
650 km. The spinel structure disproportionates to MgSiO3 with PEROVSKITE structure and MgO with a ROCK SALT structure, i.e. Mg2SiO4 > MgSiO3 + MgO. Additionally the MgSiO3.Al2O3 component transforms to an ILMENITE structure and any sodium present will transform to a high pressure form of NaAlSiO4 having CALCIUM FERRITE structure.
Lower Mantle. Below 700 km no more major transformations are possible - the minerals are as close-packed as they can get. There is thus then a slow progressive increase in density to the mantle-core boundary.
|Fig. 9. Density changes with depth in the mantle, and the changes in mineral structure that have been proposed to explain them|
A point of interest is whether this sharp increase in density at 650-700 km acts as a barrier to mantle convection. If the slab cannot penetrate this boundary, does it pile up above 700km? Are there two convecting zones in the mantle: one above, one below the 700km discontinuity? Does this also coicide with a chemical boundary? Is there any chemical interchange across the boundary layer?