Phase transitions in the deeper mantle

Refinement of seismic wave data has shown number of discontinuities (Fig. 8):

These zones are:

(1) LOW 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.



(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?