Dynamic modeling of mantle convection, thermo-chemical models.
In the first part of this lecture, we completed the presentation begun in Lecture4 on our current knowledge of how to predict seismic anisotropy from dynamics and materials physics, and to compare it with seismic observations. This exercise will enable us to discard certain crystal structure models proposed for post-perovskite, on the assumption that it is actually present at the base of the mantle.
In the second part of this lecture, we discussed the concept of ULVZs(Ultra Low Velocity Zones). These remarkable structures, of limited geographical extent and low height above the CMB (20-30 km), were discovered at the base of the mantle twenty years ago. They are characterized by very significant reductions in compressional velocity (over 10%). Two possible interpretations have been proposed:
1) presence of partial melting ;
2) iron enrichment in the form of a metallic alloy.
In both cases, the predicted reduction in shear rates reaches ~30%, and the increase in density could reach 10%. Interpretation of the seismic data, however, includes significant trade-offs. These ULVZs appear to be detected preferentially in the LLSVP regions, and there is a possible correlation with the surface distribution of hotspots. My team has identified an ULVZ of roughly circular shape and particularly large size (diameter ~ 800 km), located almost vertically above the Hawaii hotspot, at the northern edge of the Pacific LLSVP. To determine the nature of ULVZs, it would be necessary to be able to better constrain their density, a particularly difficult challenge for seismologists. In the meantime, materials physicists are looking into the possible origin of the iron concentrations (solid or partially molten) that these ULVZs could contain.
