| The last decade has seen significant advances in the seismological observation of the
lowermost mantle, with our understanding of the D” region significantly increasing in
complexity. At the same time mineral physics constraints on this region have vastly
improved: (1) increases in computing power allow us to simulate the elastic, chemical and
transport properties of geologically reasonable compositions of the major phases at finite
temperature, (2) laser-heated diamond anvil cell experiments at the high-pressures and
temperatures of the core-mantle-boundary have become standard, along with the
increasing diversity of possible in situ measurements and (3) high-pressure rheological
measurements are now possible. The discovery of the post-perovskite phase has gone a
long way to explain much of the complexity of D”. The layered nature of the SiO6
units in post perovskite results in strongly anisotropic elastic properties as well as
activating dominant slip on the (010) plane. This means that post-perovskite can match
the observed seismic anisotropy in D” with a dominantly horizontal basal mantle
flow.
However, post-perovskite is a strange beast; while it might look like a layer structure for
elastic properties, transport properties are decidedly not constrained by the SiO6 layers. Here
I describe latest results on its mass transport properties.
Experiments on analogue CaIrO3 suggest: (1) Textures developed during transformation
under non-hydrostatic stress are similar to textures developed in the diamond cell in MgSiO3
and MgGeO3 and that subsequent deformation rotates this transformation textures into a
[100]{010} deformation texture. (2) There is a weakening of 1 order of magnitude (or more)
as perovskite transforms to post-perovskite and even after the transformation is complete post
perovskite remains weak.
Ab initio simulations on MgSiO3 show that the chemical diffusivity in post perovskite is
highly anisotropic, with ~ 8 orders of magnitude of anisotropy between the fast
and slow directions, and that the fast direction is substantially faster
than any direction in perovskite. For deformation in the dislocation creep regime
this means that post perovskite will be weaker than perovskite, consistent with the
experiments.
I will discuss the implications of this weakening for the dynamics in, and seismic
observations of, the D” region. |