olelog

Can slabs of oceanic crust descend as deep as to the core-mantle boundary? Is cold mid-ocean-ridge basalt dense enough to sink so deep into the mantle?

Do mantle plumes exist? And if so, can they well up from as deep as the core-mantle boundary?

These and other core-mantle boundary related questions are occupying many earth scientists. They just want to know.

Most of our earlier knowledge of the Earth's interior came from studying seismic waves (after earthquakes). It became clear that the boundary between the core and the mantle was something special. S-waves (secondary wave or shear wave) stopped. They cannot propagate through liquids. Seismic velocity is linked to the density of the medium through which the waves travel. P-wave (primary wave) velocity in general increases with increasing density - In liquids, however, the speed will be less. The core is much denser than the mantle.

When it was discovered that a thin layer directly above the core-mantle boundary had some mysterious properties it had to get a name consistent with the naming of the earth’s layers in the middle of last century. It is now still referred to as the D" ("D double-prime" or "D prime prime"). The D" name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as 'A' and the inner core as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his "D" layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D’ (D prime) and the lower part (the bottom 200 km) was named D" (pronounced "dee double prime"). A layer that produces strange seismic properties.

Research into the mysteries of the D“ layer has advanced spectacularly since 2004, when Japanese researchers found that high temperatures and pressures like those existing in the D” layer transform perovskite, the major mineral in Earth's mantle. The publication in the journal Science of Post-Perovskite Phase Transition in MgSiO₃ is seen as a turning point. Since then many papers on this topic have followed. The latest I have seen is Radiative conductivity in the Earth's lower mantle by Goncharov et al. in today’s (13 November 2008) issue of Nature.

The discovery of post-perovskovite has profound implications for the chemical, thermal, and dynamical structure of the lowermost mantle (the D” region). Several major seismological characteristics of the D” region can now be explained by the presence of post-perovskite, and the specific properties of the phase transition provide the first direct constraints on absolute temperature and temperature gradients in the lowermost mantle. A discussion of the current understanding of the core–mantle boundary region can be found in Discovery of Post-Perovskite and New Views on the Core–Mantle Boundary Region by Kei Hirose and Thorne Lay in Elements of June 2008 (DOI: 10.2113/GSELEMENTS.4.3.183 - start downloading of the full article as large pdf-file by clicking here).

The Mg²⁺ site in post-perovskite is smaller than in perovskite, resulting in a volume reduction of 1.0–1.5%. Unlike perovskite, the post-perovskite phase has a layered structure of the SiO₆ octahedra, which may lead to a large contrast in some properties with perovskite.


Scenario for the D” region. The D” seismic discontinuity is caused by the perovskite (Pv) to post-perovskite (PPv) phase transition. Post-perovskite may transform back to perovskite in the bottom thermal boundary layer with a steep temperature gradient. The large low-shear-velocity provinces (LLSVP) underneath upwellings (forming plumes) possibly represent large accumulations of dense MORB-enriched materials. The solid residue formed by partial melting in the ultralow-velocity zone (ULVZ - thin reddish zone on image) might also be involved in upwelling plumes. Such thin (20-40 km) ULVZ-layers are mainly (but not only) found under the Pacific Ocean and under Africa.

• http://www.eurekalert.org/pub_releases/2008-11/ci-eht111008.php
• http://www.cems.umn.edu/research/wentzcovitch/highlights/science_now_040324.htm
• http://olivine.ethz.ch/~artem/NewMinerals.html
• http://www.ucsc.edu/news_events/press_releases/text.asp?pid=978
• http://www.nature.com/ngeo/journal/v1/n1/full/ngeo.2007.44.html (full paper in HTML)

For my (increasing number of) Scandinavian readers there is review in Norwegian at Geoportalen - http://www.geoportalen.no/planetenjorden/jordensindre/revolusjon/ . The article here by Reidar G. Trønnes, Natural History Museum, Universitty of Oslo, on the Earth’s Interior is somewhat easier to consume, and is well illustrated.

Maybe I ought to add that some of the topics discussed in the (review) papers are still controversial.