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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2129
Post-Perovskite in the Deep Mantle - What Can it do for You
Prof. Thorne Lay
Earth and Planetary Sciences University of California IASPEI
For several decades seismologists have observed reflections from the lowermost mantle that can be
accounted for by abrupt increases in shear velocity and/or compressional velocity. Interpretation of
these velocity changes has been difficult, as their origin may be petrological or mineralogical, and they
may or may not involve dynamically generated fabrics. The discovery of post-perovskite, a high
pressure polymorph of magnesium-silicate perovskite that should exist for P-T conditions near those of
the core-mantle boundary, provides a specific hypothesis for the origin of lowermost mantle reflectivity.
Consideration of the diversity of seismic models and observations in the context of this specific
mineralogical context indicates that the phase change may plausibly account for a subset of seismic
observations, but there are many features that are not easily reconciled with an isochemical phase
change. For example, relatively weak P wave reflectivity is predicted for the phase change, but in some
locations the compressional velocity increases are observed to be strong. In other regions, more than
one seismic reflection appears to occur, some involving velocity increases and some involving velocity
decreases. When the effects of temperature and chemistry on the post-perovskite phase transition are
considered, it is possible to develop scenarios in which two or more discontinuities might be anticipated,
either as a result of double-crossing of the phase boundary in a steep thermal gradient or the presence
of multiple phase boundaries in a mixture of distinct petrologies. It is intriguing that seemingly realistic
scenarios involving the phase change can be reconciled with significant seismic complexity, but testable
demonstration of the scenarios is rather elusive. The reason this topic is attracting so much attention is
that the occurrence of the phase change has important implications for dynamics of the thermal
boundary layer in D", for detection of chemical heterogeneity, for development of seismic anisotropy in
the boundary layer, and for determination of temperature gradients in the deep mantle. Examples will
be discussed of how the post-perovskite context leads to implications for deep mantle chemisty, the fate
of subducted lithosphere, the origin of deep mantle plumes, and the heat flux through the core-mantle
boundary.
Keywords: core mantle boundary, lower mantle, thermal boundary layer