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SUMMARY:Deforming the Earth: Runny Solids in the Deep Mantle - David Dobso
 n\, University College London
DTSTART:20101102T163000Z
DTEND:20101102T173000Z
UID:TALK26273@talks.cam.ac.uk
CONTACT:John Maclennan
DESCRIPTION:The last decade has seen significant advances in the seismolog
 ical observation of the lowermost mantle\, with our understanding of the D
 '' region significantly increasing in complexity.  At the same time minera
 l physics constraints on this region have vastly improved: (1) increases i
 n computing power allow us to simulate the elastic\, chemical and transpor
 t properties of geologically reasonable compositions of the major phases a
 t finite temperature\, (2) laser-heated diamond anvil cell experiments at 
 the high-pressures and temperatures of the core-mantle-boundary have becom
 e standard\, along with the increasing diversity of possible in situ measu
 rements 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 po
 st perovskite results in strongly anisotropic elastic properties as well a
 s activating dominant slip on the (010) plane.  This means that post-perov
 skite can match the observed seismic anisotropy in D'' with a dominantly h
 orizontal basal mantle flow.\n\nHowever\, post-perovskite is a strange bea
 st\; while it might look like a layer structure for elastic properties\, t
 ransport properties are decidedly not constrained by the SiO6 layers.  Her
 e I describe latest results on its mass transport properties.\n\nExperimen
 ts on analogue CaIrO3 suggest: (1) Textures developed during transformatio
 n under non-hydrostatic stress are similar to textures developed in the di
 amond cell in MgSiO3 and MgGeO3 and that subsequent deformation rotates th
 is transformation textures into a [100]{010} deformation texture.  (2) The
 re is a weakening of 1 order of magnitude (or more) as perovskite transfor
 ms to post-perovskite and even after the transformation is complete post p
 erovskite remains weak.\n\nAb initio simulations on MgSiO3 show that the c
 hemical diffusivity in post perovskite is highly anisotropic\, with ~ 8 or
 ders of magnitude of anisotropy between the fast <100> and slow <010> dire
 ctions\, and that the fast direction is substantially faster than any dire
 ction in perovskite.  For deformation in the dislocation creep regime this
  means that post perovskite will be weaker than perovskite\, consistent wi
 th the experiments.\n\nI will discuss the implications of this weakening f
 or the dynamics in\, and seismic observations of\, the D'' region.\n\nRefe
 rences:\nAmmann\, M.W.\, Brodholt\, J.P.\, Wookey\, J. and Dobson\, D.P. F
 irst Principles Constraints on Diffusion in Lower Mantle Minerals and a We
 ak D'' Layer. Nature\, 465\, 462-465\, 2010.\n\nHunt\, S.A.\, Weidner\, D.
 J.\, Li\, L.\, Wang\, L.\, Walte\, N.P. Brodholt\, J.P. and Dobson\, D.P.\
 ,  Weakening of CaIrO3 during the perovskite-post perovskite transformatio
 n.  Nature Geoscience\, 10.1038/NGEO663\, 2009.\n\n
LOCATION:Tilley Lecture Theatre\, Department of Earth Sciences
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