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SUMMARY:How do melts change texture and anisotropy of mantle rocks - Andre
 a Tommasi (CNRS (Centre national de la recherche scientifique)\; Universit
 é de Montpellier)
DTSTART:20160415T150000Z
DTEND:20160415T160000Z
UID:TALK65514@talks.cam.ac.uk
CONTACT:INI IT
DESCRIPTION:In a melt-free mantle\, development of crystal preferred orien
 tations (CPO or  texture) in response to deformation is the major source o
 f anisotropy of  physical properties. Measurement of seismic (elastic) ani
 sotropy is indeed the  best available tool to unravel flow patterns at var
 ious depths in the mantle.  Though it cannot be easily measured in situ\, 
 anisotropy is even more marked for  thermal diffusion and viscosity. These
  anisotropies probably induce a  memory-effect on the thermo-mechanical ev
 olution of the upper mantle. In this  presentation\, we will address how t
 he presence of melts may change the  anisotropy of physical properties in 
 the upper mantle. The presence of melts  may: (1) induce an additional (pr
 obably stronger) component of anisotropy if the  melt is concentrated in a
 ligned pockets or lenses\, (2) change the olivine  texture evolution and (
 3) the mineralogical composition. Anisotropy due to melt  alignment\, thou
 gh strong\, is only effective while melts are present in t he  system. The
  two latter processes induce weaker\, but long-term changes in the  anisot
 ropy\, which remain effective even after melt extraction or  crystallizati
 on. Observations in naturally deformed peridotites suggest all  three proc
 esses occur in the upper mantle. Analysis of the spatial arrangement  of p
 roducts of melt-rock reactions in mantle peridotites provides evidence for
   melt organization in planar lenses or layers parallel to the shear plane
  at both  the grain boundary and larger (cm to tens of meters) scales. Suc
 h an arrangement  may induce significant decrease in the shear viscosity p
 arallel to the shear  plane. Comparison of olivine crystal preferred orien
 tations within and outside  melt-focusing domains records changes in the d
 eformation processes and hence on  the resulting CPO-induced anisotropy\, 
 which depend on the nature of the  melt-rock reactions. The latter also co
 ntrols the crystallization of new  minerals\, which most often dilutes the
  anisotropy.
LOCATION:Seminar Room 1\, Newton Institute
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