Presented by:
Andrea Tommasi
Date:
Friday 15th April 2016 - 16:00 to 17:00
Venue:
INI Seminar Room 1
Abstract:
In a melt-free mantle, development of crystal preferred orientations (CPO or
texture) in response to deformation is the major source of anisotropy of
physical properties. Measurement of seismic (elastic) anisotropy is indeed the
best available tool to unravel flow patterns at various 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 evolution of the upper mantle. In this
presentation, we will address how the presence of melts may change the
anisotropy of physical properties in the upper mantle. The presence of melts
may: (1) induce an additional (probably stronger) component of anisotropy if the
melt is concentrated in aligned pockets or lenses, (2) change the olivine
texture evolution and (3) the mineralogical composition. Anisotropy due to melt
alignment, though strong, is only effective while melts are present in t he
system. The two latter processes induce weaker, but long-term changes in the
anisotropy, which remain effective even after melt extraction or
crystallization. Observations in naturally deformed peridotites suggest all
three processes occur in the upper mantle. Analysis of the spatial arrangement
of products 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. Such an arrangement
may induce significant decrease in the shear viscosity parallel to the shear
plane. Comparison of olivine crystal preferred orientations within and outside
melt-focusing domains records changes in the deformation processes and hence on
the resulting CPO-induced anisotropy, which depend on the nature of the
melt-rock reactions. The latter also controls the crystallization of new
minerals, which most often dilutes the anisotropy.
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