How do melts change texture and anisotropy of mantle rocks

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.