2012.8.24
Feedbacks between melts and deformation in the upper mantle: constraints from natural systems
Andrea Tommasi (Geosciences Montpellier, CNRS and Universite Montpellier II, Montpellier, France)
We studied the feedbacks between melts and deformation in the upper mantle through structural mapping and detailed microstructural studies in four massifs, which preserve an exceptional
record
of physico-chemical interactions in a transient lithosphere-asthenosphere boundary. The studied natural systems comprise: a 100m section in the Moho Transition Zone (MTZ) of the Oman
Ophiolite,
a section of the central Lanzo massif (Alps) characterized by anastomozed network plagioclase-rich lherzolites, which locally grades into a diffuse cm-scale compositional layering, the Lherz
peridotite massif (Pyrenees), and the Ronda massif in the Betics. Except for the refertilization front in Lherz, which is extremely convoluted, suggesting formation by coalescence of
relatively
narrow (? 10 m) melt infiltration channels and hence that melt migration preceded deformation, structural relations imply in deformation in presence of small melt fractions and deformation-
controlled melt distribution. The layering parallels the foliation, layers limits are diffuse, and mm- to cm-scale lenses of fertile material parallel the foliation of refractory layers.
Olivine (and orthopyroxene, when present) form coarse weakly elongated crystals with undulose extinction, subgrains or kinks, sutured grain boundaries. Clinopyroxene, spinel, or plagioclase
occur in elongated aggregates, but lack evidence for solid-state deformation. Crystal-preferred orientations (CPO) provide strong evidence for deformation in presence of variable melt
fractions. The most striking is the change in olivine CPO at the mm-scale in the Oman MTZ. Layers with <70% olivine show strong [100] maxima parallel to the lineation (axial-[100] pattern),
whereas layers with <50% olivine have [100] dispersed in the foliation and concentration of [010] normal to it (axial-[010] pattern). Dispersion of olivine [100] axes is also observed in the
plagioclase-rich lherzolites from Lanzo and in the refertilized domains of Lherz and Ronda. This corroborates that deformation in presence of melt results in axial-[010] olivine CPO
patterns,
probably due to localized transpression or sliding along preferentially-wetted (010) grain boundaries in melt-rich layers. Olivine CPO patterns in the lherzolites are, however, usually
intermediate those in the dunites and gabbros of the Oman MTZ, consistent with the less developed segregation in the orogenic massifs. Since the change in olivine CPO symmetry is not
associated
with dispersion, it depends on the strain cumulated in presence of melt rather than on the instantaneous melt fraction. In summary, these data imply that in a deforming partially molten
mantle,
melts tend to concentrate in bands aligned in the shear plane, forming an anastomosed network that may grade with increasing shear into a planar layering. This may create a mechanical
anisotropy by reducing the shear viscosity parallel to the bands. Our data also show that deformation in presence of melt results in axial-[010] patterns and hence in a different seismic
anisotropy signature, characterized by a lower azimuthal anisotropy in the flow plane and a slow symmetry axis.