Deformation, Annealing, Melt-rock Interaction, and Seismic Properties of an Old Domain of the Equatorial Atlantic Lithospheric Mantle

We analyzed a set of mantle xenoliths from Fernando de Noronha (FN) archipelago to constrain the role of deformation, cooling, annealing, and melt percolation in the evolution of the mantle lithosphere of the equatorial Atlantic. The peridotites are dominantly lherzolites with coarse granular or porphyroclastic microstructures. Equilibrium temperatures range between 850 and 1000 °C. Olivine crystal preferred orientations (CPO) have mainly orthorhombic patterns characterized by clear [100] and [010] point maxima or a fiber-[100] tendency; these patterns imply dominant activation of [100](010) slip system during dislocation creep. Olivine fiber-[010] patterns are less common; they probably result from recrystallization or melt-rock interaction. Annealing after deformation partially recovered the microstructures and reduced the olivine CPO strength. Pyroxene CPOs often have weak consistency with olivine CPO, implying post-deformation refertilization. Chemical compositions are systematically more fertile than those from abyssal peridotites from the Mid-Atlantic Ridge. These compositions together with the microstructures suggest two stages of melt-rock interaction. An early refertilization of the base lithosphere due to continuous percolation of small melt fractions and later, more important, but local chemical changes (both refertilization and dunitization) as well as recrystallization, due to reactive melt percolation likely associated with the Cenozoic volcanism. By comparing the calculated seismic properties with existing seismological data we argue that the moderate seismic anisotropy displayed by the FN mantle xenoliths likely records past horizontal asthenospheric flow parallel to the spreading direction frozen in the lithospheric mantle as the plate cooled.