Microstructure, texture and seismic anisotropy of the lithospheric mantle above a mantle plume: Insights from the Labait volcano xenoliths (Tanzania)

The impact of a mantle plume at the base of the Tanzania craton has modified the composition and seismic velocity of the lithospheric mantle. Lavas erupted by the Labait volcano have sampled the perturbed mantle from the lithosphere-asthenosphere boundary (>140 km) to the spinel-peridotite domain (<70 km). We have studied the microstructure, texture and seismic anisotropy of a set of xenoliths spanning these depths to investigate the effects of plume activity on the fabric and seismic properties of the lithospheric mantle. The microstructure changes with depth: first the grain-size increases significantly, and then nucleation recrystallization occurs. The deepest samples display a recrystallized equidimensional matrix embedding relicts of deformed paleoclasts. The crystallographic preferred orientation (CPO) of olivine remains clear and even tends to increase with depth. In most samples, the observed CPO is consistent with dominant activation of the (010)[100] slip system. Samples from the base of the lithosphere display more unusual CPO, suggesting increasing activity on the (010)[001] slip system. Nucleation recrystallization does not appear to modify the pre-existing CPO, since neoblasts have a crystallographic orientation close to the parent grain orientation. Seismic properties remain similar over the whole section. In particular, no weakening of the seismic anisotropy is observed with depth, either for the P azimuthal or for the S polarization anisotropies. These results are consistent with previous seismological observations suggesting a coherent seismic anisotropy over the entire thickness of the Tanzania cratonic lithosphere. Our data thus provide new constraints for interpreting shear wave splitting measurements in East Africa, and support a model of perturbed lithosphere characterized by seismic signatures transitional between the “normal” lithosphere (for seismic anisotropy) and asthenosphere (for seismic velocities).