Crystal preferred orientation in peridotite ultramylonites deformed by grain size sensitive creep, étang de Lers, Pyrenees, France

In naturally deformed upper mantle rocks a strong olivine crystallographic preferred orientation (CPO) occurs in rocks with grain sizes larger than about 15 mm. Finer grained peridotites tend to have weak to random olivine CPO. The different types of olivine CPO are usually interpreted in relation to the dominant deformation mechanisms: with a strong CPO indicating dislocation creep and a random CPO indicating grain size sensitive (GSS) mechanisms involving grain boundary sliding and diffusion creep. Here we report the occurrence of a weak but systematic olivine CPO in ultra-fine-grained (0.5e10 mm) ultramylonites from the Étang de Lers lherzolite, Pyrenees, France. The microstructures, with elongated grains and mixed distribution of phases in the ultramylonites indicate deformation by dominant GSS creep. In theory, an olivine CPO may develop in rocks deformed by GSS creep during dislocation accommodated boundary sliding or phase boundary controlled diffusion creep in pyroxene-rich rocks. Alternatively, the CPO may be a relict texture preserved because the elongated grain shapes limit rotation during grain boundary sliding. Our observations confirm theoretical and experimental studies, which predict that systematic CPO and an associated seismic anisotropy can occur in naturally deformed upper mantle rocks during deformation by grain boundary sliding and diffusion creep.