Effect of pressure on the deformation of forsterite and of iron-free enstatite

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Strength of fayalite up to 8.5 GPa

International audienceA dense polycrystalline aggregate of synthetic fayalite (Fe2SiO4) was deformed up to 8.5 GPa at room temperature in the D-DIA press installed at the European Synchrotron Radiation Facility beamline ID06. Five successive shortening-lengthening cycles were performed at different pressures and up to a final strain of approximately 25% at a typical strain rate of about 10-5 s-1. Lattice stresses were quantified from ( hkl) reflections accessible with a 55-keV monochromatic beam. Combined stress and strain data show that during each cycle, fayalite deforms elastically before yielding at an axial strain close to 2%. This yielding occurs at a macroscopic stress (taken as the average of the estimated lattice stresses) of 1.5-2 GPa, irrespective of pressure. Very moderate stress hardening takes place beyond the yield point, and the average stress becomes almost constant after a strain of 5-6%, suggesting a low-temperature plastic regime. Lattice stresses estimated with (131), (130), and (022) reflections are always higher than stresses estimated with (111) and (112) by a factor of about 1.5. In addition, the (131) lattice stress becomes progressively lower than the (130) and (022) lattice stresses with increasing pressure, which suggests a possible change in dominant slip systems around 5-6 GPa. Combining our results with data from Chen et al. (Phys Earth Planet Inter 143-144:347-356, (2004), we determined a low-temperature plasticity flow law with an activation energy of 217 ± 25 kJ mol-1 and a Peierls stress at 0 GPa, σ p0 = 3.92 ± 0.02 GPa, that is consistent with dislocation motion being limited by discrete obstacles. The pressure dependence is almost entirely accounted for by the Peierls stress, with d σ p/d P = G'/ G 0, where G' is the derivative of G 0, the shear modulus. Our results suggest that fayalite has a smaller pressure dependence of low-temperature plasticity than (Mg0.9Fe0.1)2SiO4 and that the transition between low-temperature plasticity and high-temperature creep occurs at lower temperatures and lower stresses in fayalite than in Mg-rich olivines. An increase in iron content in olivine may therefore enhance ductility and lower the effect of pressure on creep, resulting in a viscosity contrast of up to 50 between fayalite and (Mg0.9Fe0.1)2SiO4 at pressures and temperatures of the lithospheric mantle