Creep mechanisms in the lithospheric mantle inferred from deformation of iron-free forsterite aggregates at 900-1200 °C

To further constrain the plasticity of rocks in the uppermost lithospheric mantle, deformation experiments were carried out on forsterite aggregates using a gas-medium apparatus (Paterson press) at 300 MPa, 900-1200 degrees C and nearly constant strain rates of similar to 10(-5) s(-1). The starting material was a synthetic iron-free forsterite aggregate with an average grain size of similar to 2.8 mu m and similar to 2-3% of iron-free enstatite. Eight deformation experiments were performed as well as an additional static annealing to characterize grain growth. The maximum stresses obtained range from similar to 480 to 1870 MPa. Below 1000 degrees C, where stress significantly exceeds confining pressure, and based on microstructural observations, grain boundary mediated creep is observed, with evidences of sliding and cavitation (gaping) at grain boundaries. At 1050-1200 degrees C, where pseudo-steady state could be achieved, the microstructures are very different and show evidences of dislocation activity, resulting from the activation of several dislocation slip systems with increasing temperature. When compared to rheology laws previously obtained from similar experiments, the temperature dependence of iron-free olivine creep is similar to the one of its iron-bearing counterpart at high temperature (similar to 1200 degrees C); at temperatures <= 1000 degrees C, however, the strength of iron-free olivine is higher than for iron-bearing olivine. The deformation-induced textures obtained show that grain boundary sliding (GBS) lead to cavitation, which was activated in response to large differential stresses, i.e., beyond the Goetze criterion. Given these high-stress conditions, our results cannot be directly applied to deformation of the Earths mantle at large scale. Nevertheless, they highlight the key role played by grain boundaries in producing strain at lithospheric temperatures, when crystal-plastic mechanisms remain inefficient.