High‐temperature creep of forsterite single crystals

Creep of forsterite single crystals has been studied with respect to the orientation of the differential stress. Three orientations have been investigated: [110]c, [101]c, and [011]c. Specimens were deformed at high temperature (T ≥ 1400°C) and moderate stresses (15 < σ < 110 MPa) in a dead load creep apparatus at room pressure and under controlled atmosphere. Assuming that the creep law has the general form = Aσn exp −(Q/RT), two different tests were performed. Stress step and temperature step experiments gave the stress exponent n and the activation energy Q, respectively. A thorough analysis of the dislocation microstructures was carried out in the potential glide planes using the optical microscope (decorated thin section) and transmission electron microscope. Comparison of both mechanical and microstructural data allowed the determination of the different flow laws for the three orientations studied. Forsterite and olivine data are consistent; both flow laws and dislocation microstructures are similar. In light of these results, extrapolation of flow laws to the stress conditions of the mantle is not straightforward. Very high activation energies at low stresses (σ < 15 MPa) may suggest a change of flow mechanisms. Accordingly, further investigations in the low‐stress domain are required.