Deformation of Al-Cr spinel in the diffusion creep regime: mechanical results and flow laws

The flow law for diffusion creep of chromite spinel has been determined from uniaxial compressive creep experiments at temperatures of 1400° to 1600 °C and a pressure of 0.1 MPa on polycrystalline samples of Mg(Cr,Al)₂O₄ with grain sizes of ~ 10 μm and chromium mole fractions, X _Cr, of 0.25, 0.50, and 0.75. Under our experimental conditions, creep rate is dominated by grain boundary diffusion of Cr. The dependence of strain rate on composition is manifest through the compositional dependence of the activation energy. The activation energy for grain boundary diffusion can be described in terms of melting temperature, T_m, as a function of composition in the form QCrgb=gCrgbRTm(XCr) , where gCrgb is a material-dependent scaling parameter. The value obtained for this scaling parameter of gCrgb = 19.7 ± 0.5 yields activations energies of approximately 405, 416, and 427 kJ/mol for samples with X _Cr = 0.25, 0.50, and 0.75, respectively. Extrapolation of our results to conditions appropriate for chromitite pods enveloped by peridotites from the Oman ophiolite indicates that lattice diffusion, rather than grain boundary diffusion, largely dominates plastic deformation of chromite spinel, which is significantly stronger than olivine.