9:30 AM - 9:45 AM
[SIT23-09] The relationship between creep and grain growth rates in forsterite+periclase polycrystals
Keywords:Rheology of the lower mantle, Diffusion creep of a two-phase material, Grain boundary diffusivity
We synthesized highly-dense forsterite + periclase (10vol%) polycrystals from a mixture of fine powders of Mg(OH)2 and SiO2 (Koizumi et al., 2010). Grain sizes of forsterite and periclase are 0.3 and 0.2 μm, respectively. We performed uni-axial compressional creep experiments on these materials at atmospheric pressure. Prior to the deformation, the sample was annealed at 1420℃ for 12h to avoid grain growth during the experiment. We changed loads ranging from 50 to 200 MPa under constant temperatures of 1180℃~1400℃ during the experiments. At each stress level, we measured a strain rate where we could assume steady-state creep. We also performed grain growth experiments at different temperatures ranging from 1280℃ to 1400℃ for 500h using temperature gradient formed outside the central heat zone in the furnace. We observed microstructures of the aggregates after the experiments using scanning electron microscope (SEM).
Based on creep data, we obtained a relationship of dε/dt ∝ σn (n = 1.3~1.6). We observed monotonic increment of grain sizes of both forsterite and periclase grains with increasing temperature. We calculated grain boundary diffusivities from rates of creep and grain growth using theoretical models for grain growth and for diffusion creep (Coble creep), finding both diffusivities are essentially identical. The diffusivities are compared with previously measured grain boundary diffusivity of Si4+ (Fei et al., 2016) and MgO (Gardes & Heinrich, 2011) finding our values are comparable to the diffusivity of Si4+. We will discuss the flow mechanism of the lower mantle based on these results.