Silicon Grain Boundary Diffusion in Forsterite and Implications to Upper Mantle Rheology
Abstract:Plastic deformation of minerals in the Earth’s interior is controlled by diffusion creep (including Coble creep and Nabarro-Herring creep) and dislocation creep. Diffusion creep makes Newtonian rheology and seismic isotropy, whereas dislocation creep makes non-Newtonian rheology and seismic anisotropy. Determination of the dominant creep mechanism in the Earth’s interior is thus essential to understand the geodynamics. Experimental deformation studies on olivine suggested that the dominant creep mechanism in the upper mantle changes from dislocation to diffusion creep at 200-300 km depth [1, 2]. However, those studies may misunderstood the creep rate due to experimental difficulties [3, 4]. It is necessary to independently examine the creep mechanisms in the upper mantle.
Coble diffusion creep in olivine is controlled by Si grain-boundary diffusion, whereas dislocation and Nabarro-Herring diffusion creeps are controlled by Si lattice diffusion. We have already reported Si lattice diffusion rate in iron-free olivine [3, 4]. In this study, we systematically measured Si grain boundary diffusion rate in forserite aggregates at 1 atm –13 GPa, 1100-1600 K, and bulk water content from <1 up to ~500 wt. ppm using multi-anvil apparatus and ambient pressure gas-mixing furnace. The diffusion profiles, water contents before and after diffusion, and microstructures of the samples were analyzed by SIMS, FT-IR, and TEM, respectively.
The activation energy, activation volume, and water content exponent for Si grain boundary diffusion are found to be 245±10 kJ/mol, 1.8±0.2 cm3/mol, and 0.22±0.05, respectively. Our results suggest that 1) pressure does not change the dominant creep mechanism; 2) Coble creep dominates at low temperature whereas dislocation or Nabarro-Herring creep does at high temperature; (3) water effect on olivine creeps are all small. Dislocation creep dominates the entire asthenosphere, namely, the creep mechanism transition at 200-300 km depth does not occur.
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