We have determined the pressure and temperature dependencies of Si volume diffusion rates in a single crystal of Pyp75Alm15Gr10 garnet at 6-25GPa and 1400-1800°C by the 29Si tracer diffusion method. High-pressure experiments were conducted using the Kawai-type multi-anvil high-pressure apparatus. Diffusion profiles were obtained using secondary ion mass spectrometry in the depth-profiling mode. The Si diffusion coefficient in garnet (Dgt) is expressed by the Arrhenius equation: Dgt=D0 exp(-(E*+PV*)/RT), with log10D0=-7.1±1.7m2/s, E*=345±56kJ/mol, and V*=4.7±0.5cm3/mol. Si diffusion seems to be the slowest in the major constituent elements in a pyrope-rich garnet and control rates of plastic deformation. The comparison between Si diffusion rates in garnet and wadsleyite/ringwoodite suggests that garnet has similar or slightly lower strength (at most ~2times) compared with wadsleyite, and has similar or slightly higher strength (at most ~3times) compared with ringwoodite at temperatures ranging from 900 to 1300°C. Thus, strength contrast between subducted oceanic crust and underlying peridotite layer may be small at the mantle transition zone conditions. This result suggests that the separation of subducted oceanic crust from an underlying peridotite layer may not occur.
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