New constraints on upper mantle creep mechanism inferred from silicon grain-boundary diffusion rates

Diffusion Creep and Grain Growth in Forsterite +20 vol% Enstatite Aggregates: 2. Their Common Diffusional Mechanism and Its Consequence for Weak‐Temperature‐Dependent Viscosity

“…Diffusion of the slowest ions that constitute minerals is generally considered to control their creep and grain growth rates. In silicate minerals, Si is usually the slowest ion for both grain boundary and lattice diffusion, which has been confirmed in experiments on forsterite (Dohmen et al, ; Farver & Yund, ; Fei et al, , ). As discussed earlier, we can approximate the polyphase aggregate as single‐phase forsterite, with a f En ‐dependent grain size in the case of creep.…”

“…During this process, the MgO is expected to diffuse from coarser enstatite grains to finer enstatite grains, which is the opposite direction to that anticipated from a typical case of Ostwald ripening, while such diffusion still allows the coarser grains to grow by the consumptions of the finer grains due to the phase transformation. Because of D Si ≪ D O ≪ D Mg for both the forsterite grain boundaries (Chakraborty et al, ; Fei et al, ; Gardés et al, ) and the lattice (Chakraborty et al, ; Fei et al, ), the second slowest diffusion, that is D O , is predicted to determine the rates of creep and grain growth based on the model of Sundberg and Cooper ().…”

Evidence of phase nucleation during olivine diffusion creep: A new perspective for mantle strain localisation

“…For instance, recent quantifications of silicon diffusion in olivine (Fei et al, 2016) have predicted grain-boundary diffusion creep (coble creep) to be dominant -at the expense of dislocation creep -in the whole uppermost mantle. Moreover, numerical models of upper mantle convections have shown that strain localisation induced by grain size reduction during olivine diffusion creep have a great potential in promoting plate boundaries, which is a sine qua non condition for Plate Tectonics to originate (Bercovici and Ricard, 2014).…”

Identical activation volumes of dislocation mobility in the [100](010) and [001](010) slip systems in natural olivine

“…[] (1.7 ± 0.4 cm 3 /mol) in forsterite, they are slightly higher. On the other hand, the activation volume of the Si grain‐boundary diffusion in forsterite is 4.0 ± 0.4 cm 3 /mol [ Fei et al ., ], which is higher than those of the dislocation annihilation and Si lattice diffusion. The lattice diffusion, dislocation climb, and grain‐boundary diffusion are interactions between atoms and vacancy, dislocation, and grain boundary, respectively.…”