Si and O diffusion rates on polycrystalline (Mg,Fe)2SiO4 wadsleyite and

ringwoodite were determined at pressures between 16 and 22GPa and between

1400 and 1600C, using a Kawai-type multi-anvil high-pressure apparatus. Presynthesized

polycrystalline wadsleyite or ringwoodite was used as starting

materials. Diffusing sources of 29Si and 18O enriched (Mg,Fe)2SiO4 thin film were

deposited on the surface of wadsleyite and ringwoodite by pulsed laser deposition.

The diffusion profiles were obtained using secondary ion mass spectrometry in the

depth-profiling mode. All measured diffusion profiles were composed of volume

and grain-boundary diffusion regimes. Arrhenius relations for volume and grainboundary

diffusion rates of Si and O in wadsleyite and ringwoodite were

determined. The results showed that Si was the slowest diffusing element among the major elements in both wadsleyite and ringwoodite under mantle transition

zone as well as slab conditions. Therefore, Si should be the rate-controlling species

for high-temperature creep in wadsleyite and ringwoodite. Comparisons with Si

and O diffusion rates obtained here and those in olivine and silicate perovskite

suggested that Si and O diffusion rates were enhanced at both 410 and 660km

seismic discontinuities. The deformation mechanism maps constructed from Si

diffusion data in wadsleyite and ringwoodite suggested that the dominant

deformation mechanism operating in the mantle transition zone was dislocation

creep, which may be the origin of the observed global seismic anisotropy. Under

cold subduction zone conditions, grain-size sensitive diffusion creep becomes

dominant when the grain size was reduced to less than 10–100μm below the

metastable olivine wedge.

Si and O Diffusion in (Mg,Fe)2SiO4 Wadsleyite and Ringwoodite and Its

Implications for the Rheology of the Mantle Transition Zone. A.Shimojuku,

T.Kubo, E.Ohtani, T.Nakamura, R.Okazaki, R.Dohmen, S.Chakraborty: Earth and

Planetary Science Letters, 2009, 284[1-2], 103-12