Si and O diffusion in (Mg,Fe)2SiO4 wadsleyite and ringwoodite and its implication for rheology of the mantle transition zone

Si and O diffusion rates on polycrystalline (Mg,Fe)2SiO4 wadsleyite and ringwoodite have been determined at pressures between 16 to 22 GPa and temperatures between 1673 to 1873K. High pressure experiments were conducted using a Kawai-type multi-anvil high pressure apparatus. Pre-synthesized polycrystalline wadsleyite or ringwoodite were used as starting materials. Diffusing sources of 29Si and 18O enriched (Mg,Fe)2SiO4 thin film were coated on the surface of wadsleyite and ringwoodite by a pulsed laser deposition (PLD) in order to ensure the deposition of stoichiometric thin films. The diffusion profiles were obtained by a depth-profiling mode using a secondary ion mass spectrometry (SIMS). The obtained all diffusion profiles were composed of volume and grain-boundary diffusion regimes. Therefore, Arrhenius relations in volume and grain-boundary diffusion rates in wadsleyite and ringwoodite have been determined simultaneously. Their diffusion rates are characterized as follows: In (Mg,Fe)2SiO4 wadsleyite with 20-80 wt. ppm H2O, Dv(Si)= 2.79x 10-8 [m2/s] exp(-409 [kJ/mol]/RT),deltaDGB(Si)= 1.31x10-15 [m3/s] exp(-327 [kJ/mol]/RT), Dv(O)= 3.04x10-11 [m2/s] exp(-291 [kJ/mol]/RT), deltaDGB(O)= 1.62x10-17 [m3/s] exp(-244 [kJ/mol]/RT). In (Mg,Fe)2SiO4 ringwoodite with 130-220 wt. ppm H2O, Dv(Si)= 3.33x10-6 [m2/s] exp(-483 [kJ/mol]/RT), deltaDGB(Si)= 5.70x10-14 [m3/s] exp(-402 [kJ/mol]/RT), Dv(O)= 2.87x10-9 [m2/s] exp (-367 [kJ/mol]/RT), deltaDGB(O)= 7.85x10-18 [m3/s] exp (-246 [kJ/mol]/RT). The results show that Si diffusion rates are slower than O diffusion rates and previously reported Mg-Fe interdiffusion rates in both (Mg,Fe)2SiO4 wadsleyite and ringwoodite. Si is likely to be rate-controlling species in high-temperature creep involving diffusion creep and climb-controlled dislocation creep. Compared viscosities for diffusion and dislocation creep estimated from Si diffusion data with mantle viscosity inferred from geophysical observations, the mantle viscosity is explained by a grain size of 1 mm in diffusion creep regime and a stress of 0.1-1 MPa in dislocation creep regime. Consequently, plausible grain size and stress in the mantle transition zone may be 1 mm and 0.1-1 MPa.