Wright, Kathleen Valerie; (1991) The rheology of perovskites and its implication for mantle dynamics. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

A combination of experimental studies on analogues and computer simulation techniques has been used to investigate the rheological behaviour of MgSiO3, the major phase in the Earth's lower mantle. Creep experiments have been performed on two oxide perovskites, CaTiO3 and NaNbO3. Both are found to deform according to a dislocation controlled power-law mechanism. Experiments performed on CaTiO3 with applied stress parallel to two different directions, indicate a strong dependence of creep rate on orientation. Computer simulation techniques have been used to model defects and diffusion in the perovskites SrTiO3, CaTiO3 and MgSiO3. The calculated value of activation enthalpy for oxygen diffusion in SrTiO3 agrees well with published experimental data thereby justifying the use of this method for materials with the perovskite structure. In CaTiO3, Ti is predicted to be the rate controlling species. Defects and diffusion in MgSiO3 have been simulated at pressures of 0, 60 and 125 GPa. Si is predicted to be the rate controlling species at 0 and 60 GPa, but at 125 GPa, Mg has the higher diffusion activation enthalpy. The results of the experiments on perovskites have been compared to those of other studies to assess the likelihood of perovskites forming an isomechanical series. No evidence for isomechanical behaviour has been found. However, CaTiO3 is considered to be a suitable analogue for MgSiO3. The information from experiments and simulations has been combined in order to compile rheological data sets for both CaTiO3 and MgSiO3 perovskites. These data sets have been used to construct deformation mechanism maps for both and in the case of MgSiO3 to construct viscosity profiles for the lower mantle. Materials-based predictions of mantle viscosity are comparable with those inferred from geophysics. The viscosity profiles indicate that the lower mantle is best described by a power-law rheology, with an increase of a factor 10 in viscosity with depth. The models are consistent with, but do not exclusively require, whole mantle convection.

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