Experimental Study Of Si And Mg Diffusion In Single Crystal Of MgSiO₃ Perovskite Under Lower Mantle Conditions

MgSiO₃ perovskite is the most abundant and important minerals in the Earth's lower mantle. Its creep determines the rheological properties of lower mantle. The experimental diffusion coefficients of elements in MgSiO₃ perovskite are the most directviewing and reliable parameters for understanding the rheological properties. However, because of the conditions of high pressure and high temperature in the lower mantle, until now there are few experimental studies of diffusion coefficients in MgSiO₃ perovskite. Especially, there is still in controversial of Si and Mg diffusion results. In our study, Si and Mg self-diffusion coefficients were measured simultaneously in the single crystals of MgSiO₃ perovskite under lower mantle conditions using a Kawai-type multi-anvil apparatus at the Institute for Study of the Earth's Interior, Okayama University. The results showed that Mg has almost identical diffusivity of Si in perovskite. Mg together with Si is the slowest diffusing species in MgSiO₃ perovskite. It is different from the case in most silicates, which is Si. Mg and Si may together control the rheological properties in the lower mantle. Diffusion coefficients obtained in this study for both Si and Mg seemed to be isotropic. Furthermore, based on previous studies of perovskite type materials, we obtained the diffusion model of Si and Mg in MgSiO₃ perovskite. In addition, we synthesized a new pyrochlore type material MgZrSi₂O₇ under lower mantle conditions. The pyrochlore type materials have many important applications and are widely studied in materials science. The new pyrochlore MgZrSi₂O₇ can stabilize in very high pressure and temperature conditions and may have special applications in material science. The mainly contents of my docotoral work are listed as following:1. Synthesis experiments of MgSiO₃ perovskite single crystal: The single crystals of MgSiO₃ perovskite were synthesized at the conditions of 25GPa and 1500oC. It is very difficult to synthesis large grainsize crystals at high pressure and high temperature. However, in order to obtain high quality diffusion coefficients, single crystal were need. We synthesized some high quality single crystals following Shatskiy et al. (2007) and crystals were examined by Microfocused X-ray diffractometer.2. Diffusion experiments of Si and Mg in MgSiO₃ perovskite. Crystals were orientated by procession X-ray camera along a- and c- axes and polished using diamond paste and collidoal silica. The polished surfaces were coated with the 29Si- and 25Mg-enriched MgSiO₃ thin film using the pulsed laser deposition technique (PLD) at Ruhr University of Bochum, Germany. Si and Mg diffusion experiments were conducted at the conditions of 25 GPa and 1400-1800oC. Diffusion profiles were measured by SIMS in Hokkaido University. The results showed that Mg and Si almost have the same diffusivity in perovskite. This is the first time of observation in perovskite simultaneously. This is very important to understand the reological properties of lower mantle. It is helpful to understand perovskite structure properties and diffusion mechanism.3. Synthesis and crystal chemical characterization of pyrochlore type MgZrSi₂O₇ The pyrochlore type of MgZrSi₂O₇ was synthesized at 25 GPa and 1500 oC using a Kawai-type, multi-anvil apparatus. Powder X-ray diffraction and Rietveld analysis revealed that the phase assumed the pyrochlore structure (space group Fd-3m, cubic) with the lattice parameter a= 9.2883(1)? and the structural parameter x = 0.4295(4). Chemical analysis by the electron probe microanalysis (EPMA) confirmed the stoichiometry of MgZrSi₂O₇ . It was demonstrated that the eight-fold coordinated 16c site is randomly occupied by both Mg2+ and Zr4+ ions in a 1:1 ratio. The high ionic radius ratio RA/RB (where A and B denote Mg+Zr and Si, respectively) of 2.22 necessitates a relatively high pressure to stabilize the pyrochlore structure.4. The model of Si and Mg diffusion in MgSiO₃ perovskite. The situation of Mg diffusion in MgSiO₃ perovskite is different from other minerals (e.g olivine, wadsleyie and ringwoodite), in which Mg diffusion is several orders of magnitude faster than that of Si. Mg diffusion in perovskite has almost the same diffusivity as that of Si. We explained this result based on the comprehensive of the perovsktie structure. It is the first time of a new model to be used to explain the coupled motion of Si and Mg in perovskite, which is basically derived from the other perovskite-type oxides. In this model, the different atoms are related with each other and Mg diffusion through an unhindered path due to the presence of a Si and an O vacancy. The associated motion would account for similarity of diffusivity of Si and Mg, and the much faster diffusion rate of oxygen in perovskite. It is the first time to apply this model in perovskite structure under lower mantle conditions.