Mineral physics of the mantle transition zone: Solid solutions and elasticity.

Abstract

One goal of high pressure mineralogy is to determine the composition of the earth's interior. This effort must include an understanding of elasticity at high pressures, and the effects of disorder and of solid solutions on physical properties of minerals. Chapters II and III focus on wadsleyite, which is thought to be the most abundant mineral in the upper part of the earth's transition zone (410--520 km depth). The calculated elastic anisotropy of this phase is small and almost independent of pressure. The results also show that bond length and bond-angle systematics in silicates obtained from chemical variations at zero pressure do not capture the effect of pressure correctly. Wadsleyite shows a non-random iron distribution on its three symmetrically distinct octahedral sites. The results (Chapter III) provide an explanation for this observation in terms of crystal field splitting of the highly acentric M2-site. For ringwoodite, which is thought to be most abundant mineral in the lower part of the transition zone (520--660 km depth) the energetics of magnesium/silicate disorder at high pressures is investigated (Chapter IV). The results show the importance of structural relaxation for the modeling of Mg/Si disorder. The enthalpy differences are a factor 80 smaller as compared to previous studies but still too high to stabilize completely inverse ringwoodite at transition zone conditions. Ferromagnesian silicate perovskite is thought to be the most abundant mineral in the earth's lower mantle (660--2900 km depth) and should dominate the physical properties in this region. Seismic tomography commonly observes an anti-correlation of lateral variations in shear-wave and bulk-sound speed in the earth's deep lower mantle. It was suggested that lateral variations of iron content in perovskite could explain this observation. The results presented in Chapter V show that this is not the case and other explanations must be sought.