title: The Elastic Properties of Wadsleyite and Stishovite at High Pressures : Tracing Deep Earth Material Cycles
Creators: name: Buchen, Johannes
Publisher: name: University of Bayreuth
Year: 2018
Subjects: subject: mineral physics
subject: high pressure
subject: elasticity
subject: deep Earth
subject: wadsleyite
subject: stishovite
subject: 550

description: Plate tectonics on Earth is integrated into global material cycles that exchange chemical components between Earth’s surface and Earth’s mantle. At subduction zones, slabs of oceanic lithosphere sink into the mantle and carry basaltic crust and H2O stored in hydrous minerals into Earth’s interior. A fraction of the stored H2O might reach the transition zone at 410 km depth where the phase transition of the mineral olivine to the high-pressure polymorph wadsleyite, beta-(Mg,Fe)2SiO4, gives rise to a discontinuity in seismic wave speeds. Under favorable conditions, nominally anhydrous wadsleyite can incorporate up to 3 wt-% H2O as hydroxyl groups in its crystal structure. The high solubility of H2O in wadsleyite may have had consequences for deep cycling of H2O in Earth’s mantle throughout the geological past. Even small amounts of H2O in Earth’s mantle may lower the melting point and viscosity of mantle rocks and affect the geodynamical and geochemical evolution of the mantle. Despite the key role of the transition zone in deep cycling of H2O, little is known about the amount and distribution of H2O in the transition zone. Seismic waves that travel through the transition zone or reflect off the 410-km seismic discontinuity carry a signature of the compositional and thermal state of the transition zone. To read this signature, the elastic properties of relevant minerals and rocks are needed, ideally at pressures and temperatures of the transition zone, e.g. 14 GPa and 1500°C at 410 km depth. 

I performed high-pressure X-ray diffraction experiments on wadsleyite single crystals up to 20 GPa using a diamond anvil cell and determined the equation of state (EOS) from the variation of unit cell volume as a function of pressure. With Fe/(Mg+Fe) = 0.11 and 0.24 wt-% structurally bound H2O, the wadsleyite crystals had a chemical composition relevant for the transition zone. I combined the EOS of the here-studied crystals with literature data to construct a multi-end-member model for the EOS of wadsleyite solid solutions. The model shows that the bulk modulus of wadsleyite increases with the ferrous iron content but decreases with increasing contents of H2O and ferric iron. 

To determine the elastic stiffness tensors at high pressures and high temperatures, I performed Brillouin spectroscopy experiments on wadsleyite single crystals that were loaded pairwise into diamond anvil cells. The collected sound wave velocities were inverted to parameters of finite-strain theory using a novel inversion strategy. Aggregate sound wave velocities calculated from the high-pressure elastic properties of the here-studied wadsleyite crystals were compared to literature data on different wadsleyite compositions to assess the effect of structurally bound H2O on sound wave velocities. At ambient conditions, the incorporation of H2O reduces the sound wave velocities of Fe-bearing wadsleyite. The comparison with literature data on Fe-bearing wadsleyite with a high H2O content, however, reveals that both P and S wave velocities of Fe-bearing wadsleyite with high H2O content converge and potentially cross over with those of the here-studied wadsleyite with low H2O content at pressures of the transition zone. These findings imply that seismic wave speeds may be less sensitive to H2O in the transition zone than previously assumed. Instead, modeling of seismic properties based on the experimental results of the present study combined with literature data suggests that a low acoustic impedance contrast and hence a locally reduced reflectivity of the 410-km seismic discontinuity may be a better indicator for H2O in the shallow transition zone. I further present the first experimentally determined elastic stiffness tensors of wadsleyite at simultaneously high pressures and high temperatures. 

Oceanic crust that enters the mantle at subduction zones transforms into an assemblage of dense high-pressure phases that allow basaltic material to sink into the lower mantle. Stishovite, rutile-structured SiO2, may contribute with up to 20 vol-% to a basaltic rock at conditions of the lower mantle. The ferroelastic phase transition from stishovite to high-pressure CaCl2-type SiO2 strongly perturbs the elastic properties of these crystalline silica phases. I analyzed X-ray diffraction patterns that were recorded on sintered polycrystalline silica along a compression path across the ferroelastic phase transition and refined the lattice parameters of sintered polycrystalline silica at 30 different pressures between 9 and 73 GPa. Based on the pressure evolution of unit cell volumes and edge lengths, I determined the EOS and described the elastic properties of sintered polycrystalline silica using Landau theory. Unlike silica powder, sintered polycrystalline silica shows a substantial drop of the bulk modulus at the phase transition from stishovite to CaCl2-type SiO2. Together with the previously predicted shear wave softening, the drop in bulk modulus might affect the propagation of seismic waves raising the possibility to detect silica-rich rocks in Earth’s lower mantle and thereby to better constrain mantle convection patterns.