Water Solubility in Mantle Phases and its Effect on Physical Properties

Abstract

Water is essential for our species’ survival on this planet and its behavior and presence inside of the Earth has been a topic of interest for many years. On the Earth’s surface water is ubiquitous and calculating its quantity is relatively straight-forward. Determining the quantities present in the Earth’s deep interior is much more challenging. To investigate the current water content of the Earth’s mantle, a combined experimental and geophysical approach is required. The solubility of water and its effect on the physical properties of mantle minerals has to be disentangled by laboratory experiments. These results can then be compared to geophysical observations of Earth’s mantle to assess the presence of water. This work contributes to the understanding of the water presence in the interior of Earth and rocky planets in general. The first part of this thesis consists of investigating the presence of water in bridgmanite and its effect on elasticity, as will be explained in Chapter 2. Bridgmanite is the major mantle phase in the Earth’s lower mantle, and its water solubility has been a controversial topic for the past 20 years. By using a combination of Fourier Transform Infrared Spectroscopy and Atom Probe Tomography, the presence of hydrogen in the crystal lattice of bridgmanite is confirmed in this work. The first measurements of shear wave velocity in a hydrous bridgmanite sample containing ∼1054 ppm wt H2O and 3.44 wt.% Al2O3 are presented. These measurements indicate that the presence of water in bridgmanite causes the material to stiffen, expressed as a 6% decrease in shear modulus (G0), and a 5% increase in the first pressure derivative of G0 (G0’). Modeling the effect of water on the velocity profile of the lower mantle and comparing it to known geophysical reference models like the Preliminary Reference Earth model, shows that no significant amount of water is to be expected however. Secondly, the phase transition from stishovite to CaCl2-type SiO2 is studied using Brillouin Scattering Spectroscopy. The phase transition is found to take place at ∼30 GPa, which is lower than most previous studies on this topic. The possible reasons for this are examined in Chapter 3 of this work, and its implications on geophysical interpretations for this phase transition are discussed. Most importantly, this result suggests that mantle scatterers at a 1000–1200 km depth range could be explained by the phase transition. The last part of this thesis consisted in developing a method to optimize thermodynamic parameters used in geophysical modeling to better represent experimental results. Thermodynamic databases used for studies of the deep interior have thus far never ncluded the established effect of water on mantle phases’ stability and physical properties. Preliminary results of solving this issue are given in Chapter 4.