Evolution of grain boundary distributions in MG2SIO4 with temperature and water content
File(s)
Author(s)
Austin, Alexandra C.
Type
Thesis
Abstract
Olivine is one of the most geologically significant minerals, constituting at least 60 volume % of the upper mantle. As the only fully interconnected phase, it’s physical and electrical properties have a profound influence on mantle rheology, seismological observations, and conductivity measurements used to understand the dynamics of the deep Earth. Rocks are aggregates of minerals, connected by grain and phase boundaries. Many of the properties of interest: rheology, strength, and conductivity, are all affected by the presence of these boundaries. The diverse conditions and assemblages within the deep Earth give rise to a variety of observed behaviours governed by complex mechanisms and reconciling these observations is an ongoing challenge for the field.
Grain boundaries can exhibit phase-like behaviour, with the most stable (lowest energy) structure varying with a range of thermodynamic variables. In ceramic and metal systems, complexion transitions have been observed and linked to changes in macroscopic materials properties.
This work examines changes in the grain boundary populations of the magnesium end-member of olivine, Mg2SiO4 (forsterite), with varying temperature and water content, using stereological methods and electron backscatter diffraction (EBSD). By analysing the relative areas of grain boundaries with different interfacial characters, a temperature-dependent change in the most stable boundary is identified, indicative of a complexion transition. Validation of these results is achieved using multiple indexing methods for EBSD data and transmission electron microscope-based orientation mapping. Additionally, the introduction of hydrogen via water is shown to alter the grain boundary population, highlighting another potential mechanism of water-weakening influencing the strength of the upper mantle. In addition to olivine, the grain boundary population in a compositionally-complex perovskite oxide with hydrogen transport properties is also evaluated. This behaviour is believed to be associated with grain boundary processes and demonstrates the applicability of this analysis to engineering materials.
Grain boundaries can exhibit phase-like behaviour, with the most stable (lowest energy) structure varying with a range of thermodynamic variables. In ceramic and metal systems, complexion transitions have been observed and linked to changes in macroscopic materials properties.
This work examines changes in the grain boundary populations of the magnesium end-member of olivine, Mg2SiO4 (forsterite), with varying temperature and water content, using stereological methods and electron backscatter diffraction (EBSD). By analysing the relative areas of grain boundaries with different interfacial characters, a temperature-dependent change in the most stable boundary is identified, indicative of a complexion transition. Validation of these results is achieved using multiple indexing methods for EBSD data and transmission electron microscope-based orientation mapping. Additionally, the introduction of hydrogen via water is shown to alter the grain boundary population, highlighting another potential mechanism of water-weakening influencing the strength of the upper mantle. In addition to olivine, the grain boundary population in a compositionally-complex perovskite oxide with hydrogen transport properties is also evaluated. This behaviour is believed to be associated with grain boundary processes and demonstrates the applicability of this analysis to engineering materials.
Version
Open Access
Date Issued
2024-12-03
Date Awarded
01/12/2025
License URL
Advisor
Marquardt, Katharina
Publisher Department
Department of Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)