Nanoscale simulation of crystal defects with application to mantle minerals
Creators: name: Skelton, Richard
Publisher: name: The Australian National University
Year: 2018
Subjects: Dislocations
forsterite
atomic-scale modeling AND atomic-scale modelling
Peierls-Nabarro
glide lubrication
point defect

description: Minor and trace elements can influence the chemical and physical properties of the Earth's mantle, whether by forming separate minerals that can host trace elements at far higher concentrations than major mantle minerals, or by influencing physical properties of major minerals, for instance by enhancing diffusion or pinning topological defects. The present study uses atomistic computational modeling to investigate the properties of calcium phosphate minerals at mantle pressures, and to determine if cation vacancies can lubricate dislocation glide in mantle minerals. With their large, high-coordination cation sites, calcium phosphate minerals can potentially dissolve large quantities of incompatible elements, and may be an important mantle reservoirs for these elements. Quantum mechanical calculations are used to determine the variation of structure and properties of apatite, a common low-pressure phosphate, with the identity of its channel anion. The crystal structure and compressibility are both found to vary substantially with channel anion identity. Tuite, which forms by the breakdown of apatite at high pressure, may extend the phosphorous cycle beyond the stability field of apatite and into the lower mantle. DFT calculations are used to show that the pressure at which apatite decomposes to form tuite is sensitive to the identity of the channel anion. The calculated bulk moduli of the calcium-hosting sites in tuite are considerably lower than those of the M site in CaSiO3 perovskite (cpv). Atomistic calculations show that strontium and barium impurities partition strongly from cpv into tuite. The second part of this thesis considers the role that dislocations play in hosting vacancy-related defects, and influence that these segregated defects may have on the rheology of mantle minerals. Interatomic potentials are used to calculate energies for the segregation of bare and protonated cation defects to dislocation cores in MgO and forsterite. Vacancies segregate strongly to [100](010) and [001](010) dislocations in forsterite. [100](010) edge dislocations serve as particularly suitable hosts for vacancies on the M1 sub-lattice, while vacant M2 sites, which are energetically unfavorable in the unstrained bulk lattice are more common around [001](010) edge dislocations. Similarly, both {□Mg}″ and {2HMg}X defects segregate strongly to all of the major dislocation slip systems in MgO. For both minerals, cation vacancies will be far higher at dislocation core sites than in the unstrained lattice. Peierls-Nabarro calculations, parameterized using generalized stacking fault energies calculated using plane-wave density functional theory, show that protonated vacancies increase dislocation core widths and reduce the Peierls stresses in MgO. Bare Mg vacancies, especially those on the M2 sub-lattice, similarly reduce g-line energies and Peierls stresses for the dislocations in olivine. The magnitude of this decrease depends strongly on dislocation and the type of the lattice site (i.e. M1 or M2). Concentrations of vacancy related defects are likely to be elevated particularly in mantle wedge, due to the combination of oxidizing conditions and the availability of liquid water from dehydrating phases in the subducting slab, and changes in the olivine deformation fabric in these regions match those predicted in this study.