Simulations of dislocation-mediated deformation, attenuation and dispersion in the Earth's mantle
Lead Research Organisation:
University of Cambridge
Department Name: Earth Sciences
Abstract
Understanding the structure and behavior of the Earth requires both geophysical observation and studies of the properties of the materials which are believed to comprise the deep interior. Seismology is a particularly powerful way to observe the interior, but to extract the structure, temperature and composition of the mantle and core the elastic behavior of minerals must be known to extreme temperature and pressure, pushing the limits of experiment. To sidestep this problem much of the data now used is actually generated by computer simulation of minerals performed at the atomic scale and this approach has been phenomenally successful in recent years. However, these approaches have only considered the elastic response of the minerals - the fact that energy is dissipated as seismic waves pass through the Earth is ignored. A second related factor is also not provided by calculating the pure elastic response, the fact that seismic waves with different frequencies travel at different speeds. To understand these two effects, which are known as attenuation and dispersion, we must study anelasticity - the time delayed response to the passage of the seismic wave. Anelasticity is the result of the presence of imperfections or defects in the mineral structure. These imperfections are also the agents responsible for long term flow in the mantle, a process which results in plate tectonics, growth of mountain ranges, volcanism and many of the processes that make the Earth we see today. Key to the project is the ability to simulate crystals containing defects and especially a type of defect called dislocations. By studying dislocations on the atomic scale the project will allow seismologists to extract more information from the observed data and will enhance our understanding of the Earth's interior. With a solid understanding of the causes of attenuation and dispersion it will be possible to extract much more information about the nature of the Earth's interior from observation. The research will also be valuable in other fields where dislocations alter the properties of materials. For example, the problem of deformation of pharmaceuticals during the production of tablets could be addressed by the methods developed for the study of the Earth's interior. Furthermore dislocations in materials used in computer chips, catalysts, fuel cells and batteries alter the behavior of these materials and the methods can be used to tackle all these problems. Finally, dislocation emerging at the surface of crystals allow reactions to take place on the surface, one example of an environment where this could be important is the interaction between pollutants and ices in clouds in the the upper atmosphere.
Organisations
Publications
Carrez P
(2008)
Evidence from numerical modelling for 3D spreading of [001] screw dislocations in Mg 2 SiO 4 forsterite
in Philosophical Magazine
Palin E
(2008)
A computational study of order-disorder phenomena in Mg2TiO4 spinel (qandilite)
in American Mineralogist
Walker A
(2008)
Comment upon the screw dislocation structure on HKUST-1 {111} surfaces
in CrystEngComm
Walker A
(2008)
The compressibility and high pressure structure of diopside from first principles simulation
in Physics and Chemistry of Minerals
Woodley S
(2007)
New software for finding transition states by probing accessible, or ergodic, regions
in Molecular Simulation
| Description | 1) The development of a general-case numerical solution to find the elastic displacement field around dislocations. This advanced previous work using analytical solutions for the displacement field which were limited screw and edge dislocations in special directions in high symmetry crystals and allows the treatment of mixed dislocations of any orientation in triclinic crystals. 2) A detailed comparison between the structure of dislocations as predicted by full atomistic calculations and the semi-discrete Peierls-Nabbarro method, undertaken in collaboration with Cordier and Carrez (Universite_ des Sciences et Technologies, Lille). Focusing on the [001] screw dislocation in forsterite lead to the discovery of dramatic differences between results from two methods. Our explanation for this difference is that the semi-discrete approach leads to a core structure with maximal mobility, while the discrete method leads to a core with minimum energy. We proposed that dislocation cores must transform between these configurations in order for them to move, and suggest how this explains some of the deformation properties of this mantle mineral. 3) The structure and core energies of screw dislocations in wadsleyite were established by atomic scale simulation. These results explained the experimental observation that [100] screw dislocations are common (they have a low core energy and a simple mobile core structure) and that [001] dislocations are rare (they have a high energy and a complex sessile core structure). 4) A simple numerical model of kink migration over a Peierls potential has been developed in collaboration with Jackson (the Australian National University) and Hunt (State University of New York at Stony Brook). This model builds on earlier work where the potential is a simple analytical function and permits deformation rates and anelasticity to be probed for realistically complex barriers to kink and dislocation mobility. Importantly, this approach allows a direct link between atomic scale simulations of dislocations and experiments probing the onset of deformation. This has lead to the discovery of an interesting "intermediate" regime in published deformation data and work is ongoing to explain this in terms of kink migration. 5) Large scale simulation of kink formation on dislocation lines has allowed the key assumptions behind the atom-free numerical model to be tested. The intention was for these calculations to incorporate the effect of temperature making use of molecular dynamics but this proved to be impossible due to the elastic strains interfering with the atomic motions. Instead, large scale static calculations were undertaken, which provide sufficient information for ionic crystals where the Peierls barrier is large. |
| Exploitation Route | The deeper understanding of dislocation dynamics in these complex Earth materials allows a further understanding of the Earth's interior and could contribute to other fields where deformation of manufactured materials is important. |
| Sectors | Aerospace, Defence and Marine,Energy,Environment |
| Description | These findings have contributed to the creation of a large NERC research programme with significant potential for impact. |