Abstract
dc:description.abstractPartial melting is an important geological process in the deep Earth that affects physical, chemical and rheological properties of rocks. The effect of partial melting on mantle dynamics depends on both the amount of melt and how it is distributed within the crystalline matrix. A few percent of melt have potentially large effect on the physical properties of rocks. In this work, atomic scale simulations are used to study the structure and transport properties of ultrathin melt films between olivine grains, which is a simple model system of partially molten peridotite. The model system consists of 0.8 to 7.0 nm thick layers of magnesium silicate melt with a composition close to MgSiO3 (enstatite) confined between Mg2SiO4 forsterite crystals. We examine how the atomic structure, the chemistry and the self-diffusion coefficients vary across the interface and investigate their dependence on the thickness of the melt layer and the crystal orientation. The particle interactions are represented by an advanced ionic model. From the particle trajectories, we derive various properties, like charge densities, cation coordinations, chemical compositions, and self-diffusion coefficients. Interfacial layers of up to 2~nm thickness show distinctly different physical behavior than the bulk melt and the bulk mineral. The simulation results indicate that for crystal orientations with higher surface energy, the self-diffusion coefficients of all ionic species in the melt decrease at constant melt layer thickness. By increasing the melt layer thickness between the crystals, the average mobility of ions in the melt is increased. On the interfacial part the charge mobility of all species decreases due to solid-like ordering between atoms. For modeling the petrophysical behavior of partially molten rocks, the effective diameter for the conducting channels is reduced by up to two nanometers, which effects the rheological and transport properties of partially molten rocks, especially in the presence of ultra-thin melt films in well-wetted systems. In the latter case, the electrical conductivity of the confined melt in a partially molten rock could be reduced up to a factor of two due to interfacial effects. A slight difference is observed in the interfacial properties due to change in chemical composition, pressure and temperature conditions. When calcium is added to the system, the self-diffusion coefficients of all species slightly change. At different pressure and temperature, a huge difference is observed in the self-diffusion coefficients. Freezing of the system and confinement effect is clearly observed at 2000 K with pressure of 10 GPa, and at 2400 K with 10 GPa pressure. Non-equilibrium molecular dynamics simulations with constant shear rate are performed on this system showing complex rheological behavior in the vicinity of interfaces. A dependence of the viscosity on shear rate is observed which constitutes non- Newtonian behavior of the melt at the high shear rates accessible to molecular dynamics. The viscosity calculated from non-equilibrium molecular dynamics simulations is found to be somewhat higher then the viscosity calculated from equilibrium molecular dynamics simulations. The viscosity at the lowest modeled shear rate is in good agreement with the experimental viscosity.
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Gurmani, Samia Faiz
Subjects
dc:subject × 3Rights
- Licence dc:rights.uri
- Language dc:language
- eng
Identifiers
dc:identifier.*- Identifier URI
- http://dx.doi.org/10.17169/refubium-12376