Dynamic recrystallization and grain growth in olivine rocks

Abstract

A mechanism based description of the rheology of olivine is essential for modeling of upper mantle geodynamics. Previously, mantle flow has been investigated using flow laws for grain size insensitive (GSI) dislocation creep and/or grain size sensitive (GSS) diffusion creep of olivine. Generally, flow laws have been calibrated in experiments to relatively low strains. Recenty, however, it has become apparent that such low-strain calibrations do not represent true steady state flow, as the microstructure continues to evolve. Also, it has become apparent that a rheological description should account for grain size distribution. Recrystallization processes like grain boundary migration and nucleation govern microstructural evolution during large strain deformation. Few models for explaining a relationship between flow stress and recrystallized grain size include microstructural evolution. Moreover, little experimental work has been done regarding this issue, notably in the GSS creep regime. The aim of this thesis is to present an improved description of the rheology of olivine, incorporating effects of microstructural evolution up to high strain. New experiments on olivine under conditions poorly covered in previous experiments, and a theoretical treatment and numerical modeling of microstructural evolution during deformation have been performed. A model for dynamic recrystallization is developed through obtaining relations for nucleation rate and the grain growth rate, assuming distributed grain size. By combining the results, the evolution of the grain size distribution with time is determined. The model shows that the grain size evolution during dynamic recrystallization can be highly complex. Next, static heat treatment experiments and deformation tests were performed on wet fine-grained synthetic forsterite aggregates. Samples were deformed, under conditions where GSS creep and grain growth dominate. Static grain growth was found to be minor due to pinning. Deformation experiments showed continuous hardening with strain. When straining was temporally interrupted, no difference in flow stress was observed before and after the interruption. Samples showed an increase in grain size with strain. We relate the observed increase in flow stress with strain to deformation induced grain coarsening. A dynamic grain growth model involving an increase in defect fraction (fraction of non-hexagonal grains) seems best applicable. The hypothesis that deformation-induced topological changes may explain dynamic grain growth is then tested using 2D-microstructural modeling. Simulations were performed combining grain boundary migration with homogeneous or GSS straining. The simulations showed extra grain switching, an increase in defect fraction and enhanced grain growth compared to static grain growth. Finally, deformation experiments have been preformed on wet fine-grained synthetic olivine material, now incorporating a small amount of Fe. By comparison of the results with previous data on fo100, the addition of iron allows to study the effect of Fe on the flow behaviour during GSS creep, and on the microstructural evolution to high strain. Compared to the data for wet fo100 at similar conditions, adding Fe to wet synthetic olivine was found to have no significant effect on its rheology. An extra component of grain growth was observed during deformation. This enhanced grain growth can be related to a change in defect fraction.