Relationship between dynamic recrystallization, grain size distribution and rheology

Abstract

The solid state flow behavior (rheology) of materials constituting the Earth’s mantle and crust is of key importance in controlling the dynamics of large scale geodynamic processes, such as mantle convection, subduction, mountain building and basin formation. Flow laws that are calibrated using laboratory experiments can provide constraints on the rheology of rock materials under natural conditions, given that all active deformation mechanisms and microphysical processes affecting rheology are accounted for. Rocks invariably exhibit a distributed grain size, with small grains that may deform by grain size sensitive (GSS) deformation mechanisms and large grains that may deform by grain size insensitive (GSI) deformation mechanisms. Moreover, dynamic recrystallization can affect the rheology of rock materials by extensively modifying the grain size distribution. However, most conventional flow laws are either fully empirical or based on a single deformation mechanism, and if grain size is included, it is represented as a constant single value. This thesis aims to provide an improved description of the rheology of rock materials by incorporating distributed grain size and multiple deformation mechanisms into theoretical flow laws. It further aims to assess the influence of dynamic recrystallization on the evolution of grain size distribution and on composite flow behavior. The research combines theoretical work and laboratory deformation experiments. In the theoretical part, composite diffusion-dislocation flow laws for materials with a lognormal grain size distribution are derived that incorporate the distribution parameters, i.e. standard deviation and median grain size. The flow laws are used to include the distribution parameters into a model for dynamic recrystallization in which grain size and rheology are assumed to adjust itself to the boundary between the GSS and GSI fields. In the experimental part, the natural calcite rock Carrara marble and synthetic polycrystalline halite (wet and dry) have been deformed in axial compression for a systematic range of strains, strain rates, stresses and temperatures at elevated pressure. The results show that for an accurate description of the rheology of rock materials, the complete grain size distribution, composite GSS-GSI flow and the effect of dynamic recrystallization should be taken into account. Strain rate can change by orders of magnitude due to variation of standard deviation of the grain size distribution at fixed median grain size, which is unaccounted for in conventional flow laws that include a single-valued grain size. In both Carrara marble and wet synthetic polycrystalline halite, dynamic recystallization by progressive subgrain rotation and/or grain boundary migration results in a significant change in grain size distribution and minor rheological weakening. The observed rheological behavior and weakening can only be explained by considering changes in the relative contribution of GSS flow relative to GSI flow due to alteration of the grain size distribution by dynamic recrystallization. The limited weakening observed suggests that in relatively pure, single phase materials, weakening associated with dynamic recrystallization is insufficient to cause strain localization.