EBSD analysis of heterogeneous microstructures in experimentally deformed calcite: development of core and mantle subgrains, grain boundary bulges and recrystallised grains. Geologica Ultraiectina (289)

Abstract

Geodynamic processes such as subduction and rifting are mainly controlled by high temperature plastic deformation of rocks. It is known that elements of the microstructure in plastically deformed rocks, such as subgrains and recrystallised grains, are potentially useful as indicators of past deformation conditions in the Earth. The general aims of this study are (i) to establish novel methods that allow a quantitative description of microstructures in materials showing complex, heterogeneous microstructures and (ii) to analyse deformation mechanisms and the relationship between elements of the microstructure and deformation conditions, such that microstructures in geological materials can be used as indicators of (palaeo-) deformation conditions. The material of this study consists of samples of calcite experimentally deformed for different conditions: natural strain (0.15-0.90), stress (15-85 MPa) and temperature (700-900 C). Calcite is an important rock forming material often involved in crustal deformation zones and has heterogeneous microstructures consisting of recrystallised and deformed grains, the latter having grain boundary bulges and containing mantle and core subgrains. Grain boundary bulges and recrystallised grains are shown to be independent of strain. The recrystallised grain and bulge size show an inverse stress dependency, which can be affected by temperature (especially at high stress). The bulge size – stress dependency is suggested to be related to pinning of grain boundaries by sub-boundaries, and/or differences in driving forces for migration. The recrystallised grain size – stress dependency is related to the stress dependency of the bulges, because the recrystallised grains nucleate at grain boundary bulges. It is found that the mantle subgrain size has already become dynamically stable at low strain (0.15). It is suggested that this is related to a balance between formation and ‘growth’ processes (subgrain boundary migration and subgrain coalescence). In contrast, the core subgrain size decreases with increasing strain and is linked to the deformed grain size. A mechanistic model based on slip system activation and stress intensification at grain boundaries in polycrystalline materials is proposed to explain the development of core mantle substructures. The core subgrains originally are formed by single slip and are not only dependent on stress, but also on grain size, strain and temperature. The low angle mantle subgrains are formed by slip on more than one slip system. They have a low stress sensitivity, which is probably related to cross-slip as a deformation mechanism and/or geometrically necessary dislocations accumulating in the mantle. In contrast, high angle mantle subgrains mainly consist of ‘bulge subgrains’ that are relatively more stress sensitive due to their origin by bulge rotation. Based on the stress sensitivities of the recrystallised grain size, bulge size, core and mantle subgrain sizes, suggestions are made for the use of such elements of the microstructures for palaeopiezometric purposes. It is found that high angle mantle subgrains, bulges and recrystallised grains are the most sensitive to stress and therefore most useful indicators of stress. For the application of recrystallised grains for palaeopiezometry, care should be taken especially at high stress because of the effect of temperature on the recrystallised grain size.