Dynamic recrystallization by subgrain rotation in olivine revealed by electron backscatter diffraction

Abstract

We document how dynamic recrystallization by subgrain rotation (SGR) develops in natural olivine-rich rocks deformed in extension to up to 50% bulk finite strain (1473 K, confining pressure of 300 MPa, and stresses between 115 and 180 MPa) using electron backscatter diffraction (EBSD) mapping. SGR occurs preferentially in highly deformed grains (well-oriented to deform by dislocation glide) subjected to local stress concentrations due to interactions with hard neighboring grains (poorly-oriented olivine crystals or pyroxenes). Subgrains (misorientation <15 • ) are mainly delimited by tilt walls composed of combinations of dislocations of the [100] (001), [001](100), [100](010) and [001](010) systems, in order of decreasing frequency. The activation and prevalence of these systems agree with a Schmid factor analysis using data for high-T dislocation creep in olivine. The development of closed 3D subgrain cells by SGR recrystallization requires the contribution of at least three different slip systems, implying the activation of hard slip systems and high (local) stresses. The transition from subgrain to grain boundaries (misorientation ≥15 • ) is characterized by a sharp change in the misorientation axes that accommodate the difference in orientation between the two subgrains or grains. We propose that this change marks the creation and incorporation of new defects (grain boundary dislocations with different Burger vectors and, likely, disclinations or disconnections) at the newly-formed grain boundaries and that this might be favoured by stress concentrations due to increasing misalignment between slip systems across the boundary. Finally, we document the development of strong misorientations within the parent grains, due to the accumulation of low angle grain boundaries, and between them and the recrystallized grains. SGR recrystallization may thus produce strong dispersion of the crystal preferred orientation without the need for grain boundary sliding.

Intracrystalline deformation microstructures in natural olivine with implications for stress estimation

“…In these cases, the presence of the (001)[100] slip system related SGBs does not contradict with the dominant (010)[100] and {0kl}[100] slip systems in the deformation process. This discrepancy has also been recently observed in the experimentally deformed samples 34 , 37 .…”

“…4 and 5 ). However, the prevalence of the (001)[100] slip system (28–31%) over (010)[100] (12–14%), which was also recently reported by Lopez-Sanchez et al 34 , is contradictory to the observation of an A-type CPO component. The reason for this discrepancy may be related to faster or more extensive recovery of the SGBs consisting mainly of the (010)[100] slip system compared to the SGBs composed majorly of the (001)[100] slip system on the geological time scale, thus more former SGBs than the latter ones evolve into grain boundaries 28 .…”

“…Distinct from subgrain boundary (SGB), which specifically indicates that a sharp lattice curvature in a discrete plane occurs over a short-range wavelength (e.g., < 1 μm in olivine) caused by dense stacking of dislocation arrays 30 – 34 , a low-angle misorientation boundary (LAMB) is defined here as the low-angle boundary whose misorientation angle is greater than 1° and less than 10°, derived purely from EBSD data regardless of mapping step size. In other words, the descriptive term LAMB has no meaning on the physical origins of the observed low-angle boundaries.…”

“…Alternatively, the proportion of the different dislocation types in the SGBs needs not be in direct proportion to the activity of the different slip systems that accommodate the overall deformation. The reason is that (010)[100] dislocations can move unimpededly through the crystal without leaving any (or hardly any) misorientation gradients or dislocation walls (i.e., SGBs) 34 , 36 , or that grain-to-grain interaction can activate the hard-slip dislocations and thus vary the proportion of different dislocation types in SGBs locally 34 . In these cases, the presence of the (001)[100] slip system related SGBs does not contradict with the dominant (010)[100] and {0kl}[100] slip systems in the deformation process.…”

“…S2d ). In addition, more factors can affect the measurement of microstructures and the estimation of stress by employing other piezometers, including segmentation of recrystallized grains, calculation of dislocation density, and selection of analytical parameters (e.g., kernal size of KAM), as well as the effects of structural factors (e.g., grain pinning, grain-to-grain interaction, distribution heterogeneity in substructures, and sampling size) 34 , 37 , 59 , 60 . Because of the appreciable influences of various factors listed above, to derive a reliable stress for natural samples, it is optimal to apply the same parameters and similar structures that are originally used to calibrate piezometers.…”