Abstract
Plastic deformation of peridotites in the mantle involves large strains. Orthorhombic olivine does not have enough slip systems to satisfy the von Mises criterion, leading to strong hardening when polycrystals are deformed at rather low temperatures (i.e., below 1200 °C). In this study, we focused on the recovery mechanisms involving grain boundaries and recrystallization. We investigated forsterite samples deformed at large strains at 1100 °C. The deformed microstructures were characterized by transmission electron microscopy using orientation mapping techniques (ACOM-TEM). With this technique, we increased the spatial resolution of characterization compared to standard electron backscatter diffraction (EBSD) maps to further decipher the microstructures at nanoscale. After a plastic strain of 25%, we found pervasive evidence for serrated grain and subgrain boundaries. We interpreted these microstructural features as evidence of occurrences of grain boundary migration mechanisms. Evaluating the driving forces for grain/subgrain boundary motion, we found that the surface tension driving forces were often greater than the strain energy driving force. At larger strains (40%), we found pervasive evidence for discontinuous dynamic recrystallization (dDRX), with nucleation of new grains at grain boundaries. The observations reveal that subgrain migration and grain boundary bulging contribute to the nucleation of new grains. These mechanisms are probably critical to allow peridotitic rocks to achieve large strains under a steady-state regime in the lithospheric mantle.
Highlights
As (Mg, Fe)2 SiO4 olivine is the main constituent of the upper mantle, understanding its plastic behavior and deformation mechanisms is important in order to model the rheology of the mantle.From a mineral physics point of view, despite efforts by the research community for more than50 years, a lot of questions remain unsolved
We suggest that when these subgrain boundaries boundaries (SGB) pile up at grain boundaries (GB), they build up misoriented domains that leads to the formation of new grains as in the case of bulging
In this study, using orientation mapping of highly deformed forsterite samples, we carried out a detailed microstructural investigation using ACOM-transmission electron microscopy (TEM), which shed new light on the recovery and recrystallization mechanisms in forsterite
Summary
As (Mg, Fe) SiO4 olivine is the main constituent of the upper mantle, understanding its plastic behavior and deformation mechanisms is important in order to model the rheology of the mantle.From a mineral physics point of view, despite efforts by the research community for more than50 years, a lot of questions remain unsolved. As (Mg, Fe) SiO4 olivine is the main constituent of the upper mantle, understanding its plastic behavior and deformation mechanisms is important in order to model the rheology of the mantle. Minerals 2019, 9, 17 slip systems are required to allow homogeneous grain-scale deformation of polycrystalline aggregates. 1200 ◦ C, laboratory deformation experiments on olivine rapidly show significant hardening due to the increase in the internal stored energy, which rapidly leads to brittle behavior [3]. Especially under steady-state regime, is only possible if the microstructure can evolve to release this stored energy. Recrystallization is a general term that encompasses different mechanisms leading to such evolution of microstructures during deformation [4]. Other recovery mechanisms involving dislocation annihilation (by cross-slip or climb) as well as grain boundary migration can contribute
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