Final Report Summary - INSIDE-STRAIN (In situ deformation experiments to study ductile strain localization)
Plate tectonics has been the major unifying theory in geosciences for the last 40 years but until recently, mantle convection and plate tectonics were studied as independent systems. We now understand that plate tectonics is an essential and unique feature of mantle convection in Earth. However, the processes allowing convection in the mantle to produce plate tectonics at the Earth's surface remain a major open question. Strain localization at the plate scale is crucial for the generation of plate tectonics atop a convecting mantle. Nonetheless, while strain localization in the brittle field –producing faults– is a well-known process, mechanisms leading to strain localization in the ductile regime –that is, within 5/6 of the plates– are still poorly understood. The most enigmatic point in strain localization is its initiation in an originally homogeneous medium. Detailed microstructural observations on naturally deformed rocks and existing laboratory experiments did not allow assessing how the deformation itself produces heterogeneous weakening leading to localization. The existing data also do not provide quantitative relations describing the kinematics of the competing processes: development of strain and stress heterogeneity during deformation by dislocation creep leading to dynamic recrystallization, nucleation, and grain growth, as well as post-dynamic and static recovery.
In this project we aimed to obtain data, which is essential for understanding and modeling strain localization and recovery due to microstructural evolution. We carried out experiments and high-resolution characterization of the microstructural evolution during deformation and annealing on model crystalline materials: ice Ih, as well as hexagonal metals, which deform ductilely under low to intermediate temperature and low pressure conditions by activating the same crystal-scale mechanisms as silicate minerals of the deep Earth interior. For this, we used the CrystalProbe – a scanning electron microscope dedicated to the measurement of crystal orientations by indexation of electron backscattered diffraction patterns (EBSD), which special design (inclined column) allows to perform EBSD measurements on a horizontal stage (which may be cryogenic or heating, depending on the experiments). We also studied upper mantle rocks that record ductile strain localization in order to determine the processes controlling the microstructural evolution and their consequences to strain localization. Shear zones recording ductile strain localization in the Lanzo and Ronda peridotite massifs were mapped and sampled during the fellowship, and geochemical and microstructural analyses were accomplished in parallel with the configuration and testing of the cryogenic, heating and deformation stages.
During the early stage of the fellowship it was realized that zinc alloys should be favored with respect to magnesium due to their easier sample preparation, but the reconnaissance experiments highlighted that this alloy is prone to sublime under vacuum when heated. In situ experiments on ice were also not possible for the same reasons (sublimation started at -50°C under 1-3 Pa pressures in the chamber). Thus we developed an alternative protocol for ice in which we alternated annealing experiments at -2 to -5°C in an external, high-precision laboratory freezer and analysis of the microstructures by EBSD. Uniaxial compression deformation of polycrystalline columnar ice samples was completed at the Laboratoire de Glaciologie et Géophysique de l’Environnement (France) and post-mortem characterization of intracrystalline deformation by EBSD was accomplished at the host. We carried out a series of annealing experiments on these pre-deformed ice samples and obtained microstructural data during post-dynamic and static recrystallization. Preliminary ex-situ annealing experiments using Zn alloys provided data comparable to those of the annealing experiments of ice Ih.
Main results achieved so far include:
• New data on the processes allowing for strain localization under retrograde conditions in the lithospheric mantle, highlighting the role of fluids. The Ronda data show for the first time that fluid-assisted mass transfer (pressure solution) may be important in the upper mantle.
• High-resolution crystallographic orientation maps of polycrystalline ice samples characterizing the initial stages of dynamic recrystallization (nucleation sites and processes), as well as the dislocation density field and its evolution with strain.
• A protocol to carry out annealing experiments on pre-deformed polycrystalline columnar ice and anisotropic metals.
• EBSD database of microstructural evolution during post-dynamic and static recrystallization of ice.
The InSiDe-Strain project successfully obtained microstructural data on various processes that concur to strain localization. The data for ice will serve as input parameters in numerical models addressing the role of microstructural and textural evolution on the flow of glaciers and polar ice sheets. Though preliminary, data on Zn alloys helps constraining the microstructural and textural evolution due to slow cooling after rolling. The study of natural ductile shear zones shed light on the importance of fluids in ductile strain localization on Earth. For more information please refer to the project website: http://hidaskaroly.eu/insidestrain.html(se abrirá en una nueva ventana)
In this project we aimed to obtain data, which is essential for understanding and modeling strain localization and recovery due to microstructural evolution. We carried out experiments and high-resolution characterization of the microstructural evolution during deformation and annealing on model crystalline materials: ice Ih, as well as hexagonal metals, which deform ductilely under low to intermediate temperature and low pressure conditions by activating the same crystal-scale mechanisms as silicate minerals of the deep Earth interior. For this, we used the CrystalProbe – a scanning electron microscope dedicated to the measurement of crystal orientations by indexation of electron backscattered diffraction patterns (EBSD), which special design (inclined column) allows to perform EBSD measurements on a horizontal stage (which may be cryogenic or heating, depending on the experiments). We also studied upper mantle rocks that record ductile strain localization in order to determine the processes controlling the microstructural evolution and their consequences to strain localization. Shear zones recording ductile strain localization in the Lanzo and Ronda peridotite massifs were mapped and sampled during the fellowship, and geochemical and microstructural analyses were accomplished in parallel with the configuration and testing of the cryogenic, heating and deformation stages.
During the early stage of the fellowship it was realized that zinc alloys should be favored with respect to magnesium due to their easier sample preparation, but the reconnaissance experiments highlighted that this alloy is prone to sublime under vacuum when heated. In situ experiments on ice were also not possible for the same reasons (sublimation started at -50°C under 1-3 Pa pressures in the chamber). Thus we developed an alternative protocol for ice in which we alternated annealing experiments at -2 to -5°C in an external, high-precision laboratory freezer and analysis of the microstructures by EBSD. Uniaxial compression deformation of polycrystalline columnar ice samples was completed at the Laboratoire de Glaciologie et Géophysique de l’Environnement (France) and post-mortem characterization of intracrystalline deformation by EBSD was accomplished at the host. We carried out a series of annealing experiments on these pre-deformed ice samples and obtained microstructural data during post-dynamic and static recrystallization. Preliminary ex-situ annealing experiments using Zn alloys provided data comparable to those of the annealing experiments of ice Ih.
Main results achieved so far include:
• New data on the processes allowing for strain localization under retrograde conditions in the lithospheric mantle, highlighting the role of fluids. The Ronda data show for the first time that fluid-assisted mass transfer (pressure solution) may be important in the upper mantle.
• High-resolution crystallographic orientation maps of polycrystalline ice samples characterizing the initial stages of dynamic recrystallization (nucleation sites and processes), as well as the dislocation density field and its evolution with strain.
• A protocol to carry out annealing experiments on pre-deformed polycrystalline columnar ice and anisotropic metals.
• EBSD database of microstructural evolution during post-dynamic and static recrystallization of ice.
The InSiDe-Strain project successfully obtained microstructural data on various processes that concur to strain localization. The data for ice will serve as input parameters in numerical models addressing the role of microstructural and textural evolution on the flow of glaciers and polar ice sheets. Though preliminary, data on Zn alloys helps constraining the microstructural and textural evolution due to slow cooling after rolling. The study of natural ductile shear zones shed light on the importance of fluids in ductile strain localization on Earth. For more information please refer to the project website: http://hidaskaroly.eu/insidestrain.html(se abrirá en una nueva ventana)