Title: PhD Research Scholar
Country/Region: India
Period: 2023/03/01 ～ 2024/02/29
Theme: CPO development during diffusion creep of two-phase crystalline aggregates: insights from high temperature rock deformation experiments
Host: Takehiko HIRAGA
Introduction: I pursued my PhD from the Department of Geological Sciences, Jadavpur University, India. As a researcher, I am a structural geologist and an analogue modeler, with an added interest on geological field study. Mostly I prefer to work on experimental modelling of different geological processes focusing on rock deformation. Before joining ERI, in course of my PhD thesis, I worked on several aspects of strain localization with a special focus on shear surface roughness and shear band localization. A part of my doctoral thesis dealt with analogue experimental modelling using sand-talc mixture and investigate the effects of varying rheology and initial weak zone orientation on the shear surface roughness properties. Combining with extensive field studies and 1D fractal dimension analysis, my work proposed a unique type of shear surface roughness and established mechanical instability as its potential forming mechanism. Another part of my PhD thesis was aimed at understanding the effects of mechanical heterogeneities in shear band localization pattern. By using rock analogue materials my work shows that presence of mechanical heterogeneities, combining with the applied strain rate, in a system can influence the type and pattern of shear band formation from homogenously distributed sharp to localized complex composite type of shear bands.
Here at ERI, I shall work in collaboration with Prof. Takehiko Hiraga to investigate the physical properties of Earth’s interior by studying the diffusion creep behavior and CPO development of polycrystalline aggregates of rock-forming minerals, primarily focusing on two phase aggregates of Diopside + Anorthite and Diopside + Forstertite. This will be done by high temperature rock deformation experiments followed by SEM analysis. As a bonus, I am also looking forward to interact and learn from senior researchers at ERI and expecting to make my st
Research Report:
Project Report:
Experimental investigations into high-temperature creep of mineral aggregates have yielded valuable insights not easily discernible in natural settings. These include understanding the variations in rock strength under diverse geological conditions like stress, temperature, chemical compositions, and grain size. Additionally, these experiments shed light on the evolution of rock microstructure, including changes in crystallographic preferred orientation (CPO) during deformation. The conventional diffusion creep theory anticipates the migration of atomic mass happens across grain boundaries oriented perpendicular to the compressional (high) stress axis towards the boundaries oriented at low-stress direction. This deformation mechanism results in grains exhibiting anisotropic shapes, which frequently contributes to the formation of mineral lineations and foliations observed in highly deformed mantle rocks such as peridotites. Several experimental studies on fine-grained olivine aggregates have shown that selective grain boundary sliding (GBS) predominantly occurs along boundaries with low-index planes, facilitating grain rotation to align with the corresponding shear direction. This alignment leads to the development of CPO and shape-preferred orientation (SPO). It is widely recognized that the seismic anisotropy observed in the upper mantle is predominantly attributed to the CPO of olivine. The assumption that mantle anisotropy is a reflection of olivine CPO is due to the fact that olivine is the primary constituent anisotropic mineral found at this depth.
Pyroxene, on the other hand, is the second most abundant mineral in the lower crust and upper mantle, constituting approximately 20–30 modal percent, and understanding its CPO is equally crucial and challenging. Unfortunately, not much work have been done in this area. Here, at ERI, I took up this as my main research focus, as I believe that pyroxene CPO can play a critical role in bulk peridotite anisotropy, which in turn has important implications in mantle deformation and its flow dynamics. In polycrystalline materials, individual crystals are recognized to showcase diverse grain morphologies that are often influenced by the environmental conditions (e.g., presence of secondary phase) and the stages of grain growth. I conducted experiments on two phase rock aggregates of two different compositions, with Diopside (CaMgSi₂O₆) as the major pyroxene phase, combined with minor secondary phases of Anorthite (CaAl2Si2O8) and Forsterite (Mg2SiO4), respectively. The highly dense, fine-grained (<1μm) samples was synthesized following standard procedures, using constituent nanosized powders of Mg(OH)2, SiO2, CaCO3, Ca(OH)2 and Al(OH)3 in proportions as prescribed by previous workers. I synthesized fine grained highly dense samples of dimension ~4mm x 4mm x 8mm and conducted multiple high strain (~70% finite strain) experiments to supposedly reach the optimum deformation level required to study grain alignment and corresponding CPO. The samples were then cut and polished into all three coordinate axes (containing the 3 planes: XY, XZ, and YZ) to understand the grain growth and orientation in all three directions which revealed the ultimate 3D grain morphology. SEM studies showed that deformed DiAn aggregate depicts a
much smaller grain size with strong anisotropy in the X direction, describing a pencil shaped morphology where X axis is highly elongated as compared to Y and Z axes. On the other hand, DiFo does not show such strong anisotropic character. Only EBSD measurements showed a faint CPO in the DiFo samples. Pole figure analysis based EBSD studies show a stark difference in the grain orientation (CPO) of the deformed DiAn with respect to DiFo samples. These primary results show that the secondary phase has a prominent effect on the deformation behaviour as well as the deformed grain morphology and CPO, implying a probable seismic anisotropy between the two phases. I believe, further high temperature high strain pure shear experiments and precise analysis will make us understand more about the anisotropic character of the upper mantle (containing Pyroxene) and the corresponding flow dynamics.
Experimental investigations into high-temperature creep of mineral aggregates have yielded valuable insights not easily discernible in natural settings. These include understanding the variations in rock strength under diverse geological conditions like stress, temperature, chemical compositions, and grain size. Additionally, these experiments shed light on the evolution of rock microstructure, including changes in crystallographic preferred orientation (CPO) during deformation. The conventional diffusion creep theory anticipates the migration of atomic mass happens across grain boundaries oriented perpendicular to the compressional (high) stress axis towards the boundaries oriented at low-stress direction. This deformation mechanism results in grains exhibiting anisotropic shapes, which frequently contributes to the formation of mineral lineations and foliations observed in highly deformed mantle rocks such as peridotites. Several experimental studies on fine-grained olivine aggregates have shown that selective grain boundary sliding (GBS) predominantly occurs along boundaries with low-index planes, facilitating grain rotation to align with the corresponding shear direction. This alignment leads to the development of CPO and shape-preferred orientation (SPO). It is widely recognized that the seismic anisotropy observed in the upper mantle is predominantly attributed to the CPO of olivine. The assumption that mantle anisotropy is a reflection of olivine CPO is due to the fact that olivine is the primary constituent anisotropic mineral found at this depth.
Pyroxene, on the other hand, is the second most abundant mineral in the lower crust and upper mantle, constituting approximately 20–30 modal percent, and understanding its CPO is equally crucial and challenging. Unfortunately, not much work have been done in this area. Here, at ERI, I took up this as my main research focus, as I believe that pyroxene CPO can play a critical role in bulk peridotite anisotropy, which in turn has important implications in mantle deformation and its flow dynamics. In polycrystalline materials, individual crystals are recognized to showcase diverse grain morphologies that are often influenced by the environmental conditions (e.g., presence of secondary phase) and the stages of grain growth. I conducted experiments on two phase rock aggregates of two different compositions, with Diopside (CaMgSi₂O₆) as the major pyroxene phase, combined with minor secondary phases of Anorthite (CaAl2Si2O8) and Forsterite (Mg2SiO4), respectively. The highly dense, fine-grained (<1μm) samples was synthesized following standard procedures, using constituent nanosized powders of Mg(OH)2, SiO2, CaCO3, Ca(OH)2 and Al(OH)3 in proportions as prescribed by previous workers. I synthesized fine grained highly dense samples of dimension ~4mm x 4mm x 8mm and conducted multiple high strain (~70% finite strain) experiments to supposedly reach the optimum deformation level required to study grain alignment and corresponding CPO. The samples were then cut and polished into all three coordinate axes (containing the 3 planes: XY, XZ, and YZ) to understand the grain growth and orientation in all three directions which revealed the ultimate 3D grain morphology. SEM studies showed that deformed DiAn aggregate depicts a
much smaller grain size with strong anisotropy in the X direction, describing a pencil shaped morphology where X axis is highly elongated as compared to Y and Z axes. On the other hand, DiFo does not show such strong anisotropic character. Only EBSD measurements showed a faint CPO in the DiFo samples. Pole figure analysis based EBSD studies show a stark difference in the grain orientation (CPO) of the deformed DiAn with respect to DiFo samples. These primary results show that the secondary phase has a prominent effect on the deformation behaviour as well as the deformed grain morphology and CPO, implying a probable seismic anisotropy between the two phases. I believe, further high temperature high strain pure shear experiments and precise analysis will make us understand more about the anisotropic character of the upper mantle (containing Pyroxene) and the corresponding flow dynamics.
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Fiscal Year: 2022
Fiscal Year: 2022