5:15 PM - 7:15 PM
[SCG62-P02] Superplasticity of geomaterial: An example of calcareous schist from the Kamuikotan Belt, Hokkaido
Keywords:Superplasticity, Calcareous schist, Crystallographic preferred orientation, Kamuikotan Belt
Superplasticity is defined as “a phenomenon in which a material exhibits tensile elongation of at least 400% under certain conditions” in materials science (Langdon, 2009). This term is also used in geoscience to refer to the mechanism that causes superplasticity (Hiraga, 2017). It has been demonstrated that the main mechanism of superplasticity is the occurrence of grain boundary sliding (Langdon, 1994) and speculated that it also occurs in the lower crust and mantle interior (Boullier & Gueguen, 1975). However, identifying superplasticity in geomaterials is challenging due to the similarities between the microstructures of geomaterials before and after superplastic deformation (Hiraga, 2017). Nevertheless, experiments on artificial mineral polycrystals have successfully induced superplasticity, with the mechanism explained by grain-switching and dynamic grain growth (Hiraga et al., 2010). This demonstration of superplasticity in geomaterials enabled us to compare the microstructures of natural rocks with those that have undergone superplastic deformation.
In this study, we report on the characteristics of superplastic deformation observed in calcareous schist from the Kamuikotan metamorphic Belt in Hokkaido, Japan. The Kamuikotan metamorphic rocks in Hokkaido, Japan, are known as one of the typical low-temperature/high-pressure metamorphic rocks (e.g., Banno, 1986). A thin section was prepared in a plane perpendicular to foliation, and microstructural observations were conducted by scanning electron microscopy. Its crystal-fabric was analyzed by SEM-EBSD method at Nagoya University. The calcareous schist consists of fine-grained calcite and quartz grains, with an average grain size of about 30 µm for calcite and 22 µm for quartz; the area ratio of the two phases is approximately 1:1, and the calcite is characterized by a vertically connected structure to the foliation, while the quartz is characterized by a locally aggregated texture. The crystallographic preferred orientation (CPO) is random (pfJ-index less than 1.1) for both calcite and quartz, but calcite shows weak peaks at 50° and 90° in the shape preferred orientation (SPO).
The calcareous schist exhibits a microstructure similar to that of a duplex high-entropy alloy that has experienced superplasticity (e.g., Reddy et al., 2023). Moreover, the interconnected structure displayed by calcite was frequently observed in alloys and ceramics following superplastic deformation (Maehara & Ohmori, 1987; Hiraga et al., 2002). The absence of a distinct CPO within the calcareous schist indicates that grain boundary slip was the primary deformation mechanism, rather than dislocation or diffusion creep. Additionally, the interconnected structure with 50° and 90° SPO peaks in the calcite is interpreted to result from grain-switching enhanced by grain boundary sliding (Hiraga et al., 2010). Experimental studies have not extensively investigated plastic flow in the calcite-quartz two-phase system (e.g., Rybacki et al., 2003; Renner et al., 2007). However, calcite has been shown to exhibit predominant grain boundary sliding at approximately 30-40 µm grain size and 500°C (Cross & Skemer, 2007), which aligns with the average calcite grain size observed in this study and the peak metamorphic temperature in the Kamuikotan Belt (Takeshita et al., 2018). Collectively, these findings suggest that the analyzed calcareous schist experienced superplastic deformation.
Boullier & Gueguen (1975) Contrib. Mineral. Petrol. 50.
Cross & Skemer (2007) JGR Solid Earth 122.
Hiraga (2017) Jour. Geol. Soc. Japan 123(6).
Hiraga et al. (2002) Jour. Am. Ceram. Soc. 85.
Hiraga et al. (2010) Nature 468.
Langdon (1994) Mater. Sci. Eng. A 174.
Langdon (2009) J. Mater. Sci. 44.
Maehara & Ohmori (1987) Metall. Trans. A 18.
Reddy et al. (2023) Metall. Mater. Trans. A 55A.
Renner et al. (2007) J. Geophys. Res. 112.
Rybacki et al. (2003) J. Geophys. Res. 108.
Takeshita et al. (2018) Jour. Geol. Soc. Japan 124(7).
In this study, we report on the characteristics of superplastic deformation observed in calcareous schist from the Kamuikotan metamorphic Belt in Hokkaido, Japan. The Kamuikotan metamorphic rocks in Hokkaido, Japan, are known as one of the typical low-temperature/high-pressure metamorphic rocks (e.g., Banno, 1986). A thin section was prepared in a plane perpendicular to foliation, and microstructural observations were conducted by scanning electron microscopy. Its crystal-fabric was analyzed by SEM-EBSD method at Nagoya University. The calcareous schist consists of fine-grained calcite and quartz grains, with an average grain size of about 30 µm for calcite and 22 µm for quartz; the area ratio of the two phases is approximately 1:1, and the calcite is characterized by a vertically connected structure to the foliation, while the quartz is characterized by a locally aggregated texture. The crystallographic preferred orientation (CPO) is random (pfJ-index less than 1.1) for both calcite and quartz, but calcite shows weak peaks at 50° and 90° in the shape preferred orientation (SPO).
The calcareous schist exhibits a microstructure similar to that of a duplex high-entropy alloy that has experienced superplasticity (e.g., Reddy et al., 2023). Moreover, the interconnected structure displayed by calcite was frequently observed in alloys and ceramics following superplastic deformation (Maehara & Ohmori, 1987; Hiraga et al., 2002). The absence of a distinct CPO within the calcareous schist indicates that grain boundary slip was the primary deformation mechanism, rather than dislocation or diffusion creep. Additionally, the interconnected structure with 50° and 90° SPO peaks in the calcite is interpreted to result from grain-switching enhanced by grain boundary sliding (Hiraga et al., 2010). Experimental studies have not extensively investigated plastic flow in the calcite-quartz two-phase system (e.g., Rybacki et al., 2003; Renner et al., 2007). However, calcite has been shown to exhibit predominant grain boundary sliding at approximately 30-40 µm grain size and 500°C (Cross & Skemer, 2007), which aligns with the average calcite grain size observed in this study and the peak metamorphic temperature in the Kamuikotan Belt (Takeshita et al., 2018). Collectively, these findings suggest that the analyzed calcareous schist experienced superplastic deformation.
Boullier & Gueguen (1975) Contrib. Mineral. Petrol. 50.
Cross & Skemer (2007) JGR Solid Earth 122.
Hiraga (2017) Jour. Geol. Soc. Japan 123(6).
Hiraga et al. (2002) Jour. Am. Ceram. Soc. 85.
Hiraga et al. (2010) Nature 468.
Langdon (1994) Mater. Sci. Eng. A 174.
Langdon (2009) J. Mater. Sci. 44.
Maehara & Ohmori (1987) Metall. Trans. A 18.
Reddy et al. (2023) Metall. Mater. Trans. A 55A.
Renner et al. (2007) J. Geophys. Res. 112.
Rybacki et al. (2003) J. Geophys. Res. 108.
Takeshita et al. (2018) Jour. Geol. Soc. Japan 124(7).