5:15 PM - 7:15 PM
[SCG62-P03] Fracturing as a grain-size reduction process in quartz inferred from EBSD and CL-image analyses
Keywords:deformation microstructure of quartz, crystalographic orientation analysis, fluid inclusion, chessboard pattern
Quartz is a major constituent mineral of the upper crust, and its deformation characteristics play a crucial role in determining the rheology of the upper crust, including seismogenic regions of inland earthquakes. In crustal rheology, grain-size reduction in rocks induces a transition in deformation mechanisms, leading to rheological weakening. The grain-size reduction of quartz generally proceeds through dynamic recrystallization; however, brittle fracturing has also been suggested to contribute to grain-size reduction [1].
Deformation temperatures have been estimated using the titanium content in dynamically recrystallized quartz that has reached equilibrium [2]. However, if fracturing occurs during the grain-size reduction process, such temperature estimations cannot be applicable. Therefore, it is essential to examine the grain-size reduction processes in quartz.
In this study, we report crack-like structures observed in cathodoluminescence (CL) images of weakly plastically deformed quartz and discuss their origin.
The granitic porphyry dikes of the Ryoke Belt on Awaji Island, Hyogo Prefecture, are known to have undergone mylonitization, and quartz phenocrysts in the granitic porphyry exhibit plastic deformation [3]. The deformation temperature of quartz in the granitic porphyry of this region has been estimated to be ~400–500 °C [4]. In weakly deformed samples, quartz phenocrysts exhibit subgrain boundaries that intersect in a grid-like chessboard pattern. Within the quartz phenocrysts, fluid inclusion trails (inclusion segments) with high linearity and relatively short lengths, as well as inclusions (grain-boundary fluids) distributed along grain and subgrain boundaries, are observed. These inclusions are generally aligned in one direction.
The granitic porphyry dikes of the Ryoke Belt on Awaji Island, Hyogo Prefecture, are known to have undergone mylonitization, and quartz phenocrysts in the granitic porphyry exhibit plastic deformation [3]. The deformation temperature of quartz in the granitic porphyry of this region is estimated to be approximately 400–500°C [4]. In weakly deformed samples, quartz phenocrysts develop a lattice-like network of intersecting subgrain boundaries. Within the quartz phenocrysts, in addition to healed microcracks, relatively short and highly linear fluid inclusion segments are observed. These inclusion segments tend to align along the lattice-like subgrain boundaries, with their orientations varying between individual phenocrysts. Cathodoluminescence (CL) imaging reveals that the subgrain boundaries and inclusion segments exhibit dark CL emission, and some inclusions appear as crack-like features.
To analyze the crystallographic orientations within the quartz phenocrysts, electron backscatter diffraction (EBSD) measurements were conducted at Waseda University. The mean crystallographic orientation of each quartz phenocryst was determined, and pole figure plots of the c-axes showed a Type I crossed-girdle pattern. Furthermore, a comparison between the orientation of inclusion segments and the crystallographic orientation of the host quartz phenocrysts revealed that the inclusion segments are subparallel to the basal (c) plane of the quartz phenocrysts.
Numerical analysis suggests that water infiltration through diffusion from the matrix cannot explain the observed microstructures. Based on these results, we infer that quartz initially forms cracks along subgrain boundaries, which subsequently heal.
Reference: [1] Pongrac et al., 2022, JSG; [2] Bestmann et al., 2021 J. Geophys. Res. Solid Earth; [3] Kano and Takagi, 2013, J. Geol. Soc. Japan; [4] Wang et al., 2024, JSG
Deformation temperatures have been estimated using the titanium content in dynamically recrystallized quartz that has reached equilibrium [2]. However, if fracturing occurs during the grain-size reduction process, such temperature estimations cannot be applicable. Therefore, it is essential to examine the grain-size reduction processes in quartz.
In this study, we report crack-like structures observed in cathodoluminescence (CL) images of weakly plastically deformed quartz and discuss their origin.
The granitic porphyry dikes of the Ryoke Belt on Awaji Island, Hyogo Prefecture, are known to have undergone mylonitization, and quartz phenocrysts in the granitic porphyry exhibit plastic deformation [3]. The deformation temperature of quartz in the granitic porphyry of this region has been estimated to be ~400–500 °C [4]. In weakly deformed samples, quartz phenocrysts exhibit subgrain boundaries that intersect in a grid-like chessboard pattern. Within the quartz phenocrysts, fluid inclusion trails (inclusion segments) with high linearity and relatively short lengths, as well as inclusions (grain-boundary fluids) distributed along grain and subgrain boundaries, are observed. These inclusions are generally aligned in one direction.
The granitic porphyry dikes of the Ryoke Belt on Awaji Island, Hyogo Prefecture, are known to have undergone mylonitization, and quartz phenocrysts in the granitic porphyry exhibit plastic deformation [3]. The deformation temperature of quartz in the granitic porphyry of this region is estimated to be approximately 400–500°C [4]. In weakly deformed samples, quartz phenocrysts develop a lattice-like network of intersecting subgrain boundaries. Within the quartz phenocrysts, in addition to healed microcracks, relatively short and highly linear fluid inclusion segments are observed. These inclusion segments tend to align along the lattice-like subgrain boundaries, with their orientations varying between individual phenocrysts. Cathodoluminescence (CL) imaging reveals that the subgrain boundaries and inclusion segments exhibit dark CL emission, and some inclusions appear as crack-like features.
To analyze the crystallographic orientations within the quartz phenocrysts, electron backscatter diffraction (EBSD) measurements were conducted at Waseda University. The mean crystallographic orientation of each quartz phenocryst was determined, and pole figure plots of the c-axes showed a Type I crossed-girdle pattern. Furthermore, a comparison between the orientation of inclusion segments and the crystallographic orientation of the host quartz phenocrysts revealed that the inclusion segments are subparallel to the basal (c) plane of the quartz phenocrysts.
Numerical analysis suggests that water infiltration through diffusion from the matrix cannot explain the observed microstructures. Based on these results, we infer that quartz initially forms cracks along subgrain boundaries, which subsequently heal.
Reference: [1] Pongrac et al., 2022, JSG; [2] Bestmann et al., 2021 J. Geophys. Res. Solid Earth; [3] Kano and Takagi, 2013, J. Geol. Soc. Japan; [4] Wang et al., 2024, JSG