5:15 PM - 6:45 PM
[SMP24-P12] The CL characteristics of quartz fragments in the pseudotachylyte vein
Keywords:pseudotachylyte, cathodoluminescence, quartz, structural defects
Pseudotachylyte is a fine-grained rock formed by frictional melting along faults and exhibits a vein-like or network-like structure, containing variously sized fractured rock fragments. In this study, cathodoluminescence (CL) spectrum measurements were conducted on quartz fragments in the rim and core domains of pseudotachylyte veins, which were distinctly formed by frictional melting. The characteristics of each spectrum were examined and are reported here.
The samples used in this study are from two pseudotachylyte veins developed in the Asuke Shear Zone in Aichi Prefecture and the South Harris Shear Zone in the Outer Hebrides Islands. Both veins originate from granitic rocks. While the typical thickness of pseudotachylyte fault vein is less than 1 cm, the pseudotachylyte used in this study are notably thick; 11 cm from the Asuke Shear Zone and 4 cm from the South Harris Shear Zone. For each sample, we divided the veins into several zones to compare the rim and core domains. Measurements were conducted using a CL detector attached to the SEM. Additionally, we are also investigating glassy pseudotachylyte (host rock is gneiss) associated with a landslide collected from the Langtang area in Nepal, and we plan to report the results at the time of presentation.
The CL spectrum of quartz particles mainly exhibits a broad band spectrum with peaks around 3.3 eV in the blue region and 1.95 eV in the red region. The relative intensity of these two bands varies. In the core of the pseudotachylyte veins, there is a trend where the relative intensity of the red peak increases, and the overall luminescence intensity decreases. In this study, the spectrum obtained through analysis were subjected to fitting using a total of five Gaussian functions, with peaks around 1.65 eV (705 nm), 1.9–1.95 eV (620–650 nm), 2.15–2.4 eV (500–570 nm), 2.6–2.8 eV (440–480 nm), and 3.1–3.3 eV (380–390 nm), respectively (Stevens-Kalceff, 2009). The integrated intensity of each peak was determined through this fitting process. Additionally, this trend was particularly prominent in the pseudotachylyte from the Asuke Shear Zone.
Comparing the obtained five peaks, there is a trend where the ratio of 1.9–1.95 eV increases toward the core of the vein, while the ratio of 3.1–3.3 eV decreases. The peak at 1.9–1.95 eV is known to be caused by Non-Bridging Oxygen Hole Centers (NBOHC). This center defect occurs when the bond between Si and O is somehow broken, and the oxygen atom acquires an electron. Next, the peak at 3.1–3.3 eV is known to be caused by [AlO4/M+] centers. This defect occurs when trivalent aluminum ions and monovalent cations (H+, Li+, etc.) substitute for Si. The structural defects of [AlO4/M+] centers are known to transform into NBOHC through the movement of cations or electrons (King et al., 2012). The higher proportion of NBOHC and lower proportion of [AlO4/M+] centers in the core of the pseudotachylyte vein, compared to the rim and host rock, is thought to be due to the increased activity of cations under high-temperature conditions during melting. This heightened activity may led to a sequential movement of cations from [AlO4/M+] centers to NBOHC. Furthermore, the reason for the reduced luminescence intensity in the core of pseudotachylyte veins compared to the rim and host rock may be attributed to the heightened activity of cations due to the elevated temperature. Additionally, the extended duration of existence as a melt, which is thicker in the case of the veins studied in this research, might have allowed more time for defect repair. Furthermore, the reason for the reduced luminescence intensity in the core of pseudotachylyte veins compared to the rim and host rock may be attributed to the heightened activity of cations due to the elevated temperature. Additionally, the extended duration of existence as a melt, which is thicker might have allowed more time for defect repair.
The samples used in this study are from two pseudotachylyte veins developed in the Asuke Shear Zone in Aichi Prefecture and the South Harris Shear Zone in the Outer Hebrides Islands. Both veins originate from granitic rocks. While the typical thickness of pseudotachylyte fault vein is less than 1 cm, the pseudotachylyte used in this study are notably thick; 11 cm from the Asuke Shear Zone and 4 cm from the South Harris Shear Zone. For each sample, we divided the veins into several zones to compare the rim and core domains. Measurements were conducted using a CL detector attached to the SEM. Additionally, we are also investigating glassy pseudotachylyte (host rock is gneiss) associated with a landslide collected from the Langtang area in Nepal, and we plan to report the results at the time of presentation.
The CL spectrum of quartz particles mainly exhibits a broad band spectrum with peaks around 3.3 eV in the blue region and 1.95 eV in the red region. The relative intensity of these two bands varies. In the core of the pseudotachylyte veins, there is a trend where the relative intensity of the red peak increases, and the overall luminescence intensity decreases. In this study, the spectrum obtained through analysis were subjected to fitting using a total of five Gaussian functions, with peaks around 1.65 eV (705 nm), 1.9–1.95 eV (620–650 nm), 2.15–2.4 eV (500–570 nm), 2.6–2.8 eV (440–480 nm), and 3.1–3.3 eV (380–390 nm), respectively (Stevens-Kalceff, 2009). The integrated intensity of each peak was determined through this fitting process. Additionally, this trend was particularly prominent in the pseudotachylyte from the Asuke Shear Zone.
Comparing the obtained five peaks, there is a trend where the ratio of 1.9–1.95 eV increases toward the core of the vein, while the ratio of 3.1–3.3 eV decreases. The peak at 1.9–1.95 eV is known to be caused by Non-Bridging Oxygen Hole Centers (NBOHC). This center defect occurs when the bond between Si and O is somehow broken, and the oxygen atom acquires an electron. Next, the peak at 3.1–3.3 eV is known to be caused by [AlO4/M+] centers. This defect occurs when trivalent aluminum ions and monovalent cations (H+, Li+, etc.) substitute for Si. The structural defects of [AlO4/M+] centers are known to transform into NBOHC through the movement of cations or electrons (King et al., 2012). The higher proportion of NBOHC and lower proportion of [AlO4/M+] centers in the core of the pseudotachylyte vein, compared to the rim and host rock, is thought to be due to the increased activity of cations under high-temperature conditions during melting. This heightened activity may led to a sequential movement of cations from [AlO4/M+] centers to NBOHC. Furthermore, the reason for the reduced luminescence intensity in the core of pseudotachylyte veins compared to the rim and host rock may be attributed to the heightened activity of cations due to the elevated temperature. Additionally, the extended duration of existence as a melt, which is thicker in the case of the veins studied in this research, might have allowed more time for defect repair. Furthermore, the reason for the reduced luminescence intensity in the core of pseudotachylyte veins compared to the rim and host rock may be attributed to the heightened activity of cations due to the elevated temperature. Additionally, the extended duration of existence as a melt, which is thicker might have allowed more time for defect repair.