[S-CG54] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG54-P14] Development of CL Zoning Structure in Artificial Quartz Veins thorough Fluid Pressure Fluctuation Experiments

*Nishimura Hataka¹, Atsushi Okamoto¹, Astin Nurdiana¹, Kenta Yoshida² (1.Tohoku university Graduate School of Environmental Studies, 2.JAMSTEC)

The behavior of fluid pressure in the earthquake cycle is an extremely important factor in understanding its mechanism, as exemplified by the Fault-Valve model. Recent studies have suggested that quartz veins formed in seismogenic zones may record fluid pressure fluctuations corresponding to the earthquake cycle in the form of cathodoluminescence (CL) zoning structures and trace element distributions, such as aluminum (Al) [1]. However, no experimental evidence has been provided to support this hypothesis. In our previous studies, we conducted silica precipitation experiments using a flow-through hydrothermal reaction apparatus and confirmed that CL zoning structures were sometimes recorded in quartz crystals when clogging occurred. However, since intentional fluid pressure fluctuations were not applied, it remained unclear whether the CL zoning structures were directly correlated with fluid pressure fluctuations. In this study, silica precipitation experiments were conducted using a flow-through hydrothermal apparatus that artificially imposes fluid pressure fluctuations. A 110 mL autoclave was equipped with an alumina inner tube, inside which ten granite blocks (4.2 mm * 4.2 mm * 20 mm) were placed. The temperature was set at 280°Cupstream and 440°C downstream, and the three experiments were conducted. The first, second, and third experiments were run for 100, 109, and 84 (hours), respectively, all with a constant flow rate of 0.2 mL/min. Fluid pressure was controlled using two syringe pumps placed upstream and downstream. When these three experiments were conducted, the first had a constant pressure of 25 MPa applied, while the second and third had a pressure cycle that oscillated between 20 MPa and 25 MPa using square and sinusoidal waves (with a period of 4 hours). In all experiments, amorphous silica and nucleated quartz particles mostly precipitated upstream, while downstream, the growth of quartz on the granite surface was predominant. SEM-CL and EPMA analyses of the obtained samples revealed that regular CL zoning structures formed in the experiments where square and sinusoidal pressure fluctuations were applied. Furthermore, the total number of CL zoning bands was nearly consistent with the number of fluid pressure oscillations. The peak wavelength of CL emission was 640 nm for both CL-dark and CL-bright regions. The CL-dark zones had a growth width approximately 1.3 times larger than that of the CL-bright zones, and fluid inclusions were observed along the growth planes. Additionally, the CL zoning structures corresponded primarily to zonal variations in trace elements such as Al and K. By comparing the temporal changes in fluid pressure fluctuations with CL zoning structures, we found that CL-dark zones formed at 20 MPa, while CL-bright zones formed at 25 MPa. This difference in growth rates can be attributed to the lower water density at 20 MPa, which reduces Si solubility and consequently increases the crystal growth rate. Similar quartz veins found in sandstone blocks of the Shimanto Belt exhibit stretched crystal structures and CL banding patterns along with Al and K zoning. These findings suggest that similar fluid pressure fluctuations repeatedly occurred during the formation of natural quartz veins.
[1]Rainbourg, H., Famin, V., Canizares, A., Le Trong, F., 2022. Geochim. Geophs. Geosys., 23,e2022GC010346