Presentation Information
[S2-03]Formation Process of Quartz CL Zoning Structures under Fluid Pressure Oscillation
*Hataka Nishimura1, Atsushi Okamoto1, Kenta Yoshida2 (1. Tohoku Univ. Env, 2. JAMSTEC)
Keywords:
Quartz
Pore fluid pressure is a key factor controlling fault strength and the seismic cycle, and silica precipitation from abundant quartz veins in seismogenic zones is thought to drive fault healing and permeability reduction (Sibson et al., 1992). Quartz in such veins commonly exhibits distinct cathodoluminescence (CL) zoning, and correlations between CL intensity, trace-element (Al) content, and fluid-inclusion density suggest that CL zoning may record earthquake-related fluid pressure oscillations (Raimbourg et al., 2022). However, the mechanism by which fluid pressure oscillations produce CL zoning remains poorly understood. We therefore conducted silica precipitation experiments under artificially controlled fluid pressure oscillations to investigate this mechanism. In a flow-through hydrothermal reactor with a double-tube configuration, a granite block was placed in the inner alumina tube. The starting solution (Si = 292 mg/kg H2O, Al = 5-6 mg/kg H2O), prepared by dissolving granite in subcritical water, was supplied at a constant flow rate of 0.2 mL/min. A temperature gradient was imposed (280 °C upstream, 440 °C downstream), and three fluid pressure conditions were tested using a back-pressure regulator: (1) constant 25 MPa (Ex1); (2) square-wave oscillation between 25–20 MPa with a 4-hour period (Ex2); and (3) combined sinusoidal and square-wave oscillation between 25-20 MPa with an 8-hour period (Ex3). Under these conditions, the fluid evolves from subcritical to supercritical, and from subcritical to vapor, along the reaction tube, driving silica precipitation. Precipitates were characterized by optical microscopy, SEM-CL imaging, EPMA trace-element analysis, and nanoscale X-ray CT at a synchrotron facility for fluid inclusion analysis. In all experiments, the highly supersaturated upstream region developed textures of sparsely deposited amorphous silica and fine-grained quartz particles, whereas euhedral quartz crystal growth from the substrate dominated in the less-saturated downstream region. Under constant fluid pressure (Ex1), quartz crystals grown downstream showed only weak, irregular CL zoning. In contrast, under oscillating conditions (Ex2, Ex3), banded structures formed in the upstream fine-grained deposits, and distinct periodic CL zoning developed within downstream quartz crystals; the number of CL bands closely matched the number of pressure oscillation cycles, with CL-dark bands roughly 1.7-2.0 times wider than CL-bright bands. Because CL-bright zones are enriched in Al, K, and Na relative to CL-dark zones, the bright zones are attributed to [AlO4/K+]0defects. BSE imaging after EPMA mapping also showed selective depressions at CL-bright zones, consistent with greater lattice defect density and weaker electron-beam response in these impurity-rich regions. Primary fluid inclusions within the quartz crystals were aligned along CL-bright zones, and density estimates from vapor–liquid volume ratios obtained by X-ray CT indicate a trapping pressure of approximately 25 MPa. These observations suggest that fluid pressure changes alter silica supersaturation: at 20 MPa, low supersaturation promotes classical crystal growth, forming CL-dark zones, whereas at 25 MPa, high supersaturation generates nanocrystalline/amorphous particles that attach to and are incorporated into the growing quartz crystal, forming CL-bright zones. Future work will compare CL zoning and trace-element patterns in natural quartz veins with these experimental products to further elucidate the relationship between earthquake-related fluid pressure oscillations, supersaturation changes, fluid migration, and the development of quartz CL zoning.