14:45 〜 15:00
[SCG45-53] Pressure oscillations caused by silica precipitation during fluid flow in a granite fracture
キーワード：石英脈、水熱実験、流体圧振動、シリカ析出
A well-known model that links the fluid pressure change and earthquake cycle is called as fault-valve model, in which increasing fluid pressure causes a reduction in the effective normal stress, rupture occurs when the imposed shear stress becomes equal to the shear strength. The rupture leads to a sudden increase in permeability and a drop in fluid pressure. As fault healing or sealing proceeds, the fault strength gradually increases and permeability increases again. Ubiquitous occurrences of quartz veins within the seismogenic zones indicate that the contribution of silica precipitation to fault sealing and fluid pressure change during earthquake cycle; however, there are no studies that show the temporal evolution of fluid pressure by silica precipitation. In this study, we conducted a hydrothermal flow through experiments on silica precipitation in fluid flow with a constant flow rate. We showed that characteristic fluid pressure oscillation occurred during sealing of the fracture. We also showed that a preliminary results on the silica precipitation experiments with artificially-imposed fluid pressure oscillation to test whether fluid pressure change can be recorded in cathodoluminescence (CL) zoning of quartz.
The flow-through experiments were conducted at fluid pressure at 25 MPa, and to induce silica precipitation within the slit of granite (6 mm × 0.5 mm × 100 mm), we established a temperature gradient between the inlet (370 °C) and outlet (425–430 °C) of the granite core to exploit the decrease in quartz solubility with increasing temperature to ~450 °C. Flow-rate were 0.5 mL per min and 0.2 mL per min, respectively. We used the granite-dissolved high Si solution as input solution. Following an induction period, the difference in pressure between the inlet and outlet (ΔP) exhibited an increase and oscillations with a sigmoidal pattern to a peak before an abrupt decrease. As sealing progressed, the background ΔP increased until reaching a stable state. The observed oscillations in fluid pressure resulted from the repeated blockage of flow pathways by silica precipitation and the subsequent rupture of locally sealed layers, which produced characteristic quartz textures such as blocky textures and banded fluid inclusions as observed in natural veins. Our results suggest that the generation and transport of silica particles in fluid, driven by rupture events, may induce transient and local variations in fluid pressure, thereby contributing to earthquake nucleation and rupture. Therefore, although the failure events are not shear failure, but are likely mode I extension failure. However, our experimental results on silica precipitation and pressure change represent a demonstration of fault-valve model.
In the second flow-through experiments with silica precipitation, we artificially imposed fluid pressure oscillation as square waves between 20 MPa and 25 MPa every 4 hours. Based on the SEM-CL observation, we found the repeated bands composed of CL-bright and CL-dark in quartz, corresponding to the fluid pressure change. The CL-bright bands show high Al and K concentration in quartz. Similar zonings are often reported in the quartz veins within the seismogenic zones, suggesting that these repeated CL zonings represents the fluid pressure changes in the earthquake cycle.
Sibson, R.H., Robert, F., Poulsen, K.H., (1988) Geology, 16, 551-555.
Raimbourg, H, Famin, V., Canizares, A., Le Trong E, (2022) Geochem Geophys Geosys, 23 e2022GC010346.
Okamoto, A., Vinis E., (accepted) Nature Communications.
The flow-through experiments were conducted at fluid pressure at 25 MPa, and to induce silica precipitation within the slit of granite (6 mm × 0.5 mm × 100 mm), we established a temperature gradient between the inlet (370 °C) and outlet (425–430 °C) of the granite core to exploit the decrease in quartz solubility with increasing temperature to ~450 °C. Flow-rate were 0.5 mL per min and 0.2 mL per min, respectively. We used the granite-dissolved high Si solution as input solution. Following an induction period, the difference in pressure between the inlet and outlet (ΔP) exhibited an increase and oscillations with a sigmoidal pattern to a peak before an abrupt decrease. As sealing progressed, the background ΔP increased until reaching a stable state. The observed oscillations in fluid pressure resulted from the repeated blockage of flow pathways by silica precipitation and the subsequent rupture of locally sealed layers, which produced characteristic quartz textures such as blocky textures and banded fluid inclusions as observed in natural veins. Our results suggest that the generation and transport of silica particles in fluid, driven by rupture events, may induce transient and local variations in fluid pressure, thereby contributing to earthquake nucleation and rupture. Therefore, although the failure events are not shear failure, but are likely mode I extension failure. However, our experimental results on silica precipitation and pressure change represent a demonstration of fault-valve model.
In the second flow-through experiments with silica precipitation, we artificially imposed fluid pressure oscillation as square waves between 20 MPa and 25 MPa every 4 hours. Based on the SEM-CL observation, we found the repeated bands composed of CL-bright and CL-dark in quartz, corresponding to the fluid pressure change. The CL-bright bands show high Al and K concentration in quartz. Similar zonings are often reported in the quartz veins within the seismogenic zones, suggesting that these repeated CL zonings represents the fluid pressure changes in the earthquake cycle.
Sibson, R.H., Robert, F., Poulsen, K.H., (1988) Geology, 16, 551-555.
Raimbourg, H, Famin, V., Canizares, A., Le Trong E, (2022) Geochem Geophys Geosys, 23 e2022GC010346.
Okamoto, A., Vinis E., (accepted) Nature Communications.