17:15 〜 19:15
[SCG45-P13] Experimental representation of crack-sealing, silica vein formation, and permeability evolution induced by fluid pressure drop at seismogenic depths
キーワード：フロースルー実験、流体圧降下、シリカ脈
Permeability evolution of crust is important for understanding seismic cycles and is governed by cracking and its sealing with mineral vein formation[1]. Various vein formation models have been proposed in nature, including the models associated with fluid pressure drop, however, there is little experimental validation of the vein formation processes and their time scales. Recent experimental studies have shown that silica precipitates upon fluid pressure drop[2]. Meanwhile, no examples exist of silica precipitation forming veins that close cracks. Here we reproduce crack sealing behavior by generating cracks and precipitates silica to form veins within the rock.
Hydrothermal flow-through experiments were conducted under a high confining pressure (Pc) relevant to seismogenic depths (i.e., ≧100 MPa). Samples of Aji granite (diameter: 6 mm height: 12 mm) were introduced for thermal cracks at 600°C with porosities of 1.1-2.8%. Experimental conditions were set at 200°C and 300°C, respectively, with Pc = 100 MPa, upstream pressure (Pup) = 50 MPa, and downstream pressure (Pdown) = 0.1 MPa. As control experiments, Pdown = 8 MPa at 200°C and Pc = 75 MPa or Pdown = 20 MPa at 300°C, respectively. Temporal permeability changes were measured during experiments, with samples analyzed by XCT, SEM, and EPMA.
In the 200°C experiments, quartz and feldspars were mainly dissolved upstream under both conditions, while silica was not precipitated when Pdown = 8 MPa. Reticulate amorphous silica precipitated on the downstream surface in the experiment at Pdown = 0.1 MPa, and the permeability was constant at 1-5 × 10-19 m2 in both cases. At both 300°C conditions, quartz and feldspar dissolved mainly upstream as well. Silica was not precipitated at Pdown = 20 MPa, but chimney-like amorphous silica precipitated locally on the downstream end face at Pdown = 0.1 MPa; Pc = 100 MPa, and Pc = 75 MPa, the permeability decreased from 10-18-10-19 to 10-20-10-22 m2. Furthermore, in the experiment with Pc =100 MPa and Pdown = 0.1 MPa, amorphous silica veins were formed in the interior of the sample near the downstream surface perpendicular to the fluid flow direction.
At 300°C and under high Pc, multiple amorphous silica veins were about 10-20 µm wide and were connected at both ends to the chimney-like precipitates. In this experiment, about two increases and decreases in permeability were observed, which may correspond to the formation of amorphous silica veins, suggesting the following sealing processes. First, the rock dissolved upstream, and then the fluid pressure drop caused a phase change to gas phase, resulting in the formation of chimney-like precipitates on the surface, and reduced the permeability. This increased the fluid pressure near the downstream surface, and the large differential stress caused the cracking perpendicular to the flow path, again increasing the permeability. Similar precipitation-cracking behaviors repeated, which again caused an increase or decrease in the permeability.
The crack sealing process was successfully reproduced by fluid pressure drop under high Pc of 100 MPa and 300°C, which is equivalent to the conditions of the seismogenic zone. Although this experiment was conducted under an extremely high fluid pressure gradient, mineral veins with a width of several tens of µm, similar to those observed in the seismogenic zone at 300°C [3], were generated in four days. Provided that fluid pressure drop of 50 MPa continues in response to fault jog opening[4], silica veins would form in days, which could be comparable to interval of low frequency earthquakes at shallow plate interface in SW Japan. The results of this experiment may provide a constraint on a time scale for the fault-valve model. Flow-through experiments at 400°C are also now being conducted, and will be presented on-site.
[1]Sibson, 1992 Tectonophysics; [2]Amagai et al., 2019 Scientific Reports; [3]Ujiie et al., 2018 GRL; [4]Weatherley and Henley, 2013 Nature Geoscience
Hydrothermal flow-through experiments were conducted under a high confining pressure (Pc) relevant to seismogenic depths (i.e., ≧100 MPa). Samples of Aji granite (diameter: 6 mm height: 12 mm) were introduced for thermal cracks at 600°C with porosities of 1.1-2.8%. Experimental conditions were set at 200°C and 300°C, respectively, with Pc = 100 MPa, upstream pressure (Pup) = 50 MPa, and downstream pressure (Pdown) = 0.1 MPa. As control experiments, Pdown = 8 MPa at 200°C and Pc = 75 MPa or Pdown = 20 MPa at 300°C, respectively. Temporal permeability changes were measured during experiments, with samples analyzed by XCT, SEM, and EPMA.
In the 200°C experiments, quartz and feldspars were mainly dissolved upstream under both conditions, while silica was not precipitated when Pdown = 8 MPa. Reticulate amorphous silica precipitated on the downstream surface in the experiment at Pdown = 0.1 MPa, and the permeability was constant at 1-5 × 10-19 m2 in both cases. At both 300°C conditions, quartz and feldspar dissolved mainly upstream as well. Silica was not precipitated at Pdown = 20 MPa, but chimney-like amorphous silica precipitated locally on the downstream end face at Pdown = 0.1 MPa; Pc = 100 MPa, and Pc = 75 MPa, the permeability decreased from 10-18-10-19 to 10-20-10-22 m2. Furthermore, in the experiment with Pc =100 MPa and Pdown = 0.1 MPa, amorphous silica veins were formed in the interior of the sample near the downstream surface perpendicular to the fluid flow direction.
At 300°C and under high Pc, multiple amorphous silica veins were about 10-20 µm wide and were connected at both ends to the chimney-like precipitates. In this experiment, about two increases and decreases in permeability were observed, which may correspond to the formation of amorphous silica veins, suggesting the following sealing processes. First, the rock dissolved upstream, and then the fluid pressure drop caused a phase change to gas phase, resulting in the formation of chimney-like precipitates on the surface, and reduced the permeability. This increased the fluid pressure near the downstream surface, and the large differential stress caused the cracking perpendicular to the flow path, again increasing the permeability. Similar precipitation-cracking behaviors repeated, which again caused an increase or decrease in the permeability.
The crack sealing process was successfully reproduced by fluid pressure drop under high Pc of 100 MPa and 300°C, which is equivalent to the conditions of the seismogenic zone. Although this experiment was conducted under an extremely high fluid pressure gradient, mineral veins with a width of several tens of µm, similar to those observed in the seismogenic zone at 300°C [3], were generated in four days. Provided that fluid pressure drop of 50 MPa continues in response to fault jog opening[4], silica veins would form in days, which could be comparable to interval of low frequency earthquakes at shallow plate interface in SW Japan. The results of this experiment may provide a constraint on a time scale for the fault-valve model. Flow-through experiments at 400°C are also now being conducted, and will be presented on-site.
[1]Sibson, 1992 Tectonophysics; [2]Amagai et al., 2019 Scientific Reports; [3]Ujiie et al., 2018 GRL; [4]Weatherley and Henley, 2013 Nature Geoscience