Oscillations in fluid pressure caused by silica precipitation in a fracture

Domain: earth_science
Published: 2025-02-20
Editorial outcome: minor_revision

Abstract
Ubiquitous quartz veins within the crust underscore the contribution of silica precipitation to fault sealing and fluid-pressure changes during earthquake cycles. However, quantifying silica precipitation, fluid pressure fluctuations, and earthquake rupture remains challenging. Here, we present the results of hydrothermal flow-through experiments with constant flow rates that show silica precipitation reduces permeability and induces fluid-pressure oscillations in the flow of highly supersaturated fluid. Following an induction period, the difference between inlet and outlet pressures in the experiments increased and oscillated, reaching a peak before abruptly decreasing, while the background pressure difference increased gradually. Such pressure oscillations resulted from the repeated blockage of flow pathways by silica and the failure of locally sealed layers, which produced characteristic quartz textures. The results suggest that the generation and transport of silica particles in fluid, driven by failure events, may induce transient and local variations in fluid pressure, thereby contributing to earthquake rupture.

Introduction
Earthquakes result from abrupt, brittle shear failure when the imposed shear stress becomes equal to the frictional strength of a fault. In some cases, seismic rupture is believed to be influenced by pore fluid pressure, which lowers the effective normal stress1–4. A widely accepted framework for understanding of the relationship between an earthquake cycle and fluid pressure variations is known as the ‘fault valve model’1–3,5. According to this model, increasing fluid pressure leads to a reduction in the effective normal stress, with rupture occurring when the imposed shear stress becomes equal to the shear strength. This rupture leads to a sudden increase in permeability and a reduction in fluid pressure. As fault healing and sealing progress, the fault strength gradually increases and permeability decreases, causing the fluid pressure to rise once more until the next rupture event. Originally proposed to explain seismicity associated with gold mineralization in the continental crust1,2, this coupled mechanical–hydrological model has since been widely applied to various seismic phenomena in fluid-rich environments, including regular and slow earthquakes as well as fluid-driven earthquake swarms within the crust and in subduction zones3,6,7.

Some seismogenic zones contain many mineralized veins comprising mainly quartz8–12. These veins frequently exhibit banded textures with arrays of inclusions, indicative of repeated fault cracking and sealing processes8,10–13. The orientations of quartz veins and the variation in fluid density within inclusions in single veins suggest that their formation records cyclic changes in fluid pressure14–16. These observations imply that silica precipitation plays a key role in fault sealing. However, estimates of the timescale of this sealing process are highly dependent on models that consider factors such as quartz supersaturation, reactive surface area, and the transport mechanism of aqueous silica (i.e., diffusion versus advection)10,13. Consequently, the constraining of the timescale of quartz vein formation and associated changes in fluid pressure and earthquake cycles, remains challenging.

The solubility of quartz in water is strongly influenced by temperature and water density17,18. At higher pressures (greater than ~100 MPa), quartz solubility increases with increasing temperature, whereas at lower pressures (less than ~100 MPa) it has a local maximum within the sub- to supercritical conditions of water, which occurs as water transitions from high-density to low-density supercritical conditions (Fig. S1a). At pressures of <50 MPa, after a reduction near the critical point of water, quartz solubility increases again with increasing temperature (Fig. S1b). In volcanic regions with high geothermal gradients, the sudden drop of quartz solubility near the critical point of water plays a crucial role in the formation of silica sealing layers at shallow depths19–21. Silica supersaturation is also driven by the migration of fluids along pressure and temperature gradients at greater depths. Earthquake-induced pressure reductions, such as a shift from lithostatic to hydrostatic pressure within the crust, can lead to silica supersaturation of fluids1,16,21,22. Extreme supersaturation may occur in response to very large drops in fluid pressure associated with the co-seismic dilation of fault jogs, which is often associated with gold mineralization23,24.

Previous studies have involved hydrothermal experiments with silica precipitation under temperature/pressure conditions of 400–500 °C and 20–30 MPa in low-density supercritical water or superheated steam, achieving silica supersaturation25–28. Silica precipitation occurs through various mechanisms, including epitaxial overgrowth on pre-existing quartz grains at low supersaturation, and nucleation of quartz grains or the formation of metastable silica phases (e.g., amorphous silica and cristobalite) at higher supersaturation. Amorphous silica particles generated in superheated steam are readily transported, leading to reduced fracture permeability27,28. Recently, phase-field modeling has been used to investigate the microstructural evolution of crack–seal quartz veins and its effect on fluid pathways29,30. However, no physical experimental studies have successfully reproduced crack–seal quartz veins, which form via repeated cycles of quartz sealing and fracturing. Therefore, a comprehensive model is required that links the microtextures of quartz veins with cyclic changes in fluid pressure and permeability.

Here we show the results of hydrothermal flow-through experiments on silica precipitation within a slit in granite at constant flow rates to investigate microstructural evolution and dynamic changes in fluid pressure and permeability during sealing. Our results show that silica precipitation effectively reduces permeability and induces characteristic fluid-pressure oscillations in highly supersaturated fluid flow.

Results
Experiments
Two granite cores (total length 100 mm, diameter 9.8 mm) containing a slit of dimensions 6 mm × 0.5 mm × 100 mm were connected to the inlet of a reaction vessel (Fig. S2). Fluid was introduced at a constant flow rate, and the fluid outlet pressure maintained at 25 MPa using a back-pressure valve. Two experimental runs were undertaken at different flow rates: Run_02 (0.2 mL min–1) and Run_05 (0.5 mL min–1) (Table S1). To induce silica precipitation within the slit, a temperature gradient between the inlet (370 °C) and outlet (425–430 °C) of the granite core was established to exploit the decrease in quartz solubility with increasing temperature up to ~450 °C (Fig. S1b). This temperature gradient induced high supersaturation at high temperature, enhancing the rate of silica precipitation within the slit. The initial solutions had Si, Na, and Al concentrations of 280–314, 2–3, and 5–6 mg kg–1 H2O, respectively (Table S2). During the experiments, fluid pressure was monitored at the downstream and upstream ends of the granite core. After the experiments, the porosity was assessed by X-ray computed tomography (X-ray CT; Fig. S3), with detailed microstructural observations of thin sections performed by optical and electron microscopy. For Run_05, X-ray CT images were also obtained during an intermediate period of the run to confirm the progress of silica precipitation (Table S1; Fig. S3).

Fluid pressure oscillations
In the early stage of Run_02 (0–24.5 h), there was no detectable difference in fluid pressure (ΔP < 0.1 MPa) between upstream and downstream sections of the granite core (Fig. S4). However, at 24.5 h the upstream pressure began to rise while the downstream pressure remained unchanged, leading to a gradual increase in ΔP (Fig. 1a). By 32.5 h, ΔP had risen to 4 MPa, after which numerous irregular pressure oscillations occurred. In each event, ΔP increased slowly, then increased more quickly before slowing as it approached a peak. Upon reaching the peak, fluid pressure dropped abruptly within 30 s, followed by a subsequent further rise. Between 32 and 38 h, eight major oscillations (ΔP drop > 3 MPa) were recorded, during which fluid pressure dropped to baseline levels. Smaller pressure-drop events occurred between these major events. As sealing progressed, the peak ΔP increased, reaching a maximum of 9.1 MPa. After 37 h, ΔP became smaller, stabilizing at 7–9 MPa. Temperature drops coincided with pressure drops, although the temperature recovered more quickly than the fluid pressure (Fig. S4).

Similar oscillations in fluid pressure was observed in Run_05 after 11 h (Fig. 1b and S4). Sixty-two oscillations (ΔP drop > 3 MPa) were recorded over a period of 7 h. The peak ΔP gradually increased to 10.7 MPa, and ΔP became more stable after 18.2 h. While the magnitude of the pressure drops (i.e., amplitude) was comparable to that in Run_02 (Fig. 1c), the number of oscillations per minute in Run_05 was 2–3 times higher (Fig. 1d).

Microstructures of synthetic quartz veins
In Run_02, no silica precipitation was observed near the inlet (distance from inlet, x = 0–1.8 cm; Fig. S5). As the temperature increased along the slit, silica precipitation occurred in response to the decrease in quartz solubility (Fig. S2). The porosity (∅) within the slit gradually decreased from ∅ = 1.0 at x = 1.8 cm to ∅ = 0.03–0.15 at x = 3.1–3.8 cm (Fig. 2a), before increasing again. Beyond 7 cm from the inlet, the porosity exceeded 0.9, with irregular spikes. The region of lowest porosity (x = 3–4 cm) contained isolated pores (i.e., unconnected at the voxel resolution of 10 μm in X-ray CT images) of various sizes (Fig. 2b). In Run_05, porosity began to decrease beyond 3.0 cm from the inlet, and silica precipitates were distributed more broadly, with a minimum porosity of ∅ = ~0.2 at ~5 cm from the inlet (Fig. 2a).

The mineralogy and occurrence of silica minerals changed systematically along the flow path (Fig. 2c–g and S5–S7), due to the change in quartz solubility and the decrease in the Si concentration in the solutions as a result of precipitation25. Near the inlet (x = 2–3 cm) in Run_02, amorphous silica appeared as balloon-shaped or spheroidal shells on both walls of the slit (Fig. 2c). At x = 3–4 cm, silica was present as amorphous silica particles surrounded by fine-grained quartz, and as quartz grains nucleated on the walls of the slit (Fig. 2e, S5b, and S7). At x = 4–5 cm, fine quartz grains grown from both walls exhibited a banded structure (6–8 bands in total) defined by bands of inclusions (fluid inclusions as well as dusty solid inclusions) and zoning in cathodoluminescence (CL) images (Fig. 2e). The size of individual banded quartz grains was 50–100 μm wide by 100–300 μm long, and some banded layers in the inner parts of the vein comprised fine quartz grains (less than ~20 μm; Fig. 2e). At x = 5–6 cm, discrete grains of quartz nucleated regardless of the substrate mineral, and grew with a blocky to elongate–blocky texture28. Further downstream, epitaxial quartz overgrowths became dominant on pre-existing quartz grains (Fig. 2g, S5, and S6), which contributed to the development of surface roughness and a spike in porosity (Fig. 2a). Amorphous silica has an Al2O3 content of up to 6 wt%, and Na2O and K2O contents of up to 2 wt% (Table S3). The proportion of amorphous silica was greater in Run_05 than in Run_02, with quartz crystals being embedded in amorphous domains.

Mechanism of pressure oscillations during silica precipitation
The evolution of permeability during the experiments was determined from fluid pressure measurements (Fig. 3a and S8), considering the fluid viscosity, a characteristic distance of 10 mm for the clogged region, and the cross-sectional area of the slit (0.5 mm × 6 mm). In Run_02, the base permeability gradually decreased by 1–2 orders of magnitude over ~10 h, while in Run_05 it decreased over ~4 h, exhibiting numerous spikes in response to fluid pressure oscillations (Fig. S8). Figure 3b shows the temporal evolution of expected quartz solubility in water at the most-clogged position (x = 3.8 cm) in Run_02. Of note, quartz solubility showed fluctuations similar to those of fluid pressure, initially being low compared with the input solution (indicating high supersaturation) and gradually increasing, although with oscillations, to near-equilibrium levels.

Based on the microstructures of silica precipitates and pores (Fig. 2), and the temporal evolution of fluid pressure (Fig. 1), we propose the following mechanism that led to the fluid pressure oscillations induced by silica precipitation. Precipitation of amorphous silica and nucleation of quartz grains caused flow-path clogging, independently of pre-existing quartz grains (Fig. 2d and S6). This mode of silica precipitation is crucial for effective sealing20,27, given that granite contains only ~30% quartz. A saturation ratio of 7–9 with respect to quartz solubility (Fig. 3b) is sufficient to nucleate amorphous silica at ~400 °C25. Once formed, amorphous silica facilitated the heterogeneous nucleation of quartz grains on the surface of metastable phases31. The generated silica particles were transported by fluid flow and attached to the walls, generating surface roughness (t = t1 in Fig. 3c). The roughness was enhanced by ongoing precipitation, which formed thin silica layers that contributed to the pressure differential between upstream and downstream sites (t = t2). As the silica layer was thin and porous, it fractured easily when ΔP reached the tensile strength of the silica aggregates (several MPa; Fig. 1a). Upon failure, fluid pressure returned to the baseline level (t = t3). During ongoing precipitation, the flow path became clogged, with complex porosity structures comprising unconnected pores with thin pore throats (<10 μm, t = t4; Fig. 1b). In the subsequent failure event, fluid pressure did not drop to the downstream level but rather increased from background pressure (t = t5). As silica sealing progressed, the thickness and strength of the sealed zone increased, and the increase in quartz solubility at elevated fluid pressures inhibited further precipitation at this location (Fig. 3b), leading to a reduction in the frequency of fluid pressure oscillations (Fig. 1a). The primary location of silica precipitation then shifted downstream. These observations suggest that the fluid pressure patterns were strongly influenced by the mechanism and kinetics of silica precipitation. Higher flow rates may enhance the formation of broader silica-sealed zones due to increased transport of silica particles (Fig. 2a).

The occurrence of crack–seal processes in natural quartz veins has been inferred from inclusion arrays parallel to vein walls, which are interpreted as being formed by repeated cracking and sealing by silica at a given site10,13,32. The banded texture at x = 4–5 cm in Run_02 (Fig. 2e) is similar to a crack–seal texture, and the number of bands (6–8) is consistent with the number of large drops in fluid pressure (>3 MPa; Fig. 1a). However, in the case of Run_02, the crack–seal process did not occur at the location of banded quartz; rather, the opening or closing of the ‘valve’ occurred in upstream regions. This indicates that banded quartz veins may form in nature through the episodic injection of supersaturated fluids, controlled by crack–seal processes at distant sites. Further studies of the relationship between CL zoning and the trace-element compositions of quartz crystals, in response to local fluid composition and pressure changes, may provide insights into the link between natural quartz veins and fluid pressure fluctuations within the crust and subduction zones15.

Methods
Flow-through experiments
The experimental apparatus used in this study is similar to that described in our previous work25,28, with a newly developed precipitation vessel of 110 mL capacity specifically designed for X-ray CT scanning and to achieve fracture sealing. The cylindrical vessel featured an inner assembly for silica precipitation (Fig. S2a). This assembly comprised two granite cores (Aji granite, Japan) measuring 9.8 mm in diameter and 50 mm in length, separated by a slit of 0.5 mm × 6.0 mm × 50 mm, encased in a stainless-steel sleeve (0.1 mm thick) of 200 mm length, creating a flow path within the granite of length 100 mm (Fig. S2b). The granite sample comprised 37 vol% plagioclase, 24 vol% K-feldspar, 30 vol% quartz, and 8 vol% biotite, with traces of Fe oxides. Quartz and feldspar grains were 1–3 mm in size. The granite cores were cut in half, ground using a surface grinder to create the slit, and then reassembled into a single cylindrical core (Fig. S2c). The outer diameter was finely machined to fit snugly within the stainless-steel sleeve. Two alumina tubes (6 mm inner diameter, 9.8 mm outer diameter, 50 mm length) were positioned at the upstream and downstream ends of the granite cores. The inner assembly was connected to a cap at the inlet of the precipitation vessel and was open at the downstream end (Fig. S1b, d, e). The fluid pressure was maintained at 25 MPa using a back-pressure valve, ensuring that the internal and external pressures of the granite core were equal at the start of the experiment, with no confining pressure applied (Fig. S1e). Following clogging, the downstream fluid pressure remained consistent with the external pressure, while the upstream fluid pressure increased, meaning that fluid overpressure was generated within the slit.

Flow-through experiments were performed using a high-silica input aqueous solution at a constant flow rate. Two experiments were conducted with different flow rates using a syringe pump at room temperature: 0.2 mL min–1 (Run_02) and 0.5 mL min–1 (Run_05) (Table S2). Fluid flowed upward and was pre-heated. In Run_02, the temperatures at the upstream and downstream ends of the granite core were ~370 °C and 425 °C, respectively, while in Run_05 the temperatures were ~370 °C and 430 °C. Fluid pressures at the upstream and downstream ends were recorded every 30 s for Run_02. Run_05 was divided into two periods: 0–10.3 h (Run_05-1) and 10.3–19.1 h (Run_05-2), with X-ray CT analysis performed between the two periods. During Run_05-1, no pressure increase was observed. Fluid pressures were recorded every 10 s for Run_05-1 and every 1 s for Run_05-2.

Input and output solutions
The input solutions were prepared by dissolution of granite sand (30 g, 1–2 mm grain size) + albite sand (30 g, 1–2 mm grain size) in water at 360 °C and 35 MPa. The concentrations of Si, Al, and Na in input and output solutions were determined by inductively coupled plasma–optical emission spectrometry (ICP–OES; Agilent 5110, USA) at Tohoku University, Japan. The pH was measured at room temperature, yielding pH = 7.1 for the input solution and pH = 4.6–5.6 for the output solution (Table S2). The Si concentration was 280–314 mg kg–1 H2O for the input solution, decreasing to 47–52 mg kg–1 at the outlet, which is near the solubility of quartz at 430 °C (Fig. S1b). We observed no systematic trend in the composition of the output solution, except for a slight increase in Si concentration at the first sampling time (Table S2). The input solution contained Al, Na, and K (2–6 mg kg–1 H2O), with concentrations decreasing to <1 mg kg–1 in the output solution. Small amounts of impurities such as Al are important for the formation of quartz rather than amorphous silica25,26.

Analysis of porosity by X-ray CT
After each run, the granite core and stainless-steel sleeve were removed from the precipitation vessel and the porosity and structure of silica precipitates within the granite slit were examined by X-ray CT (ComScan, Scan-Xmate-D225RSS270, Japan) at Tohoku University. Three-dimensional (3D) images were reconstructed from transmitted images (3000 projections) obtained with an acceleration voltage of 200 kV and beam current of 100 μA. The voxel size was 10 μm. The 3D image from a single measurement comprised 1246 × 1246 × 2816 voxels. To cover the extent of precipitation along the flow path, four X-ray CT analyses for each sample were undertaken and the images connected. During Run_05 we conducted X-ray CT scans twice, at the intermediate period and end of the experiment (Table S1; Fig. S3).

Analyses of mineral compositions and identification of minerals
Before the experiments, we undertook X-ray mapping of Si, Al, Fe, Mg, Ca, K, and Na on the surface of the granite slit using micro-X-ray fluorescence spectrometry (M4 TornadoPLUS; Bruker Corporation, USA), with an acceleration voltage of 50 kV and beam current of 300 μA, and a map pixel size of 20–50 μm. After the measurements, mineral maps (Fig. S6) were created from element maps using the k-means method with Matlab software.

After X-ray CT analyses of experimental products, a thin section was made parallel to the long-axis of the granite core and perpendicular to the slit aperture, and used for optical microscopy and analysis of chemical compositions. Compositions of silica deposits were determined by electron-probe microanalysis (EPMA, Jeol JXA-8200, Japan) at Tohoku University with an acceleration voltage of 15 kV and beam current of 12 nA (Table S3).

Silica minerals observed in thin section were identified using a Raman spectrometer (Kaiser Optical Systems, USA, HoloLab 5000) equipped with an Olympus (Japan) BX51 microscope, at Tohoku University (Fig. S7).

Calculation of permeability change
During sealing by silica, fluid flows through porous silica precipitates, and the permeability, k (m2), can be estimated by Darcy’s law as follows:

k = (Q × μ × L) / (A × ΔP)

where Q is the flow rate (m3 s–1), μ is viscosity (Pa s), L is the characteristic distance (length of clogged region) (m), A is the cross-sectional area of the slit (m2), and ΔP is the fluid pressure difference between inlet and outlet (Pa). To consider the permeability change in the most-clogged region, we used the specific volume of water (6.005 × 10–3 m3 kg–1) and viscosity (2.929 × 10–5 Pa s) for conditions of 400 °C and 25 MPa, taken from the US National Institute of Standards and Technology (NIST) Chemistry Webbook (https://webbook.nist.gov/chemistry/fluid/). We applied Q = 0.2 g min–1 for Run_02 and Q = 0.5 g min–1 for Run_05, with L = 10 mm. The estimated permeability values have uncertainties, as Darcy’s law assumes steady-state conditions (e.g., fluid viscosity, cross-sectional area, sample length), while these parameters varied dynamically in the experiments (Figs. 3a, S8).

Solubility of quartz in the middle section of the granite core
The solubility of quartz in water was calculated using the software Lonar AP18. Figure S1b shows the solubility as a function of temperature at 25–35 MPa. At 25 MPa, the solubility of quartz increases gradually with increasing temperature, reaching a maximum of 390 mg kg–1 H2O at 350 °C before dropping sharply toward a local minimum of 28 mg kg–1 at 450 °C, and then increasing again. The rate of decrease of silica solubility decreases with increasing pressure. We could not directly measure the temperature change along the flow path within the slit. However, if the temperature is assumed to increase linearly from 370 °C to 430 °C, the sharp drop in quartz solubility would have occurred at ~3 cm from the inlet, largely consistent with the onset of silica precipitation in Run_02 (Fig. 2a).

After the start of fluid pressure oscillations, the upstream pressure increased while the upstream and downstream temperatures remained constant (Fig. S4). The temporal evolution of quartz solubility at 3.8 cm from the inlet in Run_02 (Fig. 3b) was calculated from the upstream pressure and temperature, as obtained by linear interpolation between inlet and outlet temperatures.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Dear Editor,\nThe manuscript submitted by A. Okamoto and E. Vinis, titled \"Oscillations in Fluid Pressure Caused by Silica Precipitation in\na Fracture,\" presents experimental results from flow-through experiments using fluid with controlled chemistry and a\ntemperature gradient to promote silica precipitation along fractures. The authors observe pore-pressure changes attributed to\nsilica precipitation, which could potentially occur in fault zones associated with deep fluid flow.\n----------------\nGeneral Comments:\n----------------\nThe paper is well-written, concise, and employs a well-designed experimental methodology to investigate chemical clogging\nalong fractures and its effects on fluid flow. However, further details on the methodology would help to better understand the\nexperiments and rule out other potential explanations raised below. The integration of experimental results, microstructural\nanalysis, and geochemical modeling show a convincing correlation between fluid equilibrium, mineral precipitation, and\nfluid flow. While the implications for swarm activity are debatable, this work offers valuable insights into fault sealing and\nemphasizes the interplay between mechanical and chemical effects in fault dynamics.\n-----------------\nSpecific Comments\n-----------------\nMethodology:\n***********\n• The experimental setup needs a more detailed description. The references cited (lines 274 and elsewhere) provide\ninsufficient informations.\n• The paper states that fluid pressure is 25 MPa both internally and externally on the granite sample without applying\nconfining pressure.\n◦ What do \"internal\" and \"external\" pressures signify? Are these equivalent to upstream and downstream reservoir\npressures?\n◦ Based on Figure S1a, the downstream pressure matches the confining pressure. How does the jacket prevent fluid\nshortcuts between the upstream and downstream reservoirs, particularly during dilation? A clear explanation of how leakage\nwas avoided is crucial.\n• Line 96/396: It is stated that X-ray CT scans were conducted at the end of the experiment. However, Table S1 mentions\nintermediate scans for Run_05. Could the authors clarify this discrepancy and include intermediate X-ray CT data for\nRun_05?\nPore-Pressure Oscillations:\n**************************\n• The term \"oscillation\" typically refers to smooth, cyclic changes. In this study, the pore-pressure changes appear abrupt\nand rapid. \"Pore-pressure bursts\" or \"pore-pressure pulses\" might better describe the observed phenomena.\n• The interpretation of downstream pressure increases and upstream pressure decrease during barrier breakage is\nplausible. However, could the authors discard alternative explanations, such as fluid flow along the jacket or re-opening of\nthe cohesionless slit caused by pressure buildup?\n• Ensuring the absence of fluid leakage between the sample and the jacket is crucial for accurate permeability\nmeasurements. Providing additional details on how leakage can be ruled out would further strengthen the methodology and\nhelp researcher to reproduce the experiments.\n• Line 218: Photographic images zoomed in on silica particles surrounded by quartz grains would strengthen the\ninterpretation that self-generated fine silica particles are responsible for clogging.\nPermeability Measurements:\n*************************\n• Permeability was calculated using Darcy's law (Eq. 1) with constant parameters such as viscosity, cross-sectional area,\nand sample length. However, this method assumes steady-state conditions, which are not achieved due to the dynamic\nnature of the experiment, introducing significant uncertainties.\n• Since ΔP already indicates sealing of the slit, why are permeability measurements necessary? If included, the limitations of\napplying Darcy's law under these experimental conditions should be clearly addressed.\nInput-Output Solutions:\n**********************\n• Fluid chemistry was measured for one input and 4–5 output samples. Were these output samples collected throughout the\nexperiment, or only at its end? Temporal variations in fluid composition would provide valuable insights on fluid-rock\ninteraction.\n• Did the authors attempt mass balance calculations comparing input-output fluid compositions with X-ray CT results? This\ncould strengthen the interpretation of silica precipitation dynamics.\nCrack-Seal Veins:\n****************\n• Crack-seal veins are typically associated with incremental fracture opening and sealing, whereas the veins formed here\nresult solely from sealing without fracture opening. Thus, the comparison between crack-seal veins and fracture clogging is\nnot straightforward.\n• Line 243: While comparisons to crack-seal veins are made, the experimental mechanism differs since the slit thickness\nremains constant. Higher magnification microstructural images showing banded structures would clarify similarities or\ndifferences with crack-seal veins.\nImplications for Swarm Activities:\n*********************************\n• The proposed link between pore-pressure oscillations and seismic swarms (line 207) needs further clarification. The\nmechanism involving high-temperature, over-pressured fluid, and barrier rupture is more consistent with fault-valve behavior\nand mainshock-aftershock sequences (e.g., Miller et al., 2004, Nature). Swarm activity attribution remains less\nstraightforward. Figure 4 aligns better with fault-valve dynamics than swarm-like sequences.\nMinor Comments:\n**************\n• Line 40: The increase in pore-fluid pressure leads to a reduction in normal stress, not shear strength, following the Mohr-\nCoulomb theory. Rupture occurs when shear stress reaches the failure envelope.\n• Line 133: Could the evolution of microstructures and mineralogy along the slit be linked to changes in quartz solubility ?\n• Line 202: Seismic swarms are distinct from aftershock sequences. While overlap between the two phenomena is possible,\nswarm are by definition characterized by long term seismicity without mainshock.\nFigures:\n*******\n• Figure 2b: The colors need explanation. The figure is less informative than Figure 2a and could be removed. Larger,\nclearer microstructural images, including scales, would be beneficial.\n• Figure S5: The origine of quartz, plagioclase, K-feldspar, and biotite (i.e., from the granite or precipitated during\nexperiments) is unclear. Could the authors clarify the surface topography and the surface being analyzed?\nThis paper provides significant insights into chemical processes in fractures and their implications for fault mechanics.\nAddressing these methodological and interpretative issues would greatly enhance its impact.\nSincerely,", "reviewer_2": "General comments\nThis paper presents novel experimental results that test the hypothesis that repeated cycles of (1) silica precipitation and\nassociated permeability reduction, and (2) fault rupture and associated permeability enhancement in fault zones control fluid\npressure variations, episodic flow and fault-valve behaviour.\nThe very novel approach in based on using up-temperature flow of silica-saturated water, within the low pressure (25 – 35\nMPa) retrograde silica solubility regime, to simulate the down-temperature and down-pressure flow of silica-saturated water\nin higher pressure regimes in the mid- to upper crust of the Earth. The pressure drops associated with repeated fracturing of\nsilica precipitates during the experiments also conveniently simulate pressure drops that can be\nassociated with co-seismic dilatancy during natural fault ruptures.\nThe conclusions that episodic increases in fluid pressure differentials (between the upstream and downstream positions) are\nrelated to clogging of pore structures by growth of crystalline quartz and precipitation of amorphous silica along fluid\npathways is very robust. Likewise, the interpretation that the sudden drops in fluid pressure relate to failure within the\nclogging silica deposits is also correct. The work also highlights the importance of fluid pressurisation in driving failure,\neven in the absence of differential stress (or changes in differential stress). The role of fluid pressurisation in driving fault\nfailure has long been understood from experiments, from hydraulic fracturing tests, and from natural fluid-driven earthquake\nswarms. But the Okamoto & Vinis experiments are a very neat and further demonstration of fluid-driven failure, along with the\nquantified coupling between pore clogging and pressurisation in a constant flow rate regime.\nThe study also highlights the importance of precipitation of amorphous silica particles at high supersaturation levels in\nadvective flow regimes. This is something that is not widely enough appreciated in the fault zone mechanics/hydrology\ncommunity.\nIn addition to being novel and extremely valuable science, this manuscript is very clearly written, very concise and\nbeautifully illustrated.\nDetailed comments\nLine 20: “quantifying” might be better than “linking”.\nLines 24-25: There appears to be some text missing here. Also, highlight the point that the experiments are conducted in a\nconstant flow rate regime.\nLine 29: Possibly replace “rupture” with “failure”? In my mind, at least, I associate “rupture” with shear failure. “Failure” can\nrefer to either extension failure or shear failure.\nLine 36: Better to say “In some cases, seismic rupture ….”. This is because only a subset of earthquake ruptures is driven by\nchanges in pore pressures. Many, if not most, large earthquakes are likely driven largely by tectonic loading.\n40 – 41: It is more rigorous to say “… when the imposed shear stress becomes equal to the shear strength …”.\n48: The coupled model has also been applied to explain fluid-driven earthquake swarms.\n49: “Some seismogenic zones …..”. Many exhumed fault zones do not contain any veins.\n64 - 65: “… such as a transition from an overpressured regime to hydrostatic pressure…”. Note also that there need not be an\n“impermeable barrier” in the crust. Many low permeability barriers are more likely just the sealed zones within faults that are\nthemselves embedded in relatively impermeable host rocks. The major transition in the crust is the one from deep,\nimpermeable rocks to shallower, highly permeable rocks in which fluid are near-hydrostatic pressured. At depth, in the\nimpermeable zone, fluid pressure might only be relevant in zones of fluid production and the faults or shear zones that drain\nfluids away from them. Elsewhere, fluid might not be present in significant quantities.\nLine 78: Although no physical experimental studies have produced crack-seal structures, the modelling experiments of\nSpaeth et al (2022a, b; GRL and JSG) might be relevant; they used phase field modelling to explore growth of crack-seal\nveins and effects on fluid pathways and (indirectly) permeability.\n139: It would be nice to know a little more about the banding illustrated in Figure 2E. Does the banding relate to grainsize,\nratio of amorphous to crystalline silica, or something else. Is the banding a genuine crack-seal effect, or does it reflect\nsomething to do with changes in fluid chemistry etc, as in the “crustiform” bands that are so common in epithermal veins?\n183: The evolving changes in the locations of sites of silica precipitation is very interesting and likely has some lessons for\nus to better understand the formation of fault-fill veins in both epithermal and deeper crustal, high fluid flux fasult regimes.\n210 – 211: This sentence reinforces the myth that fluid pressure fluctuations in active, fluid-containing faults range only\nbetween near-lithostatic and hydrostatic. You clarify this a couple of lines later. But at 211 it might be better just to refer to\nrupture-induced fluid pressure fluctuations.\n230: You could mention here that increased injection rates and associated pressurisation rates have implications for rupture\nrecurrence rates.\n234: “As layers that are locally-sealed by silica …” is better grammar.\n242: “… depths in cases where a pressure drop …” might also be a bit clearer.\n250: See also Cox & Munroe (2016, Amer J Sci) who, earlier than Berger and Herwegh, described cockade breccias and\ncalculated transient, rupture-related fluid flow velocities for repeated failure-sealing cycles in a natural fault zone.\nStephen F Cox", "reviewer_3": "The authors present some novel experiments, with flow fluid through a slit in granite where silica that precipitates as quartz\nalong the slit clogs and makes the flow tortuous until a fracture occurs and restores the flow, leading to fluctuations in fluid\npressure and cyclic cracking associated with fluid pressure fluctuations. These fluctuations lead to textural banding in the\nfracture. It would be useful to know what the banding is represented by. In veins I have observed, the bands are either\ndelineated by fluid inclusions (syntaxial) vs. solid inclusions of the wall rock (antitaxial). This case strikes me as epitaxial\nalong the walls until bridges form across the slit, in which case refracture in the center (syntaxial). This work could may be of\ninterest to workers who have long speculated that textural evidence for cyclic cracking and sealing is the result of\nfluctuations in fluid pressure. Although I find the paper lucid and organized and the experiments novel, there are some\nfundamental issues that I feel must be resolved before publication.\nI am confused by the notion that solubility of quartz decreases with increasing temperature (line 92-93), which is incorrect.\nSolubility of silica follows a van’t Hoff relationship (Fisher and Hirth, 2024, figure 5), with decreasing equilibrium solubility\nwith decreasing temperature. Consequently, when fluid flows down a gradient in temperature (and hence chemical\npotential), minerals precipitate along the path. To flow up temperature from 370 (at the inlet) to 450 (at the outlet), it seems\nthat the only way to get precipitation is to have fluid come in at the inlet very supersaturated at low T (and at much higher\nconcentrations than the outlet equilibrium solubility) and approach equilibrium at high T through precipitation within the slit,\nwhich is unusual in metamorphic system with single pass flow. Dissolution should happen in a shallow convecting\nhydrothermal system with increasing T and precipitation should happen as the fluid cools. Obviously the experiments are\nproducing precipitation, so I have to assume that the precipitation is driven by design through input of a supersaturated fluid?\nCalcite has retrograde solubility, but not quartz. See line 300 comment. I think that this study demonstrates that precipitation\nin a fracture can lead to fluid pressure fluctuations, but the authors need to clear up the discussion of solubility and to\ncompare the conditions of their experiments to conditions of natural systems and acknowledge the manners in which they\nare different.\nLine 18- “Seismogenic depths” are mentioned but this paper is more suited to shallow high temperature hydrothermal\nsystems as the temperatures in the slit exceed the temperatures of the seismogenic zone and there is no confining pressure\n(perpendicular to the slit) involved in the fluid pressure variations.\nLine 22- “that shows silica…”\nLine 61-62- I don’t understand. In Craig Manning’s experiments, the transition from subcritical to supercritical conditions\nresults in a dramatic increase in solubility.\nLine 91-93, 286-286- See the comment above about the relationship between quartz solubility and temperature. How can\nthere be a decrease in quartz solubility at increasing T? The inlet is at 370 and the outlet at 450? So the fluid is flowing\ntoward higher temperature and precipitating? (see comment on line 300. I think I might understand the design of the\nexperiment but am confused by the statement that the experiments take advantage of “the decrease in quartz solubility with\nincreasing T”).\nLine 136- the amorphous silica that builds up on the walls is interesting. Is this seen in natural veins, or does it readily get\nconverted to quartz? Is there any way to detect this texturally in natural vein systems? Commonly, the size of quartz grains in a\nfracture increase through time due to anisotropic growth. I wonder if the amorphous silica precipitation is a kinetic effect that\nrequires dramatic overstepping as seen in these experiments but would be less likely in natural systems, particularly\nseismogenic zones with low temperature gradients along the fluid path.\nLine 211- At depths of the seismogenic zone (10-40 km), when a hydrofracture develops, the fluid pressure cannot drop from\nlithostatic to hydrostatic because this can result in pressure drops of 100’s of MPa, which should lead to large compressive\nstresses perpendicular to the crack (the pressure drop is larger than the fracture toughness so the crack should close). I\nknow that workers in fluid inclusions in veins have argued that trapping pressure variability reflects pressure drops of that\nsize, but it strikes me as physically impossible, so I worry about variable decrepitation (Raimbourg et al., 2018) as a more\nplausible explanation. In the case of this manuscript, the constant width slit is more in keeping with shallow hydrothermal\nsystems where the background fluid pressure may be hydrostatic and effective sigma 3 can be tensile without near-\nlithostatic fluid pressures.\nLine 221- As pointed out earlier in the review, blocky textures can also occur in the case of growth of crystals into an open\ncrack. Is there any feature that is diagnostic of conversion of amorphous silica to blocky crystals?\nLine 223-224- Does opening mode cracking parallel to the slit mean that the slit gets wider over time? What is the aperture of\nthe crack and what is controlling that in this case of an initial slit that doesn’t close during fluid pressure drops?\nLine 300- Does the experiment flow fluid into the inlet that is supersaturated at a low T by design, with a much higher\nconcentration than the equilibrium solubility (at both ends of the slit but importantly, at the highT end of the slit) and then flow\nup temperature where the solubility approaches equilibrium at the outlet leaving behind silica along the way. If so, I can\nsee how the experiments achieve precipitation along the length of the slit despite the increase in equilibrium solubility from inlet\nto outlet. It is because the fluids are coming in at concentrations far greater than the equilibrium at the high temperature outlet\nbut at temperatures much lower, leading to injection of hugely supersaturated fluid into the slit at the inlet. This doesn’t seem\nlike an analog for natural systems, where fluid generally flows from high temperature and pressure to low temperature and\npressure during single pass flow, or in the case of hydrothermal systems with convective flow, dissolves on the way down\nand precipitates on the way up."}, "author_response": "Responses to reviewer comments\n(Responses are in blue)\nReviewer #1\n<A-1> General Comments:\nThe paper is well-written, concise, and employs a well-designed experimental methodology to\ninvestigate chemical clogging along fractures and its effects on fluid flow. However, further\ndetails on the methodology would help to better understand the experiments and rule out other\npotential explanations raised below. The integration of experimental results, microstructural\nanalysis, and geochemical modeling show a convincing correlation between fluid equilibrium,\nmineral precipitation, and fluid flow. While the implications for swarm activity are debatable,\nthis work offers valuable insights into fault sealing and emphasizes the interplay between\nmechanical and chemical effects in fault dynamics.\nThank you for the positive comments. We modified the manuscript following the comments,\nparticularly concerning the experimental setup and analyses, and implications for seismicity in\nthe crust.\nSpecific Comments\nMethodology:\n<A-2> • The experimental setup needs a more detailed description. The references cited (lines\n274 and elsewhere) provide insufficient informations.\nWe added a photograph that shows the connection between the inner assembly and the cap at the\ninlet (Fig. S2d). We also added an explanation of how we connected the inner assembly and cap,\nand how we avoid leakage (see the Supplementary Information).\n<A-3> • The paper states that fluid pressure is 25 MPa both internally and externally on the\ngranite sample without applying confining pressure.\nWe did not apply confining pressure. We have added a schematic illustration showing the\ndistribution of fluid pressure within the vessels before and after clogging (Fig. S2e), and added\nan explanation in the Supplementary Information.\n<A-4> What do \"internal\" and \"external\" pressures signify? Are these equivalent to upstream\nand downstream reservoir pressures?\nThe downstream end of the stainless sleeve was open, meaning it was directly connected to the\nexternal space filled with fluids. Therefore, the internal and external pressures were almost the\nsame (except for the effects of flow pressure). The distribution of fluid pressure is shown in Fig.\nS2e.\n<A-5> Based on Figure S1a, the downstream pressure matches the confining pressure. How\ndoes the jacket prevent fluid shortcuts between the upstream and downstream reservoirs,\nparticularly during dilation? A clear explanation of how leakage was avoided is crucial.\nWe added the following explanation to the Supplementary Information.\n“The inner assembly (stainless-steel sleeve covering alumina tubes and granite cores) was\nconnected to a cap at the inlet of the precipitation vessel by a nut and ferrule set made of\nstainless steel (Fig. S2b, d). After individual runs, this connection was carefully checked for\nleaks. If the seal at the joint of the inner assembly and inlet cap had been broken to form a short-\ncut to the outside of the stainless sleeve, silica precipitation should have been observed at the\nsite of leakage. However, after the runs in this study, we found no signs of silica precipitation at\nthis joint of the inner assembly.\nIn our experiments, the confining pressure was not applied to the fracture, so it may have been\npossible that fluid flowed through the space between the stainless-steel sleeve and granite core,\nor though the contact parts of the two granite half-cores (if the pressurized fluid had acted to\nopen the spaces between the two half-cores). If the fluid flow had occurred outside the slit, silica\nwould have been precipitated there. However, the X-ray CT images show no visible spaces or\nsilica precipitates between the outside of the cores and stainless steel, and no spaces between the\ntwo half-cores (Fig. S3). Instead, the slit had a high porosity even in the most-clogged part after\nthe increase in fluid pressure. This indicates that the observed pressure oscillations were caused\nby repeated sealing by silica precipitation and breakage within the slit.”\n<A-6> • Line 96/396: It is stated that X-ray CT scans were conducted at the end of the\nexperiment. However, Table S1 mentions intermediate scans for Run_05. Could the authors\nclarify this discrepancy and include intermediate X-ray CT data for Run_05?\nDuring Run_05 we conducted X-ray CT scans twice, at the intermediate period and\nend of the experiment. This point is described clearly in the revised text (lines 106–107 and\n344–345) and in the footnote to Table S1. We have added representative X-ray CT images of the\ncross-section at x = 4.8 mm of the granite core at the intermediate period and after Run_05 (Fig.\nS3).\nPore-Pressure Oscillations:\n<A-7> • The term \"oscillation\" typically refers to smooth, cyclic changes. In this study, the\npore-pressure changes appear abrupt and rapid. \"Pore-pressure bursts\" or \"pore-pressure pulses\"\nmight better describe the observed phenomena.\nThank you for this comment. We have given this much thought, and finally decided that\n‘oscillation’ satisfactorily describes our observations (almost regularly-repeated change in fluid\npressure), so have retained that term.\n<A-8> • The interpretation of downstream pressure increases and upstream pressure decrease\nduring barrier breakage is plausible. However, could the authors discard alternative\nexplanations, such as fluid flow along the jacket or re-opening of the cohesionless slit caused by\npressure buildup?\nWe have added X-ray CT images of the whole granite core within the stainless-steel sleeve (Fig.\nS3) and explained this point in the Supplementary Information.\nBased on the X-ray CT images captured after the runs, we did not find distinct spaces or silica\nprecipitation along the boundaries between the granite core and stainless-steel sleeve, or along\nthe contact between the two core halves. The porosity remained high in the most-clogged\nregions after the runs. These features indicate that silica precipitation and fluid flow occurred\nmainly within the slit.\n<A-9> • Ensuring the absence of fluid leakage between the sample and the jacket is crucial for\naccurate permeability measurements. Providing additional details on how leakage can be ruled\nout would further strengthen the methodology and help researcher to reproduce the experiments.\nWe have carefully checked the possibility of leakage and fluid flow through the spaces between\nthe granite core and sleeve. If leakage had occurred, it would have been recognized by silica\nprecipitation. We found no signs of leakage. X-ray CT images of the whole core indicate no\nspaces or precipitates between the granite core and stainless sleeve, or along the contact\nbetween the two half-cores. We added the X-ray CT images of the whole cores in Fig. S3 and\nadded a relevant explanation in the Supplementary Information. See reply A-5 for more details.\n<A-10> • Line 218: Photographic images zoomed in on silica particles surrounded by quartz\ngrains would strengthen the interpretation that self-generated fine silica particles are responsible\nfor clogging.\nWe added a close-up view of a silica particle surrounded by quartz (see Fig. 2d). We also added\nclose-up view of BSE image of this texture (Fig. S5b).\nPermeability Measurements:\n<A-11> • Permeability was calculated using Darcy's law (Eq. 1) with constant parameters such\nas viscosity, cross-sectional area, and sample length. However, this method assumes steady-\nstate conditions, which are not achieved due to the dynamic nature of the experiment, introducing\nsignificant uncertainties.\nWe agree that the application of Darcy’s law has limitations regarding the assumption of steady-\nstate conditions. In our experiments, the fluid pressure dynamically changed due to silica\nprecipitation. However, even given the approximate nature of the estimates and assumptions, we\nconsider that the evolution of permeability during the experiments (Fig. 3a) provides\nmeaningful insights into how silica seal-and-break changes the hydraulic properties in a fracture\n(see reply A-12 for more details). In the revised text, we added a description of the limitations of\nDarcy’s flow (see the Methods, lines 377–380).\n<A-12> • Since ΔP already indicates sealing of the slit, why are permeability measurements\nnecessary? If included, the limitations of applying Darcy's law under these experimental\nconditions should be clearly addressed.\nAs suggested by the reviewer, sealing of the slit can be inferred from an increase in ΔP.\nHowever, ΔP depends not only on the hydraulic properties of the slit, but also on the viscosity of\nthe fluid and the flow rate. Therefore, we consider that permeability is useful in demonstrating\nhow silica sealing controls the hydraulic properties of fractures. In addition, the temporal\nevolutions of ΔP and permeability are useful in comparing the results of our experiments with\nthose of previous studies of fault-valve behavior, as proposed by Sibson and others. In the\nrevised text, we added a description of the limitations of the application of Darcy’s flow (see the\nMethods section, Line 377-380).\nInput-Output Solutions:\n<A-13> • Fluid chemistry was measured for one input and 4–5 output samples. Were these\noutput samples collected throughout the experiment, or only at its end? Temporal variations in\nfluid composition would provide valuable insights on fluid-rock interaction.\nWe added the solution chemistries for individual samples during the experiments (Table S2).\nFor Run_05 we removed one solution point (at ~11 h) because it was close to the timing of the\ninterval period of the run for X-ray CT scan, at which the flow was topped. Consequently, the\nadded four solutions to both runs.\nIn both runs, the Si concentration in the output solution was relatively high initially, likely due\nto the low reactive surface areas of quartz grains on the wall. Then the concentration of Si in the\noutput solution was slightly decreased to a constant concentration of ~50 mg kg–1 H O. Even\n2\nafter the increase in pressure in upstream regions, silica precipitation occurred close to the level\nof quartz solubility on the sides (25 MPa) of the slit in downstream areas, resulting in a similar\nSi concentration in the output solutions to those before pressure oscillations. A description of the\noutput solution has been added to the revised manuscript (lines 328–330).\n<A-14> • Did the authors attempt mass balance calculations comparing input-output fluid\ncompositions with X-ray CT results? This could strengthen the interpretation of silica\nprecipitation dynamics.\nWe added a discussion on the mass balance to the Supplementary Information as follows:\n“We attempted to obtain the amount of silica precipitate from X-ray CT results for Run_02, and\ncompared it with the value estimated from the difference in Si concentration between the input\nand output solutions (average Si concentration). The X-ray CT based estimate of the weight of\nsilica precipitate was 170 mg, or 68% of the solution-based estimates (251 mg). This\ndiscrepancy is likely due to the underestimation of the volume of silica precipitate due to (1) the\nlow resolution of X-ray CT images (10 μm voxel size) relative to pore size in silica deposits,\nand (2) uncertainties in the segmentation of X-ray CT images in distinguishing silica precipitate\nfrom wallrock (granite), and (3) the effect of the changes in fluid chemistry during the initial\nstage of each run and during heating and cooling (Table S2). This mass balance analysis may\nhave considerable uncertainty, but it indicates that most of the silica was precipitated within the\nvessel.”\nCrack-Seal Veins:\n<A-15> • Crack-seal veins are typically associated with incremental fracture opening and\nsealing, whereas the veins formed here result solely from sealing without fracture opening.\nThus, the comparison between crack-seal veins and fracture clogging is not straightforward.\nWe agree with this comment. Incremental fracture opening and sealing differs from the process\nthat occurred in our experiments. However, both processes are similar in that repeated sealing\nand failure is likely related to fluctuations in fluid pressure. In addition, the banded quartz veins\nin our experiments contain inclusion bands, similar to some crack–seal veins. When completely\nsealed, it is not always easy to distinguish the two types of vein with inclusion bands. We added\nan enlarged view of the inclusion bands (Fig. 3e). We consider that a comparison of the two\ntextures is helpful when interpretating natural quartz veins.\n<A-16> • Line 243: While comparisons to crack-seal veins are made, the experimental\nmechanism differs since the slit thickness remains constant. Higher magnification\nmicrostructural images showing banded structures would clarify similarities or differences with\ncrack-seal veins.\nWe added an enlarged view of Fig. 3e, showing clear inclusion bands that are similar to those in\ncrack–seal veins. For details, see our reply to comment A-15.\nImplications for Swarm Activities:\n<A-17> • The proposed link between pore-pressure oscillations and seismic swarms (line 207)\nneeds further clarification. The mechanism involving high-temperature, over-pressured fluid,\nand barrier rupture is more consistent with fault-valve behavior and mainshock-aftershock\nsequences (e.g., Miller et al., 2004, Nature). Swarm activity attribution remains less\nstraightforward. Figure 4 aligns better with fault-valve dynamics than swarm-like sequences.\nWe appreciate the comment. Although mainshock–aftershock sequences do not always involve\nfluids, some such sequences clearly show evidence of pulses of high pore fluid pressures, which\ncan be related to fault-valve dynamics. We have described the aftershock sequences driven by\nthe migration of high-pressure fluids (lines 218–220).\nWe also use the term “the high fluid-flux regimes related to fault-valve dynamics” instead of\n“swarm seismicity” (lines 227–228).\nMinor Comments:\n<A-18> • Line 40: The increase in pore-fluid pressure leads to a reduction in normal stress, not\nshear strength, following the Mohr-Coulomb theory. Rupture occurs when shear stress reaches\nthe failure envelope.\nWe replaced “shear strength” with “effective normal stress”. We also modified the sentence to\n“increasing fluid pressure leads to a reduction in the effective normal stress, with rupture\noccurring when the imposed shear stress becomes equal to the shear strength” (lines 37–39).\n<A-19> • Line 133: Could the evolution of microstructures and mineralogy along the slit be\nlinked to changes in quartz solubility ?\nYes, the evolution of microstructures and mineralogy along the flow path is linked to changes in\nquartz solubility and the decrease in Si concentration in solution as a result of precipitation. This\nexplanation has been added to the revised manuscript (lines 141–143).\n<A-20> • Line 202: Seismic swarms are distinct from aftershock sequences. While overlap\nbetween the two phenomena is possible, swarm are by definition characterized by long term\nseismicity without mainshock.\nFollowing this comment, we have described swarm seismicity and aftershock sequences\nseparately (lines 218–220).\nFigures:\n<A-21> • Figure 2b: The colors need explanation. The figure is less informative than Figure 2a\nand could be removed. Larger, clearer microstructural images, including scales, would be\nbeneficial.\nWe added an explanation of the colors in the caption to Fig. 2b. We also added arrows indicating\nthe boundaries of individual connected pores, showing that the silica-sealed zone has thin walls.\nWe also enlarged individual optical and SEM-CL images (Fig. 2c–d) and added enlarged views\nof nucleated and banded quartz (Fig. 2d, e). We added the information on scale bars to the\ncaption.\n<A-22> • Figure S5: The origine of quartz, plagioclase, K-feldspar, and biotite (i.e., from the\ngranite or precipitated during experiments) is unclear. Could the authors clarify the surface\ntopography and the surface being analyzed?\nBefore the experiments, we undertook X-ray mapping of the Si, Al, Fe, Mg, Na, and K contents\non the surface of the silt in the granite using micro-X-ray fluorescence spectrometry (Fig. S6).\nThen, mineral maps were created from the element maps with the k-means method using Matlab\nsoftware. This explanation has been added to the Methods section (lines 348–353).\nAn explanation of the surface topography obtained from X-ray CT images and mineral maps has\nbeen added to the Methods section and the caption to Fig. S6.\n<A-23> This paper provides significant insights into chemical processes in fractures and their\nimplications for fault mechanics. Addressing these methodological and interpretative issues\nwould greatly enhance its impact.\nWe appreciate this comment, which has helped us to improve the manuscript.\nReviewer #2:\nGeneral comments\n<B-1> This paper presents novel experimental results that test the hypothesis that repeated\ncycles of (1) silica precipitation and associated permeability reduction, and (2) fault rupture and\nassociated permeability enhancement in fault zones control fluid pressure variations, episodic\nflow and fault-valve behaviour.\nThe very novel approach in based on using up-temperature flow of silica-saturated water,\nwithin the low pressure (25 – 35 MPa) retrograde silica solubility regime, to simulate the down-\ntemperature and down-pressure flow of silica-saturated water in higher pressure regimes in the\nmid- to upper crust of the Earth. The pressure drops associated with repeated fracturing of\nsilica precipitates during the experiments also conveniently simulate pressure drops that can be\nassociated with co-seismic dilatancy during natural fault ruptures.\nThe conclusions that episodic increases in fluid pressure differentials (between the upstream and\ndownstream positions) are related to clogging of pore structures by growth of crystalline quartz\nand precipitation of amorphous silica along fluid pathways is very robust. Likewise, the\ninterpretation that the sudden drops in fluid pressure relate to failure within the clogging silica\ndeposits is also correct. The work also highlights the importance of fluid pressurisation in\ndriving failure, even in the absence of differential stress (or changes in differential stress). The\nrole of fluid pressurisation in driving fault failure has long been understood from experiments,\nfrom hydraulic fracturing tests, and from natural fluid-driven earthquake swarms. But the\nOkamoto & Vinis experiments are a very neat and further demonstration of fluid-driven failure,\nalong with the quantified coupling between pore clogging and pressurisation in a constant flow\nrate regime.\nThe study also highlights the importance of precipitation of amorphous silica particles at high\nsupersaturation levels in advective flow regimes. This is something that is not widely enough\nappreciated in the fault zone mechanics/hydrology community.\nIn addition to being novel and extremely valuable science, this manuscript is very clearly\nwritten, very concise and beautifully illustrated.\nWe thank the reviewer for this positive comment and encouragement.\nDetailed comments\n<B-2> Line 20: “quantifying” might be better than “linking”.\nChange made as suggested. Line 19\nLines 24-25: There appears to be some text missing here. Also, highlight the point that the\nexperiments are conducted in a constant flow rate regime.\nWe added a description of the constant flow rates (line 21).\n<B-3> Line 29: Possibly replace “rupture” with “failure”? In my mind, at least, I associate\n“rupture” with shear failure. “Failure” can refer to either extension failure or shear failure.\nWe replaced the term “rupture” with “failure” in describing the experiments (line 26 and others).\nThe term “rupture” has been retained when describing the seismicity.\n<B-4> Line 36: Better to say “In some cases, seismic rupture ….”. This is because only a subset\nof earthquake ruptures is driven by changes in pore pressures. Many, if not most, large\nearthquakes are likely driven largely by tectonic loading.\nChange made as suggested. Line 33\n<B-5> 40 – 41: It is more rigorous to say “… when the imposed shear stress becomes equal to\nthe shear strength …”.\nChange made as suggested. Line 38\n<B-6> 48: The coupled model has also been applied to explain fluid-driven earthquake swarms.\nChange made as suggested. Line 45\n<B-7> 49: “Some seismogenic zones …..”. Many exhumed fault zones do not contain any\nveins.\nChange made as suggested. Line 47\n<B-8> 64 - 65: “… such as a transition from an overpressured regime to hydrostatic\npressure…”. Note also that there need not be an “impermeable barrier” in the crust. Many low\npermeability barriers are more likely just the sealed zones within faults that are themselves\nembedded in relatively impermeable host rocks. The major transition in the crust is the one from\ndeep, impermeable rocks to shallower, highly permeable rocks in which fluid are near-\nhydrostatic pressured. At depth, in the impermeable zone, fluid pressure might only be relevant\nin zones of fluid production and the faults or shear zones that drain fluids away from them.\nElsewhere, fluid might not be present in significant quantities.\nWe deleted the term “across the impermeable barrier” as suggested (line 68-69).\n<B-9> Line 78: Although no physical experimental studies have produced crack-seal structures,\nthe modelling experiments of Spaeth et al (2022a, b; GRL and JSG) might be relevant; they\nused phase field modelling to explore growth of crack-seal veins and effects on fluid pathways\nand (indirectly) permeability.\nWe added a description of phase-field modeling studies of crack–seal veins, citing Spaeth et al.\n(2022a, b). See lines 80–82.\n<B-10> 139: It would be nice to know a little more about the banding illustrated in Figure 2E.\nDoes the banding relate to grainsize, ratio of amorphous to crystalline silica, or something else.\nIs the banding a genuine crack-seal effect, or does it reflect something to do with changes in\nfluid chemistry etc, as in the “crustiform” bands that are so common in epithermal veins?\nWe added an enlarged view of Fig. 2e and a description of the banded quartz (banded silica\ncomprises only quartz) (lines 150‒152).\nAlthough the input solution remained the same during the run, the fluctuations in fluid pressure\ncould have led to changes in the local flow rate and fluid chemistry (i.e., Si and trace element\nconcentrations. The detailed relationship between fluid pressure, cathodoluminescence, and the\ntrace-element compositions of the fluids and quartz crystals is interesting (e.g., Raimbourg et\nal., 2022, G-cubed), but is beyond the scope of this study. This point is stated in lines 207–210.\n<B-11> 183: The evolving changes in the locations of sites of silica precipitation is very\ninteresting and likely has some lessons for us to better understand the formation of fault-fill\nveins in both epithermal and deeper crustal, high fluid flux fault regimes.\nWe agree with this comment. The importance of the evolving changes in the locations of silica\nprecipitation is discussed in lines 258–264.\n<B-12> 210 – 211: This sentence reinforces the myth that fluid pressure fluctuations in active,\nfluid-containing faults range only between near-lithostatic and hydrostatic. You clarify this a\ncouple of lines later. But at 211 it might be better just to refer to rupture-induced fluid pressure\nfluctuations.\nWe removed the terms “lithostatic” and “hydrostatic” as suggested (lines 239–230).\n<B-13> 230: You could mention here that increased injection rates and associated pressurisation\nrates have implications for rupture recurrence rates.\nChange made as suggested. Line 250-252\n<B-14> 234: “As layers that are locally-sealed by silica …” is better grammar.\nChange made as suggested. Line 256\n<B-15> 242: “… depths in cases where a pressure drop …” might also be a bit clearer.\nWe modified this part as “… depths where a pressure drop …” Line 264\n<B-16> 250: See also Cox & Munroe (2016, Amer J Sci) who, earlier than Berger and\nHerwegh, described cockade breccias and calculated transient, rupture-related fluid flow\nvelocities for repeated failure-sealing cycles in a natural fault zone.\nWe cited Cox & Munroe (2016, Am J Sci) (lines 272 and 531–533).\nReviewer #3:\n<C-1> The authors present some novel experiments, with flow fluid through a slit in granite\nwhere silica that precipitates as quartz along the slit clogs and makes the flow tortuous until a\nfracture occurs and restores the flow, leading to fluctuations in fluid pressure and cyclic\ncracking associated with fluid pressure fluctuations. These fluctuations lead to textural banding\nin the fracture. It would be useful to know what the banding is represented by. In veins I have\nobserved, the bands are either delineated by fluid inclusions (syntaxial) vs. solid inclusions of\nthe wall rock (antitaxial). This case strikes me as epitaxial along the walls until bridges form\nacross the slit, in which case refracture in the center (syntaxial). This work could may be of\ninterest to workers who have long speculated that textural evidence for cyclic cracking and\nsealing is the result of fluctuations in fluid pressure. Although I find the paper lucid and\norganized and the experiments novel, there are some fundamental issues that I feel must be\nresolved before publication.\nWe appreciate the comments. We agree that our synthetic quartz veins are syntaxial. We hope\nthat our study links quartz vein texture and the temporal evolution of fluid pressure, and is\nuseful for the interpretation of natural quartz veins.\n<C-2> I am confused by the notion that solubility of quartz decreases with increasing\ntemperature (line 92-93), which is incorrect. Solubility of silica follows a van’t Hoff relationship\n(Fisher and Hirth, 2024, figure 5), with decreasing equilibrium solubility with decreasing\ntemperature. Consequently, when fluid flows down a gradient in temperature (and hence chemical\npotential), minerals precipitate along the path. To flow up temperature from 370 (at the inlet)\nto 450 (at the outlet), it seems that the only way to get precipitation is to have fluid come in at\nthe inlet very supersaturated at low T (and at much higher concentrations than the outlet\nequilibrium solubility) and approach equilibrium at high T through precipitation within the slit,\nwhich is unusual in metamorphic system with single pass flow. Dissolution should happen in a\nshallow convecting hydrothermal system with increasing T and precipitation should happen as\nthe fluid cools. Obviously the experiments are producing precipitation, so I have to assume that\nthe precipitation is driven by design through input of a supersaturated fluid? Calcite has\nretrograde solubility, but not quartz. See line 300 comment.\nWe consider that the reviewer’s questions regarding solubility have arised from a\nmisunderstanding of the temperature-dependence of quartz solubility at low pressure. The\nsolubility of quartz in water is a function of temperature and water density (Manning 1994,\nGCA; Akinsieve and Diamond, 2009, GCA). At higher pressures (>100 MPa), quartz solubility\nincreases with increasing temperature, and the van’t Hoff relationship (i.e., the linear\nrelationship between 1/T and solubility) is satisfied as shown by Fisher and Hirth (2024). In\ncontrast, at low pressures (<50 MPa) near the critical point of water, fluid density decreases\nsignificantly with increasing temperature, resulting in a decrease in quartz solubility (Fig. S1).\nThis decrease has been well studied in high-temperature geothermal systems (Fournier, 1999,\nEconomic Geol; Saishu et al., 2014, Terra Nova) and has been used in our previous experiments\non silica precipitation (Okamoto et al., 2010, GCA; Okamoto and Sekine 2011, JSG; Saishu et\nal., 2012, Am Min; 2014, Terra Nova; Busch et al., 2021, JSG).\nIn addition, we confirmed that no silica was precipitated during cooling due to the slow reaction\nkinetics in our flow-through experiments (e.g., Okamoto et al., 2010, GCA). In the revised\nmanuscript, we have added a plot of quartz solubility at 200°C–600°C and 25–200 MPa\n(Supplementary Information, Fig. S1a), which shows how the solubility of silica changes at low\npressures.\nWe also added more details on silica solubility (lines 59–66).\nI think that this study demonstrates that precipitation in a fracture can lead to fluid pressure\nfluctuations, but the authors need to clear up the discussion of solubility and to compare the\nconditions of their experiments to conditions of natural systems and acknowledge the manners\nin which they are different.\nWe agree, and added this point in the text (lines 273–278).\n<C-3> Line 18- “Seismogenic depths” are mentioned but this paper is more suited to shallow\nhigh temperature hydrothermal systems as the temperatures in the slit exceed the temperatures\nof the seismogenic zone and there is no confining pressure (perpendicular to the slit) involved in\nthe fluid pressure variations.\nAbstract is shorten to 150 words according to the regulation of the journal. Following this\nchange, this sentence is changed simply as follows (Line 17-18):\n“Ubiquitous quartz veins within the crust underscore the contribution of silica precipitation to\nfault sealing and fluid-pressure changes during earthquake cycles.\nWe agree that our experimental conditions (high temperatures and no confining pressure) differ\nfrom the conditions of typical seismogenic zones. We state this point in lines 221–226 as\nfollows.\n“The conditions in our experiments are characterized by high temperatures (>400°C), low\npressures, and no confining pressure (<30 MPa), similar to high-temperature hydrothermal\nsystems in the shallow crust but different from the conditions of typical seismogenic zones.\nHowever, the temporal evolution of fluid pressure and the resulting quartz-vein texture observed\nin this study provide insights into quartz vein formation related to fluid-driven earthquakes\nwithin the crust.”\n<C-4> Line 22- “that shows silica…”\nFollowing the comments from reviewers 2 and 3, we modified this sentence as follows:\n“…we present the results of hydrothermal flow-through experiments with constant flow rates\nthat show silica…” (lines 20–21).\n<C-5> Line 61-62- I don’t understand. In Craig Manning’s experiments, the transition from\nsubcritical to supercritical conditions results in a dramatic increase in solubility.\nThe temperature dependence of quartz solubility at low pressures (<50 MPa) is quite different\nfrom that at high pressure. This trend is clearly observed if we calculate the quartz solubility\nfollowing Maning (1994). We have added a quartz solubility plot with a broader pressure range\n(Fig. S1a) and added a relevant explanation to lines 59–62.\nSee the response to comment C-2 for more details.\n<C-6> Line 91-93, 286-286- See the comment above about the relationship between quartz\nsolubility and temperature. How can there be a decrease in quartz solubility at increasing T? The\ninlet is at 370 and the outlet at 450? So the fluid is flowing toward higher temperature and\nprecipitating? (see comment on line 300. I think I might understand the design of the\nexperiment but am confused by the statement that the experiments take advantage of “the\ndecrease in quartz solubility with increasing T”).\nAt low pressures (<50 MPa), quartz solubility may decrease in response to a decrease in water\ndensity, as the density of H O (number of water molecules) is essential for the hydration of Si as\n4\nwith increasing temperature in the experimental system. Accordingly, in this experimental\ndesign, both high supersaturation and high temperature were realized, which enhanced the rate\nof silica precipitation (Okamoto et al., 2010, GCA). This point is made in lines 99–101.\n<C-7> Line 136- the amorphous silica that builds up on the walls is interesting. Is this seen in\nnatural veins, or does it readily get converted to quartz? Is there any way to detect this texturally\nin natural vein systems? Commonly, the size of quartz grains in a fracture increase through time\ndue to anisotropic growth. I wonder if the amorphous silica precipitation is a kinetic effect that\nrequires dramatic overstepping as seen in these experiments but would be less likely in natural\nsystems, particularly seismogenic zones with low temperature gradients along the fluid path.\nRelics of amorphous silica are commonly observed in epithermal quartz veins showing\ncolloform textures (e.g., Amagai et al., 2019, Scientific Reports; Marinova et al., 2014,\nMineralium Deposita). In our previous studies (e.g., Amagai et al., 2019), we found that\namorphous silica can be transformed to quartz within a few days under supercritical conditions\n(450 ˚C). Therefore, it is reasonable to consider that some blocky veins in hydrothermal quartz\nveins in high-temperature crust (i.e. related to porphyry copper deposits; Rusk and Reed, 2002)\nare formed by this process, although it is difficult to show that amorphous silica was a precursor\nto quartz in most of natural quartz veins. Cathodoluminescence imaging has the potential to\nprovide such evidence, but further systematic studies are needed (lines 206–210).\nIt is quite unclear that similar process (nucleation of amorphous silica particles, subsequent\ntransformation from amorphous silica to quartz) can occur to form blocky-veins in low-\ntemperature seismogenic zones, especially in subduction zones. When micro- to nano-size\nparticles of silica (even for amorphous silica and quartz) are formed at the instance of\nearthquake rupture, the clogging by silica particles may be realized as observed in this study. We\nadded this point in line 238.\nAt least, we consider that our study shows an example of fault-valve behavior caused by\nrepeated silica sealing and breakage, as can occur in the high-fluid-flux regime in high-\ntemperature crust. We state this point in line 221–226 and 273–278.\n<C-8> Line 211- At depths of the seismogenic zone (10-40 km), when a hydrofracture develops,\nthe fluid pressure cannot drop from lithostatic to hydrostatic because this can result in pressure\ndrops of 100’s of MPa, which should lead to large compressive stresses perpendicular to the\ncrack (the pressure drop is larger than the fracture toughness so the crack should close). I know\nthat workers in fluid inclusions in veins have argued that trapping pressure variability reflects\npressure drops of that size, but it strikes me as physically impossible, so I worry about variable\ndecrepitation (Raimbourg et al., 2018) as a more plausible explanation. In the case of this\nmanuscript, the constant width slit is more in keeping with shallow hydrothermal systems where\nthe background fluid pressure may be hydrostatic and effective sigma 3 can be tensile without\nnear-lithostatic fluid pressures.\nFollowing this comment and the comment by reviewer 2, we rewrote this sentence without\ndescribing the change from lithostatic to hydrostatic, as follows:\n“… the fluid pressure suddenly drops, and the fluid migrates into the overlying crust” (lines\n229–231)\nWe also state that our experimental conditions are similar to those of high-temperature\nhydrothermal systems in shallow crust (lines 221–226 and 273–278).\nWe consider that open cracks can be sustained even when a large pressure drop (i.e., from\nlithostatic to hydrostatic) occurs, provided there is irregular fracture roughness or overgrowth of\nquartz within the cracks. However, the magnitude of pressure drops in natural systems is not the\nmain focus of this study.\n<C-9> Line 221- As pointed out earlier in the review, blocky textures can also occur in the case\nof growth of crystals into an open crack. Is there any feature that is diagnostic of conversion of\namorphous silica to blocky crystals?\nYes, some blocky texture may form through growth from the wall. However in this study, in the\nmost-clogged regions (e.g., x = 3.5 cm in Run_02), fine quartz grains were filled with relatively\nlarge amorphous silica particles (Fig. 3d; S5b; S7a, b). These quartz grains with blocky texture\nrequire nucleation. It is difficult to say whether they were initially formed as amorphous silica\nwith subsequent transformation to quartz, or they were directly nucleated as quartz via\nheterogeneous nucleation. In our previous study, we found that amorphous silica is readily\ntransformed to quartz under supercritical conditions, within 1 day (Amagai et al., 2019,\nSciRep). At x = ~3.5 cm in Run_02, there are spaces between amorphous silica and fine-grained\nquartz (Fig. S5b). Such open spaces can be formed via the dissolution of amorphous silica and\nprecipitation of quartz (Okamoto et al., 2010), as the specific volume of quartz is smaller than\nthat of amorphous silica. Therefore, it is reasonable to consider that some fine-quartz grains\nwith blocky texture were formed from amorphous silica.\nIn either case, silica particles (amorphous silica or quartz nuclei) can flow and attach to form\nsilica-seal layers, causing the characteristic oscillations in fluid pressure.\nIn the revised manuscript, we have added an enlarged view of quartz grains with blocky texture\nin Fig. 2d, and cited Figs. 2d, S5b, S7a, b in lines 241.\nWe added labelled the quartz texture in Fig. S5a and an enlarged view of the BSE image of\namorphous silica surrounded by fine-grained quartz (Fig. 5b), and provided the above\nexplanation in the caption.\n<C-10> Line 223-224- Does opening mode cracking parallel to the slit mean that the slit gets\nwider over time? What is the aperture of the crack and what is controlling that in this case of an\ninitial slit that doesn’t close during fluid pressure drops?\nIn our experiment, we doubt that the slit width changed at the time of the pressure drop. Rather,\nthe torturous flow paths within the porous silica deposits were partly broken by the increase in\nfluid pressure. In the revised manuscript, we modified this sentence to “…and the slight opening\nof the narrow fluid pathways within the porous silica deposit” (lines 245–246).\n<C-11> Line 300- Does the experiment flow fluid into the inlet that is supersaturated at a low T\nby design, with a much higher concentration than the equilibrium solubility (at both ends of the\nslit but importantly, at the highT end of the slit) and then flow up temperature where the\nsolubility approaches equilibrium at the outlet leaving behind silica along the way. If so, I can\nsee how the experiments achieve precipitation along the length of the slit despite the increase in\nequilibrium solubility from inlet to outlet. It is because the fluids are coming in at\nconcentrations far greater than the equilibrium at the high temperature outlet but at temperatures\nmuch lower, leading to injection of hugely supersaturated fluid into the slit at the inlet. This\ndoesn’t seem like an analog for natural systems, where fluid generally flows from high\ntemperature and pressure to low temperature and pressure during single pass flow, or in the case\nof hydrothermal systems with convective flow, dissolves on the way down and precipitates on\nthe way up.\nIn the experiment, the reduction in water density was achieved by increasing the temperature,\nwhich caused the precipitation of silica (see the reply to comment C-2). This experimental\nsystem is similar to the situation in natural geothermal systems, where the downstream fluid\nflow is heated to become vapor-like under supercritical conditions (Saishu et al., 2014,\nScientific Reports). The reduction in the density of water can occur more widely in the crust if\nthe low-permeability barrier is broken by an earthquake (i.e., the ‘valve’ is opened).\nSubsequently, silica precipitation can occur through fluid flow and a reduction in silica\nsolubility (Rusk and Reed, 2002). As such, the overall arrangement of fluid moving along a\nsolubility gradient is generally applicable throughout the seismogenic regime in high-fluid-flux\nregime (e.g., Cox and Muroe, 2016).\nWe added a plot of quartz solubility over a wide range of pressures (Fig. S1a) to show the trend\nof quartz solubility in the low-pressure region. We also emphasize that our experimental\nsystems are similar to a high-temperature shallow hydrothermal system, and show a clear\nexample of how silica precipitation reduces permeability and controls repeated fluid-driven\nfailure (lines 221–226 and 273–278)."}, {"round": 2, "reviewer_comments": {"reviewer_1": "Dear Editor,\nThe authors have successfully addressed all my comments in the revised manuscript. The added sentences and figures\nprovide greater clarity, and I have no further remarks on the paper. I fully support the publication of this work in your journal.\nSincerely", "reviewer_3": "I reviewed an earlier version of this manuscript. As I said in my previous review, the experiments strike me as novel, and the\nresults, showing fluid pressure fluctuations in response to flow and precipitation in the slit are compelling and interesting. I\nfeel that the work is applicable to low pressure shallow hydrothermal systems and the observations are of interest to workers\nin vein microtextures. My most fundamental criticism of the first version was in the application to fault behavior at depths and\ntemperatures where silica solubility increases with increasing temperatures. This was due to a misunderstanding on my part\nthat the authors were claiming retrograde silica solubility across a range of temperatures applicable to all natural fault\nsystems. The text of the manuscript is improved, and the authors have cleared up my misconceptions by including the figure\nin the supplement showing that the experiments are designed for low pressure where retrograde solubility applies. My\nmisunderstanding, but the supplemental figure clears the air. The original manuscript was well written and the modifications\nin the revised version seem appropriate."}, "author_response": "Reviewer #1 (Remarks to the Author):\nThe authors have successfully addressed all my comments in the revised manuscript. The added\nsentences and figures provide greater clarity, and I have no further remarks on the paper. I fully support\nthe publication of this work in your journal.\nAnswer\nWe appreciate your acceptance to our manuscript. Your comments are very useful for improving our\nmanuscript.\nReviewer #3 (Remarks to the Author):\nI reviewed an earlier version of this manuscript. As I said in my previous review, the experiments strike me\nas novel, and the results, showing fluid pressure fluctuations in response to flow and precipitation in the slit\nare compelling and interesting. I feel that the work is applicable to low pressure shallow hydrothermal\nsystems and the observations are of interest to workers in vein microtextures. My most fundamental\ncriticism of the first version was in the application to fault behavior at depths and temperatures where silica\nsolubility increases with increasing temperatures. This was due to a misunderstanding on my part that the\nauthors were claiming retrograde silica solubility across a range of temperatures applicable to all natural\nfault systems. The text of the manuscript is improved, and the authors have cleared up my misconceptions\nby including the figure in the supplement showing that the experiments are designed for low pressure\nwhere retrograde solubility applies. My misunderstanding, but the supplemental figure clears the air. The\noriginal manuscript was well written and the modifications in the revised version seem appropriate.\nAnswer\nI am glad to hear that you understand our explanation on silica solubility, and appreciate your great help\nfor improving our manuscript."}]

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