An Experimental Study On The Process Of Pressure Solution Of Plagioclase Under Hydrostatic Pressure And Implication To Frictional Healing Effect

In some parts of the continental lower crust and deep subduction zones,there are earthquakes or weak unstable slips.Therefore,their origin may be related to the mechanical properties of mafic rocks in the brittle deformation regime.As natural faults produce detrital materials during the sliding processes,and the frictional sliding behavior of the detrital material determines the nature of fault movement,thus to understand the mechanism of lower-crust earthquake and slow slips,it is firstly required to study the frictional properties of mafic rocks and their maijor rock-forming minerals.With this background,previous studies have performed a series of experimental researches on gabbro and the major constituents.According to the results of previous experiments,friction sliding experiments of plagioclase under hydrothermal conditions(100 ~ 600°C)show a property of velocity weakening,and the experimental results show that both the direct rate effect parameter(a value)and the healing effect parameter(b value)increase with temperature,a typical characteristic for thermally-activated processes.Velocity weakening corresponds to a shear band that has a stronger friction healing effect than the direct rate effect in the rate and state friction constitutive framework,and the healing effect(b value)in constitutive relation mainly reflects the increase in contact area with time under hydrothermal conditions,with some minor effect of structural changes.Since the microphysical mechanism of feldspar minerals at the contacts is mainly brittle cataclasis for temperatures below 600°C,the significant frictional healing effect in this case can only be explained by the mechanism of pressure solution.In order to determine if the dissolution process of plagioclase actually occurs on the laboratory time scale,we conducted hydrostatic experiments on plagioclase powder samples under hydrothermal conditions whereby frequent contact switch between particles in frictional sliding experiments can be avoided,making the observation on the dissolution sites possible.Two sets of experiments were designed in this study;the first set of experimental conditions confining pressures were 150 MPa,120 MPa,and 90 MPa,the pore fluid pressure was 30 MPa,the temperature was 500°C,and the fault gouge thickness was 2 mm.The purpose of this series was designed to explore the effect of effective pressure on the pressure solution of plagioclase.The second set of experimental conditions were set to at confining pressure 120 MPa,pore fluid pressure 30 MPa,and temperatures of 400°C,500°C,600 °C,focusing on the effect of temperature changes on the pressure solution process.The results of the experiments are as follows:(1)Drainage rate of pore water in plagioclase gouge was high in the first few hours of experiment,but gradually decreases over time for both temperature and pressure series of experiments slowing down to a steady state.This feature indicates that there is a creep process that evolves inside the plagioclase gouge.In the temperature-series experiments,the drainage rate of the pore water in the plagioclase gouge at 400°C is relatively low than the cases for higher temperatures,and it was found that the same rate at 600°C is higher than the case at 500°C.Thus,the applied temperature is positively correlated with the creep of plagioclase gouge.In the pressure-series of experiments,the drainage rate of pore water increases with an increase in effective pressure.(2)Scanning electron microscopy(SEM)observations of the experimentally deformed samples were performed on thin sections cut along the sample axis.Firstly,from the images of microstructure,it was found that the degree of particle fracture became more significant at a higher effective pressure,with smaller pore volume between particles.In the temperature-series experiments it is found tha the degree of compaction of plagioclase gouge increases with increasing temperature.Precipitation of plagioclase grains in layered structures was generally observed in highmagnification images,indicating the presence of pressure solution processes.Contact points were also found to be in a state of ambiguity that seems to be a fused morphology,but the details of the structure remain to be determined by further observations.(3)We analyzed the mechanical data of our experiments through based on theoretical equations of pressure solution and obtained parameters in the exponential creep equation.At 500°C with various effective confinging pressure,analysis was tried to determine the relation between initial strain rate and effective confining pressure to estimate the true effective pressure at the contacts thus determines the stress concentration factor.The above results indicate that the pressure solution process of plagioclase particles can occur on a typical laboratory time scale,and the results of this study provide robust experimental evidences for the theory that links between pressure solution and the mechanism of frictional healing and unstable slips for plagioclase.