An Experimental Study On The Frictional Properties Of Antigorite Under High Fluid Pressure Conditions

In order to investigate the frictional behavior of antigorite in hydrothermal conditions and its significance to the slow slip phenomenon in subduction zone,we conducted frictional sliding experiments on antigorite gouge under hydrothermal conditions with high pore-water pressure.To avoid dehydration at high temperatures,we did not conduct experiments at temperatures higher than 500℃,and set the temperature in a range of 100-500℃ instead.Three series of experiments were conducted respectively with high effective normal stress,moderate effective normal stress,and low effective normal stress.We also employed identical experimental conditions as in Takahashi et al.（2011）for comparison,where the pore fluid pressure was set at 30 MPa and the confining pressure at 100 MPa.Experiments were also conducted to examine the influence of the existence of lizardite in another serpentine sample（20%lizardite+80%antigorite）at high pore fluid pressure（P_f=100 MPa）and low effective normal stress(σ_eff=30 MPa).Throughout all the tests,the axial loading rate was varied between 0.04μm/s,0.2μm/s,and 1.0μm/s in order to acquire data on the responses to velocity change.The low effective normal stress series were conducted at 30 MPa of effective normal stress and 100 MPa of pore fluid pressure.The moderate effective normal stress series were conducted with50 MPa effective normal stress and 100 MPa pore-fluid pressure.The high effective normal stress series were conducted at 100 MPa effective normal stress and 100 MPa pore-fluid pressure.A supplementary experiment was conducted to investigate the influence of further lower rate,with the axial loading rate varying between 0.008μm/s,0.04μm/s,0.2μm/s and 1.0μm/s.We also conducted a supplementary experiment at an effective normal stress of 15 MPa and a temperature of 300℃ to investigate the frictional properties of antigorite under a lower effective normal stress.All of our experiments are carried out on a gas medium high temperature and high pressure triaxial testing system.Our data analysis was based on the rate-and state-dependent frictional law,where the sign of the velocity dependence（a-b）determines the stability of frictional sliding.When the sign of（a-b）is positive（a-b>0）,the frictional behavior is velocity strengthening and stable,whereas when（a-b<0）,the frictional sliding is velocity weakening and unstable slip nucleation may occur.To investigate the stability of frictional sliding,it is essential to examine the healing effect b to observe if b>a for the occurrence of velocity weakening.We determined the constitutive parameters a,b,dc for each rate step by numerically fitting the experimental mechanical data（dc is the characteristic sliding distance）.After performing shearing experiments,we cut sample to make thin sections and observed the microstructure with a SEM.The results of the antigorite experiments are as follows:1.The friction coefficient of antigorite decreases systematically with increasing temperature in the applied temperature range（100℃ to 500℃）for all three series of effective normal stresses（30 MPa,50 MPa,100 MPa）,respectively from 0.81 to 0.41,0.77 to 0.4,and 0.72 to 0.46.Under the effective normal stress at 30 MPa,the antigorite-lizardite mixture gouge also exhibits the similar decreasing trend in friction with increasing temperature,from 0.75 to 0.37.Under high effective normal stress and low pore fluid pressure conditions（Cp=100 MPa,P_f=30 MPa）,the friction coefficient of antigorite decreases with temperature from 0.659 to 0.543 over the range of 100-400℃,but the friction coefficient sharply increases to 0.701 at 500℃.2.Under three levels of effective normal stress（30 MPa,50 MPa,100 MPa）,the velocity dependence value（a-b）exhibits a"V"-shaped trend as a function of temperature,which decreases first and then turns to an increasing trend.When the temperature is below or above 300-350℃,antigorite shows velocity-strengthening behavior,while velocity-weakening behavior occurs only under the conditions of 30MPa and 50 MPa effective normal stress and at a loading rate of 0.04μm/s（manifested as slow slip events）.This is different from the previous findings that antigorite exhibits velocity strengthening behavior at temperatures below 400℃ under lower pore pressure conditions.Numerical fitting shows that the a value under three levels of effective normal stress exhibits an initial decreasing trend and then increases with temperature.The b value under 30 MPa and 50 MPa effective normal stress conditions also exhibits a trend of initial decreasing and then turns to an increasing trend as temperature increases,while the b value under 100 MPa effective normal stress condition shows a systematic decreasing trend as temperature increases.As for the characteristic slipping distance dc value,it varies in a range of 1-46μm under 30 MPa effective normal stress and in 1-26μm under 50 MPa and 100 MPa effective normal stress conditions.3.Microstructural observations were carried out on sheared samples of antigorite.Under three levels of effective normal stress and at 100℃,the antigorite gouge samples show a pervasive shear deformation with significant particle size reduction in localities.No preferred orientation was observed in the shear direction,on the whole indicating a brittle cataclasis process during the shear deformation.As the temperature increased to300℃,local shear zones of R shear and boundary shear developed in the fault gouge.The particle size reduction is very significant in the localized shear zones,showing cataclasis-dominated shear deformation process,along with significant amount of precipitates within the shear zones.At temperatures of 400℃ to 500℃,the antigorite sample exhibited pervasive shear deformation on the whole.Zoomed microstructural observations show cataclastic features particle size reduction,but the degree of particle size reduction is much lower than that at 100℃,with a higher proportion of large particles（>10μm）.Signatures of pressure solution process were also found（manifested as blunted particle boundaries and precipitates filling voids between particles）,indicating that pressure solution widely occurred during the shearing process at temperatures above 300℃.4.The transition between velocity weakening and velocity strengthening,as revealed in this study,is different from a brittle-ductile transition.The mechanism here seems to be due to the competition between two mechanisms,namely,cataclasis and pressure solution.When the velocity is slow enough,enough duration is guaranteed for the operation of pressure solution,while slow deformation limits the operation of cataclasis.However,when the velocity increases to a higher level,there is not enough duration for pressure solution to dominate,but the displacement advancement leads to the cataclastic flow.5.For the three experimental series（30 MPa,50 MPa,100 MPa）,the friction coefficient of antigorite exhibited significant displacement/strain weakening for temperatures above 300℃.Through the analysis of mechanical data and observations of microstructure,we speculate that the combination of cataclasis that results in grain-size reduction and the creep of contact junctions by pressure solution may be the micro-mechanism causing the strain-weakening phenomenon.6.Our friction experiments on antigorite under low effective normal stress conditions reveal a mechanism of slow slip different from the previously reported velocity-weakening mechanism.Under the condition of high-pressure fluid,antigorite itself will produce slow slip without transitioning to olivine through dehydration.Based on the effect of effective normal stress revealed in this study,extremely low effective normal stress may facilitate velocity weakening of antigorite in a broader T range including higher temperatures,thus the temperature limit for occurrence of unstable slow slips associated with serpentinite in subduction zones may be higher than 300℃.