Investigation On Structural Stability Of Hydrous Minerals(Gibbsite,Talc And Serpentine) Under High Pressure And Shear Stress

The structural stability of hydrous minerals under the temperature and pressure conditions of the deep earth is related to the opening of various geological activities,the gestation of major geological events,and the evolution of the livable earth environment.The dehydration of hydrous minerals significantly reduces the melting temperature of mantle rocks,enhances the creep and diffusion rate of mantle rocks,changes the temperature and pressure of phase transitions of mantle minerals,and fosters and initiates deep earthquakes and volcanic activities.The dehydration of hydrous minerals produces proton H⁺or hydrated hydroxyl H₃O₂^-,which completely changes the oxidation/reduction state of mantle rocks,enhances its ion mobility and conductivity,promotes the material circulation and energy transmission of the mantle convection,and forms the current relatively stable interior of the earth.Structure and surface livable environment.Plate subduction is the main channel through which surface water enters the depths of the earth.The subduction zone is one of the most active geological parts in the earth and a key place for the water cycle of the earth system.The structural stability of hydrous minerals in the process of plate subduction determines the way,form and quantity of water entering the deep part of the earth,affects the depth and speed of plate subduction,affects the viscosity and melting of mantle materials,and induces island arc volcanoes and deep earthquakes.In the early ocean-ocean subduction,ocean-continent subduction,and land-continent subduction,the temperature of the subducted slab was generally low,and the structural stability of the slab carried mainly depends on its depth(positive pressure)and moving speed(shearing).In order to study the structural stability of hydrated minerals under this low temperature and high shear pressure environment,clarify the stress mechanism of plate dehydration,and deduce the deep water circulation process of the earth,this dissertation uses a rotatable diamond pressure chamber device,T301 stainless steel gasket and Capton Two types of plastic gaskets are assembled.Under normal temperature and high pressure shearing conditions,the hydrous minerals of gibbsite Al(OH)₃,talc Mg₃[Si₄O₁₀](OH)₂ and serpentine Mg₆[Si₄O₁₀](OH)₆ Minerals has carried out in-situ micro-laser Raman spectroscopy and ex-situ micro-area X-ray diffraction tests,and obtained the following innovative experimental results and theoretical understanding:1.In the high-pressure shear experiment with the T301 stainless steel gasket,the gibbsite Al(OH)₃ undergoes a structural phase change at about 2.7 GPa,and no dehydroxylation occurs.The result is the same as that of the quasi-hydrostatic experiment.The sample changed from the initial?-Al(OH)₃ monoclinic phase(SG:P21/n,Z=8)to another monoclinic structure of?-Al(OH)₃(SG:P21/b,Z=8)phase.In the high-pressure shear experiment with the Capton plastic gasket,the gibbsite Al(OH)₃ undergoes complete dehydroxylation,generating water H₂O and hydrated hydroxyl H₃O₂^-.At room temperature,Compressing to 1.5 GPa and rotating to 180°,the 4 hydroxyl stretching vibration peaks(3363,3434,3524 and 3618 cm^-1)in the high-wavelength range of the initial sample disappeared one after another,and 3303and 3560 cm^-12 new peaks with different intensities appeared.The intensity of the Raman spectrum in the low-wavelength range is significantly weakened,and there is no amorphous broad peak;Al-O-Al deformation vibration double peaks(568,539cm-1)and Al-O stretching vibration shoulder peaks(321 and 307 cm^-1),respectively Fusion into one peak;4 hydroxyl deformation vibration peaks(1052,1018,981,and922 cm^-1)are still visible.Continue to pressurize to 3.5 GPa,rotate 360°,and then unload to normal pressure.Only the two new hydroxyl stretching vibration peaks in the high wave number range and the Al-O-Al deformation vibration peak and Al-O stretching vibration in the low wave number range are still visible.Comparing the high-pressure shear experiment result of Raman spectra of T301stainless steel gasket and Capton plastic gasket,in the experiment where Capton plastic is the gasket material,gibbsite is stacked parallel to the c-axis direction(OH)-Al-(OH)coordination octahedral layer framework undergoes dehydroxylation,and new O-Al-O coordination octahedral framework,water H₂O and hydrated hydroxyl H₃O₂^-are generated.The spacing of the octahedral framework layers stacked parallel to the c-axis direction is significantly reduced,and the stretching and deformation Raman vibration peaks of the aluminum-oxygen bond Al-O,the aluminum-oxygen bridge bond Al-O-Al and the hydroxyl OH change.2.In the high-pressure shear experiment with the T301 stainless steel gasket,the talc Mg₃[Si₄O₁₀](OH)₂ undergoes partial dehydroxylation to generate water H₂O and H₃O₂^-.At room temperature,Compressing to the experimental maximum pressure of11 GPa and rotating at 360°,the initial sample's hydroxyl stretching vibration peak at3677 cm^-1 is slightly weakened,and two new peaks with varying intensities appear at the positions of 3303 and 3560 cm^-1.At low wave numbers Raman spectra,There is no obvious change in the Raman spectrum.When the pressure is depressed to normal pressure,the two new hydroxyl stretching vibration peaks in the high wave number range and the magnesium-oxygen bond Mg-O,silicon-oxygen tetrahedral peak SiO₄and silicon-oxygen bridge bond Si-O_b-Si Raman peaks in the low wave number range Coexist at the same time.In the high-pressure shear experiment with the Capton plastic gasket,the talc Mg₃[Si₄O₁₀](OH)₂ undergoes complete dehydroxylation,generating water H₂O and hydrated hydroxyl H₃O₂^-.At room temperature,compressing to 1.0 GPa in the center and rotate 360°.The only visible hydroxyl stretching vibration peak of the initial sample (3677 cm^-1)has disappeared,and two new peaks of varying intensities appear at the positions of 3303 and 3560 cm^-1.The intensity of the Raman spectrum is obviously weakened,and there is no amorphous broad peak.The original Mg-O,silicon-oxygen tetrahedron and Si-O_b-Si peaks still exist.When the pressure is relieved to normal pressure,the Mg-O bond stretching vibration peak is 192 cm^-1,the silicon-oxygen tetrahedral SiO₄ strong peak is 362 cm^-1,and the silicon-oxygen bridge bond Si-O_b-Si stretching vibration peak is still visible.Comparing the high-pressure shear experiment result of two assembly of T301stainless steel gasket and Capton plastic gasket,In the experiment with Capton plastic as the gasket material,talc Mg₃[Si₄O₁₀](OH)₂ is coordinated with Mg O₄(OH)₂ The octahedral skeleton undergoes dehydroxylation,resulting in a new O-Mg-O coordinated octahedral skeleton,water H₂O and hydrated hydroxyl H₃O₂^-.The symmetrical stretching vibration peaks of silicon-oxygen tetrahedron SiO₄ and silicon-oxygen bridge bond Si-O_b-Si are suppressed.3.In the high-pressure shear experiment assembled with the T301 stainless steel gasket,the serpentine Mg₆[Si₄O₁₀](OH)₆ undergoes partial dehydroxylation,resulting in the formation of water H₂O and hydrated hydroxyl H₃O₂^-.At room temperature,Compressing to central pressure of 8 GPa and rotating 360°,the initial sample's hydroxyl stretching vibration peaks 3668 and 3699 cm^-1 showed a slight decrease in intensity,and at the same time two new peaks with different intensities appeared at the positions of 3303 and 3560 cm^-1;There is no obvious change in the low wave number Raman spectrum.When the pressure is relieved to normal pressure,the two new Raman peaks in the high wave number range and the two hydroxyl peaks of the original structure coexist at the same time.The characteristic Raman peaks of SiO₄and Mg O₄(OH)₂ coordination octahedrons still exist.In the high-pressure shear experiment with the Capton plastic gasket,the serpentine Mg₆[Si₄O₁₀](OH)₈ undergoes complete dehydroxylation,generating water H₂O and hydrated hydroxyl H₃O₂^-.At room temperature,compressing to the center pressure of 3.0 GPa and rotating at 360°,the two hydroxyl stretching vibration peaks3668 and 3699 cm^-1 in the high wave number range of the initial sample completely disappeared,and two unequal intensities appeared at the positions of 3303 and 3560cm^-1.New peak;the intensity of the Raman spectrum in the low wave number range is significantly weakened.After the pressure is depressed to normal pressure,the symmetrical stretching peak of 375 cm^-1 of silicon-oxygen tetrahedron SiO₄ and the symmetrical peak of stretching vibration of Si-O_b-Si of silicon-oxygen bridge bond of 683 cm^-1 still exist.Comparing the high-pressure shear experiment result of two assembled T301stainless steel gaskets and Capton plastic gaskets,in the experiment with Capton plastic as the gasket material,the Mg-(O,OH)₆ of serpentine coordination octahedron undergoes dehydroxylation,resulting in a new O-Mg-O coordination octahedral framework,water H₂O and hydrated hydroxyl H₃O₂^-.The symmetrical stretching vibration peaks of silicon-oxygen tetrahedron SiO₄ and silicon-oxygen bridge bond Si-O_b-Si are suppressed.Based on the above comparative experimental results,we found that under the action of large shear differential stress,gibbsite Al(OH)₃,talc Mg₃[Si₄O₁₀](OH)₂ and serpentine Mg₆[Si₄O₁₀](OH)₆ occur Complete dehydration to produce water H₂O and hydrated hydroxyl H₃O₂^-,the original coordinated polyhedral framework structure undergoes compression and slippage between the superimposed layers.The experimental results of this paper provide important experimental evidence for the identification of the dehydration mechanism of the cold slab subduction process,and help us to explore the physical and chemical properties of the subducting slab,the speed and angle of slab subduction,and the evolution of island arc volcanoes.process.