Pressure Calibration For 3GPa Molten Salt Medium Pressure Vessel Under High Pressure And High Temperature

The rock mechanics group of the Tectonophysics Laboratory has designed and constructed a new high-temperature (max temperature of 1200℃) and high-pressure(max confining pressure of 3Gpa) experimental apparatus with triaxial pressure vessel using molten salt as confining pressure medium at the State Key Laboratory of Earthquake Dynamics. I participated in apparatus assembling fortunately when I returned to the Institute. I wrote the system software, joined system testing, as well as pr-researching in confining pressure calibration during the period of my master degree studies. The thesis introduces the new experimental system, software of controlling and data acquisition, as well as the pressure calibration method and preliminary experiments. In addation, I discussed what kinds of the experiments we could do using the new system in the future, and calculated rheology parameter of gabbro using flow lows of anorthite and diposide (or clinopyroxene) obtained from experiential tests according to the method given by Tullis et al (1991), and compared the results with new experimental data given by Zhou et al (2006).The new experimental system has the following main features: high confining pressure, high temperature, molten salt medium, differential stress load, high precision displacement and stress sensors, high-performance controlling and data acquisition software. Compared with other experimental equipments in China , this experimental equipment has a great deal of improvement both in hardware and software: servo-controlled up-lay axial pressure and confining pressure oil jams; high-precision displacement and pressure sensors, as well as digital data collection cards; using molten chloride salts or mixtures with pressure medium; high-intensity pressure vessels and new sample assembly method; using a gassy cushion as the move power of pressure vessel, to replace the track and manual handling methods.Computer software is most important in the high-pressure apparatus, which controls the apparatus's normal operation and gets high-quality experimental data. Because of a number of breakthroughs in program designing, this software could control all-round of the apparatus, and makes sure that the system is more stable and reliable. The core idea of software is determined by professor HE Changrong, and I wrote and tested the program. The major functions and innovation points of the software are:(1) Major functions are: real-time acquisition of experimental data; real-time acquisition of temperature; real-time control to device; real-time show(and plot) and saving experimental data and temperature; controling switch of pump-power; setting basic experimental parameters and the control parameters;(2) Innovation points are: Data transmission between computer and temperature table by serial communication; Controlling pump-power by software; Separation of control-state switch external and internal run; Using double-threaded operation: a thread for control, the other for real-time display and temperature reading.It is very important for solid medium pressure vessel to perform pressure calibration under high pressure and high temperature before apparatus is used, because precise pressure calibration directly determines the preciseness of measurement of experimental pressure. I referred to a lot of literatures, and summarized the methods of pressure calibrations used in previous studies. Pressure calibration includes confining pressure calibration and axial load calibration.Confining pressure calibration consists of two parts: the pressure calibration at room-temperature and at high-temperature. The main pressure calibration methods are as follows : (1) under room-temperature using some salts and metal phase calibration transition; (2) under high-temperature, the best method for calibrating confining pressure is mineral phase transition and Chloride salts partial melting, commonly used mineral phase transitions are: quartz-coesite, albite-jadeite + quartz, fayalite + quartz-ferrosilite, calcite-aragonite and farringtonite - Mg₃ (PO₄)₂-â…¡. The preliminarily decided confining pressure calibration method is: chloride salts melting curve, calcite - aragonite transformation and quartz-coesite.The best way of axial load calibration is to estimate axial friction by multi-cycle of piston-in and piston-out. There are two key points during the test: (1) Ensuring the hit-point of piston and sample: The hit-point is determined by an intersection of two beelines, one is the linear fit to the part of load-displacement curve of piston contacting with soft metal, the other is the linear fit to the part of load-displacement curve of sample's elastic deformation; (2) Determing dynamic friction: The dynamic friction which dependents on displacement is established by the linear fit to the part of load-displacement curve of piston contacting with soft metal. Then, the final axial calibration includes cutting the load-displacement curve before hit-point, and correcting the load-displacement with dynamic friction.Tests were performed for calibrating the axial friction using 2GPa confining pressure vessel. The sample is gabbro. Two experiments have been performed: (1) A number of cycle-experiments with different piston-rates under 500MPa and 820℃, 1 000MPa and 900℃, 1 000MPa and 25℃; (2)Under 500MPa and 1 000℃, firstly cycle-experiments were conducted, and then piston rate is reduced from 2×10^-4mm / s to 5×10^-5mm / s after rock sample's plastic deformation, and the rate dependence of creep is observed.The result of the experiment shows that the factors which affect the dynamic friction are confining pressure, temperature and piston rate. Confining pressure is the main factor, dynamic friction increases with the increase of confining pressure. Temperature and piston rate only influence intercept. Hence, axial calibration should be conducted under specific experimental conditions.Based on the above features, I think the new experimental system is suited to perform following studies: rheological behavior and deformation mechanism of lower crustal rocks under real lower crust conditions; partial melt under dynamic and static state of lower crust and upper mantle; and mineral phase transition. The lower crust rheology is an interesting topic, and should be the major research field in the future. So, I calculated rheology parameter of gabbro using flow lows of anorthite and diposide (or clinopyroxene) obtained from experiential tests according to the method given by Tullis et al (1991), and compared the results with new experimental data given by Zhou et al (2006). The results show that experimental data of dry samples is in agreement with results of calculation, however, the calculated lower crustal strength in wet condition is more stronger than the results based on experimental data of wet samples.