Computational Simulation On Transport Properties Of Minerals And Case Studies

The mass and energy transportations in Earth’s interior are fundamental processes for understanding the dynamic evolution of the earth. Therefore, the research involves the transport properties of the minerals has always been a major field in earth science. The mineral properties of transportation usually include the thermal conductivity, diffusion ratio and electrical conductivity. The elemental diffusion is one of the controlling factors of the mass transfer while the heat conduction is the main process of the energy transportation in the solid earth. These two phenomena not only follow the same mathematical governing equation, they also closely couples with each other during the earth evolution. For instance, the thermal conductivity of mineral is affected by the constitutive elements of the mineral, and meanwhile the temperature determines the ratios at which the elements diffuse, thus they both are the primary factors that affect the elemental distribution of the Earth’s interior, the thermal structure of basins and the process of hydrocarbon accumulation.The traditional experimental studies on measuring the elemental diffusion and the thermal conductivity of mineral are suffering from several aspects, one of which is the difficulty to reproduce the high pressure and high temperature environments of the deep earth. In addition, the preparing of the mineral samples are also affected by many factors, e.g. the impurities, defects, cracks and inclusions of the minerals have significant effects on the thermal conductivities and elemental diffusions of the minerals. With the improvements of the relevant theories, algorithm and as well as the greatly enhanced CPU power, the computation simulation technique based on statistical thermodynamics and computational chemistry has become a powerful tool in the earth sciences. Especially for high pressure and high temperature researches, the atomic simulation has its own advantage and can provides valuable information that beyond the capability of the current experiment techniques. In our research, we have conducted preliminary studies on the heat and mass transfer of the minerals by using molecular dynamic simulation and first principles calculation. We also performed numerical simulation to investigate the evolution of a special geothermal system with accurate parameters of the thermal conductivities of the minerals. This thesis contains the following contents:(1) The helium diffusion in olivine was acquired by using first principles calculation based on density functional theory. The results are in excellent agreement with previous experiments, and we further extended the pressure to cover the whole upper mantle condition. Based on our calculation, we estimated the closure temperature for helium in olivine and the possible size of the 3He heterogeneity to survive from the diffusion in the upper mantle. The closure temperature of He in olivine ranges from 143℃ to 244℃ in most earth-surface geological systems based on our calculation, suggesting that olivine has the potential to be a medium or low temperature thermo-chronometry. Taking both the pressure and temperature effects on the helium diffusion behavior in olivine, helium can diffuse dozens to hundreds of meters in 100 Ma at the conditions of the upper mantle.(2) As a key geochemical tracer for the mantle evolution, the diffusion of the helium are vital parameters for modelling the helium evolution in the mantle. We calculated the helium diffusion in MgO-periclase, MgSiO3-perovskite and MgSiO3-post-perovskite under the pressures from 25 to 140 GPa, to provide primary data on helium diffusion in the lower mantle. And based on our results, the diffusion ratio of He at the lowermost mantle can reach up to ～10-8 m2/s. We estimated the evolution of 3He reservoirs with different sizes in the lowermost mantle, and the result indicate that the reservoirs have to be larger than 50 km to persist the high content of 3He in 4.5 Ga. Thus in our model, the possibility of the ultra-low velocity zones (ULVZs) to be the reservoirs has been precluded as the thicknesses of the ULVZs are normally less than 40 km.(3) We investigated the temperature dependent thermal conductivity of the quartz by using molecular dynamic simulation. We found the anomalous temperature dependence of the thermal conductivity after the phase transition from αâ†'β quartz at high temperatures. In order to explain the discrepancy between the thermal conductivity of the ideal lattice and the experimental samples, we also evaluated the effect of the oxygen vacancies. The temperature dependence and anisotropy of the bulk thermal conductivity of quartz is obvious. With the temperature rising from 300 K to 800 K, the thermal conductivities for c-axis and a-axis decease quickly from 13.8 W/mK,11.1 W/mK at 300 K to 6.0 W/mK,4.9 W/mK at 800 K, respectively. After the transition temperature from a-quartz to β-quartz (850 K), the usually 1/T dependence of the thermal conductivity disappears which might due to the slight volume reduction of quartz with heating at high temperature. We also investigated the effects of O vacancies on thermal conductivity, and found that even as low as 0.1% vacancy concentration can reduce thermal conductivity by nearly 30% at 300 K.(4) Based on complex heat conduction model, we have investigated the thermal history and hydrocarbon generation of the source rocks intruded by two doleritic sills in the Luo-151 area, Bohai Bay Basin. The completely cooling of a single sill with thickness about 100 m intruded at the depth of 1700 m, can continue for 0.1 My. In our modeling, the peak temperatures wall rocks experienced is about 500℃ at 10 m,250℃ at 50 m,170℃ at 100 m respectively from the contacts. The simulated amount of hydrocarbon generated during the cooling of both sills is well consistent to the proven reserves, which support an atypical mechanism combining rapid hydrocarbon generation at shallow burial depth, a self-generation and self-accumulation system of intrusive and wall rocks, and the extreme high efficiency of hydrocarbon accumulation in sill fractures and metamorphic pores in hornfels.