Molecular Dynamics Study Of Diffusion Of Oxygen Isotope In Silica Melt

The diffusivity of isotope in mineral melt of mantle can affect the processes of isotope exchange and fractionation, constrain the time and spatial scale of microelement and isotope transportation between mineral melt and the surroundings. It also has an important effect on the chemical and isotopic information of the mineral that can reflect the accurate thermal evolution and growth history which kept in mineral. Except for the using of experimental simulation in getting the diffusivity of ions, theoretical calculation also gradually plays a significance role in this field. As one of the most important components of silicate melts, SiO2plays a very important role in the earth mantle structure and the dynamics process. We study the Oxygen isotope diffusion behaviors in silica melt, and calculate the changing process of Si and O ions’self-diffusion coefficient under the temperatures of3000K over a wide range of pressure by using the molecular dynamics (MD) simulation which contains4500and108000ions in the silica melt with MS-Q force field. Calculations show that the number of particles in the system is sensitive to the simulation results. In the pressure scope from0.0001to40GPa, both the self-diffusion coefficients of oxygen and silicon increase from the beginning, reach a maximum value, and then decrease with the increasing pressure. The diffusion coefficient of18O reaches the maximum value at the pressure of15GPa, while the corresponding pressure for16O and Si is15to17.5GPa, which shows that the diffusion rate of oxygen is slightly higher than silicon. Under the temperature of3000K, the diffusion coefficient of18O is greater than I6O only at the pressure of15GPa, this phenominon is also valid when the temperature is2723K, the diffusion coefficient of18O is greater than16O only at the pressure of3GPa. The defect-controlled transport mechanism and the five-fold coordination structure are very crucial for the diffusion of oxygen isotope, and the later one is the main reason for the rise of the diffusion coefficients with increasing pressure and the reason for the abnormal surpass of the diffusion coefficient of18O over that of16O. The reaching of the maximum value of the diffusion coefficient means a change in the formation mechanism of five-fold coordination of silicon in the SiCh melt. This paper also calculates the structure information of SiO2melt which coincides well with the result of experiment.