| Quantitative study of the diffusion coefficients of elements in minerals and rocks with temperature,pressure,composition and other factors has extremely important significance for us to correctly understand and deeply understand the various dynamic processes of the earth's interior and the evolution of the earth.The subduction of rare gas plays an important role in the change of the original isotope ratio of the mantle during geological periods.The study of the diffusion characteristics and sealing temperature of helium in the surface and deep earth minerals is of great significance for understanding the information of earthquake precursors and the abnormal genesis of helium in the process of fluid incubation during the earthquake,as well as for understanding the process and law of mineralization.In this paper,based on the firstprinciples calculation method of density functional theory,the diffusion coefficient of helium in aragonite under high pressure,the anisotropy of diffusion and the activation parameter(activation energy)related to diffusion are studied with temperature,pressure and crystal structure.The law of change of other factors is calculated,the diffusion path of helium in aragonite is calculated;and the important application of the calculated diffusion data in geoscience is discussed.The main research result of this paper is to obtain the diffusion data of helium in aragonite at a high pressure of 20-50 GPa.The activation energy for diffusion of helium in aragonite along the [100] crystal direction has increased from 176.02 k J/mol at 20 GPa to 278.75 k J/mol at 40 GPa,while the activation energy for diffusion along the[001] crystal direction has increased from 20 GPa.195.89 k J/mol increased to 290.43 k J/mol at 40 GPa.In the pressure range of 20-40 GPa,the diffusion activation energy increases by 100 k J/mol.The results show that the diffusion of helium in the [001] and[100] directions in aragonite is anisotropic.At 45 GPa and 50 GPa,the activation energy of diffusion of helium is abnormally high.[100] The activation energy of diffusion of helium at 45 GPa and 50 GPa of crystal orientation is 1583.76 k J/mol and1829.42 k J/mol,respectively;[001] crystal The activation energies for the diffusion of helium at 45 GPa and 50 GPa are respectively 1,645.23 k J/mol and 1945.88 k J/mol,which are 6 to 7 times higher than the activation energy of helium at 40 GPa.Regarding the influence of pressure on the diffusion of helium in aragonite,studies have found that the activation energy of diffusion of helium increases significantly with the increase of pressure,and the diffusion of helium in aragonite maintains anisotropy.This article analyzes the reason for the abnormal increase in helium diffusion activation energy in the 45 GPa and 50 GPa following stones.The reasons for the abnormal increase in helium diffusion activation energy may be the following two situations:aragonite phase changes into post-aragonite;or helium atoms form bonds in aragonite and become new Of polymers.Based on previous studies,this paper analyzes the relationship between mineral crystal structure and helium diffusion behavior.The occurrence state of elements in minerals is not only related to the temperature and pressure,but also closely related to the radius of mineral crystal particles.Therefore,in the research process,not only the temperature and pressure conditions should be paid attention to,but also the size of the mineral crystal particles should be considered.Finally,the preservation ability of other rare gas elements is compared and speculated.Quantitative analysis and systematic comparison of the occurrence capacity of rare gases in different minerals will greatly help determine the degree of influence of mineral types on geological processes in this study,and promote the development of rare gas isotope chemistry. |
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