Discrete element modeling of the micro-cracking in zircon
Zircon is rich in radioactive U and Th isotopes,and its U-Pb,U-Th-He and fission-track analyses are most commonly used in geo-thermo-chronometers.However,these ages are all dependent upon the degree of radiation damage in the host zircon crystal.The decay of U and Th in zircon leads to self-irradiation damage and changes in the physical and chemical properties of the crystal.At a macro-scale,the metamictization results in density reduction and volume expansion.Differential expansion between different growth zones with variable U and Th concentrations and self-irradiation damage generates relative expansion stresses and micro-cracks in zircon.This study aims to understand the process and mechanism of the formation of micro-cracks.On the basis of the empirical relationship between self-irradiation damage and volume expansion,we employed the discrete element modeling approach to simulate the distribution of internal stresses during the accumulation of self-irradiation damage in zircon with core-rim structures.By considering the elastic modulus and tensile strength of zircon,modeling experiments were performed to explore the effects of self-irradiation damage and confining pressure on the development of micro-cracks in two types of zircon grains,in which U and Th are concentrated in the core(Model Ⅰ)and rim(Model Ⅱ),respectively.The modeling results reveal a distinct stage-wise progression of micro-cracks in response to the increases of self-irradiation damage and volume expansion.In Model Ⅰ,radial fractures in the rim developed from a single line into symmetrical geometries displaying a hollow"x"and"*"shape.In Model Ⅱ,the multiple concentric fractures in the core developed in heavily damaged grains were formed by the expansion and connection of radial fractures developed in the early stage with relatively lower self-irradiation damage.The influence of confining pressure is dependent on the level of self-irradiation damage.Under a low self-irradiation damage condition,the increase of confining pressure inhibits the development and expansion of the fractures in both models Ⅰ and Ⅱ.However,under a high self-irradiation damage condition,increasing confining pressure leads to an increase in the density of radial fractures in the Model Ⅰ,but a decrease of concentric fractures in the Model Ⅱ.The development of micro-cracks in zircons splits the crystal into several sub-grains,changing the paths and distances of atomic diffusion out of the grains.Therefore,our study on the propagation of micro-cracking holds valuable implications for understanding the element diffusion in zircon and their relevant geological applications.
ZirconMicro-crackingSelf-irradiation damageMetamictizationDiscrete element modeling
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维普
