锆石微破裂过程的离散元模拟  

Discrete element modeling of the micro-cracking in zircon

作　　者：袁志煌 田云涛 高燕 YUAN ZhiHuang;TIAN YunTao;GAO Yan(Guangdong Provincial Key Laboratory of Geodynamics and Geohazards,School of Earth Sciences and Engineering,Sun Yat-sen University,Zhuhai 519080,China;Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai),Zhuhai 519082,China)

机构地区：[1]中山大学地球科学与工程学院,广东省地球动力作用与地质灾害重点实验室,珠海519080 [2]南方海洋科学与工程广东省实验室(珠海),珠海519082

出　　处：《岩石学报》2024年第8期2574-2584,共11页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(42172229、42488201);南方海洋科学与工程广东省实验室(珠海)(SML2023SP239)联合资助。

摘　　要：锆石中富含放射性元素U和Th,其U-Pb、U-Th-He与裂变径迹分析方法是最常用的地质热年代计,但这些年龄均与锆石的辐射损伤程度有关。锆石中放射性元素U和Th衰变所导致的晶体自辐射损伤,会改变晶体的物理和化学性质,在微观上表现为蜕晶化,宏观上则表现为密度降低和体积膨胀。不同生长环带间的差异膨胀产生相对膨胀力,导致锆石发生微破裂。为了探究微破裂的发育过程与机制,本文基于锆石自辐射损伤-体积膨胀经验关系,利用离散元模拟方法,模拟核-边双层锆石自辐射损伤过程中内部应力分布;结合锆石的弹性模量与抗拉强度,模拟自辐射损伤与围压对核部高U、Th(模型Ⅰ)与边部高U、Th(模型Ⅱ)两类锆石微破裂发育的影响。结果显示随着自辐射损伤及体积膨胀的增大,锆石微破裂过程具有显著的阶段性:模型Ⅰ边部破裂对称发育,从单条逐渐发育成空心“十”字与“米”字形破裂网络;模型Ⅱ核部发育的多个同心状破裂是由早期放射状破裂随自辐射损伤增强而拓展并逐渐相连而形成。围压的影响效果与自辐射损伤程度有关:低自辐射损伤下,围压增加会抑制模型Ⅰ边部与模型Ⅱ核部破裂的发育与拓展;高自辐射损伤下,随着围压的增大,模型Ⅰ放射状破裂密度增大,而模型Ⅱ同心状破裂数目减少。锆石中微破裂的发育将晶体分割成若干亚颗粒,改变原子扩散出颗粒的路径与距离,因此本文对锆石微破裂过程的研究,对于理解锆石中元素的扩散及相关的地质应用具有参考意义。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

关 键 词：锆石 微破裂 自辐射损伤 蜕晶化 离散元模拟 

分 类 号：P578.941[天文地球—矿物学] P597.3[天文地球—地质学]