地球内核各向异性结构成因:矿物学模型和动力学机制  

The origin of anisotropic structure of the Earth’s inner core from mineralogical model to dynamic mechanism

作　　者：何宇 孙士川 徐云帆 李和平[1] HE Yu;SUN Shi-chuan;XU Yun-fan;LI He-ping(CAS Key Laboratory of High-Temperature and High-Pressure Study of the Earth’s Interior,Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China)

机构地区：[1]中国科学院地球化学研究所,中国科学院地球内部物质高温高压重点实验室,贵阳550081

出　　处：《矿物岩石地球化学通报》2024年第2期282-291,I0002,共11页Bulletin of Mineralogy, Petrology and Geochemistry

基　　金：国家自然科学基金资助项目(42074104)。

摘　　要：地震学观测表明,地球内核具有复杂的不均一性和各向异性结构,认知内核结构的关键在于研究内核各向异性结构的矿物学组成和动力学机制。本文将介绍内核各向异性结构特征,并探讨其矿物模型和动力学机制。在内核温压下,六方相(hcp)和体心立方(bcc)相铁合金都表现出各向异性,若其快轴沿自转轴产生定向排布,则可以解释内核南北方向快、赤道方向慢的各向异性特征。轻元素的加入将显著改变铁合金的各向异性,特别是超离子态铁-氢合金的快轴方向随氢含量的增加而发生倒转。内核各向异性的动力学机制可分为3种:(1)凝固时晶体的定向生长。凝固时的组构在内核温度下会很快消失,因此该机制不是导致内核各向异性结构的主要原因;(2)外应力驱动。外应力驱动的定向排列需要考虑内核物质的流变机制,目前的模型主要基于刚性内核假设,要求内核主要发生位错蠕变且黏度大于1018Pa·s;(3)扩散内应力驱动。若内核处于超离子态,地磁场驱动轻元素定向扩散所产生的内应力可驱动晶格的定向排布。上述机制的驱动力(化学对流、热对流、地磁场)均来自外核,因此了解外核动力学与内核动力学的相互作用是认知内核组成和结构的关键。Seismological observations suggested that Earth’s inner core presents complex heterogeneity and anisotropic structure.The key to understand the structure of Earth’s inner core is to study the mineralogical composition and dynamic mechanism of the anisotropic structure of Earth’s inner core.Hexagonal close-packed(hcp)and body centered cubic(bcc)Fe alloys both have seismically anisotropic features under temperature and pressure conditions of the Earth’s inner core.When the fast axis can be oriented along the Earth’s rotation axis,the anisotropic characteristics of the Earth’s inner core,which is fast in the north-south direction and slow in the equatorial direction,can be explained.The input of light elements into Fe alloys significantly changed the anisotropy of Fe alloys.Particularly,the fast axis orientation of superionic Fe-H alloys had changed inversively with the increase of H contents in those alloys.Under There are 3 kinds of dynamic mechanisms of the Earth’s inner core.(1)The lattice preferred orientation(LPO)during the solidification period.The LPO texture and structure formed in the solidification process cannot be maintained at the temperature of Earth’s inner core for a long time,therefore the LPO is not the main reason resulted in the anisotropic structure of the Earth’s inner core;(2)The LPO due to the external stress:The directional arrangement driven by external stress needs to consider the rheological mechanism of the material in the Earth’s inner core,while the current model is mainly established based on the assumption of a rigid inner core of the Earth.It is required that the dislocation creep mainly occurred in the Earth’s inner core with the viscosity of its material exceeding to 1018 Pa·s.(3)The LPO due to the diffusion induced stress:If the Earth’s inner core was under the superionic condition,the directional diffusion of light elements driven by the geomagnetic field could result in the presence of the lattice internal stress which would then result in the LPO.The

关 键 词：地球内核 各向异性 高温高压 铁合金 定向排布 

分 类 号：O521.2[理学—高压高温物理] P57[理学—物理]