1. 北京科技大学冶金与生态工程学院 北京 100083;2. 中国科学院比较行星学卓越创新中心 紫金山天文台 南京 210023
1. College of Metallurgy and Ecological Engineering, Beijing University of Science and Technology, Beijing 100083;2. CAS Center for Excellence in Comparative Planetology, Purple Mountain Observatory, Nanjing 210023;
铁陨石记录了陨石母体所经历的熔融、分异和冷却的热历史, 研究铁陨石内部的组织结构对理解陨石母体的热历史和内部圈层结构有重要的指导意义. 分析阿勒泰铁陨石个体-乌希里克(Wuxilike)铁陨石中铁纹石和镍纹石所构造的维斯台登(Widmanstatten)纹、梳状合纹石以及云状区等组织结构来探究其各自的形成过程. 通过热动力学计算软件和数据库, 建立了Widmanstatten经过取向校正的冷却速率计算模型, 并据此计算了该陨石在695℃–400℃区间内的冷却速率; 通过研究梳状合纹石内部铁、镍纹石中Ni元素的成分分布及其位相关系, 推理得到梳状合纹石的低温马氏体分解形成机制; 通过研究云状区域中颗粒大小和局部Ni含量的关系, 得出形成云状区域所对应的铁陨石在350℃下的冷却速率. 据此模拟计算出该铁陨石在695℃–200℃范围内形成Widmanstatten纹、各类合纹石和云状区的整个热历史. 基于固态相变所建立的定量模型可望为分析铁陨石的冷却历史提供更为准确的分析手段.
In 2011, several massive pieces of iron meteorites, among which Wuxilike weighs 5 tons, were found in Aletai city, Xinjiang Province. It was immediately recognized that they are siblings of a single meteorite shower (Aletai), which broke apart when entering into the earth's atmosphere. Iron meteorites record the history of melting, differentiation, disintegration and cooling process in its parent body. Thus, the study on its internal structure is very crucial to understand both the thermal and magnetic history of its parent body. \newline \indent Several types of structures in Wuxilike, including the coarse kamacite/taenite Widmanstatten structure, the comb/spheroidal plessite and the cloudy zone, were examined. In particular, the Ni gradient across taenite lamella was corrected by considering its spatial orientation. Moreover, the kinetics modeling on the taenite-to-kamacite phase transformation was established to simulate the Ni gradients developed near the phase interface during the cooling under different rates. By the combination of measured relation of Ni content at the middle of kamacite lamella vs. its half thickness and such a relation calculated by the modeling, the cooling rate of Wuxilike between 695 and 400℃ is found to be in the range of 32–75℃/Myr with an average value of 49℃/Myr.In the comb plessite, the kamacite lamellas all have the same orientation and maintain the K-S orientation with the neighboring Ni-rich shell of taenite lamellas, in which relatively fine taenite particles were randomly present. This suggests that the taenite lamella in the comb plessite should first transform to martensite at MS temperature during cooling except for its outer shell having higher Ni content, and then martensite is decomposed to particle-like taenite and taenite at lower temperature. The cloudy zone is formed by spinodal decomposition of taenite below 350?C during the cooling rate of 7℃/Myr, which is estimated according to the measured sizes and Ni contents of taenite islands in the cloudy zone. In summary, we determined the thermal history of Wuxilike that is responsible for forming various structures in this iron meteorite, including the coarse Widmanstatten structure formed during the cooling from 695?C to 400℃ at 49℃/Myr, the spheroidal plessite by the decomposition of martensite below MS temperature, the cloudy zone developed during the cooling from 350?C to 200℃ at 7℃/Myr. We believe that our proposed modeling in the basis of solid phase transformation can be effectively employed on accurately predicting the cooling history and rate for all the kinds of iron meteorites.
周扬扬,徐伟彪,胡斌,罗海文.乌希里克铁陨石的组织结构、冷却历史和定量预测模型[J].天文学报,2021,62(6):60. ZHOU Yang-yang, XU Wei-biao, HU Bin, LUO Hai-wen. Microstructure, Cooling History and Its Quantitative Modelling of Wuxilike Iron Meteorite[J]. Acta Astronomica Sinica,2021,62(6):60.复制