中国南极球粒陨石的高精度钾同位素研究  

High precision potassium isotopic study of Chinese Antarctic chondrites

作　　者：蒋云 Piers KOEFOED[3] 王昆 徐伟彪 JIANG Yun;Piers KOEFOED;WANG Kun;HSU Weibiao(Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing,J iangsu 210023,China;CAS Center for Excellence in Comparative Planetology,Hefei,Anhui 230000,China;Department of Earth and Planetary Sciences,Washington University in St.Louis,Missouri 63130,USA)

机构地区：[1]中国科学院紫金山天文台,江苏南京210023 [2]中国科学院比较行星学卓越创新中心,安徽合肥230000 [3]美国圣路易斯华盛顿大学,地球和行星科学系,密苏里63130

出　　处：《地质学报》2021年第9期2878-2888,共11页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号41873076);国防科工局民用航天技术预先研究项目(编号D020202和D020302);中国小行星基金会联合资助的成果。

摘　　要：钾和其他中等挥发性元素亏损是类地行星普遍的全岩化学成分特征之一,能用来示踪不同的亏损过程。球粒陨石是组成行星的前体物质,研究球粒陨石中钾同位素的亏损和分异机制,对于太阳系物质或行星的起源、形成和演化具有十分重要的意义。本文利用近年来发展的高精度钾同位素分析技术,测试了14个中国南极陨石以及6个目击型陨石(Murchison、Allende、Ningqiang、Tagish Lake、Xinyang和Banma)的全岩钾同位素组成。结果显示,21个碳质球粒陨石全岩δ41K值分布范围为-0.62‰±0.05‰至0.37‰±0.08‰,平均值为-0.32‰±0.24‰(2SD),比全岩硅酸盐地球(BSE)稍重。18个普通球粒陨石全岩数据(如果异常值GRV 021603除外)的δ41K值分布范围为-1.02‰±0.05‰到-0.61‰±0.02‰,平均δ41K值为-0.81‰±0.15‰(2SD),比全岩硅酸盐地球稍轻。2个目击型陨石Murchison(CM2型)和Allende(CV3型),呈现较大的内部钾同位素差异(分别为0.22‰和0.16‰),可能反映了水蚀变导致的钾同位素在100 mg球粒陨石全岩尺度上的不均一性。小行星母体过程(水蚀变、热变质和冲击变质)不能很好地解释球粒陨石的钾同位素分馏,母体作为一个封闭体系只是平衡均一化同一个化学群内的钾同位素成分。不同化学群球粒陨石之间的钾同位素与钾元素之间没有显示明确的相关性,这一趋势与最近国际上发表的数据在误差范围内基本一致,太阳星云的单阶段热过程如蒸发或冷凝均不能很好地解释。核合成异常是一种可能的解释,大质量恒星的星风或者超新星爆发注入可以增加一些41K到太阳星云中,从而在原始球粒陨石中保存下来,然而这一观点需要更多研究支持。Potassium and other moderately volatile element(MVEs) depletion is a common feature ofplanetary bodiesand can be used to trace different depletion processes. As the precursor materials of asteroids and planets, understanding the MVE patterns of chondrites is also of great significance as without this, the origin, formation and evolution of planetary bodies cannot be fully understood. In this paper, using a recently developed high-precision potassium isotope technique, we report the bulk potassium isotopic compositions of 14 Chinese Antarctic chondrites and 6 observed falls(Murchison, Allende, Ningqiang, Tagish Lake, Xinyang and Banma). The results showa range of δ41K for the 21 carbonaceous chondrites of-0.62‰±0.05‰ to 0.37‰±0.08‰, with an average of-0.32‰±0.24‰(2 SD), which is slightly heavier than that of bulk silicate earth(BSE). The range of δ41K for the 18 ordinary chondrites(excluding one extreme GRV 021603) span from-1.02‰±0.05‰ to-0.61‰±0.02‰, with an average of-0.81‰±0.15‰(2 SD), which is slightly lighter than that of the BSE. Murchison(CM2) and Allende(CV3) have relativelylarge internal variation in δ41K(0.22‰ and 0.16‰, respectively) possibly due to the heterogeneity at the 100 mg-scale within the samples caused by aqueous alteration. Parent-body processes(aqueous alteration, thermalmetamorphism and shock metamorphism) cannot explain the observed variations in K isotopic composition of bulk chondrites becauseas a closed-system, the whole asteroid has been affected by isochemical modification. In this work, we found no clear correlation between Kdepletion and K isotopic composition among chondrites of different chemical groups, which is generally consistent with the data recently published by others. The single-stage thermal processes such as evaporation or condensation in the solar nebula cannot be well explained. Nucleosynthetic origin could be a possible explanation. Massive stellar winds and supernova ejecta would inject abundant 41K to the solar nebula, which wa

关 键 词：南极陨石 钾同位素 球粒陨石 水蚀变 热变质 蒸发/冷凝 星云 

分 类 号：P185.83[天文地球—天文学]