离子探针U-Pb体系研究识别阿波罗11号月壤及新发现  

Identification and new discovery to the Apollo 11 lunar soil using SIMS U-Pb analyses

作　　者：李秋立[1,2] 刘宇 李金华[3] 陈意[1,2] 李献华[1,2] LI QiuLi;LIU Yu;LI JinHua;CHEN Yi;LI XianHua(State Key Laboratory of Lithospheric Evolution,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;College of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing 100049,China;Key Laboratory of Earth and Planetary Physics,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China)

机构地区：[1]中国科学院地质与地球物理研究所,岩石圈演化国家重点实验室,北京100029 [2]中国科学院大学地球与行星科学学院,北京100049 [3]中国科学院地质与地球物理研究所地球与行星物理院重点实验室,北京100029

出　　处：《岩石学报》2022年第6期1804-1810,共7页Acta Petrologica Sinica

基　　金：中国科学院地质与地球物理研究所重点部署项目(IGGCAS-202101)资助.

摘　　要：本文报道对中国科学院地质与地球物理研究所博物馆珍藏的一件阿波罗月壤样品(E21)进行的玄武岩岩屑定年工作。受限于岩屑大小,不能采用前人对阿波罗样品中大岩屑构建常规Pb-Pb等时线来定年的方法,我们采取了以单个含锆矿物(斜锆石、钙钛锆石和静海石)颗粒来限定玄武岩岩屑年龄的技术方案。首先通过μ-XRF的X射线的穿透性来遴选出具有含锆矿物的岩屑,再制作树脂样品靶后精细抛光,在扫描电镜下利用Zr元素的x-射线面分布图来定位暴露到表面的含锆矿物,再以离子探针3μm小束斑加Pb信号多接收的分析技术来获得Pb同位素组成以计算年龄。共识别出4个年龄组分的岩屑,分别为3570±14Ma、3700±8Ma、3850±5Ma和3900±12Ma。其中前三个年龄与前人报道的阿波罗11号月壤中玄武岩年龄分布一致,是判定该样品为阿波罗11号月壤样品的重要证据。~3900Ma岩屑为新发现组分,据长石成分An=93,辉石Mg^(#)~79判别为苏长岩。此外,根据主矿物Pb同位素分析,绝大多数玄武岩岩屑落在~3570Ma的Pb-Pb等时线上,指示该期次玄武岩为阿波罗11号采样区域最主要火山活动事件,这对于撞击坑统计定年曲线上的年龄选择给出了依据。This paper reports the dating of basalt clasts from an Apollo lunar soil sample(E21)collected in the Geological Museum,the Institute of Geology and Geophysics,Chinese Academy of Sciences.Due to the small size of the clasts,the previous method of constructing conventional Pb-Pb isochron with a single rock chip to determine the age of Apollo samples cannot be used.We have adopted the technical method of defining the age of basalt clasts with a single Zr-bearing mineral(baddeleyite,zirconolite and tranquillityite).Firstly,the rock clasts with Zr-bearing minerals are selected according to the X-ray penetration ofμ-XRF.Secondly,the resin sample target is made for fine polishing.The Zr-bearing minerals exposed to the surface are located by means of scanning electron microscope Zr signal mapping.Lastly,the Pb isotopic compositions of Zr-bearing minerals were obtained by the analysis technology of ion probe~3μm beam spot and a multi-collection mode for Pb signal to calculate the age.Four age groups of rocks were identified,namely 3570±14Ma,3700±8Ma,3850±5Ma and 3900±12Ma.The first three ages are consistent with the age distribution of basalt in Apollo 11 lunar soil,so it can be determined that this sample is from Apollo 11 lunar soil.The last one is a newly discovered component.According to the composition of feldspar(An=93)and pyroxene(Mg^(#)~79),it is identified as magnesian suite origin norite.In addition,according to the Pb isotope analysis of the main minerals,the vast majority of basalt class fall on the Pb-Pb isochron of~3570Ma,indicating that this period of basalt is the main volcanic activity event in the Apollo 11 sampling area,which provides a basis for the age selection on the dating curve of the crater-counting chronology.

关 键 词：月壤 阿波罗11号 离子探针 U-Pb体系 镁质岩套 

分 类 号：P588.145[天文地球—岩石学] P597.3[天文地球—地质学] P691