Kainsaz(CO3)陨石中两个富Al球粒的氧同位素组成特征与形成演化  

The origins and oxygen isotopes in two Al-rich chondrules from Kainsaz CO3 carbonaceous chondrites

作　　者：戴德求[1] 包海梅 刘爽 尹锋 DAI DeQiu;BAO HaiMei;LIU Shuang;YIN Feng(Institute of Geology,Hunan University of Science and Technology,Xiangtan 411201,China;Institute of Geochemistry,Chinese Academy of Sciences,Guiyang 550081,China;Hunan Provincial Key Laboratory of Shale Gas Resource Utilization,Xiangtan 411201,China)

机构地区：[1]湖南科技大学地质研究所,湘潭411201 [2]中国科学院地球化学研究所,贵阳550081 [3]湖南科技大学页岩气资源利用湖南省重点实验室,湘潭411201

出　　处：《岩石学报》2020年第6期1850-1856,共7页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41673070);湖南省教育厅重点科研项目(18A204)联合资助。

摘　　要：富Al球粒是原始球粒陨石中一种矿物岩石学特征介于富钙铝包体(CAIs)和镁铁质硅酸盐球粒之间的特殊集合体,所以常常认为富Al球粒在认识CAIs和镁铁质硅酸盐球粒形成演化过程中的相互联系具有特殊意义。然而,对富Al球粒的初始物质组成以及形成演化过程一直存在较多争议,而氧同位素组成研究能够对球粒演化和早期星云环境等提供重要的信息。在本文中我们报导了来自Kainsaz(1937年降落于俄罗斯,CO3型)碳质球粒陨石中的2个富Al球粒(编号K1-CH1和K2-CH2)的矿物岩石学和氧同位素组成特征。K1-CH1的矿物组成主要为橄榄石、低钙辉石和富钙长石,K2-CH2为橄榄石和富钙长石。2个球粒中的矿物均具有贫16O同位素组成特征。K1-CH1中矿物的△17O组成基本上位于2个区间:-11.1‰^-8.7‰和-3.9‰~0.4‰;而K2-CH2的△17O介于-6.6‰^-0.6‰之间,且具有从中部至边部升高的趋势。矿物岩石学和氧同位素特征表明,这2个富Al球粒的初始物质组成为富CAIs和镁铁质硅酸盐。在球粒熔融结晶过程中,与贫16O同位素组成(△17O:-8.7‰^-7.8‰)的星云发生了氧同位素交换。球粒形成后,发生迁移进入陨石母体,在相对更贫16O同位素组成(△17O:-0.6‰~0.4‰)的母体中(流体参与)发生变质作用,并再次发生了氧同位素交换。Aluminum-rich chondrules are one of the most interesting components of primitive chondrites,because they have characteristics in petrography and mineralogy that are similar to both Ca-and Al-rich inclusions(CAIs) and ferromagnesian chondrules.However,their precursor and formation history remain poorly constrained,especially with respect to their oxygen isotopic distributions.In this study,we report on the petrography,mineral chemistry,and oxygen isotope ratios of two Al-rich chondrules(K1-CH1 and K2-CH2) in the Kainsaz(CO3) carbonaceous chondrites.The major phases in K1-CH1 are plagioclase,olivine and Ca-poor pyroxene,and those in K2-CH2 are plagioclase and olivine.All minerals in two Al-rich chondrules are 16O-poor.The △17 O values in K1-CH1 vary in two ranges of from-11.1‰ to-8.7‰ and from-3.9‰ to 0.4‰,while those in K2-CH2 vary from-6.6‰ to-0.6‰ with a tendency increasing from the core to rim.These observations provide a strong indication that the two Al-rich chondrules formed by the melting and crystallization of a mixture of materials from CAIs and ferromagnesian chondrules.The precursor materials of K1-CH1 and K2-CH2 are CAIs+ olivine-pyroxene-rich chondrules and CAIs+olivine-rich chondrules,respectively.The 16O-poor isotopic compositions of all components in two Al-rich chondrules can be explained by oxygen isotopic exchange between the melt and16O-poor nebular gas(△17O:-8.7‰^-7.8‰) during melting in chondrule-forming regions;whereas the minerals in two chondrules could have experienced further oxygen isotopic exchange with a relatively 16O-poor reservoir(△17O:-0.6‰-0.4‰) on the parent body,likely during fluid-assisted thermal metamorphism.

关 键 词：矿物岩石学特征 氧同位素组成 富Al球粒 初始物质 氧同位素交换 

分 类 号：P575.1[天文地球—矿物学] P691[天文地球—地质学]