三块普通球粒陨石岩石矿物学特征及其复合球粒研究

The petrological and mineralogical characteristics of three ordinary chondrites and studies of their compound chondrules

普通球粒陨石数量约占球粒陨石的80%,对其开展研究有助于揭示早期太阳系的演化。本文使用扫描电镜和电子探针对三块普通球粒陨石的岩石学、矿物学和地球化学特征进行了分析,重点讨论了复合球粒的成因。研究表明,三块陨石均具典型的球粒陨石结构,其中GRV 050191与GRV 051993分别为LL3型和H3型陨石,球粒结构清晰且丰富;NWA 15005为H5型陨石,球粒可分辨,基质出现重结晶。GRV 050191与GRV 051993中矿物成分变化较大,橄榄石有成分环带,NWA 15005中矿物成分均一。根据陨石岩石矿物学特征,GRV 050191、GRV 051993和NWA 15005冲击变质程度分别为S2、S1和S3,风化程度为W1、W1和W2。复合球粒是由两个或多个球粒熔融形成的,本次研究发现的3个复合球粒,其中2个属于包封型复合球粒(CPC-1和CPC-3),1个为伴生型复合球粒(CPC-2)。SiO_(2)-FeO-MgO相图数据表明,复合球粒形成温度窗口约为100℃。根据复合球粒中橄榄石的FeO含量变化推测越早形成的球粒,其FeO含量越低,橄榄石化学成分变化趋势表明复合球粒可能经历了类似岩浆结晶的过程。

About 80%of chondrites are ordinary chondrites.The study of ordinary chondrites is helpful to reveal the evolution of the early solar system.In this paper,the petrological,mineralogical and geochemical characteristic of three ordinary chondrites have been analyzed by using SEM and EPMA,in order to emphatically discuss the genesis of their compound chondrules.The results show that three ordinary chondrites have typical structures of chondrites.Especially,the GRV050191 and GRV051993 chondrites,with clear chondrule structure and lots of chondrules,belong to the LL3 meteorite and H3 meteorite,respectively.The NWA15005 belongs to the H5 meteorite,with distinguishable chondrules and recrystallized matrix.The GRV050191 and GRV051993 have relatively varied mineral compositions,with compositionally zoned olivine.The NWA15005 has uniform mineral composition.Based on the mineralogical characteristics of meteorites,the impact metamorphism degrees of GRV050191,GRV051993 and NWA15005 are S2,S1 and S3,respectively.The weathering degrees of them are W1,W1 and W2,respectively.Compound chondrules were formed by the melting of two or many chondrules.According to the basic structure classification method,the CPC-1 and CPC-3 are encapsulated compound chondrules,while the CPC-2 is associated compound chondrule.Our data of SiO_(2)-FeO-MgO phase diagram indicate that the temperature window range for the formation of compound chondrules is about 100℃.The change of FeO contents of olivines in compound chondrules shows that the earlier the chondrule was formed,the lower the FeO content was.The changing trend of olivine compositions in the compound chondrules indicates that the chondrules could have undergone a process similar to the magma crystallization.