碳质球粒陨石细粒边(FGR):太阳星云演化的见证

Fine-Grained Rim(FGR) of Carbonaceous Chondrites:Witness to the Evolution of the Solar Nebula

细粒边(FGR)是碳质球粒陨石的特殊组分,主要产于粗粒组分(球粒、难熔包体、岩屑和晶屑)的周围,对探索太阳星云条件演化具有重要意义。介绍了不同类型碳质球粒陨石FGR边的岩石结构、矿物组成、矿物化学、全岩成分等主要特征。与碳质球粒陨石基质的对比表明,FGR边与基质在性质上非常相近,它与基质一样属于早期星云凝聚产物,但是产状和成分特征差异表明,它的形成早于基质,代表了不同时间次序。根据蚀变矿物和全岩成分特征分析发现,所有类型碳质球粒陨石的FGR边都不同程度遭受了含水蚀变,大部分证据表明FGR边的含水蚀变发生在星云演化阶段,而另一部分证据则支持FGR边的含水蚀变发生在母体变质阶段,但是,FGR边是否存在多次含水蚀变过程还有待进一步研究。FGR边是早期太阳星云气-固凝聚吸积作用形成,但是不同类型碳质球粒陨石FGR边的特征差异说明早期太阳星云的成分不均一性和不同星云位置物理化学条件的差异。

Fine-grained rims are the special components in carbonaceous chondrites, mainly from the coarsegrained components （ehondrule, CAI, lithic fragment and mineral fragment）, providing insight for the evolution of the solar nebula. The study summarizes the petrological texture, mieral assemblages, mineral chemistry and bulk composition of FGR from different carbonaceous ehondrites. By comparison with matrices of different carbo- naceous chondrites, FGR is found similar to the matrix in petrology and mineralogy, suggesting that FGR is formed from the condensation of the solar nebula as the matrix. But the occurrence and difference of mineral chemistry of FGR indicate that as the FGR formation predates matrax, FGR is the product of different sequence of condensation of the solar nebula. Based on the altered mineralogy and bulk composition, all FGRs experience aqueous alteration in verious degrees. The majority of evidence indicates that the aqueous alteration of FGR should occur during the period of solar nebula, but another evidence supports that the aqueous alteration of FGR might be formed in the parentbody of the chondrites. However, whether multistages of aqueous alteration exist is not certain and it needs further work and more evidence. To sum up, all the features of FGR indicate that FGR is the gas-solid condensation of the early solar nebula aggregated to the coarse-grained components, and the differences among the FGRs of various carbonaceous chondrites suggest the chemical heterogeneity of the solar nebula and the difference of physicochemical condition of the solar nebula in different locations.