南极GRV 090196普通球粒陨石熔壳特征及其成因探讨

FEATURES AND FORMATION OF FUSION CRUSTS ON AN ORDINARY CHONDRITE(GRV 090196)FROM ANTARCTICA

熔壳是陨石在穿过大气层时因摩擦发热熔融所形成的表层皮壳。由于进入速度差异和大气层的结构变化,陨石可能产生不同的熔壳结构特征和类型,因此,熔壳的研究对探索陨石穿过大气层过程和反映当时的大气层结构具有一定意义。因为南极特殊的地理气候条件,很多南极陨石样品保存了原始或完整的熔壳结构,因此,南极陨石是研究熔壳的理想对象。GRV 090196是H5型普通球粒陨石,具有完整的熔壳结构,而且存在定向飞行的特征,前后存在结构不同和厚薄不一的熔壳,即四层结构的Ⅰ号熔壳(1.3 mm厚)和二层结构的Ⅱ号熔壳(0.4 mm厚),是一块研究熔壳形成过程的理想样品。本文对GRV 090196的熔壳开展了系统的岩石学和矿物学研究,对熔壳成因进行了讨论。Ⅰ号熔壳由外向里分为四个不同结构层。第一和第二层主要由辉石质玻璃质组成,可见橄榄石斑晶,说明发生了完全熔融和重结晶。这两层熔壳橄榄石斑晶的形态与成分有明显的差别,表明它们为二次熔融体的冷凝形成。第三层熔壳发生部分熔融,大部分矿物颗粒发生圆化,其中含有少量气泡。第四层熔壳产生热变质,单偏光下不透明,反光下发黑,这可能是高温还原的暗化现象,该层岩石结构特征与陨石内部相似。Ⅱ号熔壳由外向里分为三层,第一层硅酸盐颗粒发生过部分熔融且可见大量金属颗粒,第二、三层特征与Ⅰ号熔壳三、四层基本相同。通过对比研究,我们认为熔壳可以由陨石熔融物堆积作用产生并且可以利用其判断陨石降落的方向性。Ⅰ号熔壳存在两层玻璃质层且橄榄石斑晶发生了重结晶,判断在陨石降落过程中,陨石头部因摩擦发热而产生的熔融物质受到气流驱动而在陨石定向飞行的尾部产生堆积,二次熔体的形成可能反映了该陨石运行时穿过两层密度相对大的大气结构层,而Ⅱ号熔壳位于陨石的前侧部,熔化物质仅有少量保留,形成了很薄的玻璃质熔壳。

Meteorite fusion crusts are formed during atmospheric entry and show different textural characteristics based on differences in entry velocity and structural changes in the atmosphere.Therefore,fusion crust study is important to understand atmospheric entry processes and atmospheric structure at that time.Many Antarctic meteorites have retained their original or complete fusion crust because of the region’s unique geographical and climatic conditions,so they are ideal samples for fusion crust research.An ordinary H5 chondrite(GRV 090196)from the Grove Mountains region(Antarctica)was investigated in this study.It has a complete molten fusion crust with some flow lines showing its flying direction.Thin sections showed two intact fusion crusts with thicknesses of 1.3 mm and 0.4 mm.Petrology and mineralogy showed that the first fusion crust(FC-Ⅰ)could be divided into four structural layers(outside to inside).Layers 1 and 2 were melted completely and recrystallized to form porphyritic structures in a glassy matrix,but the morphology and chemical composition of olivine phenocrysts in these layers were very different.This indicates it experienced different atmospheric layers of various densities.In layer 3,partial melting was obvious and mineral residues were observed;crystals were rounded with some bubbles.Thermal metamorphism occurred in layer 4,appearing opaque and black under plane polarized light,with a darkening phenomenon that may be from fine iron precipitation.Its texture was similar to those inside the meteorite.The second fusion crust(FC-Ⅱ)was divided into three layers(outside to inside).Silicates in layer 1 were partially melted and rich in metallic grains.Layers 2 and 3 were similar to layers 3 and 4 in fusion crust Ⅰ.We suggest that fusion crusts were caused by accumulation and can be used to judge the landing direction of a meteorite.The two glassy layers and olivine phenocrysts in fusion crust Ⅰ as well as the recrystallized olivine phenocrysts indicate that fusion crust melt was moved along the meteorite body by air.The fusion crusts experienced two relatively denser atmosphere layers that formed two glassy layers,and fusion crust Ⅱ was located in the side of the front.Only a small amount of the molten material remained and formed a very thin glassy fusion crust.