A Study Of Fusion Crust In Antarctic Ordinary Chondrites

The fusion crust was formed during atmospheric entry.Because of the difference of meteorite entering velocity and the structural change of atmosphere,it may cause different structure characteristics and types of fusion crust.Therefore,the study of fusion crust is of great significance for looking into the process of atmospheric entry and reflecting the atmospheric structure at that time.Due to the special geographical and climatic conditions in Antarctica,many Antarctic meteorites have retained the original or complete fusion crust.Therefore,Antarctic meteorites are ideal samples for fusion crust research.In this study,13 Antarctic ordinary chondrites were selected as research objects,and the mineralogical characteristics and fusion crust characteristics of their main bodies were studied.The meteorites and their fusion crusts were classified.Also for GRV 090196 ordinary chondrite with special fusion crust has been studied in detail,and the causes of the fusion crust have been discussed.(1)Among the 13 ordinary chondrites,there are 4 pieces of H4 type,2 pieces of H5 type,1 piece of H6 type,and 6 pieces of L5 type.According to the impact degree: 6 pieces S1,4 pieces S2,2 pieces S3,1 piece S4 is divided.According to the degree of weathering: 3 pieces W1,8 pieces W2,2 pieces W3,the degree of weathering is low,which shows that the Antarctic is not only a meteorite enrichment area but also a natural preservation reservoir of meteorites.(2)Antarctic ordinary chondrite fusion crust is mainly divided into two different layered structures,each of which can be divided into three layers from the outside to the inside,and they mainly show the difference in the outermost layer(the first layer).The first layer is a phenocryst glass layer or a molten metal layer,which is formed by recrystallization after the material is completely cooled.The second layer is a semi-molten layer and the third layer is a high-temperature darkening layer.(3)A systematic petrology and mineralogy study was carried out on the fusion crust of GRV 090196.The thin section retained a two-stage structure and a fusion crust of different thickness(1.3 mm,0.4 mm,respectively).Fusion crust I was divided into four structural layer from outside to inside.Layer 1 to 2 have melted completely and recrystallized to form porphyritic structure in glassy matrix.The morphology and chemical composition of olivine phenocryst in layer 1 to 2 are very different.Partial melting occurred in the third layer,at the same time,the refractory residues were observed and mineral crystals were rounded some bubbles appear.Thermal metamorphism occured in the fourth layer,it is opaque and black under single polarized light,and there is a darkening phenomenon that may be formed by fine iron precipitation.its structure is similar to those inside the meteorite.The fusion crust II wasdivided into three layers from inside to outside.Silicate in the first layer were partial melted and rich in metallic grains.layer 2 and 3 is almost the same to layer 3 and 4 infusion crust I.According to comparative study,firstly proposed that the fusion crust can not only caused by accumulation but also judge the direction of the meteorite landing.There are two layers of glassy fusion crust and olivine phenocryst exist in fusion crust I and olivine phenocryst recrystallized,so that we can judge the fusion crust melt move to the tail of the meteorite body by the wind.The fusion crust has experienced two atmosphere of different density to form two glassy layer,and fusion crust II locate in the side of the front.Only a small amount of the molten material remains,forming a very thin glassy fusion crust.