Study On Petrological And Mineralogical Characteristics And Origin Of Nine Ureilites

Ureilites are a kind of achondrites rich in carbon and diamond,which not only have the characteristics of igneous fractionated,but also show the characteristics of primitive chondrites.Therefore,the formation of ureilites is of great significance to explore the evolution history of the solar nebula.The nine meteorites,M 17005,M1812007,M 1812008,M 1812009,M 1812011,M 19011601,M 19011602,M19011603 and M 19011605,selected in this work are ureilites found in the deserts of Northwest Africa.Their petrography,mineralogy,chemical composition and Raman spectra were studied in detail by means of optical microscope,Electron Microprobe,Scanning Electron Microscope and laser Raman.On this basis,their meteorite types and subtypes are classified,and their formation temperature and meteorite genesis are discussed.Through the above work,the following points of understanding are mainly obtained.1.The structures of the 9 meteorites are all coarse-grained balanced recrystallized structures,without chondrite and breccia structures.Their mineral combinations are the same,and they are all composed of olivine,pyroxene,carbon-rich interstitials,metals and troilite,and do not contain plagioclase.Their olivines all have the characteristics of a reduced edge structure and an iron-rich edge at the core of the magnesium-rich composition anti-ring band,and the ratio of Fe/Mg-Fe/Mn of the olivine is linearly related.In addition,their pigeonite Fe-Mg projections fall within the range of primitive achondrites.According to the composition Fa-Fs of olivine and pyroxene,nine meteorites were distinguished by composition type.Among them,M 17005 is a type ? ureilites,M 1812009 is a type ? ureilites,and the remaining sevev meteorites are all type ? ureilites.2.The silicate mineral particles in the nine meteorites appear wavy extinction and parallel plane lamella,and no shock melting veins and shock melting capsules are found.Among the five meteorites M 17005,M 1812007,M 1812008,M 1812011 and M19011601,diamond and graphite are found to coexist.Therefore,the impact metamorphism of these five meteorites is S3,and the impact metamorphism of the remaining four meteorites is S2.Most of the metals and sulfides in the meteorites have been oxidized to limonite.Therefore,the weathering degree of the 9 meteorites is W2.3.Interstitial glass is formed by cooling the residual material after the condensation and crystallization of pyroxene in the interstitial melt.4.The formation temperature of the nine meteorites is about 984-1154 ?,which indicates that the achondrite has undergone high-temperature metamorphism.The symbiosis of diamond and graphite indicates that diamond is transformed from graphite due to impact.The petrology mineralogical characteristics of nine ureilites indicate that their genesis supports the multi-stage partial melting-accumulation model of achondrites.