Determination Of Metallographic Cooling Rate Of Meteorites And Study Of Thermal History Of Their Parent Bodies

In this dissertation, using optical microscope and SEM, FE-SEM, EPMA, the metallographic microstructures have been studied and the metallographic cooling rates have been determined for four magmatic irons {Xinjiang(IIIE), Jianshi(IIIAB), Yingde(IVA) and Hoba(IVB)}, two impact melt irons {Mundrabilla(IAB) and Nandan(IIICD)}, two mesosiderites{Dong Ujimqin and Weiyuan}, and two ordinary chondrites{Jilin(H5) and heavenly shocked Yanzhuang(H6-7)}. Through both metallogrphic microstructrue techniques and metallographic cooling rate methods, the author has made a comprehensive research on the low temperature thermal histories of Fe-Ni metallic phases in the host meteorites. The thermal evolutionary models of different meteorite parent bodies have also been constructed in this study.The metallographic cooling rates of Chinese meteorites were .determined according to the taenite lamellar width method using the latest diffusion coefficients and Fe-Ni-P phase diagram: Xinjiang iron cooled at 30K/Ma in the 31km radius parent body, Jianshi iron cooled at 60K/Ma in the 23km radius parent body. The cooling rates ranging very little among HIE group and IIIAB group infered a linar cooling history in HIE and IIIAB parent bodies. Yingde iron cooling at 3400K/Ma had the most rapid cooling rate among melt-differentiated irons, and the cooling rates of IV A group ranged largely, the non-linar cooling history can be proposed in the IV A parent body. Nandan iron cooled at 39K/Ma in the 28km radius parent body. The cooling rates of Dong Ujimqin mesosiderite were 0.02~0.05℃/Ma and certainly less than 0.5癈/Ma as other mesosiderites, this also indicated that mesosiderites cooled slower than any iron meteorites and had an parent asteroid >210km in radius. Jilin chondrite cooled at lOK/Ma in the >52km radius parent asteroid. The shock-induced melt about 1 ~35mm width in Yanzhuang chondrite rapidly solidified at 0.8~2935癈/sec.Based on the observed microstructures and Ni concentration profiles of taenite in Xinjiangjianshi and Nandan irons, kamacite nucleation multi-stages are commonly found in iron parent bodies. There was a kamacite nucleation due to shock event in a period ~6Ma in Jianshi iron parent body core after initial avalanche of nucleations. Martensite and plessite microstructures formed due to nonequilibium of y-FeNi in a shock event. Formations of spheroidized plessite,pearlitic plessite and duplex plessite in Xinjiang,Jianshi and Nandan irons were related to the thermodynamic mechanism of plessite lamelar coarsening at low temperature. Finger plessite,cellular plessite and net plessite in Yangde iron suggest that FVA parent body was suffered a heavy impact and broke into fragmentations after crystallization of the Fe-Ni core. Taenite is characterized by a zoned structure, consisting of CT and CZ. The CZ shows a typical "island-honeycomb" microstructure in Dong Ujimqin mesosiderite or occurs as "CT2+Ms" in Jilin chondrite, the zoned structure of taenites formed through some complicated phase transformations during a slow cooling at low temperature (<400C)vin the parent bodies.According to metalldgraphic microstructures and cooling rates, the thermal evolution models are proposed for different meteorite parent bodies. When the initial chondritic parent asteriod suffered a thermal metamorphism and repeated some degree shock metamorphism, it formed the chondrites (Jilin and Yanzhuang chondrites); When the initial chondritic parent asteriod was shocked on the surface, it formed the impact melt iron(IIICD,Nandan); When the initial chondritic parent asteriod was heated by solar and radio isotope(26Al), it would differentiate into Fe-Ni core and silicate mantle, the magmatic irons(IIIE and IIIAB) formed from Fe-Ni core fragments causing by a shock after the core had solidified. But if the differentiated parent asteriod experienced a "impact-breakup-reassembly" evolutionary stage, the reassembly parent body would form IVA group,IVB group and mesosiderite after suffered another high velocity impact.