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《Geochimica》 2006-01
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Lead-lead dating of chondrules in ordinary chondrite Semarkona

Zhi XIE1,2,3and Yuri AMELIN1,31.Geological Survey of Canada, Ottawa, ON, Canada; 2.CAS Key Laboratory of Crust-Mantle Materials and Environments, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China3.Department of Geology, University of Toronto, Toronto, ON, Canada;  
Short-lived radionuclides and long-lived isotope systems are normally used to date chondrites; the former systems can provide only relative ages while the later can provide absolute formation ages of chondrules, in contrast. Among the long-lived isotope systems, Pb-Pb system is the only one that can give most precision and creditable age information until now. The ordinary chondrite Semarkona is the most unequilibrium LL3.0 type meteorite, which might be only weakly effected by later thermal metamorphism. Thus, the age of Semarkona is significantly important for the studying of the formation and evaluation of meteorite. In this paper, different leaching procedures were used for the chondrules and chondrule fragments of Semarkona, and the U-Th-Pb compositions of leachates and residues were analyzed by TIMS method. The residues normally give higher 206Pb/204Pb ratio, suggesting higher radiogenic Pb compositions. These residues form a Pb-Pb isochron, and give an age of (4 566.9±5.8) Ma with MSWD of 26, consistent to the weighted average age of 207Pb/206Pb model ages. Though the chondrite might experience later aqueous alteration after its formation, previous researchers work about metamorphic temperatures of chondrite suggested that the thermal metamorphism temperature was not high enough to reset the Pb-Pb system in the chondrules, therefore, the isochron age may represent the forming time of chondrules of Semarkona.
【Fund】: Canadian Space Agency Contract(9F007-010128/001/SR)
【CateGory Index】: P597.3
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