首页|Zircon Zr isotope fractionation during crustal anatexis
Zircon Zr isotope fractionation during crustal anatexis
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Zirconium is one of high field strength elements but its isotope behavior during geochemical processes is still uncertain because of the limited database.While Zr isotopes in magmatic rocks are often used to trace the evolution of magmas through fractional crystallization,it is intriguing how highly heterogeneous Zr isotopes were produced by the growth of zircon during crustal anatexis.We address this issue by in-situ zircon Zr isotope analyses of migmatites from two high-temperature metamorphic terranes in the South Lhasa zone and the North Dabie zone,respectively,in China.The results show highly variableδ94Zr values from-0.30‰ to+0.81‰ and from-0.58‰ to+0.49‰,respectively.In addition to the relict zircon of magmatic origin,two types of newly-grown zircons were identified in terms of their occurrences,trace elements and 894Zr values.The peritectic zircon,mainly occurring in the in-situ leucosomes,exhibits the highest Nb-Ta-Hf-U contents and variably higher 894Zr values than those of the relict zircon.The anatectic zircon,mainly occurring in the leucocratic veins,shows higher Nb-Ta-Hf-U contents than and similar δ94Zr values to those of the relict zircon.Model calculations demonstrate that the variable Zr isotope compositions of newly-grown zircons would result from decoupled release of Zr from zircon and non-zircon phases.The Zr supply of the peritectic zircon is mainly derived from the decomposition of Zr-bearing minerals in the in-situ anatectic melt(the non-zircon effect),whereas the Zr supply of the anatectic zircon is mainly from the dissolution of pre-existing zircons in the evolved melt(the zircon effect).The significant Zr isotope variations in the migmatites well illustrate the generation,migration and accumulation of the anatectic melts during the partial melting.Therefore,Zr isotopes can be used as a powerful means for distinguishing between the peritectic and anatectic zircons during crustal anatexis.
Erlin ZHU、Qiongxia XIA、Zhaoya LI、Renxu CHEN、James VAN ORMAN
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School of Earth and Space Sciences,University of Science and Technology of China,Hefei 230026,China
State Key Laboratory of Lithospheric and Environmental Coevolution,Hefei 230026,China
Frontiers Science Center for Planetary Exploration and Emerging Technologies,University of Science and Technology of China,Hefei 230026,China
Department of Earth,Environmental and Planetary Sciences,Case Western Reserve University,Cleveland OH 44106,USA
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B-type Strategic Priority Program of the Chinese Academy of SciencesNational Natural Science Foundation of ChinaNational Natural Science Foundation of China
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