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Molybdenum(Mo)isotopes have become an important paleo-environmental proxy.However,few studies have focused on the behavior of Mo isotopes during weathering of black shales.A comprehensive set of Mo isotopic composition and concentration data were collected from the weathering profiles of black shales of the Permian Maokou Formation in Enshi Prefecture,China,to investigate Mo isotopic fractionation during weathering processes.Molybdenum isotopes were determined on Nu Plasmas multiple-collector inductively couple plasma mass spectrometer(MC-ICP-MS)using 97Mo-100Mo double spike.The δ98/95MoJMC ratios were present the significant variation,ranging from –3.38‰ to 1.96‰.The total range of δ98/95MoJMC variation was 5.34‰,which showed the geological end member with more negative δ98/95MoJMC ratio could exist in nature.The δ98/95MoJMC in fresh shales(250±237mg/kg Mo,1SD,n=17)from Shadi and Yutangba drill cores varies from 1.11‰ to 1.96‰ with an average of 1.47±0.25‰(1SD,n=17),similar to the range in other Phanerozoic shales,suggesting that Mo isotopes are not strongly fractionated during Mo sequestration in the primary sedimentary environment from anoxic to suboxic condition.However,the strongly weathered black shales(19.7±5.1mg/kg Mo,1SD,n=6)from Shadi profiles are isotopically heavier with average δ98/95MoJMC values of 1.64±0.09‰(1SD,n=6),and the Mo-enriched altered black shales(459±488mg/kg Mo,1SD,n=13)from Yutangba section are isotopically lighter with average δ98/95MoJMC values of-0.70±1.70‰(1SD,n=13).These data suggest that Mo isotopes can be fractionated during weathering processes,with lighter isotopes removed preferentially during oxidative weathering of shales.Such a shift is inconsistent with the previous studies,suggesting that the geochemical behavior of Mo isotopes during weathering of black shales on earth surface would more complicate than we expected.Molybdenum isotope ratios in the different Mo associations showed that the interaction between Mo and organic matter,and the transformation of Mo speciation affected by HS-are probably the important factors constraining Mo isotopic fractionation during weathering processes,especially to former,which has not been considered as a factor for Mo isotopic variation in nature.