Fe Isotope Fractionation During Prograde Metamorphism In Subduction Zones And Among Mineral Pairs In Eclogites

This thesis applies Fe isotopes to study the Fe isotopic fractionation among different metamorphic rocks formed during prograde metamorphism in Dabie Orogen.The coexisting clinopyroxene and garnet in eclogites and garnet-clinopyroxene pyroxenite have also been studied to explore the factors that control the Fe isotopic fractionation between these two minerals.Double spike + SSB and Ni doping + SSB methods have been established to calibrate the instrumental mass bias based on the SSB method.Data of reference materials reveal that the long-term external precision for 856Fe is within 0.05‰ and 0.03‰ for four and nine times running.New recommended value for NIST SRM3126a(?56Fe = 0.363±0.006,2se,95%c.i.;?57Fe = 0.534 ± 0.009,2se,95%c.i.)is reported based on the 51 high-precision independent tests in this study.The greenschists,amphibolites and eclogites have been generated from the similar protoliths,the mafic volcanic rocks in Neoproterozoic(750-800Ma)in the Northern Yangise.The H2O+,a good indicator for metamorphic dehydration,decreases with the increasing metamorphic prograde from greenschists,through amphibolites to eclogites.The average Fe isotope compositions(856Fe)of greenschists,amphibolites and eclogites are 0.081 ± 0.075(2SD,n = 13),0.091±0.037(2SD,n = 19),0.071 ±0.060(2SD,n = 27),respectively,in a very limited range and in good consistence with the reported Fe isotope data of igneous rocks,suggesting limited Fe isotopic fractionation during the prograde metamorphism process over the whole rock scale.However,Fe isotopic variations of the same metamorphic grade like greenschists and eclogites have been observed,reflecting the protolith heterogeneity probably caused by partial melting and fractional crystallization.Systematically decrease of Fe3+/?Fe from greenschists,amphibolites to eclogites and a good correlation between Fe3+/?Fe and H suggest that Fe has been reduced with dehydration and the high fO2 fluids might be released simultaneously.The migration of these fluids from the subducted slab to the slab-mantle interface or mantle wedge may therefore provide an oxidized mantle source region for primary arc magmas.Additionally,the Fe isotopes of clinopyroxenes and garnets from the eclogites and garnet-clinopyroxene pyroxenite of Maowu,Shuanghe,Bixiling and Hong'an have been investigated to study the inter-mineral fractionation factors.The Fe isotopic fractionation of clinopyroxenes and galIets(?56FeCpx-Grt)define a range from 0.22 to 0.45,which is consistent with the reported data.No correlation of ?6Fe and ?26Mg excludes diffusion induced fractionation.Combined with the major element and Mg-O isotope data,the ?56FeCpx-Grt in this study probably recorded the equilibrium Fe isotopic fractionation at high temperatures(675-1011K).However,in the?56FeCpx-Grt-106/T2 diagram,the data of all the mineral pairs do not follow the same temperature-dependent fractionation line.Consequently,not only the temperature,but the compositions of the mineral pairs also contributed to the inter-mineral Fe isotopic fractionation.