Vanadium Isotope Compositions Of Subduction Zone Magmatic Rocks And Glacial Diamictites

Vanadium is a redox-sensitive element who has two stable isotopes that are 50V and 51V.With the development of analytical and testing technology,researches of V isotopes have been carried out in recent years.Theoretical calculation and experimental studies have shown that V isotopes can be fractionated in both high and low temperature processes.Therefore,V isotopes show the potential of studying a number of fundamental Earth science problems,including planet formation,core-mantle segregation,crust-mantle evolution,magmatism,and magmatic ore deposit formation.The prerequisite of conducting V isotope studies is to recognize V isotope fractionation mechanisms and determine the V isotope compositions of important reservoirs.There is still lack of V isotope compositions of important reservoirs,and fractionation scale and behaviors of V isotopes have not been well understood.In this thesis,we introduce the geochemical properties of V element,and review the current state of the study of V isotope geochemistry.We have carried out two systematic studies on the basis of current state.First,the V and Fe isotope compositions of igneous rocks that have genetic relationships and evolutionary sequences from 3 typical subduction zones in the world were analyzed,which is a complement of the V isotope compositions of subduction zone magmatic rocks.Primitive arc basalts from different sources have a homogenous V isotope composition with a mean value of-0.82 ± 0.10‰(2SD,n= 16),which is consistent with that of MORBs.Vanadium isotope compositions of samples from different localitities of the same arc are homogenous as well.These indicate that differences of depletion,fluid processes,addition of sediments and subducted oceanic crust,and melting degree of the sources do not affect V isotope compositions with current precision.V isotopes are fractionated mainly by fractional crystallization.A fractionation factor of-0.15‰ between minerals and melts was obtained by Rayleigh fractionation,which is slightly larger than that obtained from mid-ocean ridge lavas.While the V isotopic variations with a range of 0.38‰ of subduction zone rocks is a little bit larger than that of mid-ocean ridge lavas,it is still much smaller than that reported in a previous study.Iron isotopic variations are smaller than V isotopic variations and Fe isotope compositions do not correlate with magmatic differentionation indicators.A straight line was obtained by fitting V isotope compositions of subduction lavas and SiO2 contents,suggesting that V isotope composition could be used as an indicator of magmatic differentiation.The second study was measuring the V isotope compositions of ancient glacial diamictites to constrain the composition and evolution of upper continental crust.These glacial diamictites are from all over the world and cover all the four glacial events on the Earth before Cenozoic.The four range of ages are Mesoarchean,Paleoproterozoic,Neoproterozoic and Paleozoic.Glacial diamictites are good representatives of upper continental crust,but its SiO2 contents had been alterd by weathering and alteration processes.Based on previous studies,we suggest V could be a robust tool of possessing primitive information of sanples.The V isotope compositions f these glacial diamictites increase with age decreasing.Calculated SiO2 contents rise from 48?49 wt.%in Mesoarchean(3.0?2.9Ga)to?54 wt.%in Paleoproterozoic and then to 57?64 wt.%in Neoproterozoic,and keep stable afterwards,indicating that the composition of upper continental crust evolved from mafic to felsic.Based'on this we can infer that the global-scale plate tectonics should initiate after 3 billion years.