Vanadium Isotope Variations In Highly Differentiated Volcanic Rocks From Hailar Basin

As a multi-valence transition metal element,V is moderately incompatible during mantle partial melting,but moderately compatible in the evolution of crustal magmas.Therefore,V has great potential to trace the magmatic processes,especially the change of redox conditions.With the advent and development of MC-ICP-MS,precise analyses of V isotope compositions become practicable,in nearly two decades,allowing resolution of the small V isotope fractionation at high temperature.However,the current research on the fractionation mechanism of V isotope and the reservoir is mainly focused on the deep magmatic process.As an important geological reservoir,the effects of continental crust and crust-drived magmatism on V isotope fractionation,in comparison to mantle partial melting,are seldom studied.Vanadium isotope composition and its isotope fracitionation during melting of continental crust and subsequent fractional crystallization are crucial complements to those caused by melting of the mantle.To study the mechanism of V isotope fractionation in high-evolved volcanic rocks,a suite well-characterized volcanic rocks?mafic volcanic rocks,I-type felsic rock and A-type rhyolite?from Hailar basin?northeast China?,which have relatively close petrogenetic links and are characterized by broad compositional in a magmatic context,are selected to study the vanadium isotope fractionation and constraint the vanadium isotope composition of volcanic rocks in the continental crust.The ?⁵¹V values?-0.72‰-0.19‰?display a positive relationship with SiO₂ contents?51-77 wt.%?but negative one with V?15-223 ppm?and Fe₂O₃ contents?0.8-10 wt.%?.The influence of fluid exsolution and thermal diffusion are excluded because of the poor correlations of Rb/La vs.?⁵¹V,?⁶⁶Zn vs.?⁵¹V and ?⁵⁶Fe vs.?⁵¹V.The Rhyolite-MELTS results show higher iron content than mafic volcanic rocks,which may be more crystallization differentiation of Fe-bearing minerals,suggesting that the higher ?⁵¹V values of mafic volcanic rocks?from-0.72 ± 0.09‰ to-0.48 ± 0.07‰?relative to MORBs?-0.84 ± 0.10‰,2SD,Wu et al.,2018?are likely resulted from fractional crystallization of minerals,including megneite,incorporating low valence V,which leads to the enrichments of lighter isotopes in minerals and heavier V isotope in residual magmas.With similar SiO₂ contents,the I-type felsic rocks are characterized by lower ?⁵¹V values?from-0.59 ± 0.08‰ to 0.03 ± 0.05‰?than the A-type rhyolites?-0.04 ± 0.03‰ and 0.19 ± 0.02‰?,which are combined results of source differences?basaltic source for I-type vs.I-type-like source for A-type?and fractional crystallization during continental crustal magmatism.Based on the observation of samples and the simulation results,single process could not well explain the variation of V isotope composition of Hailar volcanic rocks,and we speculated that the fractionation of V isotope in these volcanic rocks might be the result of a combination of multiple processes.The variation of V isotope composition of Hailar volcanic rocks is the characteristic of its magma evolution,reflecting intra-continental processes.Therefore,a cartoon is illustrated to simplified the relationships between V isotopes and petrogenesis of these rocks.It is clear that partial melting of different sources and subsequent fractional crystallization are the most prominent factors fractionating V isotopes.Considering the clear and close genetic relationships among these rocks,we may be able to calculate the V isotope compositions of the different layers of continental crust using the weak positive correlation of ?⁵¹V-SiO₂: ?⁵¹V_LCC =-0.42 ± 0.12‰,?⁵¹V_UCC =-0.71 ± 0.12‰.Our result suggests that?1?with the evolution of magma,highly evolved rocks will be enriched in heavier ⁵¹V;?2?Partial melting and crystallization differentiation could produce equilibrium fractionation of V isotope;?3?V isotopic difference in the sources could be translated to the melting products;?4?intra-crustal magmatism can produce highly fractionated V isotope compositions in the continental crustal rocks.We noted,however,that these studies are based on rather limited sample collections from a single location,they may not be universal for continental crustal magmatic rocks.The calculations only serve as a first-order observation at current states,but they might be useful since there are no estimates about the V isotope composition of the continental crust.However,due to the limitation of our understanding about the equilibrium isotope fractionation between melt and mineral during inter-continental crustal magmatism,better explanations of the large V isotopic variations in these rocks require systematical investigations on mineral-melt V isotopic fractionation factors.