Study On Vanadium Isotope Geochemistry During Basaltic Magmatism

As an important medium for energy transfer and material exchange between mantle and crust,basaltic magmatism significantly influences the evolution of Earth’s spheres.The chemical compositions,spatial and temporal distribution characteristics of basalts are natural probes for understanding the Earth’s geodynamics.With the development of analytical technology,metal stable isotopes have been increasingly applied to investigate the physicochemical properties of mantle such as oxygen fugacity and mantle heterogenetiy caused by subducted materials.As a redox-sensitive element,the valence state and concentration of vanadium(Ⅴ)in basalt are influenced by oxygen fugacity,but whether V isotopes can be used as a oxygen fugacity sensor remains controversial.A prerequisite for the application of V isotopes as a oxygen fugacity sensor is to understand the fractionation mechanism and controlling factors of V isotopes during basaltic magmatism.This dissertation explores the geochemical behavior of V isotopes in basaltic magmatism through V isotope studies of volcanic clasts in the South China Sea(SCS)and Cenozoic alkali basalts in eastern China,combined with Fe isotopes and Fe3+/∑Fe ratios.The first study analyzed the V isotopic compositions of SCS volcanic clasts from the International Ocean Discovery Program(IODP)Expedition 349(Site U1431)and investigated the effects of partial melting,melt-lithospheric mantle reaction and magnetite fractional crystallization on V isotope fractionation.The volcanic clasts were formed by multiple volcanic episodes from 12.8 Ma to 7.4 Ma.Based on the eruption time and chemical compositions,the samples can be divided into two groups:earlystage volcanic clasts(>8.3 Ma)and late-stage volcanic clasts(<8.3 Ma).The earlystage volcanic clasts are chemically similar to experimentally observed carbonated melts,indicating that they were generated by partial melting of carbonated eclogite.Their δ51V values have limited variations ranging from-0.76‰ to-0.67‰,which is slightly heavier than the V isotopic compositions of bulk silicate Earth(BSE,-0.91‰±0.09‰)and mid-ocean ridge basalt(MORB,-0.84‰±0.02‰).The slightly heavier V isotopic compositions of early-stage volcanic clasts record V isotope fractionation during the partial melting of carbonated eclogite,negligibly influenced by seawater alteration and carbonate incorporation.Late-stage volcanic clasts have the chemical compositions of alkali ocean island basalt(OIB),representing the product of carbonated melts reacting with the overlying lithospheric mantle during ascent.Late-stage volcanic clasts show δ51 V values ranging from-0.62‰ to 0.29‰,which are significantly higher than those of early-stage volcanic clasts.Carbonated melts evolved into alkali basalts during reaction with the lithospheric mantle by dissolving orthopyroxene and precipitating olivine and clinopyroxene.Vanadium is highly incompatible in olivine,while clinopyroxene is more enriched in V than orthopyroxene.Both clinopyroxene and orthopyroxene are enriched in heavy V isotope than silicate melt,the net effect of consuming orthopyroxene and precipitating clinopyroxene is likely to result in a slightly elevated δ51V value in the melt.The significantly elevated δ51V values of latestage volcanic clasts with increasing SiO2 contents cannot be attributed to the involvement of lithospheric mantle.Rather,it is controlled by the fractional crystallization of magnetite during the differentiation of alkali basalts.Due to the preference for V3+and V4+in 6-fold coordination,magnetite favor light V isotope than silicate melt.With the proceeding of magnetite fractional crystallization,the V isotopic compositions of late-stage volcanic breccias became heavier and heavier.Rayleigh fractionation simulations show that the average mineral-melt fractionation factor(Δ51Vmineral-melt)during magnetite fractional crystallization is about-0.35‰.The second sudy analyzed the V isotopic compositions of Cenozoic alkali basalts in eastern China and investigated the effects of oxygen fugacity and melting degree on V isotope fractionation during mantle partial melting.The alkali basalts in eastern China were derived from oxidized carbonated mantle by a low degree of partial melting.The results show that their δ51V values(-0.85‰ to-0.61‰)are higher than those of BSE and MORB.Chemical alteration,crustal contamination and fractional crystallization negligibly affect the δ51V values of alkali basalts.Although the enrichment of light Mg and heavy Zn isotopes indicate the contribution of sedimentary carbonates in their mantle source regions,mass balance calculations show that the incorporation of carbonates with low V content(<5 ppm)is not sufficient to significantly increase the δ51V values of mantle source regions.In contrast,the observed high δ51V values probably reflect that mantle melting controls the V isotopic compositions of mantle-derived melts.The relative abundances of V5+,V4+and V3+in silicate melt are influenced by the variation of oxygen fugacity and the degree of partial melting.Basaltic melts tend to be enriched in V with high valence at the condition of high oxygen fugacity during mantle melting,which contributes to the enrichment of 51V in alkali basalts because of the affinity of high valence V and 51V.Alkali basalts are also enriched in Fe3+/∑Fe ratios(0.21 to 0.64)and heavy Fe isotopes(δ56Fe=0.11‰to 0.27‰).The synergistic variation of δ51V and δ56Fe values of alkali basalts further indicates the effect of oxygen fugacity on V isotope fractionation during mantle melting.Because of the increasing incompatibility from V3+,V4+to V5+for mantle minerals(olivine,spinel,garnet,clinopyroxene,orthopyroxene),the valence states of V in silicate melt tend to be higher at low degree of partial melting,resulting in significant fractionation of V isotopes.With the proceeding of mantle melting,the lower valence states of V released from residual mantle increases,resulting in the decrease of valence states in silicate melt and reducing the degree of V isotope fractionation.Therefore,the oxygen fugacity and melting degree are the two main controlling factors of V isotope fractionation during mantle melting.