Vanadium Isotope Geochemistry Of Peridotites,Komatiites,and Latosols

Vanadium(?)has multiple valences(V0,V2+,V3+,V4+,and V5+)in nature.Since the species and geochemical behaviors of V are sensitive to the redox state of the environment,V has been widely used to trace the redox conditions of high-and low-temperature geological processes.Vanadium has two stable isotopes:51V and 50V.The experimental studies,theoretical calculations,and natural observations have found that V isotopes can be significantly fractionated during high-and low-temperature processes,indicating that V isotopes have the great potential to study the cosmogenic irradiation,core-mantle segregation,crust-mantle evolution,paleo-environment and paleo-climate.The bases for developing V isotope studies include the establishment of high-precision V isotope analytical methods,the determination of the V isotope compositions for various Earth's reservoirs,and the identification of the V isotope behaviors in important geochemical processes.The existing V isotope studies have the following issues:(1)lack of high-precision V isotope analytical methods for ultramafic rocks with low V and high Mg concentrations;(2)the V isotope composition of the Bulk Silicate Earth(BSE)is the baseline for the application of V isotopes to study geochemical and cosmochemical processes.Previous estimate of V isotope composition of the BSE has relatively large uncertainties,which is not conducive to the comparison of the V isotope composition of each reservoir.Therefore,it calls for a more precise estimate of the V isotope composition of the BSE;(3)the difference in V isotope composition between BSE and mid-ocean ridge basalts(MORB)is still unclear.It is also unclear whether mantle partial melting would cause V isotope fractionation;(4)weathering process is an important bridge for elements from rocks to rivers and oceans,but the V isotope behavior during weathering is still unknown.In this thesis,the geochemical properties and research status of V element and V isotopes were systematically summarized.Then,based on the above issues,we established a high-precision V isotope analytical method for ultramafic rocks.Using this method,we analyzed the V isotope compositions of a set of typical mantle peridotites,komatiites and picrites.Combining fertile peridotites with komatiites,we redefined the V isotope composition of the BSE.The V isotope fractionation during mantle partial melting was also discussed.In addition,we analyzed the V isotope and Fe isotope compositions of a latosol profile developed from basalts from Zhanjiang,South China to investige the V element and V isotope behavior during oxidative weathering.We added two more purification steps before using previous analytical procedures.The anion and cation exchange resins were used to remove Fe and Mg.The ?51V values for the ultramafic-mafic reference rocks are:PCC-1,-0.99±0.07‰(2SD,n=12);BIR-1,-0.90± 0.09‰(2SD,n = 3);BCR-2,-0.75 ± 0.05‰(2SD,n = 3),which are consistent with the recommended values in the literature within the analytical uncertainties.By using the analytical method,this study presents ?51V V data for eleven peridotite xenoliths from two late Cenozoic eruption centers at Tariat in central Mongolia,ten komatiites from five localities ranging in age between 3.48 and 2.41 Ga,and four 1.98 Ga picrites from the Onega Plateau in Fennoscandia.The ?51V for the eight fertile spinel lherzolites ranges from-0.85±0.04‰ to-0.95±0.04‰,with a mean value of-0.91±0.06‰(2SD,n= 8).They show no resolvable difference in V isotope composition compared to the three moderately to highly refractory peridotite xenoliths analyzed,with ?51V ranging from-0.92±0.03‰ to-0.93±0.03‰.The ?51V for the ten komatiites analyzed range from-0.87±0.07‰ to-0.95±0.089‰,with a mean value of-0.91 ±0.05‰(2SD,n=10).This value is identical to that for the fertile peridotites,suggesting that high-degree(up to 50%)partial melting of the mantle that produced these komatiites likely did not significantly affect the V isotope composition of the komatiitic magmas.Based on the V isotope compositions of the peridotites and komatiites analyzed in this study,the average ?51V for the BSE is estimated to be-0.91±0.09‰(2SD,n=18).In contrast,the ?51V for the Onega Plateau picrites and related cumulates ranges from-0.76±0.04‰ to-0.82±0.06‰,with an average value of-0.80±0.05‰(2SD,n=4),which is identical to the ?51V value for MORB.This value is somewhat heavier than the new BSE estimate,indicating that the low-degree(<10%)partial melts have higher ?51V than their mantle sources.The new estimate of ?51V for the BSE overlaps with the current estimates for the V isotope compositions of the silicate Moon and Mars,which may imply that these bodies have a common V isotope composition.To investigate V isotope behavior during oxidative weathering,V isotopes of Zhanjiang latosol profile(South China)developed on basalts were analyzed.The latosols display a narrow range of ?51V from-0.77±0.06‰ to-0.94±0.05‰,and the weathered basalt has ?51V of-0.88±0.03‰,which are all indistinguishable from their parent basalt with ?51V of-0.79±0.06‰ within analytical uncertainty.Vanadium and iron concentrations through the profile have large variations and show a good correlation,indicating that V behavior during weathering is associated with Fe.The latosols also show a small variation of ?56Fe ranging from 0.04±0.03‰ to 0.14±0.02‰,identical to the ?56Fe for the parent basalt(0.08±0.02‰)within analytical uncertainty.In contrast with the limited V isotope fractionation,The negative ?Th,v(-57%to-11%)and ?Th,Fe(-61%to-11%)calculated from mass balance suggest significant V and Fe loss in latosols through the profile.These observations indicate that dramatic V and Fe loss did not significantly fractionate V and Fe isotopes in latosols,thus the isotope compositions of the mobile Fe and V are similar with the parent rocks.The possible mechanism is that under oxic environments,soluble Fe(?)released from mineral during weathering could be completely oxidized to insoluble Fe(?)in situ prior to Fe migration and sequentiallly form Fe(hydr)oxides colloids.The V(?)and V(?)ions released from the mineral directly came into the structures of Fe oxides and clay minerals,or were oxidized to V(?)in situ and strongly adsorbed on the colloidal substance surfaces.Thereby V was probably transported with these colloidal substances in the latosol profile.