| With the development of analytical technique based on application of MC-ICP-MS over the last two decades, high precision stable isotope analytical method has obtained a rapid development. Now the isotope ratios of the elements overlaying whole periodic table could been measured with high precision. Since the species and behaviors of V is redox-sensitive, it has been widely used to constrain variation of redox conditions during many geological processes. Theoretical investigation and experimental work has shown that isotopes of multi-valence elements can be fractionated during redox reactions. Thus V isotope compositions of terrestrial samples can be used as a new tool to fingerprint redox-state variation. The main obstacles of the geochemical application of V isotopes are that it is still challenging to perform high precise V isotope measurements. In addition, there is also lack of theoretical or experimental study on fractionation mechanisms of V isotopes.In this thesis, we systematically introduce the geochemical properties of V element, and review the current state of the study of V isotope geochemistry. On the basis of the present study, we develop a new method to measure V isotopic composition for terrestrial rocks. By using cation- and anion- exchange resin, V was efficiently separated from matrix elements. When compared with previous methods, our new separation procedure greatly improves time efficiency and avoids the expensive TRU Spec resin. V isotope ratios were measured using MC-ICP-MS, employing a sample-standard bracketing method to calibrate the instrument fractionation. Parameters that could affect the accuracy and precision of isotopic measurements were strictly examined, which include potential effects of acid molarities and concentration mismatch, and the influence on the existence of Cr and Ti on instrumental analyses. The V isotopic composition of pure V solution (BDH and USTC-V) relative to the AA pure V standard solution (defined as δ51V= [(51V/50V)sample/(51V/50V)AA-1]×1000) based on long-term analysis are -1.23± 0.08‰(2SD,n=197) and 0.07±0.07%o (2SD, n=112), relatively. The average δ51V of synthetic standards, which made by doping USTC-V with matrix elements, agrees well with their recommended values within the limit of error. V isotopic compositions of 12 reference materials, including igneous rocks and manganese nodules, were measured using this method. The reference δ51V value of these reference materials are giving below:BCR-2,-0.78±0.08‰(2SD, n=36); BHVO-2,-0.83±0.09‰ (2SD, n=22); BIR-1,-0.92±0.09‰(2SD,n=52);JB-2,-0.87±0.06‰(2 SD, n=20); W-2,-0.94±0.08‰(2SD, n=15); AGV-1,-0.71±0.10‰(2SD, n=6);AGV-2,-0.70±0.10‰(2SD, n=37); JA-2,-0.80±0.07‰(2SD, n=15); QLO-1,-0.61± 0.03‰(2SD, n=3); GSP-2,-0.62±0.07‰(2SD, n=26); NOD-P,-1.65±0.06‰ (2SD, n=10); NOD-A,-0.99±0.10‰(2SD, n=19). Long-term external reproducibility of better than ±0.1‰(2SD) for δ51V was routinely obtained. Such precision allows us to identify V isotope fractionation in high-temperature terrestrial samples as well as supergene processes.With this method, a study was conducted to investigate the behaviors of V isotopes during magmatic differentiation and alteration of oceanic crust. High-precision V isotope analyses have been conducted on basalt to dacite lavas from mid-ocean ridges, and altered basalts and gabbros recovered from IODP site 1256 on the East Pacific Rise. The results show that altered basalt and gabbro samples from IODP site 1256 have homogeneous V isotopic composition with average value of-0.85±0.11(2SD), in agreement with that of fresh oceanic basalt within the error range, irrespective of highly variable alteration temperatures and variable water/rock ratios. Such results show that limited V isotope fractionation occurred during low- and high- temperature alteration of oceanic crust at bulk rock scale. While V isotopic variations with range of about 0.3%o exists in the basalt to dacite lavas from East Pacific Rise. There are positive correlation between SiO2 content and δ51V value and negative correlation between MgO content and δ51V value of the sample. Such results reveal that obvious V isotope fractionation occurs during magmatic differentiation, which may be related to its complex redox chemistry in magmatic systems. Our results show that V isotope might be used to study planetary differentiation, which could be a robust tool since it can resist to alteration process.Theoretical studies were also conducted to investigate V isotope fractionation in solution and during adsorption. Equilibrium fractionation factors of V isotopes among tri- (V(III)), tetra-(V(IV)) and penta-valent (V(V)) inorganic V species in aqueous system and during adsorption of V(V) to goethite, are estimated using first-principles calculation. Our calculation highlights the dependence of V isotope fractionation on valence states and the chemical binding environment. In brief, our results show that the heavy V isotope (51V) is enriched in the main V species following a sequence of V(Ⅲ)<V(Ⅳ)<V(Ⅴ), and light V isotope (50V) is preferentially adsorbed on the surface of goethite. This work reveals that V isotopes can be significantly fractionated in the Earth’s surface environments due to redox reaction and mineral adsorption, indicating that V isotope data can be used to monitor toxic V(V) attenuation processes through reduction or adsorption in natural water systems.In addition, a simple mass balance model was made which suggests that V isotope composition of seawater might vary with change of ambient oxygen levels. Thus our theoretical investigations imply a promising future for V isotopes as a potential new paleo-redox tracer. |
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