高精度的钒同位素分析方法及地幔橄榄岩的钒同位素组成

Vanadium (V) is an important transition metal. It has different valence states, and is highly sensitive to chemical redox reactions that occur in different environments. Chemical reactions that produce changes in oxidation state will also produce V isotope fractionation. Therefore, V isotope fractionation should be observed in many environments, and V isotopes maybe a good tracer of terrestrial core formation, the variation in the oxidation state of the mantle, etc. However, compared to the fast development of the other metal stable isotope systems, there has been little progress with V isotope related research. The analytical challenges are (1) there are only two V isotopes, 50V (0.24%) and 51V (99.76%), with a large mass discrepancy (51V/50V>400), and (2) the existence of isobaric interferences from 50Cr and 50Ti on the minor 50V isotope. So far, there is only one laboratory in the world that can measure V isotopes accurately, but their procedure is complicated and requires a large sample size (10-15μg of V). In this project, we will improve the method for V isotope analysis by using different ion-exchange resin in the chemical procedure and by changing the resistor of Faraday cup of MC-ICP-MS. This new method can be used to analyze small sample size (1-2μg) and can provide precise and accurate V isotope data for geochemistry research. Based on this method, we will analyze V isotope compositions of mantle xenoliths from eastern China, to provide the information for depleted mantle and to test V isotope fractionation between mantle minerals.

钒（V）是重要的过渡族金属元素，具有多个价态，对于不同的氧化还原环境非常敏感。钒同位素在氧化还原反应过程中会发生分馏，因此可以用于制约一系列重要的地球化学过程，如核幔分异、地幔演化等。但比起其它金属稳定同位素体系的快速发展，钒同位素的测量及相关研究相对滞后，主要因为（1）钒仅有的两个同位素50V（0.24%）和51V（99.76%）含量相差过大，同时测量较难；（2）50V的同质异位素（50Cr和50Ti）干扰较大，对化学流程要求极高。目前国际上仅有的高精度钒同位素分析方法所需样品量较大（10-15μg），且化学流程繁琐。本项目将采用不同的离子交换树脂，并对MC-ICP-MS的电阻进行改造，把测量所需的钒降低到1-2μg，建立一个适用于不同钒含量的高精度的钒同位素分析方法。并通过分析来自中国东部的地幔橄榄岩的全岩和单矿物，来研究地幔端元的钒同位素组成及高温下地幔矿物间是否存在钒同位素分馏。

钒是一个具有多个价态的元素，其同位素在氧化还原反应过程中会发生分馏，因此可以用于制约一系列重要的地球化学过程，如核幔分异、地幔演化等。但之前由于分析方法的局限性，关于钒同位素的研究相对滞后。本项目在前人工作的基础上，建立了一个适用于不同钒含量的高精度的钒同位素分析方法。在成功建成了高精度的钒同位素分析方法之后，我们又开发了针对高Mg、高Fe的地幔橄榄岩的V同位素分析方法，进而分析了地幔橄榄岩和科马提岩的钒同位素组成，使人们对BSE的钒同位素组成有了新的认识。也为下一步将地球样品与地外样品（如陨石、月球样品、火星样品等）进行比较提供了依据。