Tracing mantle convection with high precision U isotope measurements

项目背景：潜没将地球表面改变的物质送回深处。近地表变化的一个关键特征是，它可以带来与质量相关的同位素分异，而这在地球内部的过程中通常是可以忽略不计的。利用这种同位素变化来追踪俯冲物质的去向，是研究地幔对流的经验意义。铀同位素系统在这方面具有很大的潜力。现代，蚀变的海洋地壳有一个独特的，升高的238U/235U，但我们对U在地表环境中的行为的了解意味着，这种情况只可能发生在最近~600Ma[1]。所以我们不仅有一个诊断同位素标签，而且我们知道这个自然示踪实验是什么时候开始的。该学生将绘制跨洋盆地玄武岩中238U/235U的变化图，以研究过去~ 600Ma上地幔中异常U的弥散，并以此来约束地幔对流模型。该项目的另一个目的是让学生利用在铀同位素分析中获得的专业知识，帮助开发检测环境中人为铀同位素异常的方法。项目目标和方法主要目的是描述大洋盆地中俯冲、同位素重铀的分布特征。为此，该学生将分析来自全球山脊系统多个地点的洋中脊玄武岩样本（MORB），绘制U同位素变化图，从而推断对流平台。这需要仔细制备新鲜的玻璃样品，并通过使用多收集器电感耦合等离子体质谱法的双尖峰方法进行分析。布里斯托尔[1]已经建立了分离U和进行高精度分析的程序。然而，有机会使用有价值的MORB样品进行额外的同位素分析，使用布里斯托尔目前正在使用的程序（例如Mo或B同位素）或可能开发的程序（例如K）。同位素观测结果将与在卡迪夫运行的地幔对流模型进行比较，该模型利用越来越完整的记录中施加的板块运动来预测示踪剂的分布[见2]。该项目还将实地验证先前地质观测得出的结论，即同位素异常铀循环开始于~600Ma。这将使用来自蛇绿岩的枕头玄武岩套件进行测试，范围从400毫安到800毫安。较老的样品应该没有显示在海底蚀变过程中同位素重铀的迹象，并且应该提供完全含氧海洋上升的独立时间[cf. 3]。最后，在高精度U测量中获得的技能将用于校准新型原子探针断层扫描（APT）分析的标准。APT允许对亚微米样品进行空间分辨的同位素分析[例如4]，是核取证和核粒子表征以及地质应用的理想选择[10]。

Project Background Subduction returns altered material from the Earth's surface to depth. A key feature of near surface alteration is that it can impart mass-dependent isotopic fractionations, which are typically negligible for processes in the Earth's interior. Tracking the fate of subducted material, using such isotopic variations, is an empirical meaning of studying mantle convection. The uranium isotopic system offers great potential in this endeavour. Modern, altered oceanic crust has a distinctive, elevated 238U/235U, but our knowledge of the behaviour of U in surface environments means that can have only been the case for the last ~600Ma [1]. So not only do we have a diagnostic isotopic label but we know when this natural tracing experiment commenced. The student will map the 238U/235U variability in basalts across the ocean basins to study the dispersion of anomalous U in the upper mantle over the last~ 600Ma and use this to constrain models of mantle convection. A further aim of this project is for the student to use the expertise gained in U isotopic analysis in helping to develop methods to detect anthropogenic U isotopic anomalies in the environment. Project Aims and Methods The main aim is to characterise the dispersion of subducted, isotopically heavy U across the ocean basins. To this end, the student will analyse mid-ocean ridge basalts samples (MORB) from numerous locations along the global ridge system to map U isotopic variability and thus deduce a planform of convection. This requires careful preparation of fresh glass samples and their analysis by a double spiking approach using multi-collector inductively coupled plasma mass-spectrometry. Procedures for separating U and making high precision analyses have already been established at Bristol [1]. However, there is the opportunity to use the valuable MORB samples for additional isotopic analyses using procedures currently working at Bristol (e.g. Mo or B isotopes) or that might be developed (e.g. K). The isotopic observations will be compared to models of mantle convection being run in Cardiff that predict the distribution of tracers using imposed plate motions from increasingly complete records [see 2]. The project will also ground-truth the inference from prior geological observations that isotopically anomalous U recycling commenced at ~600Ma. This will be tested using suites of pillow basalts from ophiolites ranging from 400Ma to 800Ma. The older samples should show no sign of isotopically heavy U imparted during seafloor alteration and should provide independent timing of the rise of fully oxic oceans [cf. 3]. Finally, the skills gained in high precision U measurements will be used to calibrate standards for novel Atom Probe Tomography (APT) analyses. APT allows spatially resolved, isotopic analyses to be made on sub-micron samples [e.g. 4], ideal for nuclear forensics and nuclear particle characterisation and well as geological applications [5].