Tracing mantle convection with high precision U isotope measurements

项目背景俯冲将地球表面的蚀变物质返回到深处。近地表蚀变的一个关键特征是，它可以产生依赖于质量的同位素分馏，这对于地球内部的过程通常是可以忽略不计的。利用这种同位素变化追踪俯冲物质的命运，是研究地幔对流的经验意义。铀同位素系统在这方面具有巨大潜力。现代蚀变洋壳具有独特的升高的238 U/235 U，但我们对U在地表环境中行为的了解意味着这种情况只能发生在最近的~ 600 Ma [1]。因此，我们不仅有诊断同位素标记，而且我们知道这个自然追踪实验是什么时候开始的。学生将绘制大洋盆地玄武岩中的238 U/235 U变化图，以研究过去~ 600 Ma上地幔中异常U的分散，并使用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。项目目的和方法主要目的是查明俯冲的同位素重铀在大洋盆地中的分散情况。为此，学生将分析大洋中脊玄武岩样品（MORB）从许多位置沿着全球海岭系统映射U同位素的变化，从而推导出对流的平面形状。这需要仔细准备新鲜的玻璃样品，并使用多收集器电感耦合等离子体质谱法通过双加标方法对其进行分析。布里斯托已经建立了分离铀和进行高精度分析的程序[1]。然而，有机会使用有价值的MORB样品进行额外的同位素分析，使用目前在布里斯托工作的程序（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，该模型使用来自日益完整的记录的强加板块运动来预测示踪剂的分布[见2]。该项目还将根据先前的地质观测结果得出结论，即同位素异常的U循环始于~ 600 Ma。这将使用蛇绿岩中400 Ma至800 Ma的枕状玄武岩套件进行测试。较老的样品应该没有显示出海底蚀变过程中产生的同位素重铀的迹象，并应该提供完全含氧海洋上升的独立时间[参见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层析成像（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]，非常适合核取证和核粒子表征以及地质应用[5]。

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].