Tracing mantle convection with high precision U isotope measurements

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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].