Surface meets Deep: hydrothermal alteration of the oceanic crust - NERC GW4+ DTP project

This project is one of a number that are in competition for funding from the NERC GW4+ DTP. The GW4+ DTP consists of the Great Western Four alliance of the University of Bath, University of Bristol, Cardiff University and the University of Exeter plus six Research Organisation partners. The partnership aims to provide a broad training in earth and environmental sciences, designed to train tomorrow’s leaders in earth and environmental science. For further details about the programme, please see http://nercgw4plus.ac.uk/

Background
A range of important biogeochemical processes take place during seafloor weathering, when seawater percolates through slowly cooling oceanic crust formed at mid-ocean ridges. These hydrothermal alteration processes play a major role for the heat exchange and elemental redistribution between the deep Earth and the surface, with significance for the chemical evolution of the oceans, advancement of microbial life and elemental recycling. During seafloor weathering, uranium is removed from seawater and added to the altered oceanic crust, thereby modifying the ocean uranium content and providing a way to transfer uranium from the surface and into the mantle via subduction. The non-radiogenic isotope fractionation between 238U and 235U allows new perspectives on this global cycle. In addition, the radioactive decay of uranium creates a range of short-lived nuclides, and the disturbance of these provides time constraints on alteration processes, elemental exchange and the long-term heat flux.

Project Aims and Methods
In order to fully quantify the isotopic composition of the uranium flux to the deep Earth, the pathways and time scales of uranium uptake and associated isotope fractionation in the altered oceanic crust need to be determined. This PhD project is designed to determine the complete uranium isotope budget of the altered oceanic crust, using a range of ocean-floor drilling sections and field work and geochemical analysis in an ophiolite complex (Troodos, Cyprus). The aim is to determine the overall net uranium flux and its isotope composition, and direct link to specific alteration styles and secondary mineral phases. Furthermore, uranium-series disequilibrium chronometers (238U-234U) will be applied using in situ techniques to examine time scales of secondary U uptake in the altered oceanic crust. Overall, this will lead to a model for the pathways and time scales of uranium uptake in the oceanic crust, and improved understanding of the mechanisms of differential uranium release in the shallow mantle and subduction zones during crustal recycling, with implications for the global U cycle.