Surface meets Deep: hydrothermal alteration of the oceanic crust

A range of important biogeochemical process takes 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 a range of Earth system processes, such as the chemical evolution of the oceans, advancement of microbial life and elemental recycling into subduction zones and the mantle1.
Observations of the alteration of the oceanic crust come from studying sections recovered by ocean-floor drilling expeditions1 as well as ophiolite complexes, where oceanic crust has been uplifted and emplaced on continental margins (Fig. 1). Mass and heat fluxes can be estimated by examining the transfer of elements between “fresh” and “altered” oceanic crust (e.g. Li, U, Rb). 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 cycle2. In addition, the radioactive decay of uranium creates a range of short-lived nuclides and the disturbance of these U-series chains can provide time constraints of alteration processes (Fig. 2) and estimates of the long-term heat flux and elemental exchange between the seawater and oceanic crust. However, 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 drilled sections, supplemented by 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 link this to specific alteration styles and secondary mineral phases. Furthermore, uranium-series disequilibrium chronometers (238U-234U-230Th) will be applied to examine time scales of seafloor weathering and heat exchange. 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.