Admissions > PhD by research > Research Projects > The fate of halogens in subduction zones. Implications for cycling of volatiles in the Earth's mantle.

The fate of halogens in subduction zones.

Implications for cycling of volatiles in the Earth's

mantle.

Supervisors: Dr Simon C. Kohn and Dr Ray Burgess (University of Manchester)

The contrasting geochemical behaviour of the halogens, F, Cl, Br and I, provides an exciting opportunity to learn about the processes that occur in subduction zones. Halogens dissolved in the fluids produced by the first breakdown of hydrous minerals could be released early from subducted rocks, and make their way into the overlying wedge. Alternatively, they might be transported deep into the mantle. In either case, subsequent melting could result in the halogen concentrating in a melt phase and either returning to the surface via arc magmatism, or reacting with mantle assemblages. Straub and Layne (2003), measured water and halogen concentrations in glass samples from the Izu arc and suggested that H2O, Cl and F in the samples are derived from the subducted slab. They proposed that essentially all the Cl that is subducted is returned to the surface via magmatism, whereas only 13-22% of the water and 4-6% of the F is recycled. Very little is known about the behaviour of Br and I. It should be noted that the very low recycling efficiency of F implies that there is a reservoir in a subducting slab/wedge system that contains F, and is capable of deep subduction without releasing F. The nature of this reservoir is unknown. As far as the longer term presence of chlorine in the mantle is concerned, there is evidence that chloride melts or brines could exist in some parts of the mantle and be implicated in mantle metasomatism and the genesis of diamonds.

The specific contribution of this project will be:

  1. To measure the partitioning of halogens between silicate melts, aqueous fluids, hydrous minerals and anhydrous minerals under a range of mantle conditions.
  2. To identify the likely reservoirs for halogens in the upper mantle and transition zone.
  3. To elucidate the mechanism of structural incorporation of halogens in silicate melts and minerals.
  4. To combine the new experimental data with existing geochemical data to develop a simple quantitative model of halogen fluxes between the mantle, crust and hydrosphere.

The student will be given extensive training in high-pressure experiments to simulate the pressures and temperatures in subduction zones, and use a range of analytical techniques to analyse the different phases (minerals, melts and aqueous fluids) produced. State-of-the art halogen microanalysis will be performed in Manchester using the technique of neutron irradiation and noble gas extraction. Spectroscopic techniques such as NMR will be used to elucidate the structural incorporation mechanisms of halogens in melts and minerals. The project will provide excellent preparation for further academic research in geochemistry, petrology and mineralogy or a wide range of careers in science outside academia.

We welcome applications from students form all physical sciences, including chemistry, physics and materials science as well as Earth Sciences.

References

Last updated: 1.11.11