PhD Studentship: Halogens in subduction zone lavas: tracing recycled contributions from slab to arc

Research theme: Volcanology and Geochemistry

The Department of Earth and Environmental Sciences will provide tuition fees and a tax free stipend set at the UKRI rate (£18,622 in 23/24).

This funding is for home students only.

Magmas are generated in subduction zones by the flux of volatiles from the sinking slab into the mantle wedge, causing it to melt. The resulting magmas, which can be erupted through volcanic arcs built on the overriding plate, are characteristically volatile-rich and oxidised, which influences their evolution and physical and chemical properties. Arc volcanoes are highly explosive and pose a significant threat to both infrastructure and hundreds of millions of people worldwide. Gas emissions from arc volcanoes bear chemical and isotopic hallmarks of recycled slab phases and over geological time the changing chemistry of subduction zone outgassing may be important for the evolution of global climate. In sum, it is important to understand which volatiles are liberated from the slab, under what conditions, and whether these are efficiently recycled to the surface via arc volcanism. As yet, we lack a unified understanding of how these processes vary between different subduction zones.

In this project, our focus will be on halogens (Cl, Br, I), since the relative abundance of halogens in magmas can reveal the contributions of distinct recycled slab phases (sediments, altered basaltic crust, serpentinised peridotite) to magmatic volatile budgets. In different subduction zones, we anticipate that varying slab assemblages experiencing contrasting metamorphic conditions (PT paths) on sinking will undergo different devolatilization reactions. We hypothesise that this will be reflected in the relative abundance of halogens in arc basalts. In Manchester we have unique analytical facilities to measure Cl, Br and I in magmas with ppb level accuracy.

We will begin by studying halogens hosted in olivine crystals obtained from basaltic eruption products of Mt Etna, where a recycled slab influence has been detected in recently (post-1970) erupted lavas. We will analyse halogen relative abundance in gases released from in vacuo crushing of olivine separates using neutron irradiation noble gas mass spectrometry (NG-NGMS). We will complement these analyses with measurements of volatile content and trace element geochemistry in melt inclusions hosted in the same olivine crystals, for independent constraints on slab influence. The project will also involve a detailed investigation into the compositional variation of volatiles trapped in fluid inclusions, using FTIR and Raman microscopy alongside petrographic observations. Etna is an optimal first target for our work given the relative ease of sampling and the availability of primitive olivine-phyric deposits erupted through ‘eccentric’ vents without long intervals of processing or storage in crustal reservoirs. We will contrast recent slab-influenced lavas with those erupted throughout Etna’s history. The project may develop according to the student’s interests, for example to include further fieldwork or analyses of samples obtained from contrasting subduction zones worldwide.

The successful applicant must be able to start their PhD by the 31st of July 2024 at the latest to comply with the funding conditions.

Please submit a one-page letter addressed to the supervisory team stating your interest in the project and their suitability for it. The contact email addresses are: Dr Brendan McCormick Kilbride is [email protected] , Prof Burgess - [email protected] and Dr Hartley - [email protected] .