JC149

Our overall objective is to track volatile cycling at a magmatic arc and establish:

1. An inventory of volatiles (H2O and CO2 in particular) at the Lesser Antilles an active subduction zone, including;
   How volatiles are delivered to the arc
   How and where volatiles are stored in the arc crust
   How and where volatiles control magma production and differentiation processes
2. The influence of volatiles on natural hazards and resources at subduction zones

Magma production at subduction zones is intrinsically linked to the release of volatile-rich fluids from the subducted slab which lower the solidus of the mantle above the slab, enriching it in slab-fugitive elements and species including fluid-mobile trace elements, CO2 and H2O. The magmas so generated then pass through, or are emplaced into, the lithosphere of the upper plate where they undergo differentiation. Differentiation, in turn i) dictates the potential explosivity of consequent eruptions, by controlling viscosity and volatile concentration, ii) controls how and where (and what type of) fluids are released, influencing the generation and distribution of metal ores, iii) leads to compositional variations and the production of andesitic magmas with crust-like characteristics. Subduction zones are also dynamic environments where these deep-seated volatile-mediated processes have a significant influence on our environment. 

The seismic element of our programme comprises three parts: a 2D crustal-scale active source component, an inter-island shoot and a broad-band passive source component. Thirty four broad-band OBS were deployed during JC133 in March 2016 (PS Collier, Imperial College London). During JC149 the inter-island shoot will take place with a large capacity (~4800 cu in) but just the short (300 m) streamer. Next, the broad-band instruments will be recovered. Finally we will deploy 50 short-period OBS three times on the incoming plate to record signals from a large capacity (~4800 cu in) airgun source fired at 60 s interval. Multi-channel seismic data will also be collected with a 3 km hydrophone streamer. Standard geophysical underway data (gravity, magnetics, swath bathymetry) will be recorded throughout the survey, together with XBT data and sound-velocity dips as required to constrain the water column structure.