Volatile Recycling at the Lesser Antilles Arc: Processes and Consequences

The Earth is unique in our solar system in having abundant liquid water, plate tectonics and life. These properties are not unconnected; The evolution of life has depended heavily on water, and water is pumped around the planet by the plate tectonic cycle. Plate tectonics in turn, and its capacity to generate the very continents on which we live, also depends on the existence of water.

Subduction zones are the most important "valve" in the plate tectonic system. They form where tectonic plates sink back into the mantle. Here water, along with other volatiles such as carbon dioxide and sulphur, are returned to the deep interior. However, the return is not wholesale. As the sinking plate is subjected to heat and pressure, a large fraction of the incoming volatiles is "sweated off" and added to the overlying mantle where it causes melting. These melts feed volcanoes at subduction zones which are characteristically dangerously explosive. When considered with the earthquakes triggered by the plates scraping past each other and the consequent tsunamis and landslides, it is clear that subduction zones are the most hazardous places on Earth. Yet, these regions also have benefits: the cocktail of fluids travelling with magmas at subduction zones is responsible for transporting and emplacing valuable metal deposits into the crust, and the fine ash distributed by the explosive volcanoes produces nutrient-rich, fertile soils.

The importance of cycling volatiles through subduction zones is self-evident. However we still don't really know how it works and what the budgets are of volatiles delivered to the subduction zone, versus those recycled into the lithosphere, hydrosphere and atmosphere compared with those sequestered back into the deep mantle.

We propose an innovative multidisciplinary experiment to track volatiles at a subduction zone. Questions to be answered include: How do volatiles influence the types and amounts of magmas generated? How do they control where volcanoes, such as Mt Pinatubo and Montserrat are located and how explosive they are? How do volatiles dictate where ore deposits are formed? How do volatiles mediate the seismogenic behaviour of subduction zones - whether there are large "megathrust" earthquakes like Japan and Sumatra or whether slip is less violent?

Our focus area is the Lesser Antilles Arc, which is a special case, because it is one of only two Atlantic subduction zones. Plate formation processes at the slowly-spreading mid-Atlantic ridge produce a much more pervasively hydrated plate than those in the extensively studied Pacific. Furthermore, a laterally varying capacity to carry water in the plate and sediments subducting below the Antillean arc are a likely culprit for the arc's highly variable style and intensity of seismic and volcanic activity. By mapping structural differences along the arc we will be able to pinpoint the effects of variable water input.

We plan to use novel seismic approaches complemented by geochemical analyses and integrated using numerical models to identify and quantify where volatiles are in the downgoing plate, where they are released at depth, and how they are transported from the subducting plate through the mantle wedge to the arc. We will use a unique suite of rocks from deep in the crust which have been carried up in volcanoes to help us understand how magmas evolve, and what allows them to concentrate ore metals. Mapped water pathways will be compared with seismic and volcanic activity, as well as with those inferred at other subduction zones.

This large research project will be "bookended' on the one hand by an enormous amount of resource; data, samples, expertise and results from previous studies that will provide excellent value for money, and on the other hand a special focus on the societal benefits; informing natural hazard planning, and a better appreciation of how and where economic deposits form.

The proposed research is fundamental, and will deepen our understanding of plate tectonics, and the water cycle. Non-academic beneficiaries will include: [1] regional (Caribbean) agencies responsible for mitigating potential hazards such as earthquakes, volcanoes and tsunami. [2] Mining companies interested in understanding the genesis and distribution of subduction-related ore deposits; [3] Agencies, including government, involved in the development of policies on securing the future resource base and the sustainability of resource exploitation; [4] Host nation capacity building through education and training [5] The general public, specifically individuals and groups focused on geology and lay science, and school children.

To achieve this impact the project has set up partnerships with the Seismological Research Centre of the University of West Indies, who will use our new data and seismic velocity structures to improve their hazard maps, and with the NERC-funded STREVA project that focuses on volcanic hazard in the Antilles, and includes ties with the Montserrat Volcano Observatory and regional government agencies. A student from the University of West Indies will undertake a six-month study visit to the UK to receive training in seismological methods that will allow them to be involved in the analysis of the new data.

Co-I Wilkinson has various projects with mineral exploration companies and hosts an annual symposium to communicate new scientific results to representatives from UK industry. To further engage potential industrial users, we will publish our results in academic journals and present our work at both academically focused conferences such as the American Geophysical Union Fall Meeting, and also at industry focused conferences such as the Society of Exploration Geophysicists Meeting.

We will communicate to the wider public via several routes. We will set up a project web site with specialist as well as outreach components. In addition, all involved universities participate in outreach programs to schools as well as specialist-interest adult groups and the general public, including the Seismology-in-Schools effort (Imperial), the Discover Oceanography Program (Southampton), Nature Live at the London Natural History Museum (Imperial), Science Weeks of Imperial College and Royal Society, and the "Beacons of Engagement" public outreach project (Bristol, Durham). Several of us have experience with interviewing and presenting in the media and to public audiences.

The project will be wrapped up with a workshop in the Antilles to which international scientists as well as potential non-academic beneficiaries will be invited.