How does crust form at arcs? - a Multidisciplinary Study of the Lesser Antilles Volcanic Arc

Lead Research Organisation: University of Bristol
Department Name: Earth Sciences

Abstract

The creation of continental crust has profoundly influenced the evolution of life, the chemistry of the atmosphere, hydrosphere and biosphere and the distribution of resources, so knowing how it is made is a first-order geochemical and geophysical problem. At present, and probably throughout much of Earth history, most new crust is generated above subduction zones, so subduction zone magmatism and crustal growth are intimately linked. However, the composition of the new, immature crust generated at subduction zones is controlled by a largely basaltic magma flux, whereas the continental crust is more silica and incompatible trace element rich than this (~andesitic). Additional processes occurring in the arc crust during subduction (such as differentiation and delamination of deep cumulates) or subsequent to subduction (such as intra-crustal mixing or removal of deep crust during collision) are needed in order to go from arc crust to true continental crust. The key to unravelling these potentially important mechanisms lies in actually knowing the structure and composition of the (upper plate) crust at subduction zones. To that end, we propose a novel approach integrating petrology, petrological experiments, geochemistry and seismology to characterise the crust of an intra-oceanic island arc (The Lesser Antilles).

The integration of seismic constraints with the petrological controls is a particularly innovative approach, made possible by the availability of an extensive xenolith collection, and a very large high-quality suite of seismic data. To explain clearly how this integration will be done, we offer the following analogy: Suppose you had a handful of jigsaw pieces (the xenoliths) and wanted to find out what picture was made by the whole jigsaw (in this case the geochemical/mineralogical profile of the Lesser Antilles Arc crust). The jigsaw pieces are representative of the picture - all the missing ones are the same types as the ones you already have. If you think of the jigsaw as an x-y grid, then the petrological experiments, along with geobarometry from mineral compositions, can be used to tell you what rows the jigsaw pieces occupy (how deep the xenoliths come from). The seismic data can then be used as a "pattern" telling us which pieces go where in a 2-dimensional x-y grid. Firstly this provides validation between the seismic and petrologic data (the depths determined by both methods need to agree). Secondly, since we now know what seismic data correspond to which xenolith type, we can then use the seismic data to fill in the rest of the picture and build an along-arc crustal section. The complementarity between these approaches is key; the seismic data cannot tell us the exact mineral assemblage, but only permitted combinations of minerals . The xenoliths, in turn, can only provide examples of the subjacent crust, and can only be confirmed as representative of a given crustal depth by tying to their seismic properties (calculated from the xenoliths' mineralogies) to the observed seismic profiles. So by drawing together the xenolith data, the experimental data and the seismic data we will be able to generate a model of the crustal structure along the entire arc, which will shed new light on just how crust is built above subduction zones, a critical stage in the ultimate generation of continental crust.

The project brings together a multidisciplinary and multi-institutional international team.The recent sampling and experimental campaign at Bristol makes this effort particularly timely and it builds on a wealth of existing data and prior research, integrating directly with ongoing, substantively funded research strands The resource requested therefore represents a mere fraction of the cost of the large and protracted geophysical-analytical campaign that would otherwise be needed to reveal the structure and composition of a ~30x400km section of arc crust.

Planned Impact

A major societal challenge in the UK is engaging school children in the natural sciences at a sufficiently early age that they can make informed decisions about GCSE and A-level choices. Subduction zones provide an exciting demonstration of the integrated nature of Earth processes, connecting well known topics such as volcanoes and earthquakes with less obvious ones such as the origin of the crust and the role of subduction zones in geochemical cycles (including those which control climate change).

Our principle impact activity will be to develop an online teaching topic on "subduction zones" to contribute to the "Your Planet Earth" (YPE) website, which is a widely used teachers tool. We will develop the topic (presentations and exercises) through a team of year 4 undergraduate students via a timetabled module. Furthermore we will test the topic, and obtain feedback, using a second team of (year 3) undergraduate students who visit schools as part of their module. Finally we will convene a day workshop for local teachers to introduce the topic and provide additional information, while simultaneously obtaining feedback and input from them.

Allied with this, we will use the British Geological Survey's (BGS) "Seismology into Schools" project to reinforce our web based activities, and as an opportunity to interface (via the web) schools in Bristol and Durham with selected schools in the Caribbean. The plan is to provide two seismometers from the BGS project for Caribbean schools and link these with schools in Bristol and Durham which are already participating.

This impact project will provide opportunities for;
i. Curriculum reinforcement and raising the profile of Earth Sciences
ii. Teamwork, problem-solving and decision-making
iii. The introduction of differentiated activities suitable for a range of abilities found within a typical student group, including those aimed at stretching students with strong scientific and mathematical skills.
iv. Interaction between schools in the the UK and the Caribbean, and
v. Schoolchildren learning about subduction zones and associated phenomena (such as earthquakes, volcanoes and other hazards)

Our "Pathways to Impact" is aimed at reaching thousands of school children through (1) the production of a web-based resource focussed on subduction zones through the UK "YPE" project and (2) twinning Caribbean schools with Bristol and Durham schools through the BGS "Seismology into Schools" project. These young students will also be beneficiaries, and would gain a better understanding of Earth Science issues. The PI has been involved through CHUGD, the Mineralogical Society and the Geological Society in trying to raise awareness of Earth and Environmental Sciences and its contributions to the UK economy and wellbeing. Our efforts to bring the discipline to schools will, at least in some small part, help to improve the vitality and future prospects of our discipline through recruiting from schools into higher education.


We believe that our proposal represents a novel and effective way of making our science available to schoolchildren.
 
Description Nature of the continental crust in arc settings.
Exploitation Route Future VOILA experiment.
Sectors Environment

 
Description Volatile Recycling at the Lesser Antilles Arc: Processes and Consequences
Amount £109,843 (GBP)
Funding ID NE/K010662/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 10/2015 
End 03/2021