Volcanism at convergent margins is one of the most visible and important manifestations of the circulation of the solid Earth, with importance both for our understanding of Earth processes, and for hazards to human beings from eruptions and seismicity. Understanding these processes requires comprehensive data sets that make use of the most important geochemical tools, collected in the context of a "natural experiment" provided by the Earth. New developments in analytical geochemistry and collaboration with Chilean volcanologists provide a unique opportunity for such a study. The Chilean "southern volcanic zone" provides an experiment to see how the continental crust influences volcanic behavior and products. The thickness and age of the crust change progressively from south to north, permitting two competing hypotheses to be rigorously evaluated. The first hypothesis proposes that changes in volcanism result from the influence of the crust on melting processes in the mantle below, and that erupted melts can be interpreted in terms of the processes that take place at depth to create convergent margin volcanoes. The second argues that the crust modifies magmas beyond recognition, so that their compositions reflect shallow crustal processes rather than deeper mantle processes. A comprehensive study of volcanoes along the gradient provided by southern Chile will permit a detailed test of these hypotheses, and a new understanding of what controls convergent magma compositions.
The tools that will be used to solve this problem include the necessary field work and geological understanding provided by Chilean colleagues, standard tools of major elements, trace elements, and long-lived radiogenic isotopes. The new perspectives come from the combination of this classical approach with oxygen isotopes, U-series studies, and the cosmogenic nuclide 10Be. Oxygen isotopes are particularly sensitive to crustal contamination; U-series and 10Be reflect mantle processes. Seeing how these tracers co-vary will constrain the crust and mantle contributions.
This work will answer fundamental questions about solid Earth circulation that relate to our overall understanding to the plate tectonic cycle. It also integrates efforts of US, Swiss, French and Chilean investigators. Some of the volcanoes have had important historical eruptive activity, and this work will be useful to future hazard evaluation. Undergraduates, graduate students and post-docs are all receiving training in volcanology, the logistics of managing field expeditions, and advanced tools of chemical analysis.
