On planet Earth, oceanic plates are constantly dragged down into the mantle along destructive plate margins. This process is called "subduction" and it is one of the fundamental manifestations of plate tectonics. Subduction zones function as the primary transport mechanism for materials between the surface and interior of the Earth. An understanding of chemical cycling in subduction zones provides us with the tools to better understand the mountain-building and magmatic processes that contribute to the global cycles of critical elements, the formation of valuable natural resources, such as base-metal ores, and daily hazards of subduction zones, such as earthquakes, tsunamis and volcanoes.
Traditionally, it is envisaged that fluids expelled from the subducting oceanic plate migrate into the hot overlying mantle, where they trigger melting and the generation of magmas that lead to the formation of the volcanic chains that are ubiquitous along subduction zones around the planet. These established models can explain a number of magma compositions found in subduction-zone volcanoes. However, some magma types observed in subduction-zone volcanoes cannot be explained by that traditional model and require a source region that is different from the normal mantle. New models on material transport have emerged from computer modeling, fieldwork and geochemical studies. These new models suggest that mixing between rocks derived from subducting oceanic crust and the overlying mantle leads to the formation of thick layers of mixed rocks directly above the subducting plate. These mixed-rock layers, so-called "mélanges", are predicted to buoyantly rise into the overlying hot mantle wedge, where they start to produce significant amounts of melt. In this project, researchers Marschall, Gaetani and Cruz-Uribe will investigate for the first time the composition of melts produced from natural mélange rocks under the pressure-temperature conditions prevailing in the mantle by performing high pressure experiments in the laboratory. The melts and minerals produced in these experiments will be characterized by modern micro-analytical methods that are capable of determining the chemical composition of the material very sensitively and at a very small scale. The compositions will be compared to those of lavas erupted from subduction-zone volcanoes. Results from this study may help to explain the formation of a larger range of composition of volcanic rocks compared to the traditional model, and it help us to understand the chemical and mechanical processes operating at depth that lead to the formation of the most vigorous volcanism on the planet.
This study of the formation of arc magmas by mélange melting in wedge diapirs will have important consequences for the entire solid-Earth science community and thus, the results of this work will be of interest to a broad range of scientists. This includes petrologists and geochemists who are interested in the generation of magmas along subduction zones, as well as geophysicists and numerical modelers with an interest in the mechanical processes and the thermal state of subduction zones.
