What is the Transport Mechanism of Sediments in Subduction Zones?

Veronique Le Roux , Geology & Geophysics



Subduction zones, where an oceanic plate dives underneath another plate, are heavily monitored by the scientific community because this is where earthquakes, tsunamis and explosive volcanic eruptions can be triggered. The surface expression of subduction zones are volcanic arcs (e.g. Antilles), which are produced by melting of rocks in the deep Earth. The geochemical signature of arc volcanoes indicates that oceanic sediments that were once deposited on the seafloor are recycled at depth during subduction and participate to the formation of arc volcanoes. However, the processes by which those sediments leave the subducting plate and are entrained in the source of arc volcanoes are unconstrained, due to inaccessibility of samples and inadequate resolution of geophysical tools. The widely accepted hypothesis is that subducted sediments melt and rise to the source of arc volcanoes as liquid melts. However, recent groundbreaking field observations have shown that sediments may mechanically mix with the overlying plate, creating hybrid rocks called ‘mélange’. In that scenario, mélange rocks may primarily rise as solid diapirs instead of liquid melts. Those two scenarios imply drastically different transport mechanisms of sediments at depth, as well as drastically different timing for the onset of elemental fractionations that are typical of arc lava chemistry. Determining which process mainly controls elemental transport in subduction zones is critical to provide accurate models of those systems. Here we will perform high-pressure, high-temperature experiments in the experimental petrology laboratory of WHOI to determine if interaction between solid mélange diapirs and rocks in the deep Earth can in fact reproduce the specific chemical signature of arc volcanoes worldwide. This project is a seed to foster multiple collaborations