2020 | Mélange diapirs in subduction zones as the source of arc magmas

Geochemical models invoke three-component mixing to derive the source regions of magma generation in subduction-zone: portions of altered oceanic crust and the thin sedimentary veneer of the slab are add- ed to the depleted peridotite in the mantle wedge. However, the physical processes of transport and mix- ing of these components within the subduction factory are still largely enigmatic. More sophisticated mod- els of subduction zones have recently emerged that suggest that intense mixing and metasomatism occur at the slab-mantle interface, which leads to mixing of subducted crustal rocks with hydrated and metaso- matized mantle-wedge material to produce high-pressure mélanges (from French ‘mixture’ or ’blend’). These new models further suggest that the well-mixed materials are transported into the hot corner of the mantle wedge beneath arcs by low-density mantle-wedge diapirs. Melt is produced either in the low- density diapirs from melting the mélange rocks due to heating and decompression, or in the overlying mantle-wedge peridotite due to the influx of fluids released from the dehydrating plume (e.g. Marschall & Schumacher 2012 Nature Geoscience; Nielsen & Marschall 2017 Science Advances). Numerical models of subduction zones have predicted both the mixing process between slab and mantle material at the slab–mantle interface and the formation of cold low-density diapirs rising from these mé- lange zones. Recent experimental studies have started to investigate the major- and trace-element com- position and volatile content of melts produced during partial melting of mélange rocks at intermediate mantle-wedge pressure-temperature conditions (1.5–2.5 GPa) and determined the mineralogical and chemical composition of the restitic rocks produced during dehydration and melting of mélange rocks at these conditions (e.g. Codillo et al. 2018 Nature Communications; Cruz-Uribe et al. 2018 Geology). In this project, we will employ the unique experimental equipment at Frankfurt to extend the investigated P–T range to conditions equivalent to deeper levels (up to 150 km) along the slab-mantle interface and in the mantle wedge to characterize the behavior of mélange material below the magmatic arc at depths all the way down to the slab surface (typically 80–150 km). The mineralogical and chemical compositions of metamorphic and melting products at these conditions will be determined and compared to magmas pro- duced in subduction zones. We will also start to determine the size and shape of mélange diapirs through numerical modeling, and determine what the characteristic seismic features of mélanges and mélange diapirs would be in subduction zones and how these signals would contrast with diapir-free mantle wedg- es. The study will not be completed in its entirety within this initial 9-month TeMaS project, but is instead a larger, long-term endeavor that will be an important part of the planned future TeMaS SFB project. The combined products of such a large-scale interdisciplinary study will enable us to unravel the ways in which primary magmas are produced in subduction zones. This 2020 TeMaS project is the next step in that direction.