Collaborative Research: Melange-peridotite Interactions in the Source of Arc Magmas

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Subduction zones are tectonic boundaries where water-rich oceanic plates sink back into the Earth's deep interior. This process is associated with a large array of geological hazards such as magnitude 9 megathrust earthquakes and explosive arc volcanism. Today, millions of people live in areas that are potentially exposed to subduction zone hazards, including large regions of Cascadia and Alaska. However, the mechanisms by which water-rich rocks are transferred from the down-going plate (slab) into the Earth's interior, and the consequences of this transfer process on geological hazards, remain largely unknown. The best rock samples to study material transfer in subduction zones are unique rocks called melanges ('mixed' rocks) that come from the top of the slab. Although largely overlooked in most subduction zone models, these rocks may play a significant role in the generation of arc lavas, and strongly influence the geodynamics of subduction zones. This project will combine high-pressure, high-temperature laboratory experiments on melange materials with numerical modeling to quantify this transfer process in the Earth's interior. This transformative and interdisciplinary approach will provide novel constraints on the role of melanges in the development of subduction zone hazards, as well as training opportunities for graduate, undergraduate, and K-12 students.Conventional models of subduction zones predict that trace element fractionations observed in arc magmas occur during two distinct events: (a) dehydration and release of fluids from altered oceanic crust, and (b) partial melting of sediments at the slab-mantle interface. However, no previous experimental studies have reproduced the chemistry of arc magmas by simulating this conventional model. By contrast, other studies argue that fluids, sediments, and serpentinized wedge mix together at the slab-mantle interface to form melanges, which are hybrid rocks composed of blocks of altered oceanic crust and sediments, mixed with serpentinized peridotite. As the slab descends, melanges formed at the top of the slab may partially melt and produce trace element fractionation typical of arc magmas directly at the slab-mantle interface. Alternatively, melanges may rise from the slab-mantle interface as solid diapirs and partially melt during their ascent, delaying the onset of trace element fractionation. In both scenarios, melange melts would react with the peridotite mantle wedge on their way to the surface, combining their chemical characteristics with that of the mantle. This project will combine a series of high-pressure, high-temperature experiments with numerical modeling to investigate the chemical and physical behavior of natural melange rocks in subduction zones. The combined experimental and modeling results will provide a transformative approach to assess the role of melanges in subduction zone magmatism.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.