At subduction zones, crustal material enters the mantle. Some of this material, however, is returned to the overriding plate through volcanic and plutonic activity. Magmas erupted above subduction zones show a characteristic range of compositions that reflect mixing in the magma source region between three components: hydrous fluids derived from the subducted oceanic crust, components of the thin veneer of subducted sediments and peridotite mantle rocks. The mechanism for mixing and transport of these components has been enigmatic. A combination of results from the fields of petrology, numerical modelling, geophysics and geochemistry suggests a two-step process. First, intensely mixed metamorphic rock formations—mélanges—form along the interface between the subducted slab and the mantle. As the mélange contains the characteristic three-component geochemical pattern of subduction-zone magmas, we suggest that mélange formation provides the physical mixing process. Then, blobs of low-density mélange material—diapirs—rise buoyantly from the surface of the subducting slab and transport the well-mixed mélange material into the mantle beneath the volcanoes.
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We thank J. Blundy, N. Shimizu, G. Abers and the participants of the 2010 State-of-the-Arc meeting for discussions. G. Harlow is thanked for providing unpublished whole-rock data. Careful and constructive reviews by B. Hacker, S. Penniston-Dorland and P. Agard are gratefully acknowledged. H.M. was financially supported by the J. Lamar Worzel Assistant Scientist Fund and the Penzance Endowed Fund in Support of Assistant Scientists. Financial support from NSF grant no. 1119403 (G. Harlow) is acknowledged. We would like to thank A. Hertwig, W. Maresch and H-P. Schertl for guiding us in the field.
The authors declare no competing financial interests.
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Marschall, H., Schumacher, J. Arc magmas sourced from mélange diapirs in subduction zones. Nature Geosci 5, 862–867 (2012). https://doi.org/10.1038/ngeo1634
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