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Magma Generation in Subduction Zones – Novel vs. Classical Models

Dr Horst Marschall | Wilhelm-Heraeus-Professor for Deep Earth Processes  |  Goethe-Universität Frankfurt

Abstract:  At subduction zones, crustal material enters the mantle. Some of this material is returned to the overriding plate through volcanic and plutonic activity. It has long been established that magmas erupted above subduction zones show a characteristic range of compositions that reflect this addition of material derived from the subducting slab to the mantle source of the magmas. Yet, the mechanism for mixing and transport of these components is still enigmatic and a matter of much debate. In the classic model, discrete portions of fluids and melts migrate from the slab into the mantle wedge and trigger the generation of arc magmas by flux melting of the hot mantle. Recent geochemical studies have demonstrated that this model is not consistent with the compositional record of the erupted magmas in global subduction zones [1]. An alternative model that has emerged recently based on 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. 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 [2]. In this presentation I will discuss the evidence for the mélange-diapir model from various fields of subduction-zone research, address consequences of the model for data interpretation, and evaluate further research questions that follow from the model.

[1] Nielsen SG, Marschall HR (2017) Geochemical evidence for mélange melting in global arcs. Science Advances, 3, e1602402 [doi: 10.1126/sciadv.1602402]
[2] Marschall HR, Schumacher JC (2012) Arc magmas sourced from mélange diapirs in subduction zones. Nature Geoscience, 5: 862–867 [doi: 10.1038/NGEO1634]