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Critical role of water in the formation of continental crust

Abstract

Continental arcs are the sites of production of continental crust, but the origin of these magmatic systems is not well understood. Although a number of processes have been suggested to be important, the role of water migrating from slab to surface during subduction has been underappreciated. Directly below the Moho, hot (approximately 1,100 °C), hydrous basaltic magmas fractionate as they cool to the regional geotherm at 750 to 800 °C, ultimately solidifying as mafic underplates. Cooling and fractionation cause water to exsolve and ascend, triggering fluid-fluxed melting of overlying mafic underplates and other crust. Melting of prior mafic underplates buffers temperatures and generates the voluminous, juvenile low-K magmas of Cordilleran batholiths. These granitoid magmas comprise a low-temperature slurry of melt and residue, and recrystallize into silicic mush during adiabatic ascent. Such hydrous mushes are intermittently infused by hotter, more mafic magmas, which hybridize and facilitate ascent and, potentially, eruption. Fluid-fluxed melting overcomes many of the general petrological and geochemical problems associated with models dominated by fractional crystallization. The role of water during repeated episodes of mafic underplating is critical to generate the juvenile granitoid infrastructure of the continents.

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Fig. 1: Evidence for cold storage, transcrustal magmatic conditions in Cordilleran crust.
Fig. 2: Schematic phase relations for mantle and crust during magma ascent from subducting slab (>100 km depth) to the surface.
Fig. 3: Cartoon of subduction zone thermal structure.

Figure adapted with permission from ref. 2, Annual Reviews

Fig. 4: Isobaric \(T-X_{{\mathrm{H}}_{2}{\mathrm{O}}}\) pseudosection for sample 87S35a12, demonstrating the effect of adding water to melt volumes.
Fig. 5: Harker diagrams comparing the modelling results with the global average of arc plutonic rocks.

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The data are available within the manuscript files and by request to the corresponding author.

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Acknowledgements

W.J.C. acknowledges a Curtin Research Fellowship and T.E.J. acknowledges support from the State Key Laboratory for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan (Open Fund numbers GPMR201704 and GPMR201903).

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Collins, W.J., Murphy, J.B., Johnson, T.E. et al. Critical role of water in the formation of continental crust. Nat. Geosci. 13, 331–338 (2020). https://doi.org/10.1038/s41561-020-0573-6

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