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A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites


Andesites represent a large proportion of the magmas erupted at continental arc volcanoes and are regarded as a major component in the formation of continental crust1. Andesite petrogenesis is therefore fundamental in terms of both volcanic hazard and differentiation of the Earth. Andesites typically contain a significant proportion of crystals showing disequilibrium petrographic characteristics indicative of mixing or mingling between silicic and mafic magmas, which fuels a long-standing debate regarding the significance of these processes in andesite petrogenesis2 and ultimately questions the abundance of true liquids with andesitic composition. Central to this debate is the distinction between liquids (or melts) and magmas, mixtures of liquids with crystals, which may or may not be co-genetic. With this distinction comes the realization that bulk-rock chemical analyses of petrologically complex andesites can lead to a blurred picture of the fundamental processes behind arc magmatism. Here we present an alternative view of andesite petrogenesis, based on a review of quenched glassy melt inclusions trapped in phenocrysts, whole-rock chemistry, and high-pressure and high-temperature experiments. We argue that true liquids of intermediate composition (59 to 66 wt% SiO2) are far less common in the sub-volcanic reservoirs of arc volcanoes than is suggested by the abundance of erupted magma within this compositional range. Effective mingling within upper crustal magmatic reservoirs obscures a compositional bimodality of melts ascending from the lower crust, and masks the fundamental role of silicic melts (≥66 wt% SiO2) beneath intermediate arc volcanoes. This alternative view resolves several puzzling aspects of arc volcanism and provides important clues to the integration of plutonic and volcanic records.

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Figure 1: SiO 2 contents (H 2 O-free) of melt inclusions in arc magmas.
Figure 2: H 2 O–SiO 2 contents of melt inclusions from several arc volcanoes.
Figure 3: Comparison of chemical variation in melt inclusions, volcanic bulk rocks and experimental melts.
Figure 4: Experimental liquid lines of descent in temperature–composition space at a range of crustal pressures (100–700 MPa).
Figure 5: Chemical variation in melt inclusions, plutonic bulk rocks and experimental melts.

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This work was supported by a Marie Curie Fellowship (O.R.) and an NERC Senior Research Fellowship (J.B.). We thank S. Sparks, K. Kelley and K. Roggensack for comments on an early draft of this manuscript, and P. Wallace and M. Pichavant for critical reviews. We are grateful to V. C. Smith for unpublished data and discussions.

Author Contributions O.R. and J.B. developed the discussion. O.R. took the lead in writing the paper.

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Correspondence to Olivier Reubi.

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Reubi, O., Blundy, J. A dearth of intermediate melts at subduction zone volcanoes and the petrogenesis of arc andesites . Nature 461, 1269–1273 (2009).

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