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 crust(1). 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 petrogenesis(2) 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% SiO(2)) 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% SiO(2)) 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.