Bulk chemical trends at arc volcanoes are not liquid lines of descent

Although the magmatic output of arc volcanoes defines clear chemical trends, the compositional sequence of eruption is often chaotic. At best, support for the concept of progressive evolution by fractionation/assimilation in a central chamber is ambiguous. Rather than defining a liquid line of descent, we suggest that chemical trends in arc systems are populated by discrete magma batches. These have been derived from a limited set of sources and undergone a common family of processes, but in different proportions. Crystal–liquid fractionation may occur at the mafic end of the spectrum in classical fashion by gravitational crystal extraction from melt-rich magma. However, at the silicic end where melt viscosity is high, fractionation may occur by forceful expulsion of interstitial melt from near-solidus arc plutons and adjacent crust. The andesite/dacite middle ground is commonly occupied by relatively crystal-rich magma batches with complex mixing and mingling histories. Such intermediate magmas appear to be stable residents of shallow arc crust—mechanically because of their neutral buoyancy, thermally because they are frequently recharged with basalt, and chemically because they resist large shifts in composition. Stored magma that “regresses” in an evolutionary sense by mixing or mingling during recharge is preferentially erupted and forward evolution during quiescence is inhibited by high viscosity of melt. The surface expression is the building of a stratocone or dome complex. In contrast, silicic magmas, once they have coalesced from interstitial melt in their plutonic sources through coupled porous flow and diking, tend to be erupted promptly, flushing out resident andesite/dacite magma batches in their path. The surface expression is caldera formation, which commonly terminates one episode of cone building and serves as a prelude to another.