An Evaluation of Five Models of Arc Volcanism

A first-order observation of convergent margin volcanism is that continental arc stratovolcanoes have substantially elevated incompatible element abundances and notable fractionations of certain incompatible element ratios compared to oceanic arc stratovolcanoes. Models to account for these differences include (1) a greater influence of differentiation and contamination due to thicker crust, (2) a variable slab flux controlled by the slab thermal structure, (3) heterogeneity of the ambient mantle wedge, (4) variations in extents of melting caused by wedge thermal structure, and (5) slab diapirs and subduction erosion that provide different source materials to the melting regime. An evaluation of these models using arc geochemistry, experimental results, physical models, and tectonic constraints shows that models 1, 2, and 5 are inconsistent with a variety of observations. While a slab component is always present along the arc front, variations in the slab component are not the primary cause of the continental/oceanic arc dichotomy. There is compelling evidence for models 3 and 4, wedge heterogeneity and variations in extents of melting, which in combination provide straightforward explanations for the first-order features of arc geochemistry. Oceanic arcs with back-arc basins are formed by large extents of melting of an ambient mantle that is more depleted than the normal ocean ridge basalt mantle source. Continental arcs are formed by smaller extents of melting of an ambient mantle that is usually more enriched. The apparent greater influence of slab components in oceanic arcs can be understood by the greater leverage that the slab component has on depleted mantle compositions. These conclusions apply to the first-order global and regional systematics, which is not to say that individual volcanoes or samples may be more influenced by crustal processes, slab temperature, or a sediment diapir intermittently incorporated into the mantle wedge.