A quantitative framework for global variations in arc geochemistry

Abstract

Arc magmas are generated by partial melting of a mantle wedge modified by inputs from subducting sediment and igneous ocean crust. Despite this uniform tectonic process, global arc-front stratovolcano compositions exhibit trace element abundances that can vary by more than an order of magnitude. All incompatible elements correlate well, including those that are thought to be wedge derived, “fluid-mobile”, and sediment derived. It has been suggested that global variations in arc trace element abundances occur due to the addition of variable slab components to a depleted mantle wedge, though quantitative global models that include both trace elements and isotopes have been lacking. Here we present an alternative framework in which a relatively constant proportion of slab material, consisting of melts both of altered ocean crust and sediment, is added to ambient mantle wedge that varies from more depleted than MORB to as enriched as “EM1” ocean island basalt sources. The modified mantle melts to varying extents, with melting controlled primarily by the thickness of the overlying lithosphere. Thick lithosphere leads to lower extents of melting in the garnet stability field in most continental arcs. Continental arcs also have enriched ambient mantle, likely because of contributions from enriched sub-continental lithospheric mantle. High degrees of melting of more depleted sources cause oceanic arcs to have low incompatible element abundances. Quantitative modeling of these processes shows that this framework can account for the global arc systematics exhibited by arc-front stratovolcanoes and provides a tool to examine regional variations and more unusual magma compositions. Variations in slab temperature and amount of slab input remain important for second-order variations and certain element ratios.