Volcanic arcs as archives of plate tectonic change

Processing of the oceanic lithosphere in subduction zones gives rise to arc magmatism, and strong compositional links exist between trench input and arc output. Here we address the question whether these compositional links are sufficiently strong to allow for ‘tracing’ the composition of the sedimentary and igneous oceanic crust through the chemistry of arcs. The tracing approach hinges critically on whether key characteristics of the subducted slab are transmitted to arcs. Results from forward and inverse modeling, verified by observations from modern arc settings, demonstrate that elements Sr, Pb, Nd and Hf that are associated with radiogenic isotopes may preserve chemical characteristics of the subducted slab in arc magmas. The data indicate that the much thicker igneous subducted crust dominates the recycled flux to arcs. The flux from the highly enriched, but thin sediment layer is buffered, and may be even concealed, by the concomitant contributions from igneous crust, and/or subarc mantle, despite the much better visibility of sediment components in trace element and isotope space. Arc Pb and Pb isotopes are the most promising tracers that may capture the isotopic diversity of subducted MORB-type and OIB-type crust with sufficient temporal and spatial resolution. While arc Sr is also strongly controlled by the flux from the subducted crust, arc data may allow for distinguishing among radiogenic Sr recycled from altered oceanic crust or from subducted sediment in moderately radiogenic arcs (⁸⁷Sr/⁸⁶Sr < ~ 0.7045). Co-mingling of Nd and Hf from igneous subducted crust with mantle contributions mostly hinders the isotopic identification of subducted crust through arc chemistry. However, Nd and Hf may provide complementary information about the efficiency of recycling, and recycling via subduction erosion.The tracing approach appears feasible in Cenozoic arcs where much of the original subduction context is preserved. First results from the Izu Bonin and Central American arcs show that plate tectonic events like oceanic plate formation and destruction, subduction of hotspot tracks and the closure of oceanic gateways are recorded in the chemistry of arcs. A comparative evaluation of Cenozoic global arcs may hence significantly complement the information from the modern oceanic basins, help to obtain a more complete image of the oceanic crustal composition and implicate the geochemical processes by which it formed. Possibly, the tracing approach may also be useful in ancient, inactive arcs to obtain information on the composition of oceanic crust subducted in the geological past.