Stephen J Turner, Ivan Savov, Tamsin Mather, Richard Katz, David Rees Jones, David Pyle, Cees-Jan de Hoog, Sebastian Watt

The rates and pathways of material transport from subducting plates to arc volcanoes control the long-term chemical evolution of the atmosphere, continents, and mantle. Arc magma compositions are commonly used as proxies for the state of the slab directly below a volcanic vent, under the assumption of vertical transport from the slab to the surface. Here, we present new boron (B) isotope and trace element data that challenge this assumption. Measurements of olivine-hosted melt inclusions from 900 km along and 200 km across the Southern Andean Volcanic Zone reveal remarkably coherent trends, indicating: (1) That the B isotope composition of the slab component sampled by erupted magma is invariant with slab depth, contrary to the expectation that the slab composition should progressively change with dehydration, and (2) that slab overprinting decreases with the distance from a long-lived arc-front stratovolcano in both along- and across-arc directions. These observations indicate that slab liquids generated across a large depth range are subsequently homogenized and focused both parallel and perpendicular to the trench. Similar geochemical behavior is apparent in other arc segments. We hypothesize that along-arc focusing is the consequence of periodic-in-space, solid-state, abortive upwelling from the slab surface, and that this produces persistent zones of elevated mantle melting consistent with the characteristic narrow geometry and isolated, long-lived stratovolcanoes of volcanic arcs. This framework implies that the structures of volcanic arcs have a deep origin, and can be used to better interpret global variations in subduction fluxes and their relationship with subduction parameters.