SLAB-LIQUID FOCUSING TO ARC-FRONT STRATOVOLCANOES — EVIDENCE FROM BORON ISOTOPES AND TRACE ELEMENTS (Invited Presentation)

Advances in our understanding of subduction fluxes have been enabled by a combined approach of geophysical modeling and geochemical measurements. For example, numerical models predict variable slab temperatures beneath arc volcanoes that appear to be related to individual volcano or arc-averaged lava compositions. This approach is complicated, however, by the possibility that liquids generated within the slab (aqueous fluids, hydrous melts, or supercritical liquids) do not migrate vertically from their point of origin (where the slab dehydrates or melts), which means that the slab materials sampled by an arc volcano could have been extracted from the slab across a large range of pressures and temperatures. It is also possible that slab liquids are unevenly distributed in the mantle along arc fronts.

Boron (B) data can help constrain slab transport pathways because subducting materials have high B abundances with unique isotope compositions that partition strongly into liquid phases. Here, we present B isotope and trace element measurements of olivine-hosted melt inclusions sampled from a region spanning ~900km along and ~200km across Chile and Argentina. We found remarkable regional correlations that: (1) appear to require mixing between the mantle and a homogeneous slab component, which is contrary to the expectation that the B isotope composition of the slab should progressive change; (2) slab overprinting of the mantle source is generally reduced away from long-lived arc-front stratovolcanoes, even along the arc front. These observations indicate that slab liquids generated across a large depth range are subsequently homogenized and focused both parallel and perpendicular to the trench, creating an associated variation in magma flux that accounts for intermittent stratovolcano spacing within a narrow arc front. Similar geochemical behavior is apparent using data from other arcs. We present a conceptual model to account for along-arc focusing as the consequence of a multi-stage process that begins with solid-state upwelling of buoyant material that is periodic with along-arc distance on the slab surface. Liquids emerging from the slab are then deflected by the solid upwellings and their sloped porosity gradients. Melt extraction from the slab surface leaves behind a dense residual assemblage that prevents the formation of long-lived diapirs.