Widespread slab melting in modern subduction zones

It is still a matter of intense debate to what extent partial melting of the subducting slab contributes to arc magmatism in modern subduction zones. In particular, it is difficult to differentiate between silicate melts formed by partial melting of the slab, and aqueous fluids released during subsolidus dehydration as the main medium for slab-to-mantle wedge mass transfer. Here we use delta 49/47Ti (the deviation in 49Ti/47Ti of a sample to the OL-Ti reference material) as a robust geochemical tracer of slab melting. Hydrous partial melting of subducted oceanic crust and the superjacent sedimentary layer produces silicic melts in equilibrium with residual rutile. Modelling shows that such silicic slab melts have notably higher delta 49/47Ti (+0.24 +/- 0.06 %o) than their protolith due to the strong preference of rutile for the lighter isotopes of Ti. In contrast, even highly saline fluids cannot carry Ti from the slab and hence hydrous peridotite partial melts have delta 49/47Ti similar to mid-ocean ridge basalts (MORB; ca. 0 %o).Primitive (Mg# >= 60) arc lavas from eight subduction zones that are unaffected by fractional crystallisation of Fe-Ti oxides show a more than tenfold larger variation in delta 49/47Ti than found in MORB. In particular, primitive arc lavas display a striking correlation between SiO2 content and delta 49/47Ti that ranges from island arc basalts overlapping with MORB, to primitive rhyodacites with delta 49/47Ti up to 0.26 %o erupted in the western Aleutian arc. The elevated delta 49/47Ti of these primitive arc lavas provides conclusive evidence for partial melts of the slab as a key medium for mass transfer in subduction zones. The Aleutian rhyodacites represent a rare example of slab melts that have traversed the mantle wedge with minimal modification. More commonly, slab melts interact with the mantle wedge to form an array of primary arc magmas that are a blend of slab-and peridotite-derived melt. We identify primitive arc lavas with a clearly resolvable slab melt signature in all eight subduction zone local-ities, confirming that slab melting is prevalent in modern subduction zones.