Experimental evidence for fluid-induced melting in subduction zones

Although subduction zones are the main source of seismic and volcanic hazards on Earth, the causes of melting in volcanic arcs are still not fully understood. Recent models suggested that melting in the mantle wedge is not caused by hydrous fluids, but by sediment melts ascending from the subducted slab. A main argument for these models was that hydrous fluids are too dilute to produce the trace element enrichment observed in arc magmas. Here we demonstrate experimentally that even moderate salinities enhance the partitioning of trace elements such as the light rare earths, alkalis, alkaline earths, Pb, and U into the fluid by several orders of magnitude. Our data therefore show that saline hydrous fluids released from the basaltic part of the oceanic crust may produce the enrichment in LILE and light REE elements, and the negative Nb-Ta anomaly observed in typical arc magmas.