Constraints from partitioning experiments on the composition of subduction-zone fluids

THE generation of calc-alkaline magmas in subduction zones is thought to be the most important mechanism for the growth of continental crust since the Proterozoic eon. It is widely assumed that most of these magmas are products of fluid-triggered melting in the mantle wedge above the subducted slab(1,2). Fluid transport from the subducted slab into the zone of melting has also been invoked in order to explain many of the trace element and radiogenic isotope characteristics of calk-alkaline magmas(3-7). Here I report experimental data on the partitioning of trace elements between fluids, silicate melts and minerals, which suggest that the agent responsible for the transport of trace elements in subduction zones may be an alkali-chloride-rich aqueous fluid. The data show that chemical transport by such a fluid can generate the trace element and isotope enrichment pattern typical for calc-alkaline magmas, including the enrichment of large ionic lithophile elements, lead and uranium, and the characteristic depletion in niobium and tantalum.