Serpentinites exhumed from mantle wedges are enriched in elements that are mobile at low temperatures, such as As, Sb, Pb, and Sr, based on samples from the Himalayas and the Alps. Such data provide compelling evidence that these fluid-soluble elements were incorporated into the overlying fore-arc mantle wedge from the subducting slab. Thus, serpentinites in the fore-arc mantle act as a sink for water and fluid-soluble elements, insulating the overlying mantle from further hydration.. Downward movement of the serpentinite layer, facilitated by mantle flow, causes transport of these elements to deeper, hotter levels in the mantle. Eventual dehydration of these serpentinites discharges the fluid-soluble elements with water, leading to partial melting of the mantle wedge and accounting for the observed enrichment of these elements in magmas at the volcanic front.