The low concentrations of niobium, tantalum and titanium observed in island-arc basalts are thought to result from modification of the sub-arc mantle by a metasomatic agent, deficient in these elements, that originates from within the subducted oceanic crust(1). Whether this agent is an hydrous fluid(2) or a silica-rich melt(3) has been discussed using mainly a trace-element approach(4) and related to variable thermal regimes of subduction zones(5). Melting of basalt in the absence of fluid both requires high temperatures and yields melt compositions unlike those found in most modern or Mesozoic island arcs(6,7). Thus, metasomatism by fluids has been thought to be the most common situation. Here, however, we show that the melting of basalt under both H2O-added and low-temperature conditions can yield extremely alkali-rich silicic liquids, the alkali content of which increases with pressure. These liquids are deficient in titanium and in the elements niobium and tantalum and are virtually identical to glasses preserved in mantle xenoliths found in subduction zones(6) and to veins found in exhumed metamorphic terranes of fossil convergent zones(7). We also found that the interaction between such liquids and mantle olivine produces modal mineralogies that are identical to those observed in metasomatized Alpine-type peridotites(8). We therefore suggest that mantle metasomatism by slab-derived melt is a more common process than previously thought.