Evidence for mantle metasomatism by hydrous silicic melts derived from subducted oceanic crust

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 crust1. Whether this agent is an hydrous fluid2 or a silica-rich melt3 has been discussed using mainly a trace-element approach4 and related to variable thermal regimes of subduction zones5. 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 arcs6,7. 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 zones6 and to veins found in exhumed metamorphic terranes of fossil convergent zones7. 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 peridotites8. We therefore suggest that mantle metasomatism by slab-derived melt is a more common process than previously thought.