METASOMATISM OF THE SUB-ARC MANTLE INFERRED FROM TRACE-ELEMENTS IN PHILIPPINE XENOLITHS

ISLAND arc basalts are thought to derive from the melting of the wedge of mantle overlying the subducting slab1; hence, the composition of this mantle wedge, and the nature of any metasomatic fluids or magmas introduced from the slab, have been the subject of much speculation2-6. Most of the evidence bearing on these questions has been indirect, coming from the chemistry of island arc lavas, or from mantle xenoliths originating elsewhere than immediately below the arc7,8. Here we report trace element data for mantle xenoliths from the Luzon arc (Philippines)9, which come from the mantle wedge directly below the arc-front volcanoes. The trace element patterns reflect metasomatism of a relatively depleted mantle (more depleted than the source of mid-ocean-ridge basalts), and identify the metasomatizing agent as a hydrous fluid, rather than a melt. We suggest that the depletion of high-field-strength elements in arc magmas originates from a combination of low initial concentrations in the mantle before metasomatism, and the inability of hydrous fluids to transport these elements.