High field strength and transition element systematics in island arc and back-arc basin basalts: evidence for multi-phase melt extraction and a depleted mantle wedge

The geochemical character of the mantle wedge in convergent margin settings is investigated using the high field strength and transition elements Ti, Zr, V, Sc and Y in basaltic rocks from island arcs and associated back-arc basins. Systematic differences are observed in the composition of each arc relative to the adjacent back-arc basin. The arcs have lower average abundances of incompatible high field strength elements and Y, and extend to higher ratios of Ti Zr, V Ti and Sc Y. Partial melting of a normal mid-ocean ridge basalt source accounts for the back-arc basin basalt compositions, but cannot explain the higher element ratios observed in many arc basalts. These arc magmas are consistent with derivation from sources which are more depleted in incompatible elements those that of the back-arc basin basalts. Source depletion by melt extraction, prior to arc magma genesis, explains the similar degree of Y depletion and, by analogy, the heavy REE depletion, to that of the high field strength elements in island arc basalts. The observations are not compatible with the presence of residual titanate phases (e.g. rutile). However, amphibole may be required in the source of a subgroup of arc basalts which have relatively high Zr abundances and anomalously low Ti Zr and high V Ti values. These results have implications for subduction zone geochemical budgets, as the influence of any slab-derived component may be enhanced in regions of extensive mantle wedge depletion.