HIGH MU (HIMU) OCEAN ISLAND BASALTS IN SOUTHERN POLYNESIA - NEW EVIDENCE FOR WHOLE MANTLE SCALE RECYCLING OF SUBDUCTED OCEANIC-CRUST

Major elements, trace elements, and Pb isotopic compositions were dete rmined for ocean island basalts (OIBs) from Polynesia in the southern Pacific in order to document the chemical characteristics of OIB sourc es and to understand their origin. High mu (HIMU: mu = U-238/Pb-204) b asalts, which have distinctly high Pb isotopic ratios, have systematic ally different compositions from non-HIMU basalts; HIMU basalts are mo re enriched in Fe2O3, MnO, and CaO and more depleted in SiO2, K2O, P2 O5, Ni and incompatible trace elements than non-HIMU, except for Nb. M ajor element characteristics of HIMU basalts suggest that the HIMU sou rce is more fertile, i.e., more enriched in a basaltic component, than non-HIMU sources. This is consistent with the suggestion that subduct ed oceanic crust may contribute to the formation of the HIMU reservoir . Relative depletion of incompatible trace elements in HIMU is consist ent with involvement of sedimentary components in non-HIMU sources. Ho wever, enrichment of Nb relative to other incompatible elements in HIM U cannot be explained by simple addition of the crustal component nor partial melting processes in the upper mantle, implying that lower man tle processes may contribute to the formation of the HIMU source.