RE-OS ISOTOPE SYSTEMATICS OF HIMU AND EMII OCEANIC ISLAND BASALTS FROM THE SOUTH-PACIFIC OCEAN

The Re-Os and complementary Sr, Nd and Pb systematics of 24 oceanic island basalts from the islands of Savaii, Tahaa, Rarotonga, Rurutu, Tubuai and Mangaia are investigated. Re concentrations range from 100 to 1621 ppt (parts per trillion), while Os concentrations vary from 26 to 750 ppt. The Re and Os concentration variations suggest that fractionation and accumulation of olivine, or a low Re/Os phase in conjunction with olivine, is important in determining the Os concentration and the Re/Os ratio of the erupted basalt. Os-187/Os-186 in EMII basalts from Samoa and Tahaa varies from 1.0261 to 1.1275. These ratios are mostly within estimates for depleted upper mantle, and do not constrain the involvement of recycled continental crust in the origin of the EMII signature. Os-187/Os-186 ratios in HIMU basalts from Rurutu, Tubuai and Mangaia range from 1.1159 to 1.2473, and provide strong evidence for the role of subducted oceanic crust in the HIMU source. The Pb-Pb systematics constrain the range of possible ages and U-238/Pb-204 and Th/U ratios of the subducted crust; this crust is estimated to pass through the subduction zone with Rb/Sr, Sm/Nd, Lu/Hf and Th/U ratios similar to fresh MORB. The homogeneity of the Os isotopic compositions in the Tubuai and Mangaia basalts indicates that interaction of these basalts with low Os-187/Os-186 mantle had an insignificant effect on the Os isotopic composition of the erupted magmas. This requires a network of channels, veins or cracks capable of delivering melt from the source region (plume) to the surface fast enough to avoid interaction with the depleted upper mantle and the oceanic lithosphere. The possible identification of the HIMU signature (high Pb-206/Pb-204, low Sr-87/Sr-86) with recycled oceanic crust suggests the possible presence of segments of recycled crust, with independent histories, in other oceanic mantle sources, including that of some mid-ocean ridge basalts.