Coupled Hf-Nd-Pb isotope co-variations of HIMU oceanic island basalts from Mangaia, Cook-Austral islands, suggest an Archean source component in the mantle transition zone

Although it is widely accepted that oceanic island basalts (OIB) sample geochemically distinct mantle reservoirs including recycled oceanic crust, the composition, age, and locus of these reservoirs remain uncertain. OIB with highly radiogenic Pb isotope signatures are grouped as HIMU (high-mu, with mu = U-238/Pb-204), and exhibit unique Hf-Nd isotopic characteristics, defined as Delta epsilon(Hf), deviant from a terrestrial igneous rock array that includes all other OIB types. Here we combine new Hf isotope data with previous Nd-Pb isotope measurements to assess the coupled, time-integrated Hf-Nd-Pb isotope evolution of the most extreme HIMU location (Mangaia, French Polynesia). In comparison with global MORB and other OIB types, Mangaia samples define a unique trend in coupled Hf-Nd-Pb isotope co-variations (expressed in Pb-207/Pb-206 vs. Delta epsilon(Hf)). In a model employing subducted, dehydrated oceanic crust, mixing between present-day depleted MORB mantle (DMM) and small proportions (similar to 5%) of a HIMU mantle endmember can re-produce the Hf-Nd-Pb isotope systematics of global HIMU basalts (sensu stricto; i.e., without EM-1/EM-2/FOZO components). An age range of 3.5 to <2 Ga is required for HIMU endmember(s) that mix with DMM to account for the observed present-day HIMU isotope compositions, suggesting a range of age distributions rather than a single component in the mantle. Our data suggest that mixing of HIMU mantle endmembers and DMM occurs in the mantle transition zone by entrainment in secondary plumes that rise at the edge of the Pacific Large Low Seismic Velocity Zone (LLSVP). These create either pure HIMU (sensu stricto) or HIMU affected by other enriched mantle endmembers (sensu lato). If correct, this requires isolation of parts of the mantle transition zone for >3 Gyr and implies that OIB chemistry can be used to test geodynamic models. (C) 2013 Elsevier Ltd. All rights reserved.