Constraints on HIMU and EM by Sr and Nd isotopes re-examined

Abstract

Sr and Nd isotopes together with trace elements for ocean island basalts in the Polynesian region have been analyzed in order to investigate the origin of the HIMU and EM sources. Both whole rocks and cpx phenocrysts were analyzed for isotopic composition. Cpx samples from HIMU islands show quite uniform 87Sr/86Sr ratios (~0.70274), while leached and unleached whole rock samples show variable and higher 87Sr/86Sr than those of cpx samples. These results suggest that even leached whole rock samples have been affected by secondary contaminations of sea water. On the other hand, cpx preserves a pristine isotopic signature with minimal secondary effects. Using only the cpx analyses, HIMU form a vertical linear trend in the Sr-Nd isotope diagram with small variation in ε(Nd) (+3.3~+5.5) and constant 87Sr/86Sr. This trend is explained by a mixing of the HIMU end-member and the MORB source. Since ε(Nd) of the HIMU end-member is constrained to be less than +3.3, the HIMU source should include former sediment added to oceanic crust. To explain the vertical nature of the mixing trend, the HIMU end-member should have similar Rb/Sr to the MORB source, or much lower Sr/Nd ratio than the MORB source, which favors a mixing model between extensively dehydrated oceanic crust and sediment as the HIMU source. The correlation between ε(Nd) and trace element ratios such as Pb/Ta also supports the model.