Geochemical characteristics and origin of the HIMU reservoir: A possible mantle plume source in the lower mantle

Combined Pb-Sr-Nd-Hf-Os isotopes, together with major and trace element compositions, were determined from clinopyroxene and olivine phenocrysts, along with whole rocks, for ocean island basalts with high μ(μ = ²³⁸U/²⁰⁴Pb) (HIMU) and enriched mantle isotopic characteristics from Cook-Austral Islands. Clinopyroxene and olivine separates record reliable isotopic information of the sources because of minimized in situ radiogenic ingrowth and their lower susceptibility to crustal contamination. Coherent isotopic systematics in multi-isotope spaces defined by the HIMU samples are best explained by recent mixing of melts derived from the HIMU reservoir and the local shallow mantle. The isotopic compositions of the HIMU reservoir are constrained to be low ε_Nd (≤+4), low ε_Hf (≤+3), and moderately radiogenic ¹⁸⁷Os/ ¹⁸⁸Os (0.14-0.15) in association with radiogenic Pb isotopes ( ²⁰⁶Pb/²⁰⁴Pb ≥21.5). Since ancient oceanic crust would have had exceptionally radiogenic ¹⁸⁷Os/¹⁸⁸Os, moderately high ¹⁸⁷Os/¹⁸⁸Os precludes recycled oceanic crust as the only contributor to the HIMU reservoir. Instead, mantle metasomatized with partial melts from subducted oceanic crust is a likely candidate for the HIMU reservoir. Moreover, partial melting of oceanic crust in equilibrium with Mg perovskite would fractionate U/Pb, Sm/Nd, and Lu/Hf, which are in accordance with the time-integrated U/Pb, Sm/Nd, and Lu/Hf deduced from Pb, Nd, and Hf isotopic compositions of the HIMU reservoir, respectively, with a formation age of 2-3 Ga. We thus propose that the HIMU reservoir was formed by hybridization of a subducted oceanic crust-derived melt with the ambient mantle and then stored for several billion years in the lower mantle.