ABSTRACT

To investigate the origin of the HIMU (high-μ) reservoir in the mantle, we measured Li, Sr, and Nd isotopic compositions of several oceanic island basalts (OIBs) from the Polynesian region. We used a recently developed multiple-collector inductively coupled plasma mass spectrometry method that allows precise and accurate Li isotopic determinations. This study presents the first Li isotopic data on HIMU OIBs. The measured whole-rock δ⁷Li values (δ⁷Li = [[⁷Li/⁶Li]_sample/[⁷Li/⁶Li]_{L-SVEC standard} - 1] × 1000) of the Polynesian HIMU OIBs (Mangaia, Tubuai, and Rurutu) range from +5.0‰ to +7.4‰, which are higher than those of fresh normal mid-ocean ridge basalt (N-MORB) glasses (ca. +3‰). The simultaneously measured K/Rb, Ba/Rb, and ⁸⁷Sr/⁸⁶Sr ratios indicate that the analyzed HIMU OIBs are free from significant posteruption alteration. These results suggest that the δ⁷Li value of the Polynesian HIMU source is never lower than those of the N-MORBs. Among the numerous models for the origin of the HIMU source, the most widely accepted model is that it involves subducted (dehydrated) oceanic crust. For this HIMU-origin model, our new Li isotopic results exclude the highly altered portion, that is, the uppermost part of the oceanic crust, because the δ⁷Li value of subducted highly altered MORB should be extremely low (δ⁷Li < fresh MORB). For these reasons, we propose that the Polynesian HIMU source is the relatively less-altered oceanic crust underlying the highly altered crust. Whereas Pb, Sr, and Nd isotopic signatures dominantly indicate the involvement of sediments in a source, the Li isotopic signature is more sensitive to the degree of alteration experienced by the basaltic crust and thus can be used to distinguish what part of the crust was trapped in the OIB magma. It therefore provides information complementary to that provided by the radiogenic isotopes.