The Samoan mantle plume is thought to host three isotopically (radiogenic) distinct low He/He components including EM2 (enriched mantle 2), dilute HIMU (high μ = U/Pb), and a depleted mantle (DM) component, which were sampled by shield stage lavas from the Malu, Vai, and Upo trend volcanoes, respectively.
Fig1. (a) Topographic map showing the distribution of Samoan islands and seamounts. (b) Simplified geological map of Tutuila Island.
However, it is unclear whether the isotopically distinct components are present as different lithologies. Using new Fe–Sr–Nd-Pb isotope data for Tutuila basalts (Samoa), combined with literature data for other Samoan basalts, we attempt to infer the lithological structure of the Samoan plume. The results show that “Malu trend” basalts have heavier Fe isotopic compositions (δFe = 0.15–0.24‰) than “Vai trend” and “Upo trend” basalts. The latter two groups have average δFe of 0.14 ± 0.07‰ (2SD) and 0.11 ± 0.03‰ (2SD), respectively, similar to normal midocean ridge basalts (N-MORBs, δFe = 0.15 ± 0.05‰, 2SD). The fractional-crystallization-corrected δFe values of all shield lavas are positively correlated with (Gd/Yb), Pb/Nd and Sr/Sr ratios whereas negatively correlated with Nb/Th and ε ratios, which cannot be explained by partial melting of a single garnet peridotite but point to heterogeneous source lithologies. The EM2 lavas are characterized with high δFe and (Gd/Yb), low Nb/Th, and enriched Sr–Nd isotopic ratios, requiring a pyroxenitic source component with imprints of both recycled terrigenous sediments and oceanic crust. The Vai- and Upo-trend lavas with MORB like δFe can be explained by partial melting of peridotitic sources, although different extents of refertilization by recycled crust are essential for generating their distinct radiogenic isotope signatures.
Fig2. Sr, Nd and Fe isotopic comositions of the studied Tutuila samples.
These observations highlight the lithological heterogeneity of the Samoan plume and relates the EM2 component with a pyroxenitic lithology.