Hf isotope compositions of basaltic rocks from Jeju Island, South Korea: Implications for the origin of the dichotomous isotopic distribution of late Cenozoic magmatism in East Asia

The mantle sources of late Cenozoic intraplate basalts in East Asia can be divided into two distinct domains on the basis of Sr-Nd-Pb isotope systematics: DMM-EM1 in the north and DMM-EM2 in the south. However, the origin of this dichotomous isotopic distribution remains unclear. Here, we present new Hf isotope data for basaltic rocks from Jeju Island, South Korea, and compare the data with published data for coeval basalts from East Asia to understand the nature of the enriched components. The Jeju basalts have 176Hf/177Hf ratios of 0.282861-0.282965 and epsilon Hf values of +3.2 to +6.8. In an Hf-Nd isotope plot, the basalts plot along the terrestrial array, without marked decoupling between Hf and Nd isotopes. Late Cenozoic basalts in East Asia fall mainly in the field of global ocean-island basalts. However, basalts in NE Asia tend to be much less radiogenic than those in SE Asia. In addition, some samples deviate from the terrestrial array, reflecting the involvement of various li-thologies in the source. Major-and trace-element contents of the Asian basalts indicate the presence of subducted oceanic crust (eclogite/pyroxenite) in the mantle source. However, such crust is not a plausible source for the isotopically enriched components, given the less radiogenic Pb isotope compositions and higher 207Pb/206Pb ratios (0.816-0.917) of the late Cenozoic basalts compared with the HIMU end-member. Instead, subducted sediments could have been the enriched source. The representative EM1-type Wudalianchi basalts in NE Asia have high (Zr/Zr*)N ratios that show a negative correlation with epsilon Hf values, indicating the presence of ancient zircon-bearing sediments in the source. In contrast, SE Asian basalts have high Th/La ratios, suggesting the involvement of pelagic clay-rich sediments. A lack of meaningful correlations between Th/La ratios and epsilon Hf values indicates that the sediments are relatively young and were recycled with only a short residence time in the mantle source. Pb-Nd isotope modeling shows that ancient (-2.2 Ga) sediments were involved in the mantle source of the NE Asian basalts, compared with relatively young sediments in the mantle source of the SE Asian basalts.