Upper mantle scale enrichment of Cenozoic intraplate magmatism in Northeast Asia: He-Sr-Nd-Pb-O isotope geochemistry of the basalts around the Korean peninsula

The Earth’s mantle is considered to be geochemically heterogeneous, which is reflected by the diverse composition of oceanic island basalts (OIB). The mantle enrichment resulting in this is attributed primarily to the influx of recycled crustal materials into the mantle through subduction. Additionally, the sub-continental lithospheric mantle (SCLM) complicates the elucidation of mantle heterogeneity. From this perspective, Northeast Asia, where the Pacific stagnant slab in the mantle transition zone and the SCLM distribution are presented, is the suitable site for examining the upper mantle scale enrichment. Here we report He-Sr-Nd-Pb-O isotope values of basalts that erupted around the Korean Peninsula to illustrate the source lithology and components that led to mantle heterogeneity. Our measured helium isotope ratios ranging from 5.7 to 7.3 Ra (3He/4He ratio of air, Ra = 1.39 x 10-6) are mostly within the SCLM range (6.1 ± 0.9 Ra) but lower than the mid-ocean ridge basalt range (MORB; 8 ± 1 Ra). The Sr-Nd-Pb isotope compositions of the basalts generally display a mixture of depleted MORB mantle (DMM), enriched mantle 1 (EM1), and enriched mantle 2 (EM2) components. In addition, the basalts have δ18Oolivine (vs. V-SMOW) values ranging from 4.7 to 5.7 ‰ that deviate from the DMM range (δ18Oolivine = 5.1 ± 0.2 ‰). Our isotopic analysis results highlight the role of the pyroxenite source in the metasomatized SCLM in the genesis of basalts. The low 3He/4He range of the basalts indicates a significant role in the SCLM. Moreover, the delaminated cratonic SCLM and asthenosphere-lithosphere interaction are candidate scenarios for the low 3He/4He ratios. Therefore, we propose that mixing of DMM (high 3He/4He ratio; 7 to 9 Ra) and the metasomatized SCLM (low 3He/4He ratio; 5 to 7 Ra) allowed enrichment within the upper mantle scale for the Cenozoic intraplate magmatism in Northeast Asia.