Helium isotopes and olivine geochemistry of basalts and mantle xenoliths in Jeju Island, South Korea: Evaluation of role of SCLM on the Cenozoic intraplate volcanism in East Asia

Jeju Island, South Korea, is a Cenozoic intraplate volcano located in East Asia. The Jeju basalts display ocean island basalt (OIB)-like trace element patterns and enriched mantle type 2 (EM2) radiogenic isotope compositions, but the source that enriched the mantle remains unclear. Here we report new geochemical compositions of basalts and xenoliths, including the first helium isotope and element analysis of olivine phenocrysts to constrain the source of basalt. Olivines in the Jeju basalts have 3He/4He ratios of 3.5 to 7.3 Ra while olivines in the mantle xenoliths are 2.9 to 6.3 Ra. They provide no evidence of a lower mantle plume but overlap the range of the regional subcontinental lithospheric mantle (SCLM) and OIBs with low 3He/4He values. Olivine phenocrysts included in the basalts show variations in Mn, Ni, and Ca concentrations with forsterite (Fo) contents, similar to the trend for olivines crystallized from the pyroxenite-derived melts. Elemental ratios of Ca-Fe-Ni-Mg-Mn for the olivines also indicate the pyroxenite contribution. Considering our results, it is suggested that the main source of the Jeju basalts is the SCLM containing pyroxenite. In addition, the tomography image shows the low-velocity zone extending to the asthenosphere mantle beneath the central Jeju Island. This implies that the localized asthenospheric upwelling is caused by edge-driven convection, and the high-velocity zones exist around Jeju island indicating a thick cratonic lithosphere. Therefore, we propose that the interaction between the SCLM component containing pyroxenite and the rising asthenosphere is the main mechanism to generate enriched basaltic magmas for the Cenozoic intraplate volcanism in East Asia, including Jeju Island.