[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-09] He-Sr-Nd-Pb-O isotopes of the Cenozoic basalts around the Korean Peninsula

*Donghwan Kim¹、Changkun Park²、Mi Jung Lee²、Andrea Luca Rizzo³、Hyunwoo Lee¹ (1.Seoul National University、2.Korea Polar Research Institute、3.Istituto Nazionale di Geofisica e Vulcanologia)

キーワード：Korean Peninsula , Cenozoic basalt, Intraplate volcanism , Helium isotopes , Sr-Nd-Pb isotopes , Oxygen isotopes

In East Asia, volcanic rocks are widely distributed as evidence of the Cenozoic intraplate magmatism. Previous studies have proposed various models for magma genesis mainly using trace elements and isotopes (Sr, Nd, Pb, Hf, etc.). Here we report new analysis results of He, Sr, Nd, Pb, and O isotopes of Cenozoic basalts obtained from Ulleungdo (UD), Dokdo (DD), Jejudo (JJ), Baekdusan (BDS), Jingpohu (JPH), and Hantangang (HT) that erupted around the Korean Peninsula. First, the ³He/⁴He ratios range from 5.7 to 7.3 R_A (1R_A: the helium isotope ratio in the atmosphere). Most of the samples are within the range of the sub-continental lithospheric mantle (SCLM; 5-7 R_A) and are lower than the range of the mid-ocean ridge basalt (MORB; 7-9 R_A). Sr-Pb-Nd isotopes of UD, DD, BDS, JPH, and HT display a mixture of depleted MORB mantle (DMM), enriched mantle 1 (EM1), and enriched mantle 2 (EM2), while only the JJ basalts show a mixing relationship between DMM and EM2, as previously reported. The δ¹⁸O_olivine (vs. V-SMOW) values of the basalts range from 5.06 to 5.71 ‰, similar to the DMM range (δ¹⁸O_olivine: 5.2 to 5.8 ‰), except for JJ samples with a range from 4.7 to 5.1 ‰ below the DMM range. Overall, we show the findings as follows: (1) The ³He/⁴He ratios of Cenozoic basalts in the Korean Peninsula tend to decrease as they approach the enriched source components when compared to trace element ratios (e.g., Ba/Yb and La/Yb) and Sr-Nd isotopes. (2) The comparison of ³He/⁴He ratios and Pb isotopes reveals two distinct trends: the ³He/⁴He ratios of the EM1 basalts (UD, DD, BDS, JPH, and HT) and the EM2 basalts (JJ) decrease towards the composition between EM1 and EM2. Thus, the origin of Cenozoic magmatism around the Korean Peninsula is likely to be the result of input by ⁴He-rich SCLM or crustal material into DMM. (3) Samples with elevated Sr and Nd isotope ratios have higher δ¹⁸O_olivine values, implying an influx of recycled sediments with high δ¹⁸O values. (4) The JJ basalts with low δ¹⁸O_olivine values have high FC3MS (FeO^T/CaO-3*MgO/SiO₂) values (> 0.7), indicating a contribution from pyroxenite sources formed by the interaction between subducted oceanic crust and the mantle. Therefore, according to the He-Sr-Nd-Pb-O isotope compositions, we propose that the origin of the Cenozoic magmatism around the Korean Peninsula was formed by mixing of DMM, recycled sediments, and subducted lower oceanic crust through multiple mixing processes.