Helium and Argon Isotopes of the Tertiary Volcanic Rocks, Shandong Peninsula: Significance for Crust Mantle Circulation

摘要

He and Ar isotopes of the Tertiary volcanic rocks from Jiyang basin and nearby Shanwang and Qixia areas in Shandong peninsula have been analyzed. The results show that all samples are characterized by an enrichment of radiogenic 4He leading to extremely low 3He/ 4He ratios (0.017 8- 3.225)×10 -6 . They also have atmospheric like 38 Ar/ 36 Ar ratios and slightly higher 40 Ar/ 36 Ar ratios (395.4-1 312.7) than air value. The enrichment of radioactive 4He is attributed to the long term radiogenic accumulation of U and Th in the mantle. The low concentration of 3He is considered to be due to 3He loss during magma eruption. The loss is more obvious in the Neogene than in the Paleogene. Low 40 Ar/ 36 Ar ratios of all volcanic rocks may imply that subducted materials with atmospheric components have been preserved in the mantle, thus indicating that crust mantle circulation occurred.