Mantle-Derived Helium Distribution and Tectonic Implications in the Sichuan-Yunnan Block, China

The geochemical characteristics of mantle degassing observed on the surface of the earth can indicate the origin and migration path of mantle fluids. Compared with the plate boundary tectonic environment, the intraplate tectonic environment does not have a large number of active volcanoes and active faults, and the observation of mantle volatiles in hot spring gas is relatively limited. We selected the Sichuan-Yunnan block to discuss mantle degassing based on the carbon and noble gas isotopes of the spring gases and previous studies on the fault slip rate and geophysical research. A total of five hot spring gas samples (including two free gases and three dissolved gases) were collected from the Sichuan-Yunnan block. Chemical and isotopic compositions were analyzed in N₂-dominant hot spring gases. The ³He/⁴He ratio (0.068-0.541 R _a) indicates the occurrence of mantle-derived helium throughout the Sichuan-Yunnan block, which has been diluted by a crustal radiogenic ⁴He component. The occurrence of mantle-derived helium in the study areas ranges from 0.74 to 5.67%. The lower proportion of mantle-derived helium in YNWQ and HGWQ than that in other spring gases near the Jinghe-Qinghe fault may be caused by the smaller scale of fault around YNWQ and HGWQ than the Jinghe-Qinghe fault. The correlation between ⁴He, ²⁰Ne, and N₂ concentrations implies a common trapping mechanism for ⁴He, ²⁰Ne, and N₂ in hot spring gases. The ⁴⁰Ar/³⁶Ar ratios and N₂/Ar ratios indicate that N₂ and Ar are mostly meteoric, and YNWQ and HGWQ have more crustal-derived Ar contribution (40.56 and 51.49%, respectively). The δ¹³C(CO₂)_o values calculated by Rayleigh fractionation and CO₂ concentration suggest that CO₂ has inorganic and organic origins. The plot of R _c/R _a versus δ¹³C(CO₂) indicates that the spring gas CO₂ origin in the Sichuan-Yunnan block is mainly derived from mixing of limestone and organic sediments with minor mantle CO₂. The δ¹³C(CH₄) versus CH₄/³He values indicate that the origin of methane is thermogenic and microbial oxidation. The low mantle-derived helium distribution pattern is most likely controlled by the weak fault activity rate, the small fault scale, and not obvious magmatic activity in the Sichuan-Yunnan block.